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Chapter: Electricity

Electricity has become very vital part in our day to day life. We reached such a stage that there is nothing without electricity. Earlier electricity was basically used for lighting and ventilation purpose such as lights and fans. Thereafter its applications expanded to many more appliances such as refrigerator for storage of food items, washing machine for washing clothes,  dish washer for washing utensils, mixer, grinder, juicer, blender, microwave oven for fast and even cooking, electric rice cooker, electric stove called as induction stove, water heater for heating water, cooler, AC, computer, laptop, hair dryer etc.,

But have you ever thought what electricity is? How does it flow in an electric circuit?

Battery was termed to be the basic form of electricity. It converts chemical energy into electrical energy.

Electric Current and Circuit

We can say that electric current flows through a wire or any conductor if electric charge flows through it. Electric current is expressed as the amount of charge flowing through a particular area in unit time.

Mathematically, Electric current (I) =Charge (q) / Time (t)

The electric current is expressed by a unit called ‘ampere’ (A).

One ampere is constituted by the flow of one coulomb of charge per second. Small quantities of currents are expressed in milliampere (1 mA = 10-3 A)  Or, in microampere (1 μA = 10-6 A) 1 A = 1 C/1 s. Ampere is named after the French scientist, Andre-Marie Ampere.

Ammeter is the instrument used for measuring electric current in a circuit. An ammeter is always connected in series in a circuit through which the current is to be measured.

Electric Potential and Potential Difference

The source of electricity is of two charges, positive and negative. The amount of charge on a body is measured in coulomb and is symbolized as C. These charged particles flow in a particular direction and they need some potential difference to move as they don’t move on their own.

Let us take an example to understand how charges flow inside a wire. Take some water in a tank and we can see that water doesn’t flow on its own. It needs some driving force/ pressure difference which can be obtained by connecting a tube at one end and connecting another tube at some higher level than the first tube. Then we can see the water flows due to the pressure difference between the two ends of the tube. In the same way charges do not flow in a copper wire themselves. The flow of charges in a conducting metallic wire, the gravity, though has no role to play, the charges move only if there is a difference of electric pressure called the potential difference along the conductor.

Electric potential difference is defined as the work done to move a unit charge from one point to other.

Mathematically it is expressed as

Potential difference (V) = Work Done (W)/Charge (q)

V = W/q

The SI unit of electric potential difference is volt(V). It is named after Alessandro Volta, an Italian physicist

If one joule of work is done to move a charge of one coulomb from one point to the other, in a current carrying conductor then the potential difference between two points is said to be 1 volt.

Therefore,

1 volt = 1joule/1 coulomb

The potential difference measured across the terminals of an open cell is known as emf (electro motive force). Voltmeter is used to measure potential difference. And a voltmeter is always connected in parallel across the points between which the potential difference is to be measured.

Circuit Diagram

An electric circuit contains a battery, plug key , electrical components and connecting wires. Conventional symbols used to represent some of the most commonly used electrical components are given in the following table.

Ohm’s Law

Ohm’s law explains the relation between potential difference and the current through a conductor.The current (I) is proportional to the potential difference (V). The slope of the line gives the value of the resistance R. This relationship, between potential difference and current, was first established by the German Physicist, George Simon Ohm and is known as Ohm’s law.

The current through a conductor element is proportional to the potential difference applied between its ends, provided the temperature remains constant. If the voltage V is applied to an element and a current ‘I’ is passes through it,

V ∝ I

V/I = constant

= R

(Or) V = RI

Here R is a constant for the given metallic wire at a given temperature and is called its resistance.

Units of resistance:

R= V (volt) /I (ampere) = V/I (ohm)

The unit of resistance is called the ohm and its symbol is Ω.

Limitations of OHM’S Law

1. Ohm’s law is valid only for metallic conductors provided the temperature and other physical conditions remain constant.
2. Ohm’s law is not applicable for gaseous conductors.
3. Ohm’s law is also not applicable to semiconductors such as germanium

Conductor: A component of a given size that offers a low resistance is a good conductor.

Resistor: A conductor having some appreciable resistance is called a resistor.

Insulator: A component of identical size that offers a higher resistance is called an insulator.

Resistance of a System of Resistors

A conducting material (e.g., wire) used in a circuit is called a resistor. A resistor is sometimes simply referred to as resistance. Two or more resistors can be connected in series, in parallel or in a manner that is a combination of those two.

There are two methods of connecting the resistors together.

1. Resistors in series
2. Resistors in parallel

Let us discuss about each of the above in detailed.

Resistors in Series

Two or more resistors are said to be connected in series if the current flowing through one also flows through the others. You will find that the value of the current is the same everywhere in the circuit. So, in a series connection, the same current passes through the resistors.

And the potential difference V is equal to the sum of potential differences V1, V2 and V3.

Therefore, V= V1 + V+ V3

Applying Ohm’s law V=IR, we get V1=IR1, V2=IR2, V3=IR3. Therefore, Rs=R1+ R2 + R3

We can conclude that when several resistors are joined in series, the equivalent resistance of the combination Rs equals the sum of their individual resistances, R1, R2, R3, and is also greater than any individual resistance.

Resistors in Parallel

If resistors are connected in such a way that the same potential difference gets applied across each of them, they are said to be connected in parallel.

You will find that the current i gets divided into the branches such that

I=I1+I2+I3

The total current flowing into the combination is equal to the sum of the currents passing through the individual resistors.

The currents I1, I2, I3 through the resistors R1, R2, R3 by Ohm’s law as,

Since the resistors are in parallel,

I = I1 + I2 + I3

Substituting the value of currents in the above equation,

V/Req = V/R1+V/R2 +V/R3

Thus, 1/Req = 1/R1+ 1/R2+ 1/R3

Similarly, if there are n resistors connected in parallel their equivalent resistance Req is given by

1/Req = 1/R1 + 1/R2 + 1/R3 +…………+ 1/Rn

For two resistances R1 and R2 connected in parallel

1/Req = 1/R1+ 1/R2 =(R1+R2)/R1R2

Req = R1R2/(R1+R2)

Heating Effect of Electric Current

When current flows, it produces heat. This process of production of heat by flow of electric current is called heating effect of electric current.

When electric charges move through a wire, they lose some of the energy to the atoms in the wire. The atoms in the wire start vibrating on receiving the energy and hence the wire starts to heat up.  Some of this electric energy is converted to heat energy.The amount of heat generated is in accordance with Joule’s first law:

H = I2.R.t

This heating effect is applied in industry such as soldering, welding, cutting and drilling etc.The law implies that heat produced in a resistor is

1. directly proportional to the square of current for a given resistance
2. directly proportional to resistance for a given current
3. directly proportional to the time for which the current flows through the resistor.

Practical Applications of Heating Effect of Electric Current

Some of practical applications of heating effect of electric current are

1. electric laundry iron
2. electric toaster
3. electric kettle
4. electric oven
5. electric heater
6. electric bulb
7. electric fuse
8. electric geyser

Electric Power

Electric power is the rate of consumption of energy. In simple terms, it denotes  the rate at which electric energy is dissipated or consumed in an electric circuit.

The power is given by

P = VI

The SI unit of electric power is watt (W). It is the power consumed by a device that carries 1 A of current when operated at a potential difference of 1 V. The unit ‘watt’ is very small. Therefore, in actual practice we use a much larger unit called ‘kilowatt’. It is equal to 1000 watts.