Contents
By learning Physics Topics, we can gain a deeper appreciation for the natural world and our place in it.
What is an Electric Circuit? Types of Circuits and Network
An electric circuit is a path along which electricity can flow. We can make a simple electric circuit by using a cell, a torch bulb and two pieces of wire (called connecting wires). Connect the torch bulb to the positive and negative terminals of a cell by using two pieces of wire as follows :
One end of a wire is connected to the positive terminal (metal cap) of the cell and the other end of this wire is connected to the metal tip at the base of the torch bulb [as shown in Figure (a)],
Similarly, one end of the second wire is connected to the negative terminal (metal disc) of the cell and the other end of this wire is connected to the metal case of the torch bulb [as shown in Figure (a)], On connecting the torch bulb between the two terminals of the cell, the bulb lights up or glows [(see Figure (a)].
The bulb lights up because electricity flows through its filament. Actually, electricity (or electric current) from one end of the cell flows along the wire through the bulb and then back to the cell along the other wire. The wires and bulb form a continuous path between the two terminals of the cell for electricity to flow through.
This path is called an electric circuit. Thus, the electric circuit provides a complete path for electricity to pass (or current to flow) between the two terminals of an electric cell. The bulb glows only when electricity (or electric current) passes through the circuit.
The electric circuit in which there is no gap in the connections between the terminals of the cell, wires and bulb, etc., is called a complete circuit or closed circuit. A bulb lights up when the circuit is complete (or closed). Thus, electricity flows through a complete circuit (or a closed circuit).
A closed circuit is shown in Figure (a). In a closed circuit, the electric current passes from one terminal of the cell to the other through connecting wires and bulb, etc. In an electric circuit, the direction of flow of electricity (or electric current) is taken to be from the positive terminal of the cell to the negative terminal of the cell [as shown by arrows on the wires in Figure (a)].
The electric circuit in which there is a gap in the connections between the terminals of the cell, wires and the bulb, etc., is called an open circuit (or incomplete circuit). Electricity does not flow through an open circuit because there is a gap in its path (or break in its path). A bulb will not light up if there is a gap in the circuit. In other words, a bulb will not light up if the circuit is open (or broken). An open circuit is shown in Figure (b).
In the circuit shown in Figure (b), though the wire going from the positive terminal of the cell is touching the metal tip at the base of the torch bulb but the wire going from the negative terminal of the cell is not touching the metal case of the torch bulb.
There is a gap in the end of this wire and the metal case of the torch bulb. It is due to this gap that the circuit in Figure 7(b) is said to be open (or incomplete). Since the circuit shown in Figure (b) is open (with a gap), no electricity from the cell can flow in this circuit and hence the bulb does not light up.
The case of Fused Bulb
Sometimes an electric bulb does not glow even if it is connected to a cell by wires without any gap in the circuit. This can happen if the bulb has got ‘fused’. When the filament (thin wire) of a bulb gets broken, the bulb is said to be fused. Thus, a fused bulb has a broken filament (as shown in Figure).
A bulb may fuse due to normal wear and tear of the filament or when too much electricity is passed through it suddenly. A break in the filament of a bulb means a break in the path of electricity between the terminals of the cell. So, when we have a fused bulb in a circuit, the circuit gets open (or broken) and electricity cannot pass through it. Since no electricity passes through the broken filament of a fused bulb, a fused bulb does not light up even if it is connected properly to a cell.
In Figure (a), we have made the electric circuit by using metal wires (such as copper wires) because copper wires are conductors of electricity. That is, copper wires can conduct (or carry) electricity through them. If, however, we make a circuit using cotton threads or jute strings, then the bulb will not glow.
This is because the cotton threads or jute strings are non-conductors of electricity (they are insulators). That is, cotton threads or jute strings cannot conduct (or carry) electricity through them. Thus, electricity can only flow if there is a complete circuit made of conductors (such as electric wires, bulb, etc.)
A Switch
If we want to ‘turn on’ or ‘turn off’ a bulb (or any other electrical appliance) easily, we should connect a switch in the electric circuit. The switch is a simple device which is used to ‘open’ or ‘close’ a circuit. An open circuit means that there is a break (or gap) in the circuit whereas a closed circuit means that it is a complete circuit (having no gap). So, we can also say that a switch is a simple device which breaks a circuit or completes a circuit, as desired by us. A switch works by opening and closing a gap in an electric circuit.
When the switch is in the ‘off position’, a gap opens up in the circuit due to which electricity stops flowing in the circuit and hence the electrical appliance stops working [see Figure (a)). On the other hand, when the switch is ‘turned on’, the gap in the circuit is closed and a complete circuit is made [see Figure (b)).
Due to complete circuit, electricity starts flowing in the circuit and electrical appliance will now start working. Thus, the purpose of using a switch is to ‘turn on’ or ‘turn off’ an electrical appliance by closing the circuit or opening the circuit respectively.
The switches used in the circuits of electric bulbs and other electrical appliances in our homes work on the same principle of closing or opening the circuit but their designs are more complex. Some of the electrical appliances which have inbuilt switches are : table fan, room heater, room cooler, mixer and grinder, table lamp, washing machine, hair drier, drilling machine, torch, television, radio set and microwave oven. We can also make a simple electric switch ourselves. This is described below.
Activity To Make A Simple Switch
We can make a simple switch by using a small sheet of thermocol (or a small wooden board), two drawing pins, a safety pin (or a paper clip) and two pieces of wire. This can be done as follows : Insert a drawing pin into the small ring at one end of the safety pin and fix it on the thermocol sheet as shown in Figure (a). Make sure that the safety pin can rotate freely around the fixed drawing pin. Also tie one end of a piece of wire around the drawing pin as shown in Figure (a).
Now fix the second drawing pin on the thermocol sheet at such a distance from the first drawing pin that the free end of safety pin can touch it when the safety pin is turned [see Figure (a)], A piece of wire is also tied around the second drawing pin (to connect the switch in a circuit).
So, now we have a safety pin whose one end is fixed in a drawing pin and the other end is free to turn so as to touch (or not to touch) the second drawing pin. When the safety pin arranged in this way is connected in a circuit (through the two wires attached to drawing pins), it behaves like a switch to ‘open the circuit’ or ‘close the circuit’ as desired.
Figure (a) shows the switch in the ‘off position’. There is a gap between the end of safety pin and second drawing pin. The circuit is open and hence no current can flow. When we rotate the safety pin and bring its free end in contact with the second drawing pin, then the gap in the circuit is closed [see Figure (b)].
The switch is now in the ‘on position’. Since the circuit is now complete, electricity can flow through it. We can bring the switch in ‘off position’ again by moving the tip of safety pin away from the second drawing pin. The drawing pins, the safety pin and wires, all are made of metal and hence they are conductors of electricity (they allow electricity to pass through them).
So, when the free end of safety pin touches the other drawing pin, the circuit is completed (because safety pin allows electricity to pass through it). We can bring the switch in ‘off position’ again by moving the free end of safety pin away from y^the second drawing pin (so that a gap is created between them).
We have already described an electric circuit consisting of a cell, a torch bulb and wires. We will now include a switch (which we have just made) in this circuit and see how it actually works. Before we do that, please note that it is usually very difficult and inconvenient to keep the two wires (coming from a cell) in touch with the two terminals of a torch bulb just by holding the bulb in hand or by fixing with adhesive tape.
So, for the sake of convenience, the torch bulb is fixed in a bulb holder [see Figure (a)]. The two terminals of the bulb are connected to the two terminals of the bulb holder for making connections in the circuit (A torch has an inbuilt bulb holder to hold the bulb in fixed position). Similarly, a cell can also be put in a small cardboard box so as to hold two wires in contact with its two terminals [as shown in Figure (a)], rather than using rubber bands or adhesive tape for this purpose.
Adding a Switch to the Electric Circuit
We have already described an electric circuit consisting of a cell, a torch bulb and two wires which makes the bulb light up [see Figure (a) on page 186]. Now, if we want to turn the bulb ‘on’ and ‘off’ easily, we should add a switch in the circuit. This can be done as follows :
In Figure (a), we have a cell having a positive terminal (+) and a negative terminal (-). The positive terminal of the cell is connected to one end of the switch with a wire. Another wire connects the other end of switch to one side of the bulb holder.
The negative terminal of the cell is connected to the other end of the bulb holder directly by means of a wire. Thus, the positive terminal of cell is connected to the bulb holder through a switch but the negative terminal of the cell is connected to bulb holder directly [see Figure (a)].
(a) In the circuit shown in Figure (a), the safety pin is not touching the other drawing pin, so the switch is in the ‘off’ position. There is a gap between the end of safety pin and the second drawing pin. The circuit is incomplete (or open) due to which electricity does not flow in the circuit in this case. So, the bulb does not light up.
(b) Move the safety pin so that its free end now touches the other drawing pin (see Figure (b). We will see that the bulb lights up. Actually, when we make the free end of safety pin touch the other drawing pin, the safety pin covers the whole gap between the two drawing pins.
Since the safety pin is made of metal, it completes the circuit and allows electricity to pass through it. Due to this the bulb lights up (see Figure (b)]. The switch is now said to be in ‘on’ position. As long as the switch remains in the ‘on’ position, electricity will keep on flowing in the circuit and the bulb will continue to glow.
(c) Now move the free end of the safety pin away from the second drawing pin. This will again create a gap in the circuit. The electricity will stop flowing in the circuit and hence the bulb will stop glowing [see Figure (a)]. Thus, we can turn off the lighted bulb by bringing the switch back to the ‘off’ position. In fact, the switch can be used to ‘turn on’ or ‘turn off’ the bulb again and again as desired by us.
Torch
A torch is a portable electric lamp which uses two (or more) cells to light a small bulb A torch is used to provide light when going out during a dark night or when electricity supply fails at home. The torch has a bulb that lights up when it is switched on. The inside view of a simple torch is shown in Figure.
We will now describe the working of a simple torch. A torch contains a simple electric circuit. In a torch, two (or more) cells are connected to a torch bulb through a sliding switch (see Figure). When the torch is needed to provide light, we close the sliding switch by pushing it forward so that the circuit is completed and the bulb lights up (as shown in Figure). The thick black line in Figure shows the’path of electricity flowing in the torch when it is switched on.
When the torch light is not needed, the sliding switch is opened by pushing it backwards so that the circuit breaks and the bulb is turned off (see Figure). Please note that a torch gets electricity from the cells kept inside it.
A torch has two (or more) cells. The cells in a torch have to be joined correctly. The two cells in a torch are joined like this: The positive terminal of one cell is always kept in contact with the negative terminal of the other cell. In Figure, the positive terminal of first cell is in contact with the base of the bulb. And the positive terminal of second cell is in contact with the negative terminal of the first cell.