Contents
Some of the most important Physics Topics include energy, motion, and force.
An Introduction to Electric Circuits
A cell (or battery) can make electrons move and electric current flow. But there must be a conducting path (like wires, bulb, etc.) between the two terminals of the cell through which electrons can move causing the electric current to flow. A continuous conducting path consisting of wires and other resistances (like electric bulb, etc.) and a switch, between the two terminals of a cell or a battery along which an electric current flows, is called a circuit. A simple electric circuit is shown in Figure.
In above Figure 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 bulb holder with a copper wire (called connecting wire) through a switch. The negative terminal of the cell is connected to the other end of bulb holder. In Figure (a) the switch is closed. So, the circuit in Figure (a) is complete and hence a current flows in this circuit. This current makes the bulb light up [see Figure (a)].
If we open the switch as shown in Figure (b), then a gap is created between the two ends of the connecting wire. Due to this, one terminal of the cell gets disconnected from the bulb and current stops flowing in the circuit. Thus, when the switch is open, the circuit breaks and no current flows through the bulb. The bulb stops glowing [see Figure (b)].
Symbols for Electrical Components (or Circuit Symbols)
In electric circuits, we have to show various electrical components such a cell, a battery, connecting wires, wire joints, fixed resistance, variable resistance, ammeter, voltmeter, galvanometer, an open switch, a closed switch, and an electric bulb (or lamp), etc.
Now, to draw the electric circuits by making the actual sketches of the various electrical components is a difficult job and takes a lot of time. So, the scientists have devised some symbols for electrical components which are easy to draw. They are called electrical symbols or circuit symbols. The common electrical symbols for electrical components which are used in drawing circuit diagrams are given below :
The symbol for a single cell is shown in Figure (a). The symbol of a single cell consists of two parallel vertical lines, one thin and long and the other thick and short (having horizontal lines on the sides). The longer vertical line represents the positive terminal of the cell (so it is marked plus, +), whereas the shorter vertical line represents the negative terminal of the cell (so it is marked minus, -).
Battery is a combination of two (or more) cells connected in series. In order to obtain a battery, the negative terminal of the first cell is joined with the positive terminal of the second cell, and so on. The symbol for a battery is shown in Figure (b). The battery shown in Figure (b) consists of two cells joined together in series. We can also draw the symbol for a battery having more than two cells in a similar way.
The resistance which can be changed as desired is called variable resistance. Variable resistance has two symbols shown in Figure (g). Variable resistance is also known as rheostat. Rheostat is a variable resistance which is usually operated by a sliding contact on a long coil (made of resistance wire).
A rheostat is used to change the current in a circuit without changing the voltage source like the cell or battery. It can do so by changing the resistance of the circuit. The galvanometer is a current-detecting instrument (which we will come across in the next Chapter).
The switch (or plug key) is a device for ‘making’ or ‘breaking’ an electric circuit. When the switch is open, then the circuit ‘breaks’ and no current flows in it [see Figure (k)]. But when the switch is closed, then the circuit is ‘made’ (or completed) and current flows in it [see Figure (l)].
Circuit Diagrams
Electrical circuits are represented by drawing circuit diagrams. A diagram which indicates how different components in a circuit have been connected by using the electrical symbols for the components, is called a circuit diagram. An electric circuit consisting of a cell, a bulb and a closed switch which was drawn in above 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 swich.
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. In this case, the electrons can move through the external circuit. These moving electrons form an electric current.
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 electrons cannot flow in this circuit due to the gap produced by the open switch.
Another simple electric circuit has been shown in Figure. In this circuit, a resistor R has been connected to the two terminals of a cell through a switch. An ammeter A has been put in series with the resistor R. This is to measure current in the circuit.
A voltmeter V has been connected across the ends of the resistor R, that is, voltmeter is connected in parallel with the resistor. This voltmeter is to measure potential difference (or voltage) across the ends of the resistor R. Before we go further and discuss Ohm’s law.