Direct current and alternating current
Direct current flows in one direction only. Alternating current reverses its direction at regular intervals; in India its frequency is 50 Hz.
DC vs AC
1 hertz (Hz) means one complete cycle per second. In one cycle, AC flows one way and then the other way, so it changes direction twice in every cycle.
Why AC is used for the power supply
Electricity is made at power stations that are often far from towns and cities. AC can be sent over long distances without much loss of energy. This is the main advantage of AC over DC. That is why the supply reaching our homes is AC.
Worked example: how often does AC change direction?
The AC supply in India has a frequency of 50 Hz. How many times does the current change direction in one second? How long does one cycle take?
- A frequency of 50 Hz means 50 complete cycles in 1 second.
- The current changes direction 2 times in each cycle.
- Changes in one second = 50 × 2 = 100.
- Time for one cycle = 1 ÷ 50 s = 0.02 s.
100 times each second (once every 1/100 s); one cycle takes 0.02 s
The socket at home gives AC at 220 V and 50 Hz. A mobile charger changes it into low-voltage DC for the phone battery, and a home inverter does the opposite during a power cut, making AC from its battery's DC.
1 marks: “What is the frequency of the AC supplied in India?” 50 Hz.
2 marks: “Distinguish between direct current and alternating current. State one advantage of AC over DC.” DC flows in one direction only (from a cell); AC reverses direction at regular intervals (from a power station, 50 Hz in India). AC can be sent over long distances without much loss of energy.
DC: Direction Constant. AC: Always Changing, 50 cycles a second in India, so 100 changes of direction.