Current and Charge
Electric charge is defined by the following equation:
Charge = Current × Time
Q = It
The unit of electric charge is the Coulomb (C). The unit of current is Ampere (A) and that of time is second (s), Therefore, 1 Coulomb = 1 Ampere-second (As).
If we rearrange the formula above we get I = Q/t. Therefore current is the rate at which charge flows. The current through a device is measured with an ammeter connected in series. An ammeter is polar and must be connected in the correct way in a circuit – the positive terminal (+ or red) of the ammeter must be connected to the nearest + on the battery or power supply.
E.M.F, P.D. and Voltage
Potential difference (p.d.) is a measure of the energy carried by a charge. P.d. is commonly referred to as voltage and its defining equation is:
Potential difference = Energy/Charge
i.e., V = W/Q
The unit of p.d. is the Volt (V). A p.d. is needed to drive current through a component.
Electromotive force (e.m.f.) E refers to the voltage supplied by a source, e.g. a battery or power supply. The voltage across a component is measured with a voltmeter connected in parallel. A voltmeter is also polar and must be connected correctly.
Terminal P.D. of a Battery
There is a difference in the voltage reading of a battery when it is driving current around a circuit (close circuit) and when it is not (open circuit). The p.d. of the battery when driving current is referred to as its terminal p.d. and this value is less than the e.m.f. of the battery. The difference between e.m.f. and terminal p.d. is referred to as ‘lost’ volts, i.e.
e.m.f. = terminal p.d. + ‘lost’ volts
E = V + v
Cells in Series and Parallel
If two or more cells are connected in series, the total e.m.f. of the battery is the sum of the values of the individual cells. If two or more cells of similar value are joined in parallel, the e.m.f. is the value of one cell but the battery lasts longer....