hello everyone hello everyone welcome welcome welcome to my life welcome everyone hi everyone whoa it's been a while since I've done one of these it's good to be back really good to be back hello everyone so my name is Miss Martins I'm going to introduce myself a few times I'm not crazy I just I'm going to introduce myself as more and more people join um science and math students this one is particularly for you and in particular I am doing a lesson for grade 11 and 12 science students so if you have a friend that's in grade 11 or grade 12 and they need help with science and in particular electric circuits please share this live with them I would really appreciate that um greetings from Colombia hello welcome right so my name is Miss Martens I am a South African teacher and today I'll be doing a science lesson I'll be doing grade 11 and grade 12 electric circuits so I'm going to give everyone a minute or two or three to join but remember if you're in grade 8 Grade 9 grade 7 grade 10 whatever grade you're in even if you don't do science hello South African popo if you don't do science please can you share this live with your friends it's the first live of mine for the term and I have decided to do a topic that will help grade 11s and grade 12s and that is electric circuits grade 10 math of course I do grade 10 maths as well I saw you at school today ma'am hello again right so please share this live with your friends I really appreciate it I know a lot of people are um maybe still doing sports um maybe extramurals maybe they're just arriving home so maybe we won't have a lot of people yet today but I am ready to help whoever I can comment your grade please below I like meeting new students so let me know if this is your first time on one of my lessons and also let me know what grade you're in and what topic you would like help with so I do maths and I do physical sciences so let me know if you what grade you're in and what topic you would like help with or what topic you're currently doing in class these comments are very important to me because I take notes of what topics you're currently doing what topics you're struggling with and then I obviously do my lives according to what the majority of the people want to do hello Zozo in grade 10 not the first time well welcome back okay so remember your name and then how are you not exhausted off to work I am exhausted uh can you see my face it's all red because I'm tired but I know everyone's tired and I just want to help you guys um okay so I see someone's asking me about about private tutoring I just want to get it out the way um at the beginning now to start off with if you go to the link in my bio so you do need to follow me and then go to the link in my bio if you click on the link and you scroll down through everything that I've got there in my link tree you will see that I say online workshops for grades for maths and online workshops for science if you are interested in potentially taking part in Online private tutoring it is small group tutoring so it's not one-on-one but I do small group tutoring where you can actually speak to me and interact um you can go on that link you can fill out under sign up for my workshops and then I will contact you via email okay so where can I find this live after you are done if Tick Tock lets me save it I will upload it to my YouTube channel so my YouTube channel is also linked in my bio Okay so lastly I'm gonna get going because I see that we're already three minutes in my name is Mr Martins I do maths I do physical sciences um please share this video with your grade 11 and 12 science friends I'll be doing electricity today um as the live goes on I would really appreciate it if new people join or just share it or whatever I'm going to have electricity okay that's what we're doing today yay okay awesome let's get going so I will be flipping the camera hopefully oh you see this is the problem with this I don't know Tick Tock has done this thing where um I used to be able to flip the camera and now it doesn't let me flip the camera okay let me just double check to see if I can do this oh there we go I can flip the camera yay okay I'm gonna flip the camera and we're gonna get started I can still see the comments so please carry on commenting let me know what you want to see I'll probably do physics for grade 10 next I'll probably be focusing on things like balancing and a little bit of Stoichiometry all right let's get started remember to share the video with your friends okay ducks so flipping the camera now please comment down below if you are in grade 11 you take science and if you're in grade 12 and you take science please let me know do I have any grade 11 and grade 12 physical sciences Learners in the live at the moment please let you know let me know I see someone says when am I doing physic what time will I be doing physics for grade 10. I do my lives all at the same time so I do it at four o'clock it's not on a set date however I don't know I will let you guys know in advance so you just need to keep a look out on my Tick Tock accounts please let me know if there are any grade 11 and 12 science Learners okay I see summer tricks hello Matrix I see some grade 11's fantastic okay let's get going what I have in front of me or what you can see on your screen and I hope it's not too blurry please let me know um as I carry on going if you're in grade 11 or grade 12 just so I can see how many of you I have in here okay so as you see in front of me hi Claire I have the examination guidelines this is for grade 12. now some of the grade 11s that are in this lesson might think okay but why is ma'am showing us the exam guidelines for grade 12 if we're in grade 11. I do lessons for grade nines yes and the reason why is even though you are in grade 11 you need to access these things these are called exam guidelines and I'm sure you've heard about them you can find them just Google exam guidelines grade 12 physical sciences and they will pop up right now why they are important is because when they set your exams at the end of the year they use these exam guidelines and they're basically everything that they say here this is what you need to be able to do for your exams everything listed here all the definitions listed here so Define power is the rate at which work is done all of the stuff is stuff that they can ask in your final exam paper in grade 12 but your grade 11 teaches the those of you that are in grade 11 they also use this and they have this helps them set the exam papers Okay so if I look at the exam guidelines for grade 12 remember the supplies to grade 11s as well you can see that you need to know Ohm's law you need to be able to work with series and parallel circuits you need to be working with all these different things you need to be working with internal resistance now the grade 11s that are in the chats or in my in this lesson please comment if you have learned about internal resistance at the school where I work we teach grade 11 science they do internal resistance but I know at other schools they only do internal resistance in grade 12. so let me know if you have done it as grade 11s and grade 12 so if you've not done it yet then you need to know about power and energy and you need to know how to calculate the cost of electricity okay right so I'm going to start off with Ohm's law then I'm going to go on to power then we're going to first look at circuits without internal resistance and then to end off I'm going to look at circuits with internal resistance and the reason I'm doing it like that is so that the grade 11's in the chat they can still take part in most of the lesson and if they if if they want to stick around for internal resistance it's only something that some schools test and teach in grade 12 but you are welcome to stay around for it I'll go through it briefly at the end so these are the definitions that I've extracted from the exam guidelines that you need to know the first one being Ohm's law the second one being EMF and the third one being power I will come back to these definitions as I go through the topic these are my formulas and I made the summary for grade 11 and 12 students if you have not done internal resistance you will not know what this formula is over here but everything else you should be familiar with so this is a Formula I use to calculate potential difference across two points in a circuit it's the work done per coulomb charge to work in joules charge in coulombs then I've got the formula to calculate current which is the amount of charge that passes a certain point per unit time we can't see it all unless I zoom out a bit okay let me just there we go is that a bit better okay there we go so current is the amount of charge that passes a point per unit time that's why I use charge and time to work give me current then we've got Ohm's law now this is what I'm going to start going over Ohm's law is essentially the relationship or helps explain the relationship between voltage current and resistance and I'm going to show you this triangle we're going to go over that then you need to know how to calculate resistance in series versus parallel and then we are finally going to look at Power and energy we're going to use these formulas to help us calculate the cost of electricity which you should be doing in grade 11 but grade 12s it's something that you generally learn in grade 11 but it is tested in metric so look at the exam guidelines there we go you need to be able to calculate the cost of electricity this is from the grade 12 exam guidelines so we're going to go over that now these are the formulas that I showed you on my summary I'm not going to spend too much time going through this because this is something that you should have done in grade 10. if you don't know what's going on here I can upload a sheet for you let me know what your thoughts are on that I've got a sheet called electric circuits where I Summarize each formula I give the formula I tell you the units I tell you how the formula works and I give you some definitions would you like me to upload the sheets to the link in my bio the Google Drive and my bio if you would like it please comment let me know I'm more than happy to do that for you I've got some stuff on internal resistance over here I've got some stuff on Ohm's law so I can upload that for you guys but you need to know how to use this formula and when to use what formula let's go on to the units now grade 11s and twelves this is important and the reason why I say that this is so important okay I see a lot of you want the notes I will upload for you okay the reason why I'm taking time out of this lesson to tell you about units which is something that you say oh that's easy I just learn it people don't put their units in their science papers and you lose marks I don't know if you're aware but every answer that does not have a unit you lose a mark and if you put the incorrect unit it doesn't matter if you did the calculations correct and you got the correct number if you for example said that charge was not in coulombs let's say you said a or J or whatever you get it wrong you need to know the quantities and the units Associated and you always need to quote your units so from this table I just want you to also note that work and energy we use it interchangeably and the unit is dual power the unit is Watts those are new you learned it in grade 11 charge resistance current potential difference you should have learned that in grade 10 already okay so it's the small things that people don't know how to do now Ohm's law is something that again the grade 11s are currently learning that so if you're in grade 11 and you know what Ohm's law is or you've learned to please let me know in the comments I'd like to know what other schools are doing because obviously I always go according to my school but my school might not do what your school does so it'll help me to know what your school does so I know how long to focus on certain things so Ohm's laws if we think about the definition of Ohm's law and I do have a definition for you here it is Ohm's law okay the potential difference we should know what potential difference is it's another word for voltage the potential difference across a conductor is directly proportional to the current so current is I in the conductor at a constant temperature now something that I always emphasize to my students my grade 11s and my grade 12s is that when you're mentioning a relationship so here I'm mentioning the relationship between potential difference and current you need to tell me what type of relationship it is so it's directly proportional which means if the one goes up the other one goes up okay so if current increases potential difference increases that's what directly proportional means also when you are mentioning a relationship you need to tell me which variable is constant so here I'm saying that it's at a constant temperature another way to State Ohm's law and grade 12 you need to know this for your exam is by this relationship over here I don't know if your teachers have shown you this symbol but that symbol is another way of writing directly proportional so that symbol is another way for writing directly proportional so what this is is voltage is directly proportional to current at a constant temperature and the formula that can help illustrate this is Ohm's Law and we know you should know that you can rewrite this formula in a variety of ways so for those of you that have seen my Tick Tock videos you would have seen that I put up one where I show this triangle v i r this triangle helps me get the formula it's V is equal to I times R so voltage if I'm looking for voltage it equals to current times resistance if I'm looking for resistance cover resistance it's equal to voltage divided by current like that and if I'm looking for current cover current it's equal to voltage divided by resistance it's the same formula just rearrange basically but what I want you to understand is Ohm's law says that current which is this value is directly proportional to voltage so if this goes up this goes up but the temperature must be constant now if I plot that on a graph it should look like this voltage current so if voltage is on your y-axis and current is on your x-axis you get a graph that looks like this and we all do maths all science students should do maths if you do math you'll be able to tell me how to calculate the gradient of that graph so if we look at it I'm going to do it from scratch if I have this how do we calculate gradient everyone so this is voltage it's on my y-axis voltage this is current measured in amperes amperes it's on my x-axis how do I calculate radians so this is a straight line graph if I have a straight line graph I can calculate the gradients the gradients is equal to for those of you that do Mass hopefully your maths teacher taught you it's change in y over change in X that is how you should have learned gradients in maths and in science change in y over change in X now if you look at this graph what is on your y-axis change in y is voltage so change in voltage and on your x-axis I've got current now what do we know about voltage divided by current voltage divided by current what does that give me voltage divided by current gives me resistance there we go voltage divided by current gives me resistance someone asked is this supposed to be grade 12 this is grade 11 but it is taught again in grade 12. in grade 12 it gets more complicated but for now we're going to focus on the more Basics we're going to build up to more difficult Okay so the gradient will give you resistance now just take note that if I swap the axes so yeah it was voltage on the Y and current on the X if I swap them and put current on the Y and voltage on the X then this is going to swap then my gradient will be changing currents over change in voltage and that no longer gives me resistance what does that give me what does that give me changing current of a change in voltage that gives me one over resistance so um Tamsin are you going to go over internal resistance in this lesson I definitely will if I have time the idea was to help the grade 12s that have problems or struggle with the basics so the basics from grade 11 so my main goal for this lesson is to do grade 11 work which is also tested in grade 12 and at the beginning of the lesson I showed you the exam guidelines all of the grade 11 stuff is Tessa there's Ohm's law all of this stuff that I'm doing now you learned in grade 11 but it's tested in grade 12. so I wanted to do the foundations and then build up towards internal resistance if I have time okay I do chemistry yes I do donkey okay right so if voltage divided by current is equal to resistance voltage over cards is resistance then current over voltage is 1 over R so we can actually ask that in a variety of ways let me show you here's a graph illustrating Ohm's law you can see it's a line of best foot if we get a line of best fit through the origin It means that my conductor is omic omic omic conductor if it does not obey Ohm's law you see straight line omic this is not a straight line through the origin then we've got non-omic you need to know that for your metric exams and grade 11 exams right if we look at this grade 12 exam question for example what we can see here and I'm gonna have to zoom in a little bit so I'm going to cut off some of the notes that's fine I just need to zoom in this graph looks quite bad but it's just the printing they want to know the independent variable now I don't know if you guys can see but this is how I remember dependent versus Independence dependent is the one that we measure and it's generally the one we put we plot on the y-axis dependence and then the independent in dependence is the one that I change and I plot it on the x-axis so the dependence D for dependence is the one that we measure and the independent is the one that we change okay so the independent variable would be potential difference in volts and the dependent variable would be current okay now the variable that must be controlled would be the temperature okay temperature remember Ohm's law basically says that the the temperature must be constant the potential difference across the conductor is directly proportional to the currents in the conductor at a constant temperature we can see that this graph obeys Ohm's law it's a straight line going through the origin we con we can conclude this from the graph so that's what I did there current is directly proportional to voltage if temperature is constant then they want you to use the gradient of the graph to calculate resistance now what I did is I first calculated the gradient and when you calculate gradients you need to use two points when you calculate gradients you need to use two points or two coordinates I listed the coordinates that I chose from the graph over there the one being the origin zero zero that coordinate and the other one being this coordinate over here which is very difficult to read obviously from where you guys are sitting and from what you can see on the phone but I promise you that it's that okay then you work out gradients gradients is change in y over change in X my Y is my current my X is my potential difference and I get that value but now remember the gradient in this case is not equal to resistance the gradient in this case is not equal to resistance and that's because the gradient is change in y over change in X in this case my Y is current okay my X is voltage I divided by V that is not equal to R it's equal to 1 over r so when I get my gradient which is zero comma 3 6 I make that equal to 1 over R and then I solve for R which is resistance so basically the reciprocal now this is grade 11 work but once again it can be tested in metric does everybody understand and I know that there are some people in the lesson that feel like this is easy and I say this in every single one of my lives if you find what I'm doing boring because it's too easy I'm actually so happy for you and I genuinely mean it the reason I go slowly and the reason why I do maybe more of the foundation things is because there are some people that don't understand there's no way you're going to be able to do the difficult stuff if you can't do the foundation things okay right understand so I need you to please let me know if you understand this concept I I see someone is saying listen kids this teacher's good at pointing out mistakes I hope that's not sarcasm I do Mark matric papers so I know where Learners go wrong I know where you go wrong because I have experience marking so I see like silly mistakes you know what I mean okay moving on now this is going to seem extremely easy for the people in the lesson but I need you guys to answer this okay I need you to answer this someone said I'm in grade nine why is this on my for you page I teach grade nine as well okay that's why it's on you for you page and I don't only do maths and science I do study tips and stuff as well okay so this is going to be super easy so for the people that know this I'm sorry that I'm being boring let's go over this in series something splits and something stays the same this is how we go over it in series what can I say about the currents what can I say about the current in series the current stays the same I do grade 10 as well yes current stays the same this is very important and some people are going to laugh at why this is so easy if you don't know this you can say goodbye to electricity and you can skip the question and I don't want you to do that so what that means is in series the total current flows through this Branch over here that means that the reading on A1 is equal to that reading on A2 which is the same as the reading on A3 which is the same as the reading on A4 which is the same as the reading on A5 the current is the same everywhere everywhere okay the voltage what about the voltage in series it splits so what that means is the voltage across the battery V1 how do I get V1 the voltage across the battery I take this voltage that voltage and that voltage and I add those together to give me the voltage across the battery so V2 plus V3 plus V4 those three added together gives me the voltage across V1 exactly at all voltages one thousand percent let's go into series I mean parallel by the way for the people that are currently in the lesson I just want to say it again in case you didn't hear it in the beginning if you are interested in smaller group private tutoring that I do not on Tick Tock I do on Google meet please go to the link in my bio and find the link where I say workshops you can sign up and I'll send you an email about it okay let's do maths and science um and the grades that I do that I'm currently doing they listed in my um they are listed in my buyer poor math teaching no wonder if you are interested okay well if you're not interested you can leave because I don't know why you would take time out of your day when I'm actually helping people okay moving on Parallel so what can I say about the current in parallel the current in parallel splits so that means A1 measures the total current and that current will split between A2 A3 and A4 okay so A1 the total current is equal to the current in this Branch plus the current in this branch plus the current in this Branch the total current is equal to the current in all three branches add together and you need to understand that when the total current so the total current will flow through here when it gets to this point over here there's a split okay it can go either through R2 or through R3 or through R4 okay I see some people are telling me okay top load the notes I can do that moving on the voltage what about the voltage in parallel the voltage is the same however be careful there's another resistor over here so here's the voltage across the battery let's pretend that it reads 10 volts the voltage across the battery there's a resistor over here connected in series Let's Pretend the voltage across that resistor let's call it V2 let's say that reads five no let's choose another number let's say six volts so across the battery is 10 volts that over here this resistor Let's Pretend the voltage across that resistor in series reads 6 volts what would the voltage across the parallel combination read 10 minus 6 is 4 volts what that means now this is very important is that the voltage for resistor 2 is 4 Volts for resistor 3 is 4 Volts for resistor 4 is 4 volts because resistor 2 3 and 4 are connected in parallel the resistance across those resistors are 4 volts okay thank you so much for the encouragement some of you very very sweet I appreciate it so much so resistor two three and four because those three are connected in parallel the voltage across those is the same but because this one is in series this one resistor this component in the circuit is six volts plus these four volts gives me the 10 volts right let's move on to easy example when I say easy I mean that the there's no internal resistance no internal resistance so the battery's resistance is negligible right it's a pleasure guys I really love helping you guys and I really hope I am helping not wasting my time or whatever okay calculate the effective resistance of the circuit so how we do that is we always always look to see yes I am left-handed always look to see if I have a parallel combination first and yes I do there is a parallel combination then what I'm going to do is I'm first going to work out the effective resistance for the parallel combination once I have that whatever that value is for resistance I'm going to add it to the 2.6 okay so let's go 1 over RP is equal to 1 over R1 plus 1 over R2 this if you are not aware that is the formula this is for grade 11 and grade 12. that is the formula used to calculate the overall or the effective resistance of resistors in parallel so these two so we're going to go one over four plus one over six and now remember this is 1 over RP so 1 over 4 plus 1 over 6 is 5 over 12. but that is for one over RP I don't want one over RP I want to know what resistance in parallel is RP so I need to take the reciprocal so what I do is I flip that fraction around then I flip that fraction around yes this is grade 10 stuff actually and I get 12 over 5 over 5 which is 2.4 ohms then what I need to do is to get the total effective resistance of the circuit R total I'm going to take what I got for the parallel so 2.4 and I'm going to add 2.6 okay 2.4 add 2.6 and I get 5 ohms that's the effective resistance of the circuit why do you add the series resistance okay good good question so the question asked me to calculate the effective another word for effective is total or overall resistance of the circuits so because I want of the entire circuit I use I calculate the resistance for the parallel piece which is 2.4 and then I add the resistance series okay you have to where did the five come from so what I did is you can do this without a calculator but I just use my calculator one over four plus one over six five over twelve okay five over twelve but that's one over RP so because I flip this to get RP I must flip this okay next question calculate the total current in the circuit now we just calculated that R total was 5 ohms and what is the total voltage it is 20 volts someone also the series is both this circuit is both series and parallel yes 100 this circuit contains a parallel combination or parallel part and a series part well done okay so my total resistance was 5 ohms which I got from the previous question and my voltage is 20 volts so V equals 20 volts they want me to calculate the total current in the circuit now my question to you guys is which of these are meters and meters measure current which of these ammeters will measure the total current that should be relatively straightforward it should be this ammeter because it's connected in series over here it's not in a branch is what I mean okay A1 measures the total current so we've got I is equal to V over R 20 over five so the total current is four amperes four amperes now calculate the reading on A2 A2 now let's look at where A2 is why does A1 calculate total okay so A1 calculates the total current because it's connected in series like this the only time it doesn't calculate the total current is if it is in a branch so you see it splits off here and A2 is in the top branch that means that this calculates the split current this one A1 is in series so it calculates the total current okay so calculate the reading on A2 now I don't know how you guys do it there's lots of different ways to do it but the one way that we can do calculate the the current in A2 is to use ratios to split the current so let's call this resistor 1 and resistor 2. okay resistor one so R1 to R2 what is the ratio of resistance over there it's four to six four to six and if I simplify four to six what am I going to get I'm going to get two to three okay that's the ratio of the resistance that I'm going to put an R there now if you look at these two resistors 4 Ohms and 6 ohms which resistor would get more current so if you look at these two resistors which one the four ohm or the 6 ohm which one will get more current the 4 ohm or the 6 ohm so the rule is the smaller the resistor the bigger the current okay so the smaller resistor gets the bigger current and that is why this is the ratio of resistance the ratio of current is the opposite okay this is how we do it so the smaller resistor which is the 4 ohm gets the bigger current you see it's got a smaller resistance the two to three two is smaller so it must get the bigger current so it must be three to two now to work out what current goes through the four ohms what I do is I use ratios so the four ohms is this side of my ratio so the three how do I split using ratios so my total current is two no sorry it's four amperes so to split using ratios I add the parts of the ratio so three plus two is five so you can either go like this you can either go well I'm looking for the current through the resistor one which is the four ohms so I go three over five times four that's one way to do it three over five times four so the top resistor will get 2.4 amperes of current another way to do it is to take the total current four divided by the parts of your ratio so there's my part of my ratio three and two so five is my total parts so 4 divided by 5 and I'm looking for the current in R1 so I need to times it by three you get the same answer yes you can use the current divider formula yes there are other ways to do this okay if you want more examples like this I can upload a video on my YouTube channel but this is something that you should have done in grade 10 actually okay let's move on now I want to vote in the comments and I'll go with the most voted option would you guys like me to do power and calculating the cost of electricity so power and electricity or internal resistance so I'm going to give you guys like 30 seconds to vote because I have notes for both but I want to know which one to start with because um yeah they can take long so power and cost of electricity or internal resistance now remember grade 12s internal resistance um okay oh I'm getting a lot of comments okay I'm gonna have to scroll up to read this remember grade 12s you will be tested on both you will be tested on Power and cost of electricity and internal resistance Matrix I mean grade 11s will not be tested on internal resistance okay oh I'm not sure which one's winning yeah I'm seeing quite a lot of both of them okay I'm going to start off with power because I think I see more power but I'm going to quickly move on to internal resistance okay power power please internal resistance power okay let's start off with power let's not waste time so these are the formula used to calculate power so power is I will get to internal resistance I promise power is the rate at which work is done and when I say rate so rate at which work is done every time you see the word rate you know I'm going to be dividing my time another symbol for energy is work so if I take work divided by time or energy divided by time I'm going to get power oh remember to share this live with your friends please your science friends so maybe they can catch the last bit of the life to help them with power and internal resistance okay so it's the rate at which work is done these are the other formulas used to calculate power power is the voltage associated with the component or whatever multiplied by the current flowing through it and from these two formula we can get these other formulas now here's another summary that I can upload for you guys but essentially it goes through what power is it goes through how to measure power and it goes through the equations so I will put that up the thing that I want to show you is power is energy divided by time so energy would be equal to power multiplied by time thank you so much to everyone who's sharing the live I appreciate it um the goal is to help as many people as we can okay so power is energy divided by time so energy if I want to isolate energy you say power times time now when it comes to calculating the cost of electricity which is what we care about mostly which is what we're working towards we're going to calculate the amount of energy transferred to a device in kilowatt hours now why people get thrown off with this is because kilowatt hours people think that that is or actually let's see what you you guys say kilowatt hours what is that a unit for what is that a unit for it's a unit four tell me is it a unit for what is it a unit for okay I'm tell me tell me tell me what you think kilowatt hour is it a unit for energy is it a unit for time is it a unit for power oh okay I'm seeing lots of different answers so most of you are saying it's a unit for power and you see this I completely understand why you would think kilowatt hours is a unit for power because it's got what's in it and what's is power but look at this power we generally measure in Watts correct time we generally measure in seconds but what if I tell you that I want you to measure power in kilowatts kilowatts and I want you so I'm going to put kilowatts and brackets out of my power and I want you to measure time in hours if I take my power which is my kilowatts and I multiply it by time which is my hours I get kilowatt hours and power multiplied by time is energy so kilowatt hours is actually a unit for energy not for power Okay so when we calculate the amount of the cost of electricity we need you should stop jumping from topic to topic um last time I checked I was doing energy equals Power Times time and I've been doing that for a few minutes so energy is power multiplied by time as I was saying now when we calculate the cost of electricity so let's say the cost that a Giza uses the cost the kettle uses the cost that a microwave uses we quote that in terms of kilowatt hours so that's how electricity is sold Let's do an example and this should make sense but basically my steps for this what I want you to do is when we calculate costs I want you to calculate the amount of energy energy is Power Times time power must be in the units kilowatts time must be in the units hours now just quickly how do I go from Watts 2 kilowatts how do I go from watts to kilowatts watts to kilowatts I divide by a thousand what's te kilowatts I divide by a thousand okay that's very important so I convert my power to kilowatts I convert my time to hours I get kilowatt hours I take whatever that is and I multiply by the cost an example calculate the cost of electricity to use a geyser you know what a geysers at heat water for 12 hours note the power of the geyser is 2500 watts now step number one I need to calculate the energy and I need my units to be kilowatt hours okay so energy is power multiplied by time now my power I said 2500 watts I need my power to be in kilowatts so how do I convert that watts to kilowatts it will be divided by a thousand so 2.5 this is easy I gave you time and hours already so 2.5 times 12 2.5 times 12 is 30. kilowatts hours okay now look what I said at the top electricity is sold in kilowatt hours and this was the price I don't know like a yo whatever ago um it differs so it's two Rand per kilowatt hour therefore that means at one kilowatt hour is two Rand therefore to get the total cost I take the number of kilowatt hours so kilowatt hours multiplied by the cost what I would say by cost is tariff it's this number over here per kilowatt hour so it's 30 times 2 which is 60 Rands so to run the Giza for 12 hours it costs 60 Rand well done guys should we try one more quickly 60 Rand yeah so it's two Rand per kilowatt hour but I've got 30 kilowatt hours so kilowatt hours is basically the unit of energy it's the amount of kilowatts used per hour right let's do one more example calculate the cost of using a kettle if you take five minutes to boil a 2 000 watt Kettle the cost of electricity is two Rand per kilowatt hour South from South Korea that's awesome so remember this cost can change they can give it to you it differs so what do I need to do first I need to work out power so power is equal to energy times time and remember because my tariff or my cost is kilowatt hour I need my power to be in kilowatt hours so my I'm doing this wrong not wrong I just messed up my formula energy is Power Times time I need my power to be in kilowatts and I need my time to be in hours okay so my power elements is yeah I did make a mistake sorry I um swap my formula around my power is 2 000 Watts so it's going to be 2. okay kilowatts okay someone said I must include the units that can be helpful and my time must be in hours now five minutes how do I convert minutes to hours how do I convert minutes to hours oh I see someone's from Greece hello so to convert minutes to hours I need to divide by 60. so to convert five minutes two hours you divide by 60. so 5 over 60 is 1 over 12 hours okay 1 over 12. so you times two kilowatts by your hours which is 5 over 60 or 1 over 12 hours then 2 times 1 over 12. is one over six kilowatt hours see kilowatts times hours gives you kilowatt hours you can write this in decimals as well but don't round off because we're not at the end of the question so the total cost is the kilowatt hours multiplied by your tariff we call it a tariff it's basically how much money you pay per kilowatt hour so if my kilowatt hours is one over six and my tariff is two Rand what is one over six times two Rand I'm gonna get zero comma three three Rand and obviously if you want to you can convert that to cents so it's 33 cents there's a hundred cents in a Rand okay I hope that makes sense so just to recap what you do is you get energy energy is Power Times time your power has to be in kilowatts your time must be in hours that gives me energy in the unit of kilowatt hours the reason I want the unit in kilowatt hours is because that's what my cost or my tariff is quoted in you take your kilowatt hours that you calculated you times it by the price per kilowatt hour and you get your total cost okay right now to end off I'm going to go over see guys I have a whole lot of questions that I want to do so if you would like to be part of lessons where I go over this not on Tick Tock but other lessons where you guys can unmute your mic ask me questions whatever whatever go to the link in my bio and sign up but I'm going to do a quick quick introduction into um internal resistance okay internal resistance now what I need you guys to understand is that if I connect a voltmeter across the battery and the switch is open that voltmeter reads the EMF so I'm going to write it in over there so grade 11's you don't have to stay for this but I mean you're going to be doing it next year if you haven't done it already that reads the EMF when the switch is open current is zero so I is equal to zero and the voltmeter connected across the battery reads the EMF right then if I close the switch that voltmeter no longer reads the EMF so the EMF is the maximum energy provided by the battery per unit charge see there's my definition for EMF the maximum energy provided by the battery per unit charge so when the switch is open it measures EMF when a switch is closed it reads what we call V external other words for the external V load or V terminal whatever you want to call it now I'm going to do I'm going to use actual examples to illustrate this if my EMF let's pretend that the EMF was 12 volts this is when the switches open then the switch is closed and V external is 10 volts so this is when the switch is closed so switches open no current the voltmeter across the battery reads 12 volts switches closed V external is 10 volts what happened to the 2 volts what happened to the two vaults what do I call that does anyone know in the chat what do I call those two volts over there so EMF is 12 V external is 10. obviously what happened is there were some vaults that were lost and we call this the official name for it is V internal but a lot of schools refer to it as V lost or lost vaults and the reason why we call it lost faults is because the battery has an internal resistance called Baby R this is called internal resistance and because of the internal resistance some of the energy is used inside the battery okay because of the internal resistance some of the energy is used inside the battery to get the charges to move through the battery so the battery heats up and that energy is lost or dissipated due to heat inside the battery okay now one way that we can represent this is we can say Okay cool so what happened was I had 12 and that was made up of the 10 and the two so I hope you were following the 12 was the EMF the tin was V external the 2 was V internal okay the 12 was the EMF the 10 was V external the 2 was V internal so in other words V external and V internal together give me the EMF this is one of the basic formula I can use to calculate EMF however I know from Ohm's law that V is equal to I times r so instead of V external I can write I which is current times R external so that's the external resistance of the circuit then instead of V internal remember V is equal to I times R so what do you think V internal is equal to V is equal to I times R so for V external it was I times Big R grade 11 you are more than welcome to leave this will be in your exam thank you Josh for the finger heart this will be in your exam next year so I mean you can learn it with me now if you want V internal is equal to I times little r now are yes I do say it like that now what is the common factor in this expression over here I this is total current that is total current the total current flows through the circuit the total current flows through the battery so I can take that I as a common factor and I'm left with r plus baby r that's equal to EMF okay and that that is the formula that we now know for EMF okay I R plus little r now when you do calculations your calculation so there I did it again I hope that helped some people there is another explanation that's when the switch is open that's when the switch is closed EMF minus the external gets you V internal all of that stuff then I broke up each one I took I out as a common factor all of that stuff and then we can answer questions now this is a very basic circuit that we can do very basic what it says is I have a battery with the EMF 20 and an internal resistance of one ohm and it's connected to three other resistors there 510 those are in parallel I hope you can see that and then the 8 ohm now they want to know the current in the 8 Ohm resistor there how I would approach this question is if you see a circuit and you have EMF and you have internal resistance by the way this is how we draw a battery more or less most of the time if we have internal resistance so we draw the cells and we draw a little baby resistor to represent Baby R internal resistance and we draw a dotted line around it so this entire thing is basically the battery now how I do how I would do this is if you see EMF and internal resistance I would write out my EMA formula so EMF is equal to I open bracket Big R plus baby r now what I always do is I look at what I have look at what I'm looking for I have EMF it's given to me it's 20. I have baby r it's given to me it's 1 ohm I'm looking for currents now note what they say they want the current and the 8 Ohm resistor in other words the total current how do I know that because the total current will flow through the 8 Ohm resistor so they want Total current so this is what they're looking for in order to use this formula I first need to calculate Big R now how I would calculate Big R is like we did in the previous example that I did this lesson so what you would do is you would go okay cool I need to calculate Big R which is R external in other words this part plus this part so we start with the parallel combination 1 over RP is equal to 1 over R1 plus 1 over R2 now grade 12s you should know that you need to put your Formula First in order to get the marks no formula no mark one over five one over five is my one resistor one over ten I get three over ten so 1 over RP is three over ten but remember I don't want one over RP I want RP so I flipped that over to get RP I need to flip this over so it's ten over three ohms and just if you prefer decimals which is fine it's three comma three three three dot dot ohms remember don't round off yet that is the resistance over there then I need to add this resistor over here so therefore the total R external Big R is 10 over 3 Plus 8. so that's going to be 11.333 dot dot dot ohms another way to write that could be 34 over 3. now the reason why I'm leaving it as a fraction now is because I'm not done with the question what I did in blue over here is I worked out Big R are external so just remember Big R is the external resistance of the circuit so the parallel plus that now I still need to find my current so I go and I substitute into my EMF formula so one more time EMF is equal to I Big R plus little r big I is what I'm looking for EMF is 20. Big R is 34 over 3. yes you can substitute 11 comma 33333 in it but I just left it as you know the fraction and little r is one okay can everyone still hear me then what I do is I say 20 20 divided by that term okay so 20 divided by that term and I get my current to be 1 comma 6 2 amperes okay did everyone follow that example I know that was so tough quick and I know that it was also a sort of easy example but remember what I said at the beginning of the lesson I really wanted to do the lesson for grade 11's and grade 12s I really wanted to go over majority of what was in the exam guidelines and a lot of it was stuff that you learned in grade 11 so Ohm's law we did power energy calculating the cost of electricity and then we started introducing EMF the EMF formula and I did a basic calculation I do want to do more with you guys but unfortunately let me just swap my I just want to talk to you guys for a second unfortunately my time has run out and Tick Tock does this thing that like if I go over the hour um for my life it just stops the live which sucks but anyway um basically what I wanted to know is if you enjoyed this live and you are Keen to maybe join lessons of mine do you work at kids beach no I work at a school a high school um if you want to join lessons that are similar to this but not on Tick Tock they will be on Google meet and it'll be smaller groups so you can unmute your mind you can talk to me you can say when you don't understand like it's more you know one-on-one-ish sort of not one-on-one but you know what I mean then go to the link in my bio and go to the little tab where I say workshops you can sign up on there do you even post on YouTube so I don't I do but I don't um I post my lives on YouTube and every now and then I do want to try and upload a video it is very difficult because I do work as a full-time teacher and um the lessons that I record for my students in class we're not actually supposed to upload those onto YouTube it's actually not um legal because I'm recording it for the students in my class they pay me as a teacher for that school so if I upload it to YouTube then I'm technically I could make money off of that of YouTube it gets complicated guys don't stress about it but basically I don't really want to upload videos of stuff that I recorded for my with my students in class onto YouTube um also because those videos aren't very good quality anyway so I do these Tick Tock lives they go on my YouTube channel and I am trying to find more time to do YouTube um but yeah there's only so many hours in a day I'm still trying to figure out my time management and stuff um do you have a patron no I don't um yeah that's it this is just me yeah so uh please keep an eye out on my Tick Tock page if you follow me you should see when I do lives I do other grades and I'll probably do grade 10 science next maybe some grade some maths not sure but anyway thank you so much when is when you're doing organic chemistry that's a good question I don't know if I'll do it live on that on Tick Tock but I might do one on my Google meets thing such a pleasure everyone hi my mom emailed about me emailed you about my brother's mask okay I will go check my inbox some of my emails have been going to junk for some reason okay I wanted to be one of your students but your English is too much and you're fast oh I'm so sorry okay awesome oh thank you guys you all mean so much to me um keep watching my Tick Tock videos I will try my best to post study tips and stuff um you teach me more physics than my own teacher oh I really love that I can be um a teacher for you guys on this platform so remember although you're not sitting in my physical classroom you are in my classroom you're always welcome to ask me questions okay when you're doing physics for grade 10 I'll probably do it next week okay guys goodbye thank you for joining me remember to follow me and comment on my tech talks what you want to see next if you comment I might make a video answering you so Victoria what grade is your brother in um just so you know um yeah if you comment over tick tocks um tell me what you're struggling with tell me what you want to see notes for what you want to see lessons for or just ask a question in general I might just onto you okay awesome goodbye everyone okay great eight okay cool bye guys love you all oh my goodness my thing is going weird okay are you going live tomorrow again no I'm not going live next week next week see you guys next week bye guys