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Work, energy and power Questions
1. Given that a lamp is rated 45W 240V. Calculate the resistance of the heating element.
2. An electric bulb is rated 40W, 240V. What is the resistance of its filament?
3. An electrical immersion heater is rated 3kW, 250V. Choose a suitable fuse from 3A, 5A, 10A,
12A, and 20A that can be used in such an appliance.
4. An electric kettle is rated 3KW, 250V. Determine the resistance of the coil
5. An electric kettle rated 3.0Kw, 240V is filled with water. If the water boiled after 8 minutes
of heating, determine the energy used in boiling the water.
6. (a) An electrical heater is rated 3.45KW. The heater is immersed in 2.4kg of water.
Calculate the minimum time it takes for the temperature of the water to rise from
23.0oC to 69.0oC. (Specific heat capacity of water = 4.2Jg-1K-1)
Work, energy and power Answers
1. D = 1V P I2R
I = P R = P
V I2
= 45
1 mk = 45
240 (0.1875)2
1 mk
= 0.1875A = 45 = 1280
1 mk
0.03515
2. P = V2
R
R = (240)2
40
= 1440
3. P = I
V
3000 = 12A
250
suitable fuse 13A
4. An electric kettle is rated 3KW, 250V. Determine the resistance of the coil
P = IV = V2
R
300 = 2502
R
R = 62500
3000
= 20.83
5. Energy E1 = Power x time
= 3000 x 8 x 60
= 1440000J
6. = 2.4 x 4.2Jg-1K-1 x 46K (1mk)
= 2.4 x 4200JKg-1 K-1 x 46K (1mk)
= 463 6805J
Let the rime be t
Energy H = Pt = 3450W x t
3450t = 463 680J
t = 134.4s
