Concept of Alternating Current (AC) and Direct Current (DC)

 

Concept of Alternating Current (AC) and Direct Current (DC)

 

An electric current is a flow of charged particles. Inside a copper wire, current is carried by small negatively-charged particles, called electrons. The electrons drift in random directions until a current starts to flow. When this happens, electrons start to move in the same direction. The size of the current depends on the number of electrons passing per second. Current is represented by the symbol I, and is measured in amperes, or ‘amps’, A.


One ampere is a flow of 6.25 × 1018 electrons per second past any point in a wire. That’s more than six million million million electrons passing per second. This is a lot of electrons, but electrons are very small and each carries a very tiny charge. In electronic circuits, currents are most often measured in milliamps, mA, that is, thousandths of an Ampere.

 

One terminal of a cell or battery is positive, while the other is negative. It is convenient to think of current as flowing from positive to negative. This is called conventional current. Current arrows in circuit diagrams always point in the conventional direction. However, you should be aware that this is the direction of flow for a positively-charged particle.

 

In a copper wire, the charge carriers are electrons. Electrons are negatively-charged and therefore flow from negative to positive. This means that electron flow is opposite in direction to conventional current. Current flow in electronic systems often involves charge carriers of both types. For example, in transistors, current can be carried by electrons and also by holes, which behave as positive charge carriers.  When the behavior of a circuit is analyzed, what matters is the amount of charge which is being transferred. The effect of the current can be accurately predicted without knowing about whether the charge carriers are positively or negatively charged.


A cell provides a steady voltage, so that current flow is always in the same direction. This is called direct current, or d.c. In contrast, the domestic mains provide a constantly changing voltage which reverses in polarity 50 times every second. This gives rise to alternating current, or a.c., in which the charge carriers move backwards and forwards in the circuit.

 

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