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Monday, November 15, 2021

Electric power worksheet and answers



Power and Energy

  1. 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,


  1. 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?

  1.  Question 2 asks `what is its resistance IN USE', Explain why the last two words have been added.


  1.  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)

  1.  A current of 2 A flows through a resistor of 10 Ω for 2 min, how much energy is transferred?


  1. 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)

  1. 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?


  1. 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.

  1. 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?

  1. What is the total cost of using four 100W bulbs for 6 hours, if a kWh costs 9p. The equivalent low energy bulb has a rating of 21W. How much would it cost to run 4 of these for 24 hours.

  1. A 3kW immersion heater is used for 4 hrs. How much will this cost if 1 kWh costs 9.7p

  1. A 2.5Kw kettle boils in 13 minutes. How many kWh does it transfer?

  1. A TV rated at 102 W uses 0.6 W on standby. How many kWh will it transfer in a day if it is used for viewing for 4 hours in the evening? How much does this cost at 9.3p per kWh.

  1. An LCD TV is rated at 134W. A similar plasma TV is rated at 281W. How much will it cost to watch 3 ½ hours on each set if electricity costs 9.5p per kWh?

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 !)







Power and Energy

  1. 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,


  1. 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?

  1.  Question 2 asks `what is its resistance IN USE', Explain why the last two words have been added.


  1.  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)

  1.  A current of 2 A flows through a resistor of 10 Ω for 2 min, how much energy is transferred?


  1. 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)

  1. 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?


  1. 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.

  1. 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?

  1. What is the total cost of using four 100W bulbs for 6 hours, if a kWh costs 9p. The equivalent low energy bulb has a rating of 21W. How much would it cost to run 4 of these for 24 hours.

  1. A 3kW immersion heater is used for 4 hrs. How much will this cost if 1 kWh costs 9.7p

  1. A 2.5Kw kettle boils in 13 minutes. How many kWh does it transfer?

  1. A TV rated at 102 W uses 0.6 W on standby. How many kWh will it transfer in a day if it is used for viewing for 4 hours in the evening? How much does this cost at 9.3p per kWh.

  1. An LCD TV is rated at 134W. A similar plasma TV is rated at 281W. How much will it cost to watch 3 ½ hours on each set if electricity costs 9.5p per kWh?

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 !)

 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.      21.6p  19.1p

11      £1.16

12      0.54kWh

13      .42 kWh        .112p + 3.79p = 3.9p

14      4.46p            9.34p

Extra  50kW  0.072W









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