You can classify materials as either conductors that allow charge to flow or insulators that do not allow charge to flow. In this lesson we will look at the idea of resistance. Resistance is the opposition to the flow of charge through a conductor; it is measured in Ohms (Ω). All conductors allow charge to flow, but in some it flows more easily than in others. Some of the factors that contribute to the resistance of a conductor are the distance the charge tries to move, the thickness of the material, and how tightly bonded are the electrons within the material.

The following simulation looks specifically at a piece of wire. You can control the length—L, the cross sectional area—A, and the resistivity—ρ. The resistivity is a property of the material that the wire is made of, and it is related to the ability of the material to oppose the flow of electric current. From the values of these characteristics, you can find the resistance—R.

Resistance in a Wire. The simulation should open with R = 1.25Ω, L = 10 cm, and A = 4.01 cm².

Use the mouse to increase and decrease the value of L.

Click on the link to check your answers for the following questions.

What happens to the value of R —

1. When L increases?

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   R increases when L increases.

2. When L decreases?

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   R decreases when L decreases.

3. What happens in the picture of the wire?

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   The wire gets longer.

4. Why do you think R an L are related in this way?

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   When the wire gets longer, the charge has a greater distance to cover.

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Use the mouse to increase and decrease the value of A.

What happens to the value of R —

5. When A increases?

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   R decreases when A increases.

6. When A decreases?

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   R increases when A decreases.

7. What happens in the picture of the wire?

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   The wire gets thicker.

8. Why do you think R an A are related in this way?

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   When the wire gets thicker, there are more possible paths that the charge can follow.

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Use the mouse to increase and decrease the value of ρ.

What happens to the value of R —

9. When ρ increases?

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   R increases as ρ increases.

10. When ρ decreases?

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    R decreases as ρ decreases.

11. What happens in the picture of the wire?

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    More dots appear in the wire.

12. Why do you think R and ρ are related in this way?

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    There are more atoms closely packed together making it more difficult for the charge to flow.

13. Most wires are made with metals with a value of ρ between .00000002 and .00000003. Why is it valid to assume that the resistance of a short piece of wire is zero?

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    The values of the resistance would be so low, that it is nearly zero.

Electric circuits use electricity and flow of charge to perform a specific task. To control the flow of charge, electrical engineers use resistors. The value of the resistance in any given resistor is given by three colored stripes painted on them. Click on this link to see how the colored stripes show the values of resistors.