AQA AS Physics

Topic Questions

5.2 Resistance & Resistivity

1a4 marks

State what is meant by a superconducting material. Explain, therefore, why this material would be useful in improving the efficiency of power stations and hence reduce carbon dioxide emissions.

1b2 marks

With the aid of a sketch graph, explain the term transition temperature.

1c3 marks

Niobium has the highest transition temperature of any superconductor. 

Sketch on the axes in Figure 1 the variation of resistance with temperature for Niobium that becomes superconducting at 9.2 K. 

Figure 1

5-2-s-q--q1c-medium-aqa-a-level-physics

1d3 marks

Explain why superconductors are useful for applications which require strong magnetic fields and name two such applications.

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2a2 marks

A cable consists of seven straight strands of copper wire each of radius 30 µm as shown in Figure 1. 

Figure 1

5-2-s-q--q2a-medium-aqa-a-level-physics

The cable is 1.5 m in length and has a resistance of 1.3 Ω. 

Calculate the resistance of one strand of copper wire.

2b4 marks

Calculate the resistivity of copper and state an appropriate unit in your answer.

2c2 marks

One strand of the wire is now stretched to three times its original length by a process that keeps its volume constant. 

If the resistivity of the copper wire remains constant, show that the resistance increases by 9R.

2d1 mark

State one advantage of using a stranded cable rather than a solid core cable with copper of the same total cross-sectional area.

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3a2 marks

Explain the required conditions for a material to become superconducting.

3b3 marks

Figure 1 shows the cross-sectional area of a cable consisting of parallel filaments that can be made superconducting, embedded in a cylinder of copper.

 Figure 1

5-2-s-q--q3b-medium-aqa-a-level-physics

The diameter of the copper in the cable is 0.48 mm. 

Calculate the resistance of 2.2 m length of the cable. 

            Resistivity of copper = 1.7 × 10–8 Ωm

3c2 marks

Explain, with a calculation, how the resistance would change for a copper wire in the cable with a diameter twice as wide as that in part (b).

3d3 marks

State and explain what happens to the resistance of the cable when the embedded filaments of wire are made to be superconducting.

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4a3 marks

A thermistor is a type of sensory resistor commonly used in thermostats to control the central heating in houses. 

State how the resistance of a negative temperature coefficient (ntc) thermistor changes with temperature and explain why this occurs.

4b4 marks

Figure 1 shows part of a miniature electronic circuit with two small resistors connected in parallel. 

The material from which each resistor is made has a resistivity of 2.6 × 105 Ω m and both resistors have dimensions of 15 mm by 2.3 mm by 1.3 mm. 

Figure 1

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Calculate the total resistance of the electronic circuit in MΩ.

4c2 marks

The designer increases the size of the circuit including the base by making every dimension larger by a factor of 10. The potential difference across the resistors is unchanged. 

Show this increase in dimensions results in the resistance of each resistor decreasing by a factor of 10.

4d1 mark

The resistors are replaced with wires with a circular cross-sectional area with diameter d and a resistance per unit length r. 

Sketch a graph of how the resistance per unit length r varies with diameter d of one of the wires.

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5a4 marks

An electrical heating element, made from uniform nichrome wire, is required to dissipate 600 W when connected to the 230 V mains supply. 

The radius of the wire is 0.16 mm. 

Calculate the length of nichrome wire required. 

            Resistivity of nichrome = 1.1 × 10–6 Ω m

5b2 marks

Suggest two properties that the nichrome wire must have to make it suitable as an electrical heating element.

5c3 marks

Explain why the resistivity of the nichrome wire changes with temperature.

5d3 marks

An engineer wants to use the same uniform nichrome wire to use in another identical electrical heating element which is also required to dissipate 600 W when connected to the 230 V mains supply. 

The only nichrome wire they have available has a radius of 0.08 mm. 

Calculate the length needed for this new nichrome wire to produce the same current through the heating element.

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