The student is given the circuit shown in Fig. 1.1. The coil of wire is the small electric heater. Draw on the circuit in Fig. 1.1 to show a voltmeter connected to measure the potential difference across the heater.
Fig. 1.2 shows the volume of water W in the measuring cylinder. Record the volume of water W.
The student pours the water from the measuring cylinder into the beaker containing the heater, checks that the heater is covered by water, closes the switch, notes the voltmeter and ammeter readings, and then opens the switch. Fig. 1.3 shows the voltmeter and ammeter readings when the switch is closed. Record the potential difference V and the current I.
Fig. 1.4 shows the initial temperature $\theta_0$ of the water. Record the initial temperature $\theta_0$ of the water in the beaker.
The final temperature $\theta_F$ is 24.5^{\circ}C. Calculate the temperature change $\Delta\theta$. Use the equation $\Delta\theta = \theta_F - \theta_0$.
Explain why the student keeps stirring the water for a further minute after the heater is switched off.
The energy $Q_H$ supplied by the heater is given by $Q_H = I \times V \times t$, where $t = 300\,\text{s}$. Calculate $Q_H$. Show your working.
The energy $Q_W$ gained by the water is given by $Q_W = W \times 4.2 \times \Delta\theta$. Calculate $Q_W$ using your value of W from (b)(i) and your value of $\Delta\theta$ from (c)(ii). Show your working.
The heater’s efficiency is given by $\text{efficiency} = \frac{Q_W}{Q_H}$. Calculate the efficiency of the heater. Show your working.
Suggest one alteration to the apparatus used in this investigation that would increase the heater’s efficiency.