8.3.6 Direct Current and Alternating Current

1. Direct current is a uniform current flowing in one fixed direction in a circuit
2. Direct current (d.c) is usually supplied by acid-based batteries or dry cells. 
3. A common example of acid-based (electrolyte) batteries is the car battery. 
4. Figure below shows the graph of current supplied by a dry cell over time.
   

Alternating Current

1. Alternating current is an electric current in which the direction of flow of the electrons reverses periodically
2. Alternating current (a.c) is generated from alternating current generators such as hydroelectric power generators. 
3. The electricity supplied to households is alternating current.
4. Household electricity (alternating current) changes direction 50 times every second. Its magnitude also changes with time.

Period And Frequency

1. The time taken for one complete cycle is known as the period, T. 
2. The frequency f is defined as the number of complete cycles in 1 second. 
3. The relationship between the frequency and the period is:

4. In SPM, you need to know the effect of both the direct current and alternating current on
   1. a bulb
   2. a capacitor
   3. a moving coil loudspeaker
5. Table below give the summary of the comparison of the effect of direct current and alternating on a bulb, a capacitor and a moving coil loudspeaker.

The Effective Voltage for a Sinusoidal Alternating Current

1. The maximum potential difference supplied by an a.c source is known as the peak voltage V_P. 
2. The effective potential difference for an a.c is equal to the potential difference of a alternating current if both results in the same heating effect. 
3. The effective potential difference for a.c is known as the root mean square voltage (r.m.s) of the a.c. and is given y the following equation:

4. The root-mean-square (r.m.s) value of an alternating current is the value of the steady direct current which produces the same power in a resistor as the mean power produced by the alternating current. 
5. The r.m.s current is the effective value of the alternating current.
6. The r.m.s. current can be calculated by using the following equation: