in this video we're going to focus on the concept of impedance what is impedance before we can answer that question let's talk about resistance so what does a resistor do in a circuit a resistor opposes the flow of electrical current here's a symbol of a resistor and the unit for resistance is ohms whenever you increase the resistance of a circuit the current flowing in that circuit decreases and so resistance provides opposition to the flow of dc current or direct current it can also oppose the flow of ac current as well now impedance is very similar to resistance impedance is represented by the letter z and like resistance the unit is ohms but impedance represents the opposition to the flow of electrical current in ac circuits as opposed to dc circuits now the elements that impede the flow of ac current include the resistor inductors and capacitors the opposition that a capacitor provides to the flow of ac current is known as capacitive reactants the opposition of an inductor towards ac current is known as inductive reactants both capacitive reactants and inductive reactants are measured in the same unit as resistance that is in ohms the formula for impedance is as follows it's equal to the square root of r squared plus the difference the square difference of the inductive reactants and the capacitive reactants the inductive reactance is two times a means two pi times the frequency times the inductance the capacitive reactance is one over two pi fc so what you need to know is that as the frequency increases the inductive reactance increases while the capacitive reactance decreases and as the frequency decreases the reverse is true so at high frequencies inductors offer a very high impedance and capacitors offer a very low impedance at low frequencies inductors offer or provide low impedance to the flow of ac circuit whereas capacitors they oppose high i mean low frequency signals so capacitors have high impedance towards low frequency signals and inductors have low impedance towards them now there is a middle ground and this middle ground is known as the resonant frequency at the resonant frequency the inductive reactance is equal to the capacitive reactants so that's the only impedance provided by the circuit is the resistance of the circuit now let's work on some example problems so let's say we have an ac signal as the power source of this circuit and it's connected to a resistor and a capacitor now let's say the capacitance is 5 microfarads and the resistance is 400 ohms and we have a 60 hertz 120 volt ac signal calculate the current flowing in this circuit feel free to pause the video if you want to to calculate the current we need to take the rms voltage and divide it by the impedance of the circuit so we have the rms voltage it's 120 volts what we need to calculate is the impedance and so we could use this formula to do so now there are no inductors in this circuit so xl is zero but we need to calculate xc the capacitive reactants it's one over two pi times fc the frequency is 60 hertz and the capacitance is 5 microfarads which is 5 times 10 to the minus 6 ferrets so the capacitive reactance is 530.5 ohms so now that we have that we can calculate the impedance of the circuit so it's going to be the square root of 100 squared plus 0 minus 530.5 squared so the impedance of the circuit is going to be 539.8 ohms so most of the impedance of the circuit is due to the capacitor of the circuit as we can see at low frequencies the capacitor has a high reactance towards the low frequency signals now that we know the impedance we can now calculate the current in the circuit so it's going to be the rms voltage of 120 volts divided by the impedance and so that's going to be 0.222 amps which is 222 milliamps so that's the current that's flowing in this circuit now for the sake of practice let's work on another example so this time we're going to have three elements a resistor a capacitor and we're going to introduce the inductor to it so we have an rlc circuit so the resistance is going to be a hundred ohms the capacitance 20 microfarads and the inductance 200 millihenries and this is going to be a 120 volt signal at the same frequency of 60 hertz so go ahead and calculate the current that is flown in this circuit first we need to calculate the capacitive reactance and that's one over two pi times fc so it's one over two pi times the frequency of 60 hertz times 20 microfarads or 20 times 10 to the sixth ferrets and so that's going to be 132.6 ohms next we need to calculate the inductive reactants and that's 2 pi f l so 2 pi times 60 times 200 millihenries or 200 times 10 to the minus 3 henrys so then that's going to be 75.4 ohms now to calculate the impedance we could use this formula so it's going to be the square root of r squared so that's 100 squared plus xl which is 75.4 minus xc which is 132.6 squared so you should get 115.2 ohms so that is the impedance of the circuit now once you know the impedance you can calculate the current the current is going to be the rms voltage divided by the impedance and so it's going to be 120 volts divided by 115.2 ohms and that's equal to 1.04 amps so that is the current that is flowing in this particular circuit now let's calculate the frequency at which the inductive reactance equals the capacitive reactants and as was mentioned before that frequency is known as the resonant frequency and it's equal to one over two pi times the square root of lc so l is going to be 200 millihenries so that's 200 times 10 to the minus 3 which is 0.2 henrys and then times the capacitance of 20 times ten to the minus six so you should get 79.6 hertz so at that frequency xl will equal xc so now you know how to determine the resonant frequency of an rlc circuit you also know how to determine the impedance of the circuit and also the current that is flowing in the circuit so that's it for this video thanks for watching