The Flow of Electric Current

Although wires are actually very different from water pipes, electricity flowing through wire can be compared to water flowing through pipes. When electricity is flowing, there is said to be electric current.

Current (I) is the rate of flow of electrons through the wires. It is measured in amperes (called amps, A) which is a measure of the number of electrons passing through a given length of wire. This is similar to the rate of flow of water though pipes (i. e. litres per second). Current flowing in one direction is called direct current (DC), while current which changes direction of flow is called alternating current (AC). In a 12-volt DC system, a 12 watt lamp draws about 1 ampere of current.

Voltage, or potential difference, is the difference in potential energy between the ends of a conductor (e. g. a wire) that governs the rate of flow of current through it. Voltage is measured in volts (V). In basic terms, it is the amount of energy each electron has to move about and is similar to the pressure pushing water through a pipe. Grid electricity is supplied at 240 volts AC, while the electricity from automotive batteries is at about 12 volts DC.

Circuit

A pathway which electricity flows through (e. g. the wires, batteries, lamps, switches, etc.) is called a circuit. Current flows from a source of electricity (a battery, generator or solar cell) through wires to loads (lamps, motors, electric coils) and back again. When there is an uninterrupted pathway for electricity to flow, the circuit is said to be closed. When there is a point where electricity cannot pass (i. e. switch turned off), the circuit is said to be open. Thus, when you turn on a light, you close the circuit, and when you turn off a light, you open the circuit. Current cannot flow through an open circuit. Circuit diagrams are pictures of electric circuits with special symbols for switches, batteries, resistive loads, diodes and other electric equipment that help electricians to understand and plan circuits.

Series and Parallel Circuits

When a number of electrical components are wired up end to end in a continuous chain, they are joined in series. If lamps are joined in series and one of them fails, then the circuit will be broken and all the lamps will fail. If batteries or solar cells are joined in series, the voltage increases according to the number of units joined. For example, three 1.5 volt dry cells joined in series will produce a voltage of 4.5 volts.

When components are wired so that one path can be broken without affecting the flow of electricity through the others in the circuit, the components are said to be wired in parallel. The lamps in a mains-wired house are in parallel, and you can turn one off without turning off the rest of the lights in the circuit. When batteries or solar cells are wired in parallel, the available current increases but the voltage stays the same. For example, if the above three 1.5 volt dry cells were wired in parallel, the voltage would remain at 1.5 volts, but the amount of current available would increase.

Basic Electric Laws

In all basic electrical work, the understanding of two formulae is required. Once they are understood, most electrical problems encountered in low-voltage systems can be easily solved. These formulae are simple:

Power Law: watts (W) = volts (V) X amps (A)

Ohm’s Law: volts (V) = amps (A) X ohms (Q)

Power Law

Power (P) is the amount of work the electricity is doing at a given instant. It is measured in watts. The power rating in watts of a light fixture, for example, is a measure of the power it will consume to produce light. Power is calculated by multiplying the voltage (V) by current (I):

Power (P) = Voltage (V) X Current (I) or watts = volts X amps

Examples

1 A globe lamp is connected to a 12-volt battery. When it is turned on, 3 amps of current are flowing through the wire. What is the power of the lamp?

2 A 24-watt DC globe lamp is connected in a 24-volt DC system. When the globe is turned on, what current will be flowing?

Solutions

1 Example 1 is asking for the power of the globe lamp.

Power (P) = Voltage (V) X Current (I) = 12 volts X 3 amps = 36 watts

2 Example 2 is asking for the current flowing through the 24-volt wire.

Current (I) = Power (P) +- Volts (V) = 24 watts +- 24 volts = 1 amp

Ohm’s Law

Resistance (R) is the property of a conductor (i. e. wire or appliance) which opposes the flow of current through it and converts electrical energy into heat. It determines the amount of current that can flow for a certain voltage. Resistance is measured in units called ohms, which are given the symbol Q.

The formula that relates these three electrical measures is called Ohm’s Law:

Voltage (V) = Current (I) X Resistance (R) or volts = amps X ohm

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