Electronics Online

The unit of electrical power is the watt (W), named after James Watt (1736-1819). One watt of power equals the work done in one second by one volt of potential difference in moving one coulomb of charge.

Remember that one coulomb per second is an ampere. Therefore, power in watts equals the product of amperes times volts.
Power in watts = volts x amperes

P = E x I

Example: A toaster takes 5 A from the 240V power line. How much power is used?

P = E x I = 240 V x 5 A
P = 1200 Watts

Example: How much current flows in the filament of a household 75 watt light bulb connected to the normal 240 Volt supply?

Ohm’s Law describes the relationship between current, voltage and resistance in an electric circuit.
Ohm’s Law states:

The current in a circuit is directly proportional to voltage and inversely proportional to resistance.

Let:

I = current
E = voltage
R = resistance

Part of Ohms Law says: current is directly proportional to voltage.
Using the symbols given, we can write an equation to show a direct proportion between current and voltage.

I = E

Normally the above equation is read I ‘equals’ E. It can just as easily and more understandably be read as: I is directly proportional to E.
I know I harp on the direct proportion and inverse proportion stuff a lot. I do so because it is so important to thoroughly understand this when we come to more complex equations.

I = E

When components in a circuit are connected in successive order with the end of each joined up to the other end of the next as shown below in figure 1, they form a series circuit.

An electric current consists of an ordered movement of electrons. In the schematic shown in figure 1, the current leaves the negative terminal of the battery and flows through R1, R2 and R3.

It does not matter where we measure the current in a series circuit as we will always get the same value of current everywhere, as:

Current is the same in all parts of a series circuit.

The total resistance of any number of resistances in series is simply the sum of the individual resistances:

Rt = R1 + R2 + R3.. etc