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file_3177 - Episode 112 Resistivity In this episode...

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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 1

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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 ρ R = ρ l / A The units of resistivity can be derived from the equation: m . Emphasise that this is ‘ohm metre’, not ‘ohm per metre’. Discuss the great range of resistivities amongst materials. Values for metals are very small. The resistivity of a material is numerically equal to the resistance between opposite faces of a one- metre-cube of the material; although this is not a good definition of resistivity, imagining such a block of metal does indicate why its value should be so low (~10 -9 m). Student experiment: Measurement of resistivity Complete this section by asking your students to measure the resistivity of several metal wires. This experiment provides an opportunity for a detailed discussion of the treatment of experimental errors. TAP 112-3: Measuring electrical resistivity 2
Using these ideas Problems involving resistivity. Students often get confused between cross-section area and diameter. Make sure they are able to convert mm

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file_3177 - Episode 112 Resistivity In this episode...

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