What is current?
We all have seen a battery. It is in our car, mobile etc. Headlights of car or bike come on when we press a switch. Something goes from battery into the lights which causes bulbs to glow. This is current. How did this happen?
We connected two terminals of the battery to two terminals of the bulb through an ON/OFF switch. Battery has one positive (red) and one negative (black) terminal. Connecting these to bulb looks like this:
If we move from red terminal A of battery we go to red terminal B of bulb, we go inside the bulb, come out at black terminal C of bulb and back to black terminal D of battery. Duniya gol hai!
This is called a closed circuit. We easily see that unless the circuit is closed the bulb cannot light up. Making switch OFF means that circuit is no longer CLOSED circuit, hence bulbs go off.
Battery is like overhead water tank in our house; bulb similar to bathroom tap. Unless there are pipes connecting there cannot be any water reaching the bath room. If water tank also was inside the bathroom can water still flow to the tap? Or if tank is at ground floor and we live on first floor can water flow in our bathroom? NO.
We learnt from our lesson on Atom that if we could supply some additional energy to an electron rotating in the outermost orbit (shell) we could dislodge it causing it to jump out of its orbit and become suspended in air (nothingness) for some period of time and remaining FREE. Free to do my work.
What work? – “ carry charge and deliver it to…”.
We told you that an electron is like an ant carrying a head load of charge which it delivers and then dies. A new ant enters from source to replace this and that died. What does it mean: number of Free electrons remain constant; a dead ant must be replaced with a new ant.
We know how much charge an ant can carry. LOOK UP IN YOUR BOOK.
If there were 100 ants they would carry 100 e units of charge.
1000 ants would carry 1000 e coulombs of charge, and so on. It is like data in MB or GB in your mobile.
If it is only electrons that are travelling from one side to another how can I define current?
Think of stream of electrons ( ants) moving from left to right. Suppose they have to pass through a door. Suppose we could count number of electrons passing through the door in ONE SECOND and every next second we could tell teacher “ 1000 electrons passed the door in one second” OR “1000 e units of charge passed… in one second”
NOTE ONE SECOND.
We call this value as CURRENT in Amperes. In other word the above statement means “ a current of 1000e AMPS was flowing when we counted the electrons in ONE second”
We note a few things:
1. Unless current flows in the bulbs they cannot glow. Electrons inside the battery are running at high speeds but are not able to escape because we have not connected the wires (path for electrons) to bulbs. Thus, circuit must be a closed circuit.
2. Electrons, unless they MOVE are of no use to us. They must work. OK, How do we move them?
We know that opposite charges attract each other. We know electron is negative. If only we could provide a connection with a positive thing and provide a PATH like connecting wires then electrons being at a LOWER potential ( -1 is lower than 0; 1 is < 2; etc) they will be attracted by HIGHER potential thing – THEY now MOVE by attraction force. Bigger this attraction force faster they will move; more electrons will pass through the door in one second; hence more current.
3. Current in AMP is a unit of measuring number of electrons crossing a door in ONE SECOND and every second. Thus, now we have more current, now less. 4. If we OPEN the switch circuit no longer remains CLOSED and current stops and bulbs stop glowing.
5. NOTE again: unless there are FREE electrons available to do the work, no current can flow. Electrons are like drops of water that flows in pipes from the tank. Water is current. By closing or opening the tap we can control amount of water coming through the shower.