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
