Fig. 1 shows a plug-in power meter used to compare two ways of boiling water. A small travel kettle has a power rating of 1000 W. It runs for 2.5 minutes to...
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Fig. 1 shows a plug-in power meter used to compare two ways of boiling water. A small travel kettle has a power rating of 1000 W. It runs for 2.5 minutes to boil one cup of water. The meter measures energy in kWh. The student switches the kettle off as soon as the water boils and keeps the lid closed while heating. These actions reduce unnecessary energy transfer to the surrounding air.
(a) Calculate the time in hours for 2.5 minutes. [1]
(b) Calculate the energy used to boil one cup in kWh. [2]
(c) Give two ways to reduce the energy used when boiling water. [2]
A family installs a new electric oven in a kitchen. Fig. 1 is a circuit diagram for three appliances supplied from the consumer unit: an oven, a fridge and a lamp. Each appliance is connected across the live and neutral wires. During a power cut caused by a faulty lamp, the oven and fridge should still be able to operate once their own switches are on. The electrician says that this arrangement is needed in homes because each appliance is designed for the full mains potential difference.
(a) What type of circuit connection is shown? [1]
(b) Give one advantage of connecting household appliances in this way. [2]
(c) Draw a switch in the live wire for the lamp circuit. [2]
Answer Details
Travel kettle
- (a) \[2.5\text{ min}=\frac{2.5}{60}\text{ h}=0.0417\text{ h}\] [1]
- (b) First convert the power: \(1000\text{ W}=1.0\text{ kW}\).
\[E=Pt=1.0\times0.0417=0.0417\text{ kWh}\]
The energy used is \(0.0417\text{ kWh}\). [2] - (c) Boil only the amount of water needed. Also use a lid, switch the kettle off promptly once boiling, or descale it. These reduce unnecessary energy transfer to the surroundings. Any two are required. [2]
Household circuit
- (a) This is a parallel circuit. Each appliance is connected across the same live and neutral supply wires. [1]
- (b) In a parallel circuit each appliance receives the full 230 V supply. Appliances can also be switched independently, and a fault in one branch does not stop current in the other branches. Any one explained advantage gains both marks. [2]
- (c) The switch must interrupt the live wire in the lamp branch, so that switching off disconnects the lamp from the live supply. [2]
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