Current, resistance, and resistivity review (article) | Khan Academy
Current, Resistance, Voltage, and Power. Current Current is a measure of the flow of electric charge through a material. A material that can The temperature- dependent resistivity ρ(T) can be found using the formula, AP Physics Notes. Mr. Andersen describes the relationship between voltage, current and resistance in an electric circuit. Ohm's Law is introduced through a circuit simulation. 1 point. Example: The full battery current passes through both A and D, so they have the same current. Because they have the same resistance, Δ. Δ. = A. D. V.
Current I is the amount of charge per time that passes through an area perpendicular to the flow: Current is measured in SI units of amperes Aand This definition for current can be applied to charges moving in a wire, in an electrolytic cell, or even in ionized gases.
In visualizing charge flowing through a circuit, it is not accurate to imagine the electrons moving very rapidly around the circuit.
At this rate, the time to travel 10 cm is about 11 minutes. It is obvious from experience that it does not take this long for a bulb to glow after the switch is closed. When the circuit is completed, the entire charge distribution responds almost immediately to the electric field and is set in motion almost simultaneously, even though individual charges move slowly. The battery provides a voltage V between its terminals.
The electric field set up in a wire connected to the battery terminals causes the current to flow, which occurs when the current has a complete conducting path from one terminal of the batter to the other—called a circuit.
Introduction to circuits and Ohm's law (video) | Khan Academy
By convention, the direction of current in the external circuit not in the battery is the direction of motion of positive charges. In metals, the electrons are the moving charges, so the definition of the direction of current is opposite the actual flow of the negative charges in a wire.
Electric fields are not found in conductors with static charges as shown by Gauss's law, but electric fields can exist in a conductor when charges are in motion. The potential difference between the terminals of the battery when no current is present is called the electromotive force emf. The historical term emf is a misnomer because it is measured in volts, not force units, but the terminology is still commonly used. Resistance and resistivity Experimentally, it was found that current is proportional to voltage for conductors.
The proportionality constant is the resistance in the 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.
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. 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. So the current in this example is going to be our voltage which is 16 volts, divided by our resistance which is eight ohms. And so this is going to be 16 divided by eight is equal to two and the units for our current, which is charge per unit time, coulombs per second, you could say two coulombs per second, or you could say amperes.
And we can denote amperes with a capital A. We talked about these electrons flowing, and you're gonna have two coulombs worth of electrons flowing per second past any point on this circuit. And it's true at any point, same reason that we saw over here.
Current, resistance, and resistivity review
Even though it's wider up here and it's narrower here, because of this bottleneck, the same amount of water that flows through this part of the pipe in a second would have to be the same amount that flows through that part of the pipe in a second. And that's why for this circuit, for this very simple circuit, the current that you would measure at that point, this point, and this point, would all be the same.
But there is a quirk. Pause this video and think about what do you think would be the direction for the current? Well, if you knew about electrons and what was going on, you would say, well, the electrons are flowing in this direction. And so for this electric current, I would say that it was flowing in, I would denote the current going like that. Well, it turns out that the convention we use is the opposite of that. And that's really a historical quirk.
When Benjamin Franklin was first studying circuits, he did not know about electrons. They would be discovered roughly years later. He just knew that what he was labeling as charge, and he arbitrarily labeled positive and negative, he just knew they were opposites, he knew something like charge was flowing. And so, in his studies of electricity, he denoted current as going from the positive to the negative terminal.