Physics Topics can help us understand the behavior of the natural world around us.
What is Electric Circuit With Symbols, And Components Used
Electricity is a form of energy. The flow of electricity through a conductor (such as a metal wire) is called electric current (or just ‘current’). Actually, in everyday life, the words ‘electricity’ and ‘electric current’ are used in the same sense.
Electricity (or electric current) is useful because it can run many appliances (or machines) in our day to day life which make our work easier. The electric bulb, tube lights, radio, television, stereo systems, refrigerator, washing machine, fans, coolers, airconditioners, computers, electric iron, electric kettle, room heater, electric oven, mixer and grinder, geyser, water pumps and trains, etc., all work with electricity (or electric current).
Electricity is produced at power stations (also known as power plants). The big electric generators at power stations produce electricity. From the power stations, electricity is brought to our homes through thick wires atop tall electric poles. Underground electric wires (called cables) are also used to bring electricity into our homes.
Though electricity is very useful but it can also be very dangerous. If electric wires and electrical appliances are handled carelessly, then a person can get an electric shock which may cause severe burns and even death. Improper handling of electricity can also cause fires.
The electricity produced by portable electric generators and inverters (used in our homes and shops during power cuts) is equally dangerous. This is because electricity from electric mains (power supply line), portable generators as well as from inverters is at a very high voltage of 220 volts.
We should never use electricity (or electric current) from electric power supply line (which is called ‘mains electricity’) through sockets in our home or school, or that produced by portable generators or inverters, for performing science experiments because it is very unsafe to do so [see Figure (a)]. It may give us an electric shock, which may be dangerous. We can use an electric cell to obtain electricity (or electric current) that is safe for performing science experiments [see Figure (b)].
An electric cell is usually called just ‘cell’ but it should not be confused with the ‘cells’ which we study in biology. An electric cell provides much less electricity than that provided by the electric supply line (or power supply line). For example, a single electric cell usually provides only 1.5 volt of electricity whereas the electricity from the power station which we get in our homes is at a very high voltage of 220 volts !
We should use only electric cell (or battery made of electric cells) for performing all the experiments and activities based on electricity (or electric current). In order to obtain electricity (or electric current) from a cell or battery, we have to connect it into a circuit.
So, we will now study electric circuits. Before we do that please note that electric current is actually a flow of charged particles called “electrons” in a conductor (such as a metal wire). We will study electrons in higher classes.
A cell (or battery) is a source of electric current but this current cannot flow out on its own. There must be conducting materials (like wires, bulb, etc.) between the two terminals of the cell through which electric current can flow. A continuous conducting path (consisting of wires, bulb, switch, etc.) between the two terminals of a cell or battery along which an electric current flows, is called a circuit. A simple electric circuit is shown in Figure (a).
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 piece of copper wire. The other end of switch is connected to one end of bulb holder with another piece of copper wire.
The negative terminal of the cell is connected directly to the other end of bulb holder with a copper wire. In Figure (a), the switch is in ‘on’ position (or closed). Due to this the circuit in Figure 2(a) is complete and hence a current flows in this circuit. This electric current makes the bulb light up (or glow) [see Figure (a)].
If we open the switch as shown in Figure (b) so that it comes in the ‘off’ position, then a gap is created between the two ends of the copper wire (or connecting wire). Due to this, one terminal of the cell gets disconnected from the bulb and hence the current stops flowing in the circuit. Thus, when the switch is open (or in ‘off’ position), the circuit breaks and no current flows through the bulb. The bulb stops glowing.
When the switch in a circuit is ‘closed’, then the switch is said to be in the ‘on position. And when the switch in a circuit is ‘open’, then the switch is said to be in the ‘off’ position. In an electric circuit, a bulb lights up (or glows) only when the switch is in the ‘on’ position and the electric circuit is complete [see Figure (a)]. On the other hand, the bulb does not light up (or does not glow) when the switch is in the ‘off’ position and the electric circuit breaks [see Figure (b)].
Sometimes, however, the bulb does not light up (or glow) even when the switch is in the ‘on’ position (or closed). This can happen only if the bulb gets fused (and its filament breaks). In an electric bulb (say, a torch bulb) there is a thin wire called filament which becomes white-hot and glows when electric current passes through it. When the bulb gets fused, then its filament breaks.
The filament of bulb is a part of the electric circuit. So, when the filament of bulb breaks, then the electric circuit also breaks (and becomes incomplete). Under these conditions, the bulb will not light up (or glow) even if the switch is in the ‘on’ position (or closed). Please note that a switch can be connected anywhere in a circuit. Suppose we have two (or more) bulbs connected in the same circuit.
Now, when we bring the switch of such a circuit in the ‘on position, then electric current will flow instantly through the whole circuit due to which all the bulbs will light up (or glow) together at once. When the switch is in the ‘off’ position, then no current can flow in any part of the circuit and hence no bulb can light up (or glow).
Symbols for Electrical Components (or Circuit Symbols)
In electric circuits we have to show a cell or battery, switch, bulb (or lamp) and connecting wires, etc. To draw the electric circuits by making the actual sketches or drawings of the cell, battery, switch, bulb (or lamp) and connecting wires is a difficult job and takes a lot of time.
So, the scientists have devised some symbols for electrical components like the cell, battery, switch, electric bulb (or lamp) and connecting wires, etc., which are easy to draw. And these symbols are used to draw the electric circuit diagrams easily and clearly. Some of the commonly used symbols for electrical components in the circuit diagrams are given in Figure.
The symbol for a cell is shown in Figure (a). The symbol of a cell consists of two parallel lines, one thin and long, and the other thick and short (having horizontal lines on the sides). The long and thin line represents the positive terminal of the cell whereas the short and thick line represents the negative terminal of the cell. The symbol for a battery is shown in Figure (b).
The battery shown in Figure (b) consists of two cells joined together. Please note that in the symbol for a battery, the positive terminal of one cell is joined to the negative terminal of the other cell. We can also draw the symbol for a battery having more than two cells in a similar way but in every case the positive terminal of one cell will be connected to the negative terminal of the adjoining cell.
Thus, the symbol for representing a battery of three cells will have three pairs of long and short parallel lines connected to one another. The symbol for a battery of four cells will have four pairs of long and short parallel lines joined to one another, and so on.
The symbol for an electric bulb (or lamp) is shown in Figure (c). The symbol of electric bulb consists of a circle having a semi-circle inside it. The semi-circle represents the filament of the bulb. The switch is a device which is used to ‘make’ or ‘break’ an electric circuit. The symbol of an open switch is shown in Figure (d) whereas the symbol of a closed switch is given in Figure (e).
Please note that a switch is sometimes also called a key. The symbol for a connecting wire (copper wire) is shown in Figure (f). The connecting wires (which are used to connect the various components of a circuit like cell, switch, bulb, etc.) are represented just by drawing straight lines. These straight lines can be horizontal as well as vertical.
From the above discussion we conclude that it is very convenient to represent various electrical components (such as a cell, battery, bulb, switch and connecting wires) by symbols. And by using these symbols, electric circuits can be represented conveniently by drawing their circuit diagrams. This is discussed below.
A diagram which tells us how the various components in a circuit have been connected by using the electrical symbols of the components, is called a circuit diagram. It is much easier to draw a circuit diagram by using symbols. So, we usually represent an electric circuit by its circuit diagram.
An electric circuit consisting of a cell, a bulb and a closed switch which was drawn in Figure (a) can be represented by drawing a circuit diagram shown in Figure (a). In the circuit diagram shown in Figure (a), a bulb has been connected to the two terminals of a cell by copper wires through a closed switch.
The electric circuit consisting of a cell, a bulb and an open switch which was drawn in Figure (b) can be represented by drawing a circuit diagram shown in Figure (b). In the circuit diagram shown in Figure (b), a bulb has been connected to the two terminals of the cell by copper wires through an open switch.
The circuit shown in Figure (a) is complete (due to closed switch). Since there is no gap, therefore, current flows in this circuit and the bulb lights up. The circuit given in Figure (b) is broken (due to a gap because of open switch), so no current flows in this circuit and bulb goes off.
The electric cell is a common source of electric current to run a number of devices in our everyday life. For example, electric cells are used in torches, radios, TV remote controls, cassette players, electric clocks and watches, toys and laboratory experiments. A single electric cell gives 1.5 volts of electricity.
Many devices do not work properly with just one cell because they require higher voltage (than 1.5 volts) for their working. The higher voltage can be obtained by combining a number of cells in series. When the positive terminal of one cell is joined with the negative terminal of the other cell, then the cells are said to be joined in series.
A group of cells joined in series is called a battery. We can now say that many devices need a battery (of cells)-for their working. A battery of two cells gives 1.5 × 2 = 3.0 volts ; a battery of 3 cells gives 1.5 × 3 = 4.5 volts ; a battery of 4 cells gives 1.5 × 4 = 6.0 volts of electricity, and so on.
Thus, in many appliances more than one electric cell is used (to obtain the desired voltage). For example, in a torch, usually two cells are used. And in a radio, usually four cells are used together (see Figure). We will now describe how the cells are joined together to make a battery.
When joining two (or more) cells to make a battery, the positive terminal of one cell is always kept in contact with the negative terminal of the other cell. We know that the positive terminal of a cell is the brass cap and the negative terminal is the bottom of the zinc container.
So, the cells are joined in such a way Figure. A battery of that the brass cap of one cell touches the zinc bottom of the other cell (as shown two cells. in Figure). Figure shows a battery made of two cells A and B.
In Figure, the positive terminal (brass cap) of cell B has been kept in contact with the negative terminal (bottom of zinc container) of cell A. The positive terminal of cell A and the negative terminal of cell B are free to draw current from this combination (or battery) of two cells.
If, however, we combine two cells by keeping the positive terminal of one cell in contact with the positive terminal of the other cell, then the battery obtained will not work. It will not give any current when connected in a circuit. For example, in Figure (a), the positive terminal of one cell is in contact with the positive terminal of the other cell. This is a wrong way of joining cells to make a battery.
Similarly, if we combine two cells by keeping the negative terminal of one cell in contact with the negative terminal of the other cell, even then the battery will not work. In Figure (b), the negative terminal of one cell is in contact with the negative terminal of the other cell. This is also a wrong way of joining cells to make a battery.
In Figure we have shown the combination of only two cells to form a battery. We can, however, join together any number of cells in a similar way to form a bigger battery. For example, a battery made of four cells joined together is shown in Figure.
In a torch, the cells are placed one after the other (in one straight line) as shown in Figure. In many devices, the cells are not placed one after the other, the two cells are placed side by side (parallel to each other). For example, in a TV remote control, the two cells are placed side by side. When the two cells are placed side by side, they are joined to make a battery as shown in Figure (a).
In this case, the positive terminal of one cell cannot be in touch with the negative terminal of the other cell. So, the positive terminal of one cell is joined to the negative terminal of the other cell by a piece of wire [see Figure (a)]. Actually, when the two cells are to be placed side by side in an appliance, then there is a special arrangement to connect the positive terminal of one cell to the negative terminal of the other cell inside its battery compartment (or cell holder).
There is usually a thick wire or a metal strip in the battery compartment of the appliance which connects the positive terminal of one cell to the negative terminal of the other cell automatically when the cells are placed inside it [see Figure (b)], Moreover, in order to help us to place the cells correctly in the battery compartment, the symbols plus (+) and minus (-) are printed in the two sides of the battery compartment.
The battery compartment of an appliance to join two cells kept side by side is shown in Figure (b). The devices such as torches, radios, TV remote controls and toys use batteries (made of two or more cells).
The batteries used in cars, buses, trucks, tractors, and inverters, etc., are also made up of cells. The cells of these batteries are, however, different from the ordinary cells which we use in torches, etc. These cells are called storage cells.
A car battery has 6 cells joined to one another in series. Each cell of car battery produces 2 volts of electricity. So, the electricity produced by a car battery is at 2 × 6 = 12 volts. A cut out of the car battery showing its six cells is given in Figure.
A special feature of car battery is that its cells can be recharged. Ordinary cells, however, cannot be recharged. Once exhausted, they have to be thrown away. We will now discuss the effects of electric current.