CURRENT ELECTRICITY (II)
ELECTRIC CURRENT AND POTENTIAL DIFFERENCE
A basic electric circuit comprises electrical components, i.e., bulbs, cells, etc, connected together by copper wires to enable electric charges to flow from one terminal of the electrical source, through the components, to the other terminal. For proper working of electrical devices, specified currents and voltages are used and hence the need to measure them.
An electric current is the rate of flow of charge through a conductor. An instrument called an ammeter measures the electric current flowing through an electric device or a circuit. Figure shows common moving coil ammeters used in school laboratories.
The operation of a moving coil ammeter is based on the fact that a coil carrying current experiences a force when placed in a magnetic field. The deflection of the pointer attached to the coil is a measure of the current flow.
Using an Ammeter
- Before connecting the ammeter in the circuit, ensure that the pointer is at zero mark on
the scale. If this is not the case, use the zero adjusting screw to move it to the correct
position, see figure 5.1 (b)
- The ammeter is an instrument of low resistance. It is thus connected in series with other
components in the circuit so that conventional current enters the ammeter through its
positive terminal and exits through the negative terminal,
If the terminals are interchanged as in figure 5.2 (b), the pointer moves away from the scale in anticlockwise direction. This can damage the instrument.
An appropriate scale should be selected to safeguard the coil of the meter from blowing
- If, say, a scale of 5 A is selected, the meter can safely read up to a maximum of 5 A.
With such a scale, ten divisions represent 1 A. For a scale of2.5 A, ten divisions represent
The readings on the ammeter are 2.45 A when using 0 – 5 A scale, or 1.225 A for 0 – 2.5 scale.
It should be noted that more accurate digital ammeters are available in the market.
This is the work done in transferring the one coulomb of charge from one point to another
The potential difference between two points A and B (V) of a conductor is defined as the work done in moving a unit charge from point B to A of the conductor
It is the difference in electrical potential energy between the terminals of a cell that enables it to drive charges
The SI unit of the potential difference is volt (V) and it is measured using voltmeter
Using a Voltmeter
(i) The pointer is adjusted to zero as with the ammeter.
(ii) A voltmeter is always connected across (in parallel to) the device across which the voltage is to be measured. This is because it is an instrument with high resistance to flow of current, hence takes little current in the circuit. Note that the positive terminal of the voltmeter is connected to the positive terminal of the electrical power source
(iii) The appropriate scale should be selected, and, when taking the reading, parallax error should be avoided