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

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 = ..................................

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

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)

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.

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 = ......................................

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.

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

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