Lesson Video: Electrical Resistance Science

In this video, we will learn what electrical resistance is and how it affects the flow of charge within a circuit.

11:44

Video Transcript

In this video, we will learn what electrical resistance is and how it affects the flow of charge in a circuit.

Thinking of an electric circuit, say that we have a battery and that the ends or the terminals of the battery are connected by a wire. This forms an electric circuit. Charge will flow through this circuit. That’s electric current. We can even measure this current using a device called an ammeter. If we insert the ammeter into the circuit, it will read out some measured current value. This value will be in amperes, maybe two or three amperes or some number like that.

Now, let’s compare this electric circuit with this one. We have the same kind of battery, the same wire, and the same ammeter in the second circuit. So the only difference between the two is this bar of iron. We’ve drawn it big for clarity, but we can imagine that this bar is of equal thickness to the wire in the circuit. When charge flows through this second circuit, we would see the ammeter reading a smaller current than the ammeter in the first circuit. In other words, there’s something about this iron bar that decreases the current in the circuit.

As we said, current is the flow of electric charge. As electric charge enters one end of the iron bar and then passes through, the bar resists that motion. It makes it harder for charge to move across the bar. Less charge passes through the bar every second, and that means less current exists in the circuit. This decrease in the flow of charge is caused by the electrical resistance of the bar.

In general, electrical resistance is opposition to the flow of electric charge. Any material or component that makes it harder for charge to flow has resistance. In fact, there are certain components in electric circuits that are designed to provide resistance. These are called resistors. And our iron bar is an example of a resistor.

If we make some space on the left of our screen, we can sketch in a circuit diagram of this electrical circuit. Here is the circuit symbol for our battery, while this is the circuit symbol for an ammeter. The Ammeter and battery are connected up by wires, and right here is where our resistor goes. The resistor circuit symbol looks like this, like a bunch of jagged lines. And with that, our circuit diagram here matches our actual circuit over here.

We’ve seen that when we add a resistor to a circuit, it decreases the flow of charge. As electric charge slowly passes through a resistor, another thing that happens is the resistor heats up. With a large enough battery, this iron bar would get too hot to touch. The larger the current in the circuit, the hotter the resistor gets.

Now say that we had another iron bar that was the same as our first one, except it was one-half as long. If we switch the shorter bar into the circuit, there will be less resistance to charge flow. The overall current in the circuit will increase. The reason a shorter bar has less resistance is that there are fewer obstacles for moving charges to run into. That is, it’s easier for charge to flow through a shorter bar. By making this switch, we’ve decreased the resistance of our circuit.

In the same way that electric potential difference and electric current have units in which they are measured, electrical resistance does too. The unit of resistance is called the ohm. And it’s represented using this Greek letter 𝛺. So, for example, a resistor might have a resistance of five ohms or 10 ohms or 100 ohms or something like that.

Just as current is measured by an ammeter, electrical resistance is measured by an ohmmeter. Something interesting about ohmmeters, devices that measure electrical resistance, is that they aren’t inserted into a circuit to make a measurement. If we want to measure the resistance of some component, say our iron bar, we would disconnect the bar from the circuit and then wire it up by itself to the ohmmeter. This is how we measure the resistance of some electrical component. The circuit symbol for an ohmmeter looks like this. To measure the resistance of our resistor, we would disconnect it from the circuit and, as we’ve seen, connect it to the ohmmeter.

Now let’s say we happen to know that our battery provides a potential difference of one volt. And say we also know that when our entire circuit is connected, the ammeter reads a current value of one ampere. If these numbers were accurate when the circuit was connected, then if we went and measured the resistance of our resistor, we would find a value of one ohm. That is, a circuit that has one volt of potential difference and carries one ampere of current has a resistance of one ohm.

Now, think about this. What if we wanted to decrease the current in our circuit so that it was some value less than one ampere? We’ve seen that we can do that by replacing this iron rod with one that’s longer. That longer rod would have greater resistance to the flow of charge. That would make it harder for charge to flow through the circuit and would decrease the overall current. Instead of having to switch out and switch in resistors to change the resistance in a circuit, there’s actually a single resistor we can use to do this called a variable resistor.

The resistance of a variable resistor can be changed, often, say, by turning a knob on the resistor. The circuit symbol for a variable resistor looks like the circuit symbol for a resistor, except there’s a diagonal arrow drawn through it.

Knowing all this about electrical resistance, let’s look at a few examples.

Which of the following sentences describes what will happen to a wire in a circuit if a very large current is passed through it? (A) The wire will heat up. (B) The wire will cool down. (C) The wire will neither heat up nor cool down.

Alright, let’s say that we have a battery. Actually, let’s make that a gigantic battery. And say we decide to take a bit of conducting wire, and we connect up the ends of this battery. What will happen is charge will flow through the wire, lots of charge. As these charges move though, they will run into particles in the wire. This causes a transfer of energy from the moving charges to the wire. The result is that the temperature of the wire goes up. Option (A) is the correct answer choice. With all those moving charges in a very large current interacting with particles in the wire, energy is transferred to the wire, and it heats up.

Let’s look now at another example.

Any wire will have some amount of electrical resistance. There are two unequal lengths of identical wire. Which one of the following sentences is correct? (A) Both wires have the same resistance. (B) The wire of greater length has the greater resistance. (C) The wire of greater length has the lower resistance.

Okay, let’s say we have two bits of wire here. And in every way, these wires are the same. That is, they’re made of the same material, they had the same diameter, and so on. Except, as we can see, one wire is longer than the other. We want to compare the electrical resistances of these wires. Imagine we get two identical batteries, and we connect up one battery to each wire. This will cause electrical charge to flow to each wire and then through each one.

But here’s the thing. The wires resist the flow of charge. They make it harder for current to pass through the circuit. We can think of this resistance being caused by the material in the wire getting in the way of the moving charge. The more material there is in front of the moving charge, the harder it is for the charge to move forward. That means the charge passing through the shorter wire has to get through this amount of resistive material. But then, the charge passing through the longer stretch of wire has to get through this much material. The longer section of wire is harder for charge to pass through. That means it has a greater resistance. We choose answer option (B). The wire of greater length has the greater resistance.

Let’s look at another example.

Which of the following is the correct circuit component to measure resistance? (A) Ammeter, (B) voltmeter, (C) ohmmeter.

Each one of these three circuit components measures some quantity. To see which of the devices measures resistance, let’s recall what are the units of electrical current, electrical potential difference, and electrical resistance — first the units of current. The standard unit of current is the ampere. The standard unit of potential difference is the volt. And the standard unit of electrical resistance is the ohm.

Ohm, by the way, is the last name of the person who developed a law for electrical resistance. The name of this unit for resistance suggests that the ohmmeter is a device for measuring resistance. This is true. An ammeter measures current, a voltmeter measures potential difference, and an ohmmeter measures resistance. We choose answer option (C).

Let’s look now at one last example.

The diagram below shows an electric circuit consisting of a cell, a bulb, a voltmeter, and an ammeter. The readings of the ammeter and the voltmeter are shown on the diagram. What is the resistance of the bulb?

Okay, we’re told that this circuit consists of a cell, a bulb, a voltmeter, and an ammeter. The cell is right here. That supplies potential difference for the circuit. Then, this is where the bulb is located. We know that because the circuit symbol for a bulb is this circle with an X through it. A voltmeter, a device for measuring potential difference, is symbolized by a circle with a V. And an ammeter, for measuring current, is represented as a circle with an A.

Just to be clear on how this circuit works, conventional current travels from the positive terminal of the cell and follows this path with essentially no current traveling down this branch. The current then passes through the bulb up through the ammeter and then to the negative terminal of the cell. The ammeter is in position to measure current in this circuit. It measures a current of one ampere. The voltmeter, we can see, is connected across the ends of the bulb. What it does then is measure the electrical potential difference across the bulb. We see that that measured value is one volt.

Knowing all this, we want to solve for the bulb’s resistance. The unit of resistance is the ohm, represented by the Greek letter 𝛺. Any time the component of an electrical circuit has one volt of potential difference across it and one ampere of current passing through it, that component will have an equal number of ohms of resistance. In other words, one volt across it and one amp through it means there’s one ohm of resistance. Having one of two of these quantities means that there is one of the third. Our final answer is one ohm.

Let’s finish this lesson by reviewing a few key points. In this video, we learned that electrical resistance is the opposition to the flow of charge in a conductor. Any component with resistance is called a resistor. The circuit symbol for a resistor looks like this. We also learned that there’s such a thing as a variable resistor. This is a resistor whose resistance can change. Resistance is measured in units of ohms, represented using the Greek letter 𝛺. And resistance is measured using a device called an ohmmeter. We learned further that a circuit component with one volt of potential difference across it and one amp of current through it has a resistance of one ohm. Lastly, we saw that longer wires have greater resistance and large currents heat up wires. This is a summary of electrical resistance.

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