Resistance, Resistivity & Potential Dividers (Edexcel A Level Physics)

Exam Questions

27 mins6 questions
1a
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3 marks

Analogue ammeters were used before digital meters became widely available.
The analogue ammeter shown will measure a maximum current of 1.0 mA and has a resistance of 18 Ω.

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The analogue ammeter can be adapted to measure a larger current by adding a resistor, known as a shunt, in parallel with the ammeter. The arrangement is shown below. The analogue ammeter is represented by the 18 Ω resistor.

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The maximum current through the 18 Ω resistor remains as 1.0 mA.

Show that the shunt would need to have a resistance of about 0.01 Ω to adapt this ammeter to read up to a maximum current of 2.0 A.

1b
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3 marks

A shunt of this resistance was usually made from Manganin wire.

Calculate the length of Manganin wire of radius 0.95 mm required to make this shunt. resistivity of Manganin = 4.55 × 10−7 Ω m


Length = ......................................................

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2a
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3 marks

A “metre bridge” is a circuit which can be used to measure an unknown resistance accurately. The metre bridge includes a metre length of nichrome wire.

Calculate the resistance of a 1.00 m length of the nichrome wire.

resistivity of nichrome = 1.12× 10–6 Ω m
diameter of wire = 4.00× 10–4 m




Resistance = ..................................

2b
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4 marks

This metre length of wire, labelled AB, is connected to a 1.50 V cell of negligible internal resistance and a switch as shown.

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i)
Explain how the potential along this wire varies with distance from A when the switch is closed.

(2)

ii)
Show that the potential difference between A and a point 75.0 cm along the wire from A is about 1.1 V.

(2)

2c
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2 marks

The metre bridge circuit is shown. The circuit includes a resistor of resistance 3.30 Ω, a very sensitive ammeter and a resistor of unknown resistance R.

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A metal slider S can be moved along the nichrome wire and pressed firmly against it to make an electrical connection.

When the switch is closed and S is 75.0 cm along the nichrome wire, the ammeter reads 0 A because the potential difference across the ammeter is zero.

Calculate R.




R = ......................................

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3a
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3 marks

A student is experimenting with different combinations of springs and recalls that in physics it is often possible to model different physical situations in similar ways.

The student suggests that a parallel combination of springs could be a model for a parallel combination of resistors in a circuit.

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Derive an expression for the effective resistance Reff of two resistors R1 and R2 connected in parallel in a circuit.

3b
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6 marks

Assess the validity of the student’s suggestion by considering the effective stiffness of two identical springs in parallel.

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