Power and Energy
- 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 2 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 !)
