Basics of Electricity

Circuits can be drawn using symbols to give a visual representation of what the desired circuit should include. Symbols allow scientists to visualise a circuit without a language barrier as symbols are universal. Here is some of the circuit symbols you will come across in this unit:

Electric current, which is measured in amperes \text{(A)} is defined as “the rate of flow of charge”.

As an equation, this can be written as:

I=\dfrac{\Delta Q}{\Delta t}

• I= current in amperes \text{(A)}
• \Delta Q= the charge in coulombs \text{(C)}
• \Delta t= the time in seconds \text{(s)}

Conventional current is defined as the flow of charge from the positive terminal to the negative terminal within a circuit. However, this definition was given before scientists had a full understanding of electricity and is actually opposite to the flow of electrons which flow from negative to positive.

To measure current, an ammeter is often used. For measuring very small currents, a microammeter may be used. These pieces of equipment can only work if they are inserted in series into a circuit.

Potential difference (p.d) is defined as “the work done per unit of charge” and is measured in Volts \text{(V)}. This can be written as an equation in the form:

V=\dfrac{W}{Q}

• V= the potential difference in volts \text{(V)}
• W= work done in joules \text{(J)}
• Q= charge in coulombs \text{(C)}

The potential difference is a result of the power source which causes one end of the circuit to become positively charged, whilst the other end of the circuit becomes negatively charged.