Physics 9702 · AS & A Level · Resistance and resistivity
Resistance and resistivity — practice question
An electric heater, which has constant resistance, is rated at $1.20\,\text{kW},\ 230\,\text{V}$. It is attached to a $230\,\text{V}$ supply through a cable that is $9.20\,\text{m}$ long, as shown in Fig. 8.1. The cable contains two copper wires, and each wire has a circular cross-section of diameter $0.900\,\text{mm}$. Copper has resistivity $1.70 \times 10^{-8}\,\Omega\,\text{m}$.
(a(i))[2]
Demonstrate that the heater’s resistance is $44.1\,\Omega$.
(a(ii))[2]
Demonstrate that the cable’s total resistance is $0.492\,\Omega$.
(b)[3]
When the current in the cable and heater is switched on, determine the power dissipated in the heater to three significant figures.
(c)[2]
Suggest two disadvantages of connecting the heater to the $230\,\text{V}$ supply with a cable made of two thinner copper wires.
Worked solution & mark scheme
This 9-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Apply $P = V^2/R$ or $I = P/V$” …