Electricity and

Magnetism
Grade 7
Contents :
• Law of Magnetism
• Test for a magnet
• Electromagnet – its principle
• Uses of electricity
• Sources of electricity (the electric cell, the mains, the generator and solar
cells)
• Symbols and functions of various components of an electric circuit
• Series and parallel circuits
• Define conductors and insulators of electricity
What is a magnet?
• An ore of iron called magnetite.
• Capable of guiding direction.
• Two types- Natural and artificial Magnets
➢ Natural magnets – irregular in size, weak in
magnetic strength
➢ Artificial Magnets – bar magnet, magnetic
needle, horse shoe magnet, compass etc.
Properties of Magnets :
1. Attractive property – attracts small pieces of
iron, nickel, cobalt etc.
2. Directive property – If suspended freely, it
always rests in the north-south direction
Attractive property of a magnet :
The property of attraction is not the same everywhere along the length of a bar
magnet but is maximum near the ends of a bar magnet.
The ends of the magnet where the attractive property is maximum are called the
poles of the magnet.
Directive property of a magnet
• If a magnet is suspended freely with a silk
thread from a wooden stand, it swings for
some time and then comes to rest in a
particular direction-north-south direction.
• The end of the magnet which points towards
the north is called the north seeking pole or
north pole(N). Sometimes a red dot is etched
at the north pole.
• The end of the magnet which points towards
the south is called the south seeking pole or
south pole(S).
Magnetic materials :
Materials which are attracted by a
magnet are called magnetic materials.
Eg. Iron, steel, cobalt and nickel.

Non-magnetic materials :
Materials which are not attracted by a
magnet are called non-magnetic
materials.
Eg. Paper, wood, brass, plastic, copper,
aluminium etc.
Law of Magnetism :
• Like poles repel and unlike poles attract.
• Two like poles(both north poles or both south poles) repel each other.
• Two unlike poles(one north pole and the other south pole) attract each other.
Do you know ?

• The magnetic poles always exist in pairs. It is

not possible to separate the two poles of a
magnet.
• If a bar magnet is broken at the middle in two
parts, each part is found to be a
magnet(means it can attract small pieces of
iron). Each part rests in the north-south
direction when suspended freely. This shows
that new poles are formed at the broken
ends. If these pieces are broken again and
again, each part will still be a complete
magnet. Thus, the two poles of a magnet exist
simultaneously.
Repulsion is the sure test for a magnet
• Resting in north-south direction cannot be a sure test for being a magnet
because at times a non-magnetized metal may also rest in north-south direction.

• Attraction can also not be the sure test of being a magnet as a bar magnet
may attract opposite poles of other magnet or it can attract a piece of non-
magnetized magnetic substance.

• But any pole of a magnet can repel only the like pole of the other magnet. So,
repulsion is the surest test of being a magnet.
 Magnetic field:
• The space around a bar magnet in which the
magnetic compass gets influenced is called its
magnetic field.
• We know that when we place a magnetic
compass on a table, its needle rests in the north-
south direction. If we place a bar magnet near
that magnetic compass, we observe that the
magnetic needle of compass swings and then
rests in a direction other than north-south. Now if
we place the magnetic compass at different
points around the bar magnet, the direction in
which the magnetic needle of compass rests,
changes. This shows that the space around the
bar magnet has a property to influence the
magnetic needle of the compass.
Electromagnet:
• Principle : If an insulated copper wire is
wound around a cylindrical card board
or soft iron of length much more than its
diameter, we get a cylindrical coil (called
solenoid).
• When the ends of the coil are joined to a
dry cell and current is passed through it,
the cylindrical coil carrying current
behaves like a magnet(it produces a
magnetic field around it). This magnet is
called an electromagnet.
Uses of electromagnets:
• Electromagnets are used in fans, motors , mixers, air conditioners, electric bell, electrical instruments,
MRI scans etc. They are also used to lift heavy loads. They behave as temporary magnets.
Magnetic polarities at the ends of an
electromagnet:
• It depends on the direction of
current in the coil.
• Clock rule:
• The end of the electromagnet
where the direction of current is
anti-clockwise becomes the north
pole N and the end where the
direction of current is clockwise
becomes the south pole S
For a temporary electromagnet, soft iron
is used as a core whereas for a
permanent electromagnet, steel is used
as a core.
Ways of increasing the magnetic field of an
electromagnet:
1. By inserting a rod of soft iron or steel (also called core)inside the
cylindrical tube.

2. By increasing the total number of turns of the coil.

3. By increasing the strength of the current passing through it.

Earth’s Magnetic declination:
• A freely suspended magnetic needle always
rests in geographic north-south direction due
to the magnetic field of the Earth.
• The earth’s magnetic South pole is in
geographic north and the North pole is in the
geographic south. Since unlike poles attract ,
therefore the north pole of the needle is in
geographic north(though not exactly along
the geographic north) and south pole is in
geographic south.
• Magnetic declination is the angle of the
horizontal plane between the magnetic north
and geographic north. This angle is shown by
the symbol θ.
Earth’s Magnetic declination:
• The angle of declination is different at different places on earth’s surface and it
also changes at a place with time.
• The declination is taken positive if the magnetic north is towards the east of true
north and is negative if the magnetic north is towards the west of true north.
Uses of electricity :
• To light our home, school,
office
• To run various appliances like
fan, television, computer,
heater, air conditioner etc.
• To run movies in the cinema
hall
• To run machines in factories
• To run trains
• The main sources of electricity
are :
1. The electric cell and battery
2. The mains
3. The generator
4. The solar cells
The electric cell :
• An electric cell is a device, which
converts chemical energy into electrical
energy. It has two terminals, which are
made up of metal: one terminal is
positive, while the other one is negative.
When the two terminals are connected
to an electrical device, electric current
flows through it.
• It consists of a vessel with two metal rods
(called electrodes) and a chemical
substance(called electrolyte) either in
the form of solution or paste. The copper
rod becomes the positive electrode (or
anode) and the zinc rod becomes the
negative electrode(or cathode).
Dry cell:
• A dry cell consists of a zinc container
whose base acts as the negative
electrode . It has a carbon rod
placed at the centre (which acts as
positive electrode) with a brass cap.
• The rod is surrounded by a mixture of
manganese dioxide and charcoal in
a muslin bag. The electrolyte used is
mostly a paste of ammonium
chloride , plaster of paris, flour.
• The outer body is insulated with a
thick cardboard or plastic material.
Dry cell :
• When the cell is connected in a circuit, the chemical reaction takes place
and current flows in the circuit from anode of the cell towards its cathode
and the bulb glows.
• Advantages:
1. Lightweight and small in size.
2. Can be easily carried.
3. No fear of leakage.
4. Can be easily used to run simple electrical devices.
Note : A dry cell cannot be recharged.
Uses of dry cell in a torch:
• Dry cells are used in a torch as a source of
electricity.
• They are joined in such a way that the anode
of one cell is in contact with cathode of the
other cell.
• As we switch on, the torch lights up.
• If we remove the cells and then try switching
on, the torch does not light up showing that
the cells provide electricity to the torch.
Uses of a dry cell:
• Dry cells are used as a source of electricity in many appliances such as car
headlight, calculator, wrist watch, remote of TV etc
• If we use a group of two or more cells, it is called a battery. It is used when more
electricity is required.
• To form a battery, the positive terminal of one cell is kept in contact with the
negative terminal of the second cell, then the positive terminal of second cell is
kept in contact with the negative terminal of the third cell and so on.
• The bulb or appliance with which battery is to be used is then connected with
the negative terminal of the first cell and the positive terminal of the last cell.
Sources of electricity:
• The mains:
• Electricity which is produced by the generators at the power station is
distributed to homes and offices. This is known as the mains.

• This electricity obtained at the mains is used to run electrical appliances at

home and machines in factories.

• They provide much more electricity as compared to the cells or batteries.

How do we receive electricity from power stations?
• The electricity produced at a power station
is first changed to high voltage .
• It is then carried by cables or wires to city
sub-stations.
• It is then distributed through the transformers
to decrease the voltage (upto 220 volts)
using electric poles.
• Then from the pole, it is carried to the main
board of the home by means of cable.
• From the main board, it is distributed to
different rooms in the house through electric
wires.
Note : An electric meter is fixed at the main
board which measures the electricity
consumed.
Sources of electricity:
The Generator:
• Whenever there is an electric failure or shortage of power supply, we use an
electric generator to obtain electricity.
• In an electric generator, the mechanical energy converts to electrical
energy when in use.
The Solar Cells:
• A solar cell is a device which converts the light energy received from the
sun to electrical energy.
• They are used as a source of electricity in satellites.
Flow of electricity in a circuit:
• The path along which the electricity flows
is called the circuit.
• Electricity flows if the circuit is complete.
This happens when all parts of the circuit
are connected with wires made of
conductors. If there is an insulator in the
path of the circuit, it becomes
incomplete.
• Electricity does not flow if the circuit is
incomplete or broken.
Conductors and Insulators:
• Conductors – Substances which allow electricity to flow through them.
• All metals are conductors. Eg. Copper, Aluminium, silver, iron, brass, steel
• Human body is also a conductor of electricity.
• Impure water is also a conductor of electricity.
• Insulators - Substances which do not allow electricity to flow through them.
• Examples : cotton, rubber, plastic, wood, paper, glass, leather
• Distilled water is also an insulator of electricity.
Flow of charges constitutes current :
• We know that heat flows from a body at high temperature to the body at low
temperature when two bodies at different temperatures are kept in contact..
The heat flows till both the bodies attain the same temperature.
• In the fig shown below the direction of heat flow is from body A at
temperature 50°C to the body B at temperature 20°C.
Flow of charges constitutes current :
• Fig below shows two jars A and B. Jar A is narrow while jar B is wide, both containing
water and connected by a tube fitted with a stopcock. The level of water in jar A is
higher than the level of water in jar B , although jar B contains more volume of water
than jar A. On opening the stopcock, we find that the water flows from jar A in which
the water level is higher to jar B in which water level is lower. The flow of water continues
till the levels of water in the two jars become equal.
Flow of charges constitutes current :
• If we join two charged conductors, the electrons flow from a conductor
having more electrons to the conductor having less number ( or no )
electrons. The movement of electrons stops only when the number of
electrons in both the conductors become equal.
Flow of charges constitutes current :
In fig 7.24 a positively charged conductor A is
joined to an uncharged conductor B by a
metallic wire. The electrons move from the
uncharged conductor B to the charged
conductor A to balance the deficit of
electrons in conductor A. This movement of
electrons continues till both the conductors
have equal number of electrons.

In fig 7.25 a negatively charged conductor A

is joined to an uncharged conductor B by a
metallic wire. The electrons move from the
Conductor A to uncharged conductor B This
movement of electrons continues till both the
conductors have equal number of electrons.
Flow of charges constitutes current :
In fig 7.26 a positively charged
conductor A is joined to a negatively
charged conductor B by a metallic
wire. The electrons move from the
conductor B to the conductor A. This
movement of electrons continues till
both the conductors have equal
number of electrons.

The moving electrons constitute an electric current. The direction of current is taken opposite
to the direction of flow of electrons. To keep an electric current flowing between the two
conductors, it is necessary to maintain an excess of electrons on one conductor and deficit
of electrons on the other. In an electric cell, this is done by a chemical reaction in the
electrolyte which creates deficit of electrons on anode and excess of electrons on cathode.
Electric current :
• Current is defined as rate of flow of charge i.e. the amount of charge
flowing in one second.
• It is measured in the unit ampere (A)
• In metals, current flows due to the motion of free electrons but in liquids
current flows due to the motion of ions.
Symbols and functions of various components
of an electric circuit :
• The various electric components are :
1. Source of electricity
2. Switch or key
3. Bulb and
4. Connecting wires
Symbols and functions of various components
of an electric circuit :
• An electric circuit is represented by a line diagram for which we use
symbols for the various electric components.
• Source of electricity – A cell or a group of cells is generally used as a
source of electricity. A group of cells connected in series is called a
battery. A cell has two terminals : A positive (+), and a negative (-). It is
represented by the two vertical lines of unequal length. The long vertical
line represents the positive terminal, and the short line represents the
negative terminal.
Symbols and functions of various components
of an electric circuit :
• To use two or more cells, the positive terminal of one cell is
joined to the negative terminal of the other cell and so
on. This is called the series combination of cells. In series
combination, the same current flows through each cell. In
a torch, we use two or more cells in series.
• A group of cells is called a battery.
• In parallel combination of cells, the positive terminals of all
the cells are connected together and the negative
terminals of the cells are connected together. Here the
current is divided amongst cells.
Symbols and functions of various components
of an electric circuit :
• Switch or key: A switch or key is
used to put the circuit on and off.
• Bulb and resistor: It is the
component which opposes the
flow of current in the circuit.
• Connecting wires: They are used to
connect the various components in
the electric circuit.
 Direction of conventional current in a circuit
• When the circuit is complete, a current flows
through the various components connected in a
circuit from the positive terminal of the cell to its
negative terminal. The direction of current in the
circuit is indicated by marking an arrow from the
positive terminal of cell to its negative terminal.
• Figure 7.32 – (a) shows a circuit containing a cell,
switch, and bulb. The line diagram is shown in
7.32 – (b). The arrows in these diagrams represent
the direction of current in the complete circuit.
 Series and Parallel circuits
• Series circuit: In a circuit, if appliances are connected in series, they all will
work together i.e., on opening the switches, all appliances will become
operative simultaneously. If any one appliance goes out of order, the rest of
the appliances also will not operate because the circuit then becomes
incomplete. Thus in a series circuit, the appliances in use are dependent upon
each other.
• Parallel circuit: When the circuit is in parallel, the appliances work
independently. If any one appliance goes out of order, the rest of the
appliances will not be affected. Every appliance when switched on works on
its own without the interruption of other appliances. Due to this reason, in our
household wiring system, all the circuits are in parallel. If we switch on a tube
light in a room, the fan and the other gadgets in the room are not disturbed.
Precautions to be taken before the circuit is
switched on
1. See that all the components of the circuit are properly connected.
2. See that the connecting wire is tightly connected to each appliance.
3. Do not touch the switch or any component with wet hands.
4. See that the connecting wire is nowhere naked, it should be properly
insulated everywhere.
5. See that the source of electricity is properly joined.