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.
