hello everybody my name is Iman welcome back to my YouTube channel today we're going to do some MCAT practice in preparation for the General Physics section and specifically we're going to do some thermodynamic problems so let's get started this problem says if an object with an initial temperature of 300 Kelvin increases its temperature by one degree Celsius every minute by how many degrees Fahrenheit will its temperature have increased in 10 minutes the important thing to know is a few conversions between Kelvin Celsius and Fahrenheit here but more importantly it would be important to know that the Kelvin unit and the Celsius degree are the same size so what that means is a change of 10 Kelvin is going to be equal to a change of 10 degrees Celsius and so we really just want to focus on how many degrees Celsius were changing in the 10 minutes and not worry about our initial temperature in Kelvin and converting that properly all right there's no worry here what we want to focus on on is well we're increasing by one degree Celsius every minute and we're trying to be focused on a 10 minute change what that means for us is just focusing on how many degrees Celsius will change um in 10 minutes one degree Celsius every minute for 10 minutes means we're going to have a change of 10 degrees Celsius in those 10 minutes now all we need to worry is what that is in Fahrenheit what is a 10 degrees Celsius change in 10 in Fahrenheit well two things that two ways we can do this is we can know that the factor from Celsius to Fahrenheit in terms of this change is just by 1.8 degrees or conversely you might know this as a factor of nine over five regardless it's the same right or just one is in a fraction one is just an integer all right we can simply multiply ten by 1.8 and what we'll get is 18 degrees Fahrenheit how many degrees Fahrenheit will the temperature change in 10 minutes 18 degrees Fahrenheit that means the correct answer here is C fantastic let's do two now two says which of the following choices correctly identifies the following three heat transfer processes one is heat transferred from the Sun to the Earth two is a metal spoon heating up when placed in a pot of hot soup and three is a rising Plum of smoke from a fire what we want to do is look at each one of these examples and be able to properly connect them to their appropriate Concepts the three words the three concepts we're dealing with here are radiation conduction and convection we need to figure out whether one is radiation conduction or convection whether two is radiation conduction or convection whether three is radiation conduction or convection and that way we can appropriately figure out which one goes to which and then figure out the correct answer now because there is essentially only empty space between the Sun and Earth what that means is the only means of heat transfer is going to be by radiation by electromagnetic waves that are going to propagate against space and so one the heat transfer from the Sun to the Earth is an example of radiation fantastic now let's look at two a metal spoon heating up when placed in a pot of hot soup what a metal spoon is placed in a hot in a pot of hot soup the molecules in that soup are gonna collide with those on the surface of this spoon what that means is that transfer of heat is going to happen by can conduction all right so two is going to be conduction fantastic now finally the fire example a rising Plum spoke from a fire fire warms the air above it and the warmed air is going to be less dense than the surrounding air warm air rises all right so this less dense warmed air is going to rise a rising column of warm air means that heat is being transported into the air mass which is the definition which is an example of a process of convection all right so three is going to be convection that means the proper answer Choice here is going to be a one is an example of radiation two an example of conduction and three an example of convection fantastic three says a 20 meter Steel Rod at 10 degrees Celsius is dangling from the edge of a building and is 2.5 centimeters from the ground if the rod is heated to 90 degrees Celsius will the will the will the rod touch the ground fantastic so first we're going to have to find the change in length due to thermal expansion that means we're going to have to make use of a change in length equation all right for thermal expansion this is going to be multiplied by a which is just um a factor associated with the material you deal with all right L is going to be the length and um the delta T here is going to be our change in temperature we are given all this information Alpha is going to be 1.1 times 10 to the minus 5 inverse Kelvin all right our length here is going to be 20 meters right that is the length of our steel rod and the change in in temperature is going to be um uh 10 from 90 to 10 degrees Celsius 90 miles final 10 is initial subtract that so 80. all right that's what our change in temperature is 80. all right plug this into a calculator we're going to get one uh 0.0176 meters we can also write this in centimeters 1.76 centimeters because the rod is originally 2.5 centimeters from the ground above the ground and its length only increases by 1.76 centimeters we're going to be able to conclude that it's not going to be able to touch the ground after thermal expansion process is completed it does not expand enough for it to be able to touch the ground and so the answer here is d fantastic let's do four four says what is the final temperature of a five kilogram silver pendant that is left in front of an electric heater absorbing heat energy at a rate of 100 watt for 10 minutes assume that the pendant is initially at 20 degrees Celsius and the specific heat of silver is 233. all right fantastic to answer this question we're gonna first have to remember that Watts are equal to joules per second in other words in other words power is energy transfer over time all right we know this we've seen this in our previous chapters and in our previous MCAT problems in 10 minutes um not over okay yeah in 10 minutes sorry I thought it said 20 for a second in 10 minutes the pendant is going to absorb some amount of energy how are we going to figure that out well we're going to use our power formula and we're simply going to move time over we're going to multiply each side by time all right and what we're going to get is a definition of energy then it's just Power Times time perfect we can figure this out we know what the power is it is 100 watts and we know what the time is it's 10 minutes now of course we want this to be in seconds so that we can get our answer in joules or in Watts uh in joules I'm sorry so that things cancel out and we can get our answer in joules we need this to be in seconds so that Watts the unit of Watts which is joules over second cancels out with the seconds and we get our answer simply in joules so we're going to convert 10 minutes to seconds we know that one minute is 60 seconds so we're going to do some dimensional analysis here plug this into a a calculator for energy we're going to get 6 times 10. to the fourth power joules now what we can do is find the final temperature all right we can find the final temperature of this for this problem what we're going to use here is we're going to use our heat equation our MCAT equation all right and we're going to find the final temperature from here what we're going to do is we're going to plug in our heat we know that that's going to be what our energy what we calculated for energy is 6 times 10 to the 4th Joule is going to equal our Mass which is 5 kilograms all right times our specific heat 233 joules per kilogram per Kelvin times change in temperature all right now what we're going to get as an answer here if we plug this into it into a calculator to solve for temperature all right we're going to divide by 5 kilograms on both sides and 233 so that we can move this to the other side to solve for change in temperature change in temperature then is just 6 times 10 to the 4 Joule over 5 kilogram times 233 joules per kilogram per Kelvin and what we get for change in temperature is going to be 52 degrees Celsius what that means is we start off with an initial temperature of 20 degrees Celsius it's going to change by 52 degrees Celsius and our final temperature is going to be an addition of those two that means our final temperature is going to be 72 degrees Celsius that is answer Choice C fantastic let's do five how much heat is required to melt 500 gram gold earrings given that their initial temperature is 25 degrees Celsius we know that the melting point of gold is 1064 degrees Celsius its heat of fusion is 6.37 times 10 to the fourth joules per kilogram and its specific heat is 126 joules per kilogram per Kelvin first things first we need to figure out all right we need to figure out um how much heat is required to raise the temperature of the gold earrings to the melting point of gold all right then we can calculate the heat required to actually melt the gold earrings this is known as the latent heat so the total heat required to melt the gold earrings completely is going to be the sum of two heats the heat required to raise the temperature of the earrings from this 25 degrees Celsius to its melting point and then the second step to melt the the um the earrings completely let's do this first step all right first step is to figure out the heat required all right from 25 degrees Celsius to the melting point of 1064 degrees Celsius we're going to use our heat equation our MCAT equation all right we have all this information we know we're dealing with 0.05 0.5 kilograms all right we're giving 500 Kilograms that's just equal to 0.5 kilograms we want to use that so the units cancel out for the specific heat we were given which was in kilograms 126 joules per kilogram per Kelvin the kilograms will cancel out which is good all right we want our final answer for heat in just joules all right the change in temperature here we're going from um initial 25 degrees to 10 64 degrees Celsius we're just simply going to subtract 1064 from 25 remember you don't have to convert from Celsius to Kelvin to be able to do the subtraction for the MCAT it handles itself all right so if you subtract that what you're going to get here for temperature is let's see I didn't do this beforehand so I'm going to have to do this on the Fly it's going to be um 25 35 39 so 10 39. all right to Greece Kelvin all right perfect it doesn't matter if you use Kelvin or Celsius here in our MCAT equation they are interchangeable all right perfect and so we get our final answer um we get our final answer in joules and it's going to be 600 um sixty One Two Three Sixty thousand joules which is also 60 kilojoules that is the heat required for the first part all right the first part the heat required to raise the temperature of the earrings to the melting point now we want to calculate the next step the next step is to calculate how much heat is you needed to melt the earrings completely we only figured out how much it's needed to get to the melting temperature in order to figure this out we're just going to use the heat of fusion all right we don't need to use our MCAT we just need to use our heat of fusion because this is the latent heat to melt the gold completely multiplied by the amount of gold we have so here it's going to be mass times our latent heat that's simply going to be 0.5 kilograms times what we were given here for um the heat effusion all right 6.37 times 10 to the 4 joules per kilograms kilograms cancel out we're going to get a our joules here all right this is going to be 32 000 joules which is also 32 kilojoules now for the total heat all right we're going to have to figure out our total heat we just need to add 60 kilojoules plus 32 kilojoules to get 82 kilojoules total that is answer choice I'm uh um 92 that is 92 that is answer Choice d 92 approximately that's the closest answer Choice fantastic awesome let's do six six says that the total work uh given the cycle shown what is the total work done by the gas during the cycle now the total work done by the cycle is going to be the sum of the work of paths a b and c or the area within that cycle now because the area bounded by a b and c is a triangle with a base of five meters cubed all right and the height is our pressure it's three pascals we can calculate the area really easily one half we know the we know how to calculate the area for a triangle one half all right this volume this this um base which is five meters cubed times our pressure three Pascal what we get is a seven is get 7.5 Joule all right now usually clockwise Loops tend to be positive work on the environment while counterclause clockwise Loops do negative work all right this looks like a clockwise circulation and so this is gonna be positive work and the answer is going to be C fantastic let's do seven in an adiabatic compression process the internal energy of the gas blink to answer this question we have to make sure that we understand what adiabatic means we Define this in our lecture video adiabatic process means there's no exchange of heat all right there's no heat transfer in other words q is going to equal zero and so when we write our definition of internal energy change in internal energy is equal to Q Plus work that Q is just going to be zero so it's just going to be work energy here our internal energy definition is only going to be based off of work when there is an adiabatic process why because there is no exchange of heat now when a gas is compressed all right positive work is being done on the gas all right so the surroundings is doing work on the system that means that work is going to have a positive value all right based on this I'm sorry the gas is compressed so positive work is being done on the gas that's correct so the value of work done by the gas itself is going to be negative work based on this we can determine how the internal energy of the gas changes simply by using this first law of thermodynamics all right work is going to be negative and then it essentially changes and change in energy is going to be positive so the correct answer is B fantastic let's do eight eight says the entropy of a system can blink the entropy of a system can decrease as long as the entropy of its surroundings increased by at least as much on the other hand the energy of a isolated system increases for all real processes this adheres to our second law of Thermodynamics which says that the energy will be dispersed and energy of the universe will remain constant or increase during all processes with that in mind the answer choice that makes sense is B decrease when the entropy of the system increases by at least as much now nine says a student making a coffee cup calorimeter fails to use a second coffee cup and inadequate inadequately inadequately seals the lid what was her initial goal and what was the result of this mistake we talked about calorimeters and their purpose they are the best approximations of an isolated system where neither energy or matter is exchanged with the environment so when she fails to ins to to add that insulating layer all right and fails to fully contain the system then guess what heat can be exchanged with the environment you no longer have a isolated system heat can be exchanged with the environment matter can be dispersed and what you actually have now since you never properly sealed your system is an open system where heat and matter can be exchanged all right and so the correct answer Choice here is answer Choice a all right our last and final problem says a certain substance has a specific key of one Jewel per mole Kelvin and a melting point of 350 Kelvin if one mole of the substance is currently at a temperature of 349 Kelvin how much energy must be added in order to melt it to find this amount of heat needed to bring the substance to it this melting point you can just use the specific heat to heat one mole of the substance one unit Kelvin is just going to take one joule of heat and after that substance reaches its melting point additional heat is gonna need to is needed to actually induce the phase change so the problem is only asking us how much energy must be added in order to melt it well it's only going to need um one Jewel and the total amount of heat required to to keep it melting and not just reach the melting temperature is going to be is going to need to be more than that one Jewel and so the correct answer here is a and that is all I have for you guys today I hope this was helpful let me know if you have any questions or comments down below other than that good luck happy studying and have a beautiful day future doctors