Electrical Power and Energy in Circuits

Energy and power in electrical circuits

1.       An energy saving bulb takes a current of 0.05A. If the mains voltage is 240V calculate its power. Calculate the energy transferred by this bulb in 10 minutes.

E = VIt = 240 x 0.05 x (10 x 60) = 7200 W

2.       An electric Kettle takes a current of 11A. If the mains voltage is 240V calculate its power. It boils some water in 4 minutes.

Calculate the energy transferred? E = VIt = 240 x 11 x (4 x 60) = 633600 W = 6.34 x 10⁵ W = 6340 kW Is all the heat energy useful? No. all of this heat is not transferred to the water, some escapes to the surroundings.

3.       A USB port has an output voltage of 5V and supplies a current of 0.5A to charge a phone. Calculate the power transfer. P = IV = 0.5 x 5 = 2.5W

Calculate how much energy passes into the battery if it is charged for 3 hours

E = Pt = 2.5 x (3 x 60 x 60) = 27000 J = 27 kJ = 2.7 x10⁴ J

4.       Copy and complete the table below

	Voltage	Charge	Current	Time	Energy
1	12 V	Q = It = 2 x 20 = 40 C	2 A	20 s	E = VIt 12 x 2 x 20 = 480J
2	2 V	360C	3 A	2 min	720J
3	6 V	40 C	2 A	t = E/VI = 240/6x2 = 20s	240 J
4	20 V	Q = 0.1(0.5 x 60 x 60) = 180C	100 mA	½ hour	E = VQ = 20 x 180 =3600J
5	100 mV	200	I = Q/t = 10A	20 s	20J
6	V= E/Q = 5V	120C	2 A	60 s	600  J
7	V = E/It = 8.3kV	20 C	4A	5 s	10 kJ (10 000J)
8	1000 V	5kC	0.5 kA	10 s	E = VQ = 1 x 5 = 5MJ
9	2 kV	120 C	I = 120/2x10-3 = 60kA	2 ms	240kJ
10	133V	150 C	20 A	7.5 s	20 kJ

   

5.       A 3kW cooker runs for 5 minutes. How much energy does it transfer?

E = Pt = 3000(5 x60) = 900kJ

6.       A 2 kW heater runs for 2 hours on a cold evening. How much energy does it transfer?

E = Pt = 2000(2 x 60 x60) = 14.4 MJ

If it is connected to the mains at 230V what current passes through it?

I = P/V = 2000/230 = 8.70 A

Questions on the Volt

Volt

Voltage = energy ÷ charge                           V = E/Q                 1 Volt = 1 Joule / 1 Coulomb

1.       Calculate the voltage across a bulb if 20 C transfers 4 J of electrical energy. Draw an energy transfer diagram.

V = 4 J ÷ 20 C = 0.2 V                        electrical à light + thermal

2.       Calculate the voltage across a dynamo if 600 J of energy is transferred by 25 C of charge. Draw an energy transfer diagram. (How is it different from q1?)

V = 600 J ÷ 25 C = 24 V                    kinetic à electrical + heat            here another form of energy is transferred to electrical wheras in q1 energy is transferred from electrical to other forms

3.       A bulb has a voltage of 6V across it. Calculate the energy is transferred by 90 C.

E = V x Q = 6 V x 90 C = 540 J

4.       A train motor operates on a voltage of 25 kV (1kV = 1000 V). Calculate the charge that transfers    800 kJ (1kJ = 1000 J) of electrical energy.

Q = E ÷ V = 800000 J ÷ 25000 V = 32 C

5.       A torch bulb runs off a 1.5V cell (battery). Calculate the energy transferred by a charge of 0.27 C

E = V x Q = 1.5 V x 0.27 C = 0.405 J

6.       A bulb takes a current of 0.25 A. How many Coulombs per second? 0.25 C per second.

It transfers 60W of power. How many Joules per second? 20 J per second

Calculate the voltage of the supply. V = E ÷ Q = 20 ÷ 0.25 = 80 V

Electrical Energy and Electric Power

Current in Series and Parallel Circuits

Q=It Questions, workings and Answers

Award yourself

1 mark for the correct substitution into the formula
(1 mark for converting minutes in s etc).
1 mark for calculating the correct answer.
1 mark for the correct unit

1.  A current of  2 amps flows in a circuit.  How much charge passes through a point in 30 seconds?  60 C

2.  A current of  5 amps flows in a circuit.  How much charge passes through a point in 50s?

250 C

3.  A current of  0.5 amps flows in a circuit.  How much charge passes through a point in the circuit in 2 minutes. Q = It            Q = 0.5 x (2 x 60) s = 60 C

4. A current of  0.25  amps flows in a circuit.  How much charge passes through a point in the circuit in 1  hour. Q = It     Q = 0.25 x (1 x 60 x 60)s = 900 C

5. A current of  20 mA  flows in a circuit.  How much charge passes through a point in the circuit in 1 min  30  sec. Q = It       Q = (20 ÷ 1000)  x ( 60 + 30)s = 1.8 C

6.   A  charge of 60 coulombs passes a point a circuit in 10 secs.  Calculate the current flowing.  6A

7. A  charge of 14 coulombs passes a point a circuit in 7 sec.  Calculate the current flowing. 2A

8. A  charge of 10 coulombs passes a point a circuit in 12 sec.  Calculate the current flowing.  0.83A

9. A  charge of 120 coulombs passes a point a circuit in 1 min.  Calculate the current flowing.

I = Q  ÷ t = 120 ÷ 60s = 2A

10. A  charge of 6000 coulombs passes a point a circuit in 1 hr.  Calculate the current flowing.

I = Q  ÷ t = 6000 ÷ 3660s = 1.67A

11.   A current  of  2 amps flows in a circuit.   How long does it take for 8 coulombs to flow  past a certain point? 4s

12. A current  of  10 amps flows in a circuit.   How long does it take for 50 coulombs to flow  past a certain point? 5s

13. A current  of  0.5 amps flows in a circuit.   How long does it take for 100 coulombs to flow  past a certain point?  200s

14. A current  of  4 amps flows in a circuit.   How long does it take for 1000 coulombs to flow  past a certain point? 250s

15. A current  of  0.3 amps flows in a circuit.   How long does it take for 15 coulombs to flow  past a certain point? 50s

Electric Current

Uses and Dangers of Static Electricity