Question

The most common component of a circuit is a resistor. A resistor is a device which resists electrical flow in a circuit. If you think of water flowing, a water resistor is a waterwheel; it reduces the flow while getting something useful out of the flow.

We’ll make a few predictions before we start. Answer these questions on your whiteboard.

Imagine a resistor in a circuit with current flowing around the circuit. The resistor is simply a wire of a certain type of material. What could you do to change the wire that would change the resistance to electrical flow?

What do you think would happen to the resistance to flow if the wire were longer? Why?

Predict the shape of a Resistance vs. Length graph.

What would happen if the wire were thicker? Why?

Predict the shape of a Resistance vs. Area graph.

The actual resistance depends on three factors: the length, the area, and the material the wire is made of. The material affects a variable called the “resistivity”. It’s kind of like density; every material has a different value for resistivity. That means you could determine the type of material if you could calculate the resistivity for a sample. That’s what we’ll do today. We want to know the type of material in our wire boards.

The equation we will use today is this:

R = ?L/A

where R is resistance in ohms, ? is resistivity, L is length, and A is area.

You will be making measurements to see how resistance depends on length and area. Then you will use your measurements to determine the resistivity and the material.

If we want to determine resistivity, what do we need to measure?

We can’t measure area directly; figure out what you could do to determine the cross-sectional area of a wire.

Match up the equation for resistance above and try to come up with a graph that will give you resistivity.

Follow the class discussion and take notes as needed.

Go ahead and do it! Once you know the resistivity, figure out what material the wires are.

READ THIS CAREFULLY! You are going to make two additional graphs today. For the wires of (approximately) equal area, graph the resistance vs. the length. (This should be only three or four wires.)

For the wires of (approximately) equal length, graph the resistance vs. the area. (This should be only three or four wires.)

Compare these two graphs to your predictions. What do the graphs tell you about how the length and area of a wire affect the resistance of a wire?

The usual lab report today. Three graphs should be included. Your results should include: your value of the resistivity, the type of material you think we are working with, how length affects resistance, and how area affects resistance.

Answer 1