CURRENT Electricity
In this Blogs you will learn
. Conventional Current . Use of A.C AND D.C.
. Potential Difference . Hazards of Electricity and
Precautionary Measures
. Ohm;s Law
. Resistance . Electrical Measuring Instruments
. Components of a Circuit . Analogue and Digital Meters
Electricity is very common from energy. which we use in our homes and work places everyday. It has facilitated the availability of our need od our needs of life. Just turn on the switch and the electricity starts its work.
Electricity is used in four important ways. It Causes fans. electric motors and machines to move. It provides light through bulbs, tubes and television. In a loud speaker it is converted into sound and in iron. heater and toaster it assumes the from of heat.
Electric Current
You know that electric current is the flow of charges. The charges are the free electrons, which are available in conductors. In some conductors like liquids and gases, the current also flows due to the motion of positive and negative ions. But a source of energy is required to force the charges into motion. The source pushes the charges through the circuit.
The amount of charge that passes through any
cross-section in one second is called current
If charge Q passes through any cross-section in time t seconds then the current I can be written mathematically as
I=Q-t ..............(8.1)
The unit of current in system International (SI) is ampere. It is denoted by A. Current can be measured by connecting an ammeter in series with the circuit.
For your information
1 mA = 10-3 A
1 uA = 10-6 A
Conventional Current
From the very beginning after the discovery of electricity, it was assumed that the electric current is due to flow of positive charges, which move from positive terminal of the battery towards the negative terminal. This is called the conventional current. But, now, we know this fact very well that the charge carrying particles in metal conductors are the electrons having negative charge on them. Electrons move from negative terminal of the battery towards positive terminal. Thus in fact, the current is due to the flow of negative charges.
Keep in mind that electrons do not flow through a conductor like a stream but their motion is different. A number of free electrons are always bumping among the atoms of the conductor, Which is called random motion (FIG: 8.2). In the absence of a battery , the number of electrons passing towards left is the same as through any cross-section in one direction is zero.
When one end of a conductor is connected to the positive terminal of the battery and the other end connected to the negative terminal, the electrons experience force. Due to this force electrons start drifting towards the positive terminal of the battery in addition to their random motion. As the electrons have negative charge, so they carry negative charge while moving through the circuit. However we can assume that
the amount of positive charge flowing in one direction is equivalent to the
same amount of charge flowing in opposite direction.
let us consider the following example to understand this.
Suppose that the body A carries +10C charge and the body B carries -10C charge. Let both of them are connected with each other through a wire for a small duration and then put them apart. If -4C charge is transferred from body B to A during this interval. then what should be the charge on bodies A and B after they are separated again. Fig. 8.3
In this case:
Charge on body A = + 10C + (-4C) = + 6C
Charge on body B = - 10C - (-4C) = - 6C
If we assume that +4c charge is transferred from body A to B, them
Charge on body A = + 10C - (+4C) = + 6C
Charge on body B = - 10C + (+4C) = - 6C
We see that in the both cases result remains the same.
In future, we will use the word ( current ) for ( conventional current ). The words (electronic current ) is used to indicate the direction of flow of electrons.
Potential Difference
When an electric circuit is completed by closing the switch the negative terminal of the battery pushes the free electrons in the circuit towards positive terminal. This causes the flow of current.
The current flowing through a conductor can be considered analogous to the flow of water through the pipe. In Fig: (8.5). the water level is higher at position A and it is lower at position B. In this case the water flow from position A to B. The water will stop flowing when level on both sides will be the same. Now, a pump has to be used to maintain the flow of water .
The pump will lift the water from B and will put it into the pipe at A. In this way the flow A to B because the level of water at A is higher than that at B.A battery also acts like a pump. The chemical reaction in the battery transfers electrons from positive terminal to negative terminal. In this way the potential energy of the electrons, reaching the negative terminal , increases. This is the energy due to which electrons move towards the positive terminal in he outer circuit. The potential energy of the electrons decreases as they reach the positive terminal. The battery pushes them again towards the negative terminal. The energy required is provided by the chemical reaction in the battery.
If we refer to the conventional current flows from positive to negative terminal. Therefore, like water the level of potential energy of the charge at positive terminal should be higher than that of potential energy at negative. The level of potential energy is also called simply potential. Hence
Current flows from higher potential towards lower potential
In order to maintain the current, charges gain energy from the battery and expend it while passing through the circuit. The capacity of supplying energy for different batteries is different. It depends upon the potential difference of the battery.
The amount of energy supplied to one coulomb charge by a battery is called its potential difference.
potential difference is also called as voltage. The SI unit of potential difference is volt(V). The potential difference (V) between two points in a circuit can be measured by connecting a voltmeter parallel to the points.
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