6. Electricity and Magnetism

Electric circuit: Electric circuit is the combination of source of current, conducting wires, and loads. They are of two types:

Closed circuit:

The circuit in which switch is closed and the electric current passes in the circuit is called closed circuit.

Open circuit:

The circuit in which switch is open and the electric current does not flow in the circuit is called open circuit.

Combination of cells

Arrangement of the cells which is designed to get the maximum potential difference and current supply is called combination of cells. In general they can be grouped as:

Series combination of cells:

In this combination the positive terminal of one cell is connected to the negative terminal of the other cell.

Total potential in this case (V) = sum of the individual p.d of the cell.

The current in the external resistance is the number of times the current due to t e single cell

Parallel combination of cells:

The cells are said to be parallel if the positive terminal of all cells are connected at one common point and the negative terminal are connected to one common point.

In parallel combination of cells the total voltage between any two points is always constant this is equal to the voltage of one cell.

Combination of the resistors:

Series combination

When the resistors are connected end to end then it is called series combination of resistor.

If R1 R2 R3 R4 are connected as shown in figure then

R= R1 + R2 + R3 and if V1 V2 V3 are the voltage across resistors respectively.

V= V1 + V2 + V3 

Characteristics

The value of the current is same in each resistor but the p. d is different. If the one of the resistor do not work then it effects the whole combination.

Parallel combination:

When the resistors in the circuit are connected between the two common points, then the combination is called parallel combination. The voltage in each branch is same.

In this case total current (I) = I1 + I2 + I3 …………………..1

From the ohm law

I1 = V/R1

I2 =V/R2

I3 =V/R3

Putting the value in 1

V/R = V/R1 + V/R2 + V/R3 

1/R = 1/R1 + 1/R2 + 1/R3 

We get, the reciprocal of effective resistance in parallel combination is equal to the sum of the reciprocal of the each resistance.

Electric power:

Electric power of the device is defined as the rate at which electrical energy is converted into other form of energy.

The electrical power of device is the product of the current (I) and voltage (V) across.

P = VI

P = V*V/R   from ohm’s law V = IR

P = V2/R

P =RI2

Fuse:

Fuse is a safety device to protect a circuit from excessive heating. A fuse wire is a short metal wire having low melting point made of an alloy of tin and lead. It is connected in series with a circuit.

Short –circuiting:

The overflow of the current due to the low reactance path in the circuit is called short-circuiting.

Transformer:

A transformer is a device which is used to convert   low ac voltage  into the high a.c voltage or vice –versa.

A step -up transformer is used to convert low a-c voltage to high ac voltage

A step –down transformer is used to convert high a-c voltage to low ac voltage

Principle of transformer:

When an alternating emf is applied to the primary coil, a changing current flowing in it produces an alternating magnetic flux in it. This causes to change the magnetic flux linked with the secondary coil. An alternating emf is then induced in the secondary coil. It is called the principle of mutual inductance on which transformers are based.

Uses:

Transformers are used in computer, television, air condition, doorbells etc.

Electrolysis of water:

The process of the decomposition of the electrolytic solution into constituent when the electric current is passed through it is called Electrolysis of water.

When dil. H2SO4 is mixed with water 

H2O(HOH)          H++  OH-

At anode

OH-- OH + e-

4O      2H2O + O2

At cathode

H+   + e- H

4H  2H2

Electroplating:

The process of depositing a thin layer of a metal over a conducting surface by the process of electrolysis is called electroplating. The metal to be electroplated is connected to the negative terminal of the battery and the metal which is deposited over is connected to the positive terminal of the battery.

Here CUSO4 solution is taken as electrolyte. When the electric current passes through the circuit, thin layer of the copper is deposited over the metal.

Chemical reaction involved

‘When cuso4 dissolves in water

CuSO4 →   Cu++ + SO4

At anode:      Cu   +   SO4   →    CuSO4 + 2e

At cathode:    Cu++ +2e      →      Cu

Application of electrolysis

Purification of metal

Electroplating

Extraction of metal

Production of hydrogen and oxygen gas

Lighting effect of current:

Some electricaldevices converts electrical energy into the light energy this is called lighting effect of electric current

Lighting effect of current is used in:

Florescent lamp:

A florescent lamp consists glass tube filled the mercury vapor and coated with florescent powder. It converts 30% of the electrical energy to the light energy and very useful for the household purposes.

Filament lamp:

Filament lamp has tungsten filament with inert gases filled inside the bulb. These inert gases prevent from the oxidation and evaporation of tungsten metal at high temperature. It converts 10% of the electrical energy into light energy.

Fleming right hand rule:

Fleming’s right hand rule states that “If the first three fingers of the right hand are held mutually perpendicular to each other, with the index finger in the direction of the magnetic flux and the thumb in the direction of motion of coil, the middle finger points to the direction of the induced current.”

Fleming’s right hand rule is used to determine the direction of the induced current in a generator.

Ac and Dc circuit:

Alternating current is one which changes its magnitude continuously and reverses its direction periodically and if the polarity of an electrical source does not change with time it is called direct current.

Graph of ac and dc circuit

Electromagnetic induction

The phenomenon of inducing the emf in a closed circuit whenever there is change in the magnetic flux is called electromagnetic induction. The resulting current is called induced current.

Faraday laws of electromagnetic induction

Faradays laws of electromagnetic induction are:

1. Whenever magnetic flux linked with a closed circuit changes, an emf is induced in the circuit.

2. The induced emf lasts as long as the change in the magnetic flux is taking place.

3. The magnitude of the induced emf is directly proportional to the rate of change of the magnetic flux.