Current Electricity - Part I

Lights, electric fans, refrigerator or a computer, washing machine, water heater or a television (and the list is endless), all run because of the electric current which is flowing. The electric current or electricity is generated at a site and transmitted across overhead pylons to domestic or industrial use. Electric current is nothing but the flow of electrons. We know that electrons are negatively charged particles. They will flow towards a physical entity that is positive. When the electrons flow, they carry energy that is the electrical energy. This electrical energy is converted into other forms of energy by appliances that we utilize either at home or in the industry.

Effects of current electricity are quite different from static electricity. In current electricity because the charges move, they have many more applications and manifestations. One big difference is that current electricity is able to give rise to magnetic effects. This is completely absent in case of static electricity. The second difference is that electric current flows through good conductors of electricity like metals. The static electricity, on the other hand develops in bad or poor conductors of electricity.

What we will study in this chapter :

1. Potential difference and how is electric current generated
2. Electrical resistance and the Ohm’s law  
3. Power

1. Potential difference and how is electric current generated
Take a dry battery and a small bulb (may be a torch bulb) with two wires. Connect the two wires at the two ends of the bulb. If you see the battery closely, you will see that one end has a positive mark and the other end has a negative mark. Now touch the two wires at the same end of the battery. The bulb will not light up. Next, touch the two wires on the different ends of the battery : the bulb will light up. This is a very simple demonstration of flow of electricity or electric current.  

The reason why the bulb lights up is this : the dry battery has chemicals which makes one terminal more negative than the other. The negative side has more electrons stacked up and hence it is marked (-). The other side has positive charges or ions stacked up and it is marked (+). The electrons want to go to the other side and neutralize the charges. When the wires from the bulb touch the same side the electrons are not flowing. On the other hand when the wires touch two different ends of the battery, electrons are able to flow from one side of the battery to the other side, via the bulb. The bulb has a coiled filament that heats up when the electrons pass through it. The filament becomes red or white hot and glows to give light as we know it.  

There will be two questions arising in your minds : why does not the positive charge flow in the opposite direction of the electrons? And if we touch the two sides of the battery, why don’t we feel the current pass through our fingers? The positive ions are more bulky than the electrons. The mobility of positive ions is very low. The electrons move more quickly. Therefore when we talk of current electricity, we say that the electricity is generated because of the flow of the electrons. Also when we touch both the ends of the battery, the current does flow in our fingers, but the strength of the current is very small, so we do not feel it. If the strength were large, we would get an electric shock. Also you may realize that when the battery is used, there is a continuous neutralization of charges; this is the reason why a battery has a certain life.

The difference between the strength of the positive and negative sides of a physical entity (like a battery or a capacitor) is known as a potential difference. Unit of potential difference is a Volt (V). (see box for knowing about Volta)

We have already seen that a unit of electric charge is a coulomb (C). 1 Volt is also defined as follows : if a charge of 1C moved between two points and the work done is 1 joule, then the potential difference between the two points is 1 V. Volt is measured by an instrument called a voltmeter.

An electric circuit is built as shown in the adjacent figure. A battery is connected to a bulb via a key or a switch. (This is the familiar circuit for a torch battery). If potential difference has to be measured between any two points, a voltmeter can be connected across the points, as shown.  

The electrons generate an electric current. Current is the rate at which the electrons flow.

Current (I)   = charge flowing per unit time = Q/t, 

where Q is the charge flowing in the circuit and t is the time duration of the flow of charge. The unit of measuring electric current is an Ampere (A). 1 A  of electric current is generated when a charge of 1 C flows across a point in a circuit in 1 sec.  If the charge flowing in 1 sec is more, say 5 C, then we say that there is a current of 5A flowing through the circuit. Current is measured by an ammeter.  An ammeter has to be connected within the circuit to know the current flowing in the circuit.  

In our domestic electric supply, the power plugs will show a potential difference of about 220V. The current flowing through the small plugs is about 5 A and through the large plugs is about 15 A.

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