A marine centre keeps seawater tanks at a stable temperature using an immersion heater. Fig. 1 shows the heater circuit, including an ammeter and a 13 A fus...
Question 1 Report
A marine centre keeps seawater tanks at a stable temperature using an immersion heater. Fig. 1 shows the heater circuit, including an ammeter and a 13 A fuse in the plug. The heater is connected to a 230 V supply and the ammeter reads 10 A. It operates for 15 minutes after a cold delivery of water is added to a tank. The centre manager wants to calculate the energy cost and also checks that the fuse rating is suitable. The heating element transfers energy mainly to thermal energy in the water, although the tank also warms the surrounding air.
(a) What does the ammeter measure? [1]
(b) Calculate the resistance of the heater. [2]
(c) Calculate the power of the heater. [2]
(d) Calculate the electrical energy transferred in 15 minutes. Give your answer in kWh. [3]
(e) Which fuse rating is more suitable, 5 A or 13 A? [1]
(f) Describe why the selected fuse protects the circuit. [2]
Answer Details
(a) An ammeter measures electric current. [1]
(b) Use \(R=V/I\):
\[R=\frac{230\ \mathrm{V}}{10\ \mathrm{A}}=23\ \Omega\]
The heater resistance is 23 \(\Omega\). [2]
(c) Use \(P=VI\):
\[P=230\ \mathrm{V}\times10\ \mathrm{A}=2300\ \mathrm{W}=2.3\ \mathrm{kW}\]
The heater power is 2300 W, or 2.3 kW. [2]
(d) The time must be in hours because power is in kW: \(15\) minutes \(=0.25\ \mathrm{h}\).
\[E=Pt=2.3\ \mathrm{kW}\times0.25\ \mathrm{h}=0.575\ \mathrm{kWh}\]
The energy transferred is 0.575 kWh. [3]
(e) The suitable fuse rating is 13 A. [1]
(f) A 13 A fuse allows the normal 10 A current to pass. If a fault causes a dangerously high current, the fuse wire heats, melts and breaks the circuit, stopping the current. [2]
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