Resistivity
Episode 112: Resistivity
In this episode, students learn how and why the resistance of a wire depends on the wire’s dimensions. They learn the definition of resistivity and use it in calculations.
Summary
Discussion: Variation of resistance with length and area. (5 minutes)
Student experiment: Variation of resistance with length and area. (30 minutes)
Discussion: Variation of resistance with length and area. (10 minutes)
Student experiment: Measurement of resistivity. (30 minutes)
Student questions: Using these ideas. (30 minutes)
Discussion:
Variation of resistance with length and area
The analogy to water flow will be useful here - ask them how they think the flow rate will be affected if you increase the cross-sectional area or length of the pipe along which the water has to flow. This should lead to two predictions about the resistance of a wire: resistance increases with length resistance decreases with diameter or cross-sectional area
It will be worth reminding them that doubling the diameter quadruples the cross-sectional area; many students get confused about the distinction and expect a wire of double diameter to have half the resistance.
Student experiment:
Variation of resistance with length and area
You could ask them to do one or both of the following experiments. Both reinforce the idea that resistance depends on material dimensions:
TAP 112-1: How the dimensions of a conductor affect its resistance
TAP 112-2: Introduction to resistivity using conducting paper
Discussion:
Variation of resistance with length and area
Follow up with some theory suggesting:
Resistance is proportional to length l
Resistance is inversely proportional to cross-sectional area A
R= constant x length / cross-section area
The constant is a property of the material used - its resistivity R = l / A
The units of resistivity can be derived from the equation: m.
Emphasise that this is ‘ohm metre’, not ‘ohm per metre’.
Discuss
In this episode, students learn how and why the resistance of a wire depends on the wire’s dimensions. They learn the definition of resistivity and use it in calculations.
Summary
Discussion: Variation of resistance with length and area. (5 minutes)
Student experiment: Variation of resistance with length and area. (30 minutes)
Discussion: Variation of resistance with length and area. (10 minutes)
Student experiment: Measurement of resistivity. (30 minutes)
Student questions: Using these ideas. (30 minutes)
Discussion:
Variation of resistance with length and area
The analogy to water flow will be useful here - ask them how they think the flow rate will be affected if you increase the cross-sectional area or length of the pipe along which the water has to flow. This should lead to two predictions about the resistance of a wire: resistance increases with length resistance decreases with diameter or cross-sectional area
It will be worth reminding them that doubling the diameter quadruples the cross-sectional area; many students get confused about the distinction and expect a wire of double diameter to have half the resistance.
Student experiment:
Variation of resistance with length and area
You could ask them to do one or both of the following experiments. Both reinforce the idea that resistance depends on material dimensions:
TAP 112-1: How the dimensions of a conductor affect its resistance
TAP 112-2: Introduction to resistivity using conducting paper
Discussion:
Variation of resistance with length and area
Follow up with some theory suggesting:
Resistance is proportional to length l
Resistance is inversely proportional to cross-sectional area A
R= constant x length / cross-section area
The constant is a property of the material used - its resistivity R = l / A
The units of resistivity can be derived from the equation: m.
Emphasise that this is ‘ohm metre’, not ‘ohm per metre’.
Discuss