A 3 kW immersion heater is designed for use on 240 V mains. Find (a) the current taken from the mains, (b) the resistance of the heater, and (c) the energy used in 1 min,
A 2 kW electric fire is designed for use on 250 V mains. What current does it take, and what is its resistance in use?
Question 1 asks `what is its resistance IN USE', Explain why the last two words have been added.
How much energy is stored in a 12 V car battery of capacity 38 A h ? (ie it is capable of delivering 38A for 1 hour before it is flat)
A current of 2 A flows through a resistor of 10 Ω for 2 min, how much energy is used?
How many lamps marked 175 W 240 V could be used in a circuit fitted with a 5 A fuse if they are connected in parallel? (Remember all components in parallel have the same voltage)
An immersion heater marked 3 kW is designed for use on 240 V. If its resistance remains constant, what is its power if connected to a 200 V supply?
A fire of resistance 30 Ω is connected to a 240 V supply by (a) cables of negligible resistance, (b) cables of total resistance 2 Ω. Find the power consumed by the fire in each case.
A 60 W heater is to be made from manganin wire with a resistance of 3.75 ohms per metre. What length of wire is needed if the heater is to operate on a 12 V supply?
Extra
Find the power wasted in the cables when 120 kW is supplied through cables of resistance 0.2 Ω (a) at 240 V, (b) at 200 000 V.
( Hint answer for (a) is 50 kW, and (b) is much smaller !)
Answers
1 (a) P = VI P
power in W
V p.d. in V
I current in A
3000
= 240 x I
I=3000/240
=12.5A
(b) V = IR
240=12.5R
R
= 240/12.5 = 19.2 Ω
(c) 3 kW = 3000 W = 3000 joules
per second
Energy used in 1 min = 3000 x 60 = 180 000 J
2 8A 31.25Ω
3 Resistance depends upon temperature
4 1 641 600 J
5 4800J
6 6
7 2.083 kW
8 1920W 1687.5 W
9 .64m
10 50kW 0.072W
