31
Power of electric current
Textbook: pp. 90–93
GoalKnow the power of current P = A/t = IU = I²R = U²/R, the watt and kilowatt-hour, and read a meter.
New words
power of current · tokning quvvatiwatt · vattkilowatt-hour · kilovatt-soatelectricity meter · elektr hisoblagichi
Explanation
Power is the rate of doing work: P = A/t. From A = IUt we get P = IU, and with Ohm’s law also P = I²R and P = U²/R. The unit of power is the watt: 1 W = 1 J/1 s = 1 V · 1 A. Large powers are measured in kilowatts (1 kW = 1000 W) and megawatts (1 MW = 10⁶ W). Since A = Pt, electrical energy is measured in the common units watt-hour (W·h) and kilowatt-hour (kW·h): 1 kW·h = 1000 W · 3600 s = 3.6 MJ. An electricity meter shows the energy used in kW·h: the difference of two readings is the consumption in that period. The marking on a device (for example 2000 W) tells its power.
Worked examples
A device has U = 220 V, I = 0.5 A: P = I·U = 0.5 · 220 = 110 W. Its consumption over 10 hours: 110 W · 10 h = 1100 W·h = 1.1 kW·h.
A family uses about 2 kW·h a day. In 30 days that is 2 · 30 = 60 kW·h, and the meter reading grows by 60. In joules: 60 · 3.6 = 216 MJ.
Class activity
With an adult, write down your home meter reading (just look), write it again the next day and find the difference. Do not touch the meter, wires or sockets.
Practice
1
What is power and what is its unit?
The ratio of work done to time (P = A/t); its unit is the watt.
2
A kettle has a power of 2000 W. How many kW·h does it use in 3 hours?
6
3
A 100 W lamp burns 5 hours a day. How many kW·h does it use in 10 days?
5
4
Why is electrical energy usually measured in kW·h rather than joules?
The joule is very small; home consumption is millions of joules, so the kW·h is handier.