Electric Circuits – Definition and Explanation

Key Concepts

1. Light Bulb connected to a cell
2. Electric circuits
3. Fused Bulb
4. Circuit of a torch

Electric Circuits

Introduction

A torch is one of the simplest electrical devices which can be studied to understand the working of a simple electric setup. In this section, we will look at the same concept in detail and come across new terms in this chapter, such as electric circuits, circuit components and fused bulbs. 

Explanation

Light bulb connected to a cell: 

A light bulb can be connected to a cell by following the steps given below. 

• Take four pieces of wires of differently colored covers and of roughly equal lengths. 
• Remove a small piece of plastic cover from both the ends of each piece of wire. This exposes the thin metal wire inside it. 

• Fix the exposed part of two wires to the two terminals of the cell and the exposed part of the rest of the two wires to the terminals of the bulb. 
• The wires can be stuck to the cell and the bulb using rubber bands or adhesive tapes. 

• Now, connect the wires fixed to the cell to the wires fixed to the bulb in all possible ways. 

The ways in which the bulb and the cell can be connected are shown below. 

The bulb glows in the setups shown in (a) and (f) but not in any other setup shown. This is because,  

• In (a) and (f), there is a continuous path for the electricity to pass from the cell to the bulb. 
• In (a) and (f), both the terminals of the cell are connected to both the terminals of the bulb. 

However, in the rest of the setups, the following are seen. 

• In (b) and (c), the path offered to the electricity to pass from the cell to the bulb is broken, as the wires from the cell and the bulb are not connected on one side. 
• In (d) and (e), the positive and the negative terminals of the cell are not connected to the terminals of the bulb, respectively. 

Electric circuits: 

The arrangement shown in (a) and (f) is called an electric circuit. An electric circuit provides a complete path for the electricity to pass. Also, both the terminals of the cell should be connected to the bulb.  

The arrangements shown in the rest of the setups do not show a complete path for the electricity to pass.  Moreover, both the terminals of the cell are not connected to the terminals of the bulb in the latter two cases. Thus, these arrangements do not form electric circuits, as continuity is missing. In all these cases, the electric circuit is said to be broken. 

The various parts of an electric circuit are called the components of the electric circuit. Here, the cell, the wires and the bulb are the components of the electric circuit shown. 

An electric bulb glows only when an electric current flows through it. The direction of the electric current in an electric circuit is taken to be from the positive terminal to the negative terminal of the cell. The electric current is supplied by the cell and is taken to the bulb by the connecting wires, which makes the bulb glow. 

Fused electric bulb: 

Sometimes the bulb does not glow even though the electric circuit is complete. This might be because the bulb is fused. This means the filament inside the bulb is not intact but broken. This, in turn, breaks the electric circuit of which the bulb is a component. Hence, no current flows through the bulb. 

Electric circuit of a torch: 

To make a torch, we need a piece of wire, a small bulb and a cell. 

Take out small pieces of the plastic covering from both ends of the wire. 

Stick its one end to the negative terminal of the cell and wrap the other terminal around the base of the bulb. 

Now, bring the tip of the base of the bulb in contact with the positive terminal of the cell. 

The bulb would glow as the electric circuit is closed. There is a continuous path for the electric current to flow. A similar setup lies inside a torch. 

Now, if the bulb is moved away from the cell so that it gets disconnected from the cell, it stops glowing. 

This is because the circuit now breaks. There is no continuous path for the electric current to flow. 

The bulb would glow even for the setup shown below, as this is also a closed circuit. 

Question and answer: 

In which of the following circuits does the bulb not glow? Give reasons for your answer. 

Answer: 

In all cases, the light bulb will glow. This is because all the circuits are closed, allowing the flow of electric current.

Summary:

1. In a torch a battery of two or more cells is connected to the bulb using wires to lend
   electricity to the bulb.
2. An electric circuit provides a complete path for the electricity to pass.
3. An electric circuit is said to be complete if it lends a continuous path for the electric
   current to flow. Such a circuit is called a closed circuit.
4. On the other hand, if there is a discontinuity in the path in an electric circuit, it is said to
   be broken.
5. The direction of the electric current in an electric circuit is taken to be from the positive
   terminal to the negative terminal of the cell.
6. In a torch, the electric current runs through the bulb, making it glow.
7. A bulb whose filament is broken is said to be fused. Such bulbs do not glow.
8. The broken filament of a fused bulb breaks the electric circuit which does not let the
   electric current flow through the bulb.