Magnetism | 9

Question 1

Properties of Magnets

Answer 1

2 ends of the magnet are called 2 poles

They are North pole (North seeking pole)
And South pole (South seeking pole)

Like poles repel (N-N repel, S-S repel)
Unlike poles attract (N-S attract)

Question 2

How to identify Magnet sides

Answer 2

A freely suspended magnet (magnet tied
to a string) will have one end pointing in
Northern direction. This is called the
north pole (N)

See how it behaves next to a labeled magnet

Question 3

Magnet Material

Answer 3

Material can get attracted to a magnet
Material can be made into a magnet

Examples: Iron, Cobalt, Nickel, alloys
like Steel, Alnico etc.

Question 4

Hard Magnets

Answer 4

Can be made into a magnet
(magnetized)
Retains its magnetism permanently
Used for making permanent magnets
Example – Steel

Question 5

Soft Magnets

Answer 5

Can be made into a magnet
(magnetized)
Retains its magnetism only for a short
time.
Used for making temporary magnets

Example – Iron

Question 6

Where does magnetism come from

Answer 6

Electrons in an atom carry negative
charge, and also orbit the positve nucleus.

This gives the atom a tiny magnetic
property.

So, every atom behaves like a tiny
magnet, called DOMAIN.

Question 7

How to Magnetize magnets

Answer 7

Attaching the material to a magnet

Stroking the material in the same
direction on a magnet

By passing current through a wire
(electromagnets) – we will learn this later.

All of this unifies the directions of the domain in the magnet , making it magnetic

Question 8

De - magnetizing magnets

Answer 8

Hitting / hammering the magnet repeatedly

Dropping the magnet from a height

Heating it to a high temperature

All the above actions disturb the domain
arrangement, causing the magnet to lose its
property.

Question 9

Earth’s magnetic field

Answer 9

The earth behaves likes a huge, but weak bar magnet.

Studies show that it is because of the iron core and movement of charges in it.

The magnetic south pole in the compass gets attracted to the geographic north pole

Question 10

Magnetic Field

Answer 10

The region around a magnet till which it’s force of attraction or repulsion can be felt.

Question 11

Magnetic field lines of force

Answer 11

We know that any force can be represented as a line with an arrow.
Likewise, magnetic force can also be represented using arrows.

Are semi-circular (roughly) in shape

Always point from N pole to S pole

Never intersect (criss cross)

Crowded near poles, spaced out at the middle

Strong magnets – more lines of force, weak
magnets – less lines of force.

Strong magnets – large field, weak magnets - small field.

Question 12

Magnetic effect of electric current

Answer 12

When current flows through a wire, it creates a magnetic field around it.

Concentric circles

Closer near the wire and spaced out as we move far away

Right hand grip rule

Question 13

Factors affecting magnetic field

Answer 13

Current / Voltage

Length / Area of wire

Question 14

Solenoid

Answer 14

coil of wire , has magnetic field around it

Question 15

How to find north and south pole of a solenoid

Answer 15

Imagine that you hold the coil with your right hand.

Your fingers should point in the direction of
current in the side of the coil you face.

The position of thumb gives the N-pole, the other end is the S-pole.

Question 16

Making a electromagnet

Answer 16

By placing a nail (called the core) inside the
solenoid, the nail can become a magnet.

This is called an Electromagnet.

It can be stronger by:

Increasing current / voltage

Increasing number of turns in solenoid

(eg. Dump yard crane , miniature circuit breaker , MRI scan)

Question 17

Advantages of electromagnet

Answer 17

Magnetism can be turned on / off ( put magnet in a.c solenoid and it’s magnetism will get shuffled)

We can decide the strength of the magnet.

Cheaper

Question 18

Magnetic relay

Answer 18

A relay is a switch in a low power circuit that
is used to turn on a high power circuit.

Used to ensure safety for operators.

Question 19

Thrust

Answer 19

Thrust on the wire in between magnetic field - causing wire to move

Question 20

How do you find out the direction of the thrust

Answer 20

Using Fleming’s left hand rule.

Place three fingers (Thumb, forefinger, and second finger) of your left hand mutually perpendicular to each other

Forefinger – should point in Field direction
second finger – should point in the direction of Current (in this case towards you / out of paper)

Then…
Thumb – points out the thrust (movement)
direction on the wire.

Question 21

Thrust factors

Answer 21

Can be increased by

Increasing strength of magnetic field

Increasing current

Increasing area of wire

(no thrust if wire is along the magnetic field)

Question 22

What causes thrust

Answer 22

The wire carrying current has a circular
magnetic field around it.

This magnetic field interferes with the
magnetic field from pole pieces.

This interference causes the lines of force to
realign, thereby pushing the wire.

Question 23

Rotational effect on a coil

Answer 23

When a coil is left on a magnetic force field , it flips over , turning Electrical energy to kinectic enery

Similar to a motor

this is not useful as it does not give us a uniform turning motion

Question 24

How to make motor power by electricity

Answer 24

the battery rotates with the coil.
This causes the current direction to flip in the left and right arms, causing the motor rotation to flip.
If we can isolate the battery from the coil, this problem can be solved.
To do that, we introduce 2 components – split ring commutators, and carbon brushes

Question 25

In the motor which parts move and which parts dont

Answer 25

parts move
Coil and split ring commutator

parts do not move
Magnet pole pieces, carbon brushes, battery

Question 26

Split ring commutator

Answer 26

Looks like a ring split into 2 parts.

Rotates along with the coil

Isolates the battery from the rest of the circuit.

Ensures that left side of the coil and right side of the coil always carry current in the same direction

Ensures uniform direction of rotation in motor.

Question 27

Carbon brushes

Answer 27

Establishes contact between the coil and the
commutator.

Question 28

Electromagnetic induction

Answer 28

When a wire is moved inside a magnetic field, the wire cuts the magnetic field lines.

This causes a voltage (called induced emf) to be produced.

If this wire is part of a complete circuit, the
induced emf will cause a current to flow.

This is called induced current.

The whole phenomenon is called Electromagnetic induction.

Question 29

How do we find the direction and magnitude of the induced current?

Answer 29

Fleming’s right hand rule

Forefinger – Field
Thumb – Thrust
second finger – induced Current

The value and direction of induced current is measured using a Galvanometer (Centre-zero ammeter + can measure current both sides).

Question 30

What can increase the induced emf and current?

Answer 30

Stronger magnetic field

Faster movement of wire

More area / length of wire / windings (coil)

Question 31

Lenz law

Answer 31

The induced current always opposes the
movement of the coil