Ohm's Law
The relationship between voltage, current and resistance is defined by Ohm's Law.
- The potential of a battery is measured in Volts M.
- The flow of current in a circuit (I) is measured in Amperes.
- The power rating of a consumer is measured in Watts (W).
- The resistance (R) of a circuit is measured in Ohms (Ω).
Ohms law, for practical work can be described as:
Voltage/Current = Resistance
Power is calculated by multiplying Volts x Amps:
Watts = Volts x Amps
By transposing either of these formulae, the value of any unit can be calculated if the other two values are known.
For example, if a battery of 1 2V is connected to a bulb of 60W:
- the current flowing in the circuit can be calculated by using:
- W/V = I
- 60/12 = 5
- the bulb resistance can be calculated by using:
- V/I = R
- 12/5 = 2.4
To use either of the following triangles, put your finger over the value you want to find. Multiply the remaining values if side-by-side, or divide if one is over the other.
Basic Electrical Circuits
Basic Circuit Diagram
In the above circuit an electrical reservoir (the battery) is connected via a cable to a terminal on the controlling device (the switch) whose contacts are either open or closed. The other terminal on the switch is connected via a cable to the consumer (the bulb), and the other side of the bulb filament is connected to ground (earth) by another cable. The ground point is usually a part of the frame or engine, to which the battery negative terminal is also connected.
When the switch contacts are open (as shown in the diagram), the circuit is broken and no current flows. When the switch contacts are closed the circuit is made and current flows from the battery positive terminal through the switch contacts and bulb filament to ground. The frame completes the circuit to the battery negative terminal and the bulb illuminates.
Although some circuits on the circuit diagram may at first seem more complicated, it will generally be found that they can be broken down into sections which do not differ greatly from the basic circuit above.