s4t2 - magnetism · objectives! at the end of the lesson, students are able to state the properties...

MAGNETISM Electricity and Magnetism

Objectives!

At the end of the lesson, students are able to

✤ state the properties of a magnet ✤ describe induced magnetism ✤ describe the electrical methods of magnetisation and demagnetisation. ✤ draw the magnetic field pattern around a bar magnet and between the poles

of two bar magnets ✤ describe the plotting of magnetic field lines with a compass ✤ distinguish between the properties and uses of temporary magnets (e.g. iron)

and permanent magnets (e.g. steel)

Properties of Magnets

Must have two magnetic poles North South

Magnetic strength is the strongest at the two poles

North-pole (North seeking pole) always point to the earth’s Magnetic North.

Properties of Magnets

Law of Magnetic poles

Strength of Magnetic force (Attraction or repulsion force)

1. Magnetic strength of the magnets!2. Proximity of the magnets

Like poles will repel each other. Unlike poles will attract each other.

Magnetic Materials Non-magnetic materials

Can be attracted by a magnet Cannot be attracted by a magnet

Can be made into a magnet (Can be magnetised)

Cannot be made into a magnet (Cannot be magnetised)

Iron, cobalt, nickel and alloy (e.g. steel and alnico) Wood, copper, paper or glass

Magnetic Materials and Non-magnetic Materials

Not all metals are magnetic materials

Can be attracted by a magnet Cannot be attracted by a magnet

Can be made into a magnet (Can be magnetised)

Cannot be made into a magnet (Cannot be magnetised)

Iron, cobalt, nickel and alloy (e.g. steel and alnico) Wood, copper, paper or glass

Test for a Magnet

Use another magnet to test with the unknown object.

If they attract:In conclusive yet: The unknown object could be a magnetic material

or a magnet (The two ends next to each other happens to be unlike poles)

Reverse the unknown object and test the other end of the unknown object with the same pole of the magnet.

If they repel: The unknown object is a magnet as only two like-poles magnet repel each other.

Repel: Magnet Attract: Magnetic Material

If they neither attract or repel: Non-magnetic Material

Magnetic Field

Magnetic Field is a region in which a magnetic object, placed within the region experiences a magnetic force.!

A magnetic field can be illustrated by drawing magnetic field lines

Magnetic Field

✦ Magnetic field is a vector.A magnetic field can be illustrated by drawing magnetic field lines

The direction of the magnetic field is defined as the direction of the magnetic force acting on a tiny North-pole magnet.

The direction of the magnetic field is also the direction indicated by the north pole of a small plotting compass (which is itself a magnet)

OUTSIDE the magnet: The magnetic field lines points away from the North-pole and towards the South-pole of the magnet.

Plotting a Magnetic Field (Plotting Compass Method)

1. Draw the outline of the magnet on a piece of paper.!2. Place the compass to one end of the magnet and let the

compass to come to rest.!3. Mark the two end of the compass needle with two dots

(e.g. X and Y).!4. Move the compass to a new position such that one end of

the compass needle is exactly over dot Y.!5. Mark the other end of the compass needle with a dot (e.g.

Z)!6. Repeat the process until the plotting compass reaches the

other pole of the magnet.!7. Join the dots together to represent the magnetic field line.

The direction of the magnetic field line is in the same direction as the arrow of the compass needle.!!

Magnetic Field

Magnetic field lines form a complete loop

OUTSIDE the magnet:Points away from the North-pole and towards the South-pole





Magnetic Field

Points away from South-pole and towards North-pole

✦ Magnetic field lines never cross one another.!✦ The strength of the magnetic field is represented by how close

magnetic field lines are to one another.

OUTSIDE the magnet:Points away from North-pole and towards South-pole

INSIDE the magnet:

Magnetic Field

Example of Magnetic Field Patterns!

A single bar magnet

Magnetic Field


Two bar magnets

Region between two unlike poles

Region between two like poles

Neutral PointNo magnetic field !⇒ no magnetic force

Magnetic Field


Two bar magnets (Side by side parallel to each other)

Magnetic Material and Magnetic Field

The presence of a magnetic material disturbs a magnetic field.!

Magnetic field tends to pass through magnetic material (rather than air).

The magnetic field will choose to concentrate through the magnetic material .

The magnetic field will choose to concentrate through the magnetic material .

Magnetic Shielding

What happen when a magnet is place above an iron sheet?✦ Magnetic field lines will enter the

iron from the middle section.!✦ It will then choose to flow through

(concentrate through) the iron instead of exiting through the opposite side of the sheet of iron, which will enter into air. !

✦ The magnetic field lines finally have to exit from the end of the iron, back to the south pole to form a closed loop. !

Magnetic Shielding

✦ No magnetic field lines at the other side of the sheet of iron.!

✦ Any magnetic materials placed at the other side of the sheet of iron, will not experience any magnetic forces !

Magnetic shielding

What is the implication?

Magnetic Shielding

Since magnetic field lines tend to concentrate through magnetic material, a closed loop of magnetic object can be used to shield off magnetic field.!

The region within the closed loop of the magnetic object, X does not experience any magnetic field lines and hence will not experience any magnetic effect.!

Theory of Magnetism

A magnet is made of ‘atomic’ magnets (Magnetic domains)

An object is a magnet (magnetised) - Magnetic domains are aligned with their North poles pointing in the same direction. The magnetic domains within the magnet cancel each other effect, leaving only the domains at the ends to become the poles of the magnet.An object is not a magnet - Magnetic domains point in random directions. The magnetic domains cancel out the effect of one another.

Induced Magnetism

Magnetic induction is the process where magnetic materials become magnetised when they are near or in contact with a permanent magnet.

Permanent magnetS N Magnetic Material!(e.g. Iron)S N

Induced Magnet

Since magnetic materials are induced into induced magnets, they will attract permanent magnet (unlike poles attract)

Magnetic induction always precedes attraction

Induced Magnetism

Permanent magnetN S Magnetic Material!(e.g. Iron)N S

Induced Magnet

Induced Magnetism

N

S

N

Hard Magnetic Materials and Soft Magnetic Materials

Soft Magnetic Materials!(e.g

Hard Magnetic Materials!(e.g.

Does not retain its magnetism Retain its magnetism

Easily magnetised into a strong magnet

Not easily magnetised and is a weak magnet

e.g. Iron e.g. Steel

Used to make electromagnets Used to make permanent magnets

Method of Magnetisation and Demagnetisation

Only magnetic materials can be magnetised.During magnetisation: The magnetic domains in the magnetic

material are aligned in the same direction.

During magnetisation: The orientation of the magnetic domains in the magnetised magnetic material are randomised.

(a) Stroking Method (b) Electrical Method (D.C)

(a) Heating or Hammering Method (b) Electrical Method (A.C)


Magnetisation

Single Touch Double Touch

(a) Stroking Method

The end of the magnetic material where the strokes finishes has the opposite polarity to that of the end of the stroking permanent magnet.

When current flow through the solenoid, the solenoid will be magnetised and will become an electromagnet.


Magnetisation(b) Electrical Method (Direct Current)

Note: Wire is made of copper which is a non-

magnetic material.

(coil of wire)

Determine the polarity of the electromagnetMethod 1 - Direction of current flow

clockwiSe aNti-clockwise

North Pole South Pole



Method 2 - Right-hand Grip Rule Fingers pointing in the same direction as the flow of the current in the solenoid

Thumb will point to the North polarity

of the electromagnet


Magnetisation(b) Electrical Method (Direct Current)Determine the polarity of the electromagnet

When current flows in a solenoid, the solenoid becomes an electromagnet .!The magnetic field will then align the magnetic domains in the magnetic material.!Thus inducing the magnetic material into an induced magnet.



When current flows in a solenoid, the solenoid becomes an electromagnet .!The magnetic field will then align the magnetic domains in the magnetic material.!Thus inducing the magnetic material into an induced magnet.



It is NOT A MUST to have a magnetic material in the solenoid for it to become

an electromagnet. By adding a magnetic material it will strengthen the

magnetic strength at the poles.

✦ Heat will cause the magnetic domains to vibrate vigorously, thus loosing their alignment.

✦ Same as heating, hammering will cause the magnetic domains to vibrate, thus loosing their alignment.


Demagnetisation(a) Heating or Hammering Method

✦ The most effective method to demagnetise a permanent magnet.!✦ Alternating current is used instead of Direct current.

An alternating current (a.c.) is an electric current that changes its direction of flow at a regular interval.!

✦ What effect does alternating current has on the polarity of the electromagnet and on the magnetic domains of the permanent magnet placed in the solenoid? The polarity of the electromagnet (direction of the magnetic field) changes with the direction of the current at a regular interval.!This changes the direction of the magnetic domains in the permanent magnet at the same regular interval


Demagnetisation(b) Electrical Method (Alternating Current)

✦ Place the magnet in the solenoid connected to an alternating current supply.!

✦ Slowly pull the magnet out of the solenoid in the east -west direction.

✦ The alternating current will constantly change the magnetic field direction, which will constantly change the direction of the magnetic domains in the permanent magnet.!

✦ When the magnet is slowly pulled out of the solenoid, its magnetic domains will be in disarray, thus losing its magnetism.


Demagnetisation(b) Electrical Method (Alternating Current)

Uses of Permanent Magnets

Uses of Permanent Magnets -Refer to GCE ‘O’ Physics Matters, Page 409

s4t2 - magnetism · objectives! at the end of the lesson, students are able to state the properties...

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