Current, resistance, and resistivity review (article) | Khan Academy
AP Physics C Electricity Help» Electricity and Magnetism Exam» Electric .. To use this equation, we need to find the equivalent resistance of the circuit. Circuits. AP Physics C Since the capacitor is in SERIES with the resistor the current will decrease variables on different sides of the equation and integrate . Electric current, resistivity, and Ohm's law. Introduction to electricity, circuits.
A resistor is a device that is used in electric circuits, and has a certain fixed resistance. Resistors are made by choosing a piece of material with a certain resistivity, length, and area, and wrapping it in an insulator with wires leading out of each end. In circuit diagrams, it is represented with the symbol, Voltage Voltage is a difference in electric potential between two points. A voltage source is a device used in electric circuits that has a fixed potential difference between its ends.
A voltage source can be a battery, or another source of direct current with a fixed potential difference. In circuit diagrams, it is represented with the symbol, If the ends of a voltage source are connected through a circuit with any number of resistors or other components, a complete circuit is formed, and current can flow from one terminal to the other. If current is flowing, it will be the same on both terminals of the voltage source.
For an ideal source, the electromotive force is equal to the voltage difference, Real sources like batteries are not ideal, and so there is some amount of internal resistance. If the internal resistance of a battery is r, then the voltage difference across the battery is, This is also called the terminal voltage of the battery. Power Power P is a measure of the rate at which energy is delivered or used by a circuit element.
Voltage sources deliver power, while resistors use power by dissipating it as heat. That potential energy is gonna be converted to kinetic energy.
And you could look at a certain part of the pipe right over here, right over here.AP Physics C - Resistors and Resistance
And you could say, well, how much water is flowing per unit time? And that amount of water that is flowing through the pipe at that point in a specific amount of time, that is analogous to current. Current is the amount of charge, so we could say charge per unit time.
Q for charge, and t for time. And intuitively you could say, how much, how much charge flowing, flowing past a point in a circuit, a point in circuit in a, let's say, unit of time, we could think of it as a second. And so you could also think about it as coulombs per second, charge per unit time. And the idea of resistance is something could just keep that charge from flowing at an arbitrarily high rate.
And if we want to go back to our water metaphor, what we could do is, we could introduce something that would impede the water, and that could be a narrowing of the pipe. And that narrowing of the pipe would be analogous to resistance. So in this situation, once again, I have my vertical water pipe, I have opened it up, and you still would have that potential energy, which is analogous to voltage, and it would be converted to kinetic energy, and you would have a flow of water through that pipe, but now at every point in this pipe, the amount of water that's flowing past at a given moment of time is gonna be lower, because you have literally this bottleneck right over here.
So this narrowing is analogous to resistance. How much charge flow impeded, impeded. And the unit here is the ohm, is the ohm, which is denoted with the Greek letter omega. So now that we've defined these things and we have our metaphor, let's actually look at an electric circuit.
So first, let me construct a battery. So this is my battery. And the convention is my negative terminal is the shorter line here.
Current, Resistance, Voltage, and Power
So I could say that's the negative terminal, that is the positive terminal. Associated with that battery, I could have some voltage. And just to make this tangible, let's say the voltage is equal to 16 volts across this battery.
And so one way to think about it is the potential energy per unit charge, let's say we have electrons here at the negative terminal, the potential energy per coulomb here is 16 volts.
These electrons, if they have a path, would go to the positive terminal.
And so we can provide a path. Let me draw it like this. At first, I'm gonna not make the path available to the electrons, I'm gonna have an open circuit here.
I'm gonna make this path for the electrons. And so as long as our circuit is open like this, this is actually analogous to the closed pipe. The electrons, there is no way for them to get to the positive terminal.
But if we were to close the circuit right over here, if we were to close it, then all of a sudden, the electrons could begin to flow through this circuit in an analogous way to the way that the water would flow down this pipe. Now when you see a schematic diagram like this, when you just see these lines, those usually denote something that has no resistance.
Introduction to circuits and Ohm's law
But that's very theoretical. In practice, even a very simple wire that's a good conductor would have some resistance. And the way that we denote resistance is with a jagged line. And so let me draw resistance here. So that is how we denote it in a circuit diagram. Now let's say the resistance here is eight ohms. So my question to you is, given the voltage and given the resistance, what will be the current through this circuit?
What is the rate at which charge will flow past a point in this circuit? Pause this video and try to figure it out. Well, to answer that question, you just have to go to Ohm's law.
We wanna solve for current, we know the voltage, we know the resistance.
- Current, resistance, and resistivity review
- Current, Resistance, Voltage, and Power
So the current in this example is going to be our voltage which is 16 volts, divided by our resistance which is eight ohms.