Electric Circuit

An electric circuit is a complete path through which negative charges flow from the negative terminal to the positve of the cell. Such one directonal flow gives rise to what we call direct current (d.c).

A d.c. circuit connects a d.c. source like a cell (or a collection of cells call a battery) to various output devices. The load, like lamps, tools or resistors in heating element, as well as switches, fuses, ammeter and voltmeter are connected via metalic wires, normally copper wires.

An opened and closed circuit

Charges must travel in a complete path of a circuit. If the circuit is broken, there will not be any potential difference between  the terminals of a cell and the charges will not move from one terminal of the cell to the other.

A closed circuit is a complete conducting path round which charges can flow continously, while an open circuit is one or more points in the conducting path so that current does not flow anywhere in the circuit.

                                          
Figure 1. A opened circuit

Figure 2. A closed circuit

Series circuit

When a battery is connected to two lamps as shwon below, these lamps are said to be connected in series.

For convenience of discussion, the direction of current in this chapter prefers to conventional current (from positive terminal to negative terminal of a cell).

Figure 3. A series circuit.

Note that charges that flow through one lamp (lamp A) must go through the other lamp (lamp B) as well, for both lamps to work in a series circuit. Thus, the current at lamp A is the same size as that lamp B. In fact, current at any point in a series circuit is the same size throughout.

I₁ = I₂ = I₃

Since there is only one single conducting path in a series circuit, when one of the lamps blows, the other lamp will not light up even though the switch the two lamps in a series circuit is dependent on each other.

Parallel circuit

When battery is connected to two lamps, B and C as shown, we say that the two lamps are connected in parallel.

                                          
Figure 4. A parallel circuit

When lamp C blows, lamp B will continue to light up. This is because the other branch with lamp B is still a closed circuit. So lamps in parallel work independently of one another.

In parallel circuit, current at lamp A is greater than that at lamp B or C (current in separate branches). In fact, the current at lamp A is sum of the current at lamps B and C.

However, it is noted that lamp A is not connected in parallel to lamp B; neither is lamp A parallel to lamp C. if lamp A is ‘blown’, both lamps B and C will not light up, since it is open circuit.

I₁ = I₂ + I<sub>3


Download Virtual Labs Electricity here</sub>