Fig. 1 shows a museum conservator using a tiny electric heater to dry a damp display case after a leak. The heater transfers 18 000 J of electrical energy i...
Question 1 Report
Fig. 1 shows a museum conservator using a tiny electric heater to dry a damp display case after a leak. The heater transfers 18 000 J of electrical energy in 5.0 minutes. Its potential difference is 12 V. The conservator records the energy because heat could damage an old paper object. A battery pack supplies the heater through a short cable. The diagram shows that the heater is separate from the small fan, which moves warm air around the case.
(a) Complete the calculation of the heater power. [2]
(b) Give the current supplied to the heater. [2]
(c) Name the unit of electrical energy used in this question. [1]
Fig. 1 shows a school caretaker replacing a faulty doorbell transformer. The transformer reduces the 230 V mains potential difference to 8.0 V for the bell circuit. The bell takes a current of 0.40 A and rings for a total of 25 s during a busy morning. The transformer has two coils wound around an iron core. This makes the doorbell safer to use near a metal entrance panel than connecting the bell directly to the mains.
(a) Name the component that reduces the potential difference. [1]
(b) Give the power transferred to the bell. [2]
(c) Complete the calculation of energy transferred while the bell rings. [2]
Answer Details
Electric heater
(a) \[5.0\text{ min}=5.0\times60=300\text{ s}\] \[P=\frac{E}{t}=\frac{18000\text{ J}}{300\text{ s}}=60\text{ W}\] [2]
(b) \[I=\frac{P}{V}=\frac{60\text{ W}}{12\text{ V}}=5.0\text{ A}\] [2]
(c) The unit of electrical energy is the joule, J. [1]
Doorbell transformer
(a) The component is a transformer. [1]
(b) \[P=VI=8.0\text{ V}\times0.40\text{ A}=3.2\text{ W}\] [2]
(c) \[E=Pt=3.2\text{ W}\times25\text{ s}=80\text{ J}\] [2]
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