Simple Electrical Circuits

Simple electrical circuits can be represented by circuit diagrams, in which the various components in the circuit are shown by using standard symbols like those illustrated below. These diagrams are usually restricted to showing the power source (battery), switches, loads (typically bulbs), and the path of the wiring. To answer these questions you need a basic understanding of how electricity flows around a circuit.

Mechanical Aptitude Tests Electrical Circuit Components

In order for a circuit to work, it needs to be complete and there should be no ‘short circuit’. To check for a complete circuit, the wire coming out of the battery should return to the opposite side of the battery after passing through any intermediate components like switches and lamps. If there is any break in the circuit then electricity cannot flow, this is known as an ‘open circuit’. A ‘short circuit’ exists if you can follow the circuit from one side of the battery to the other without going through any other component.

You should also know the difference between series and parallel connections in circuits.

Mechanical Aptitude Tests Series and Parallel Circuits

The diagram on the left shows a series circuit. There are two bulbs (labeled B1 and B2), connected in a chain from one terminal of the battery to the other. As you can see, there is only one path for the current to flow.

The diagram on the right shows a parallel circuit. This time, the two bulbs are wired in such a way that there is more than one continuous path for electrons to flow. Each individual path (through B1 and B2) is called a branch. This means that in a parallel circuit, all of the components are connected between the same set of electrically common points.

In the series circuit example (on the left) removing one of the bulbs will create an ‘open circuit’ and no current will flow. In other words, the remaining lamp will not light up. In the parallel circuit (on the right) removing one of the bulbs will only create an ‘open circuit’ in that particular branch of the circuit and the other branch will not be affected, meaning that the remaining light will light up.

When two or more components are connected in series, the same current flows in each component but the total potential difference of the supply is shared between them. This means that if you add together the voltages across each component connected in series, the total equals the voltage of the power supply. For example, if two identical bulbs are connected in series to a 12V battery then the potential difference across each of them is 6V.

When two or more components are connected in parallel, the total current flowing in the circuit is shared between the components but the potential difference across them is the same. This means that if a voltage across a lamp is 12V, the voltage across another lamp connected in parallel is also 12V.

Mechanical Aptitude Tests Electrical Circuit Conventions

You may also see wires crossing in these diagrams. If the wires are connected then they will be shown as in the example on the left. If they are not connected then they will be shown as in the example on the right.

Key Points

  • You need to know the standard symbols for a power source (battery), switches, loads (typically bulbs), and the path of the wiring.
  • In order for a circuit to work, it needs to be complete and there should be no ‘short circuit’.
  • In a series circuit, all components are connected end-to-end, forming a single path for electrons to flow.
  • In a parallel circuit, all components are connected across two sets of electrically common points.
  • Crossing wires may be shown either connected or not connected.