Unit 17 Magnetism And Electromagnetism Physics

Front 1

What is magnetite?

Back 1

Magnetite is a naturally occuring iron oxide mineral. It is a natural magnet ththat attracts certain materials like cobalt, nickel and steel.

Front 2

What are magnetic or ferromagnetic materials?

Back 2

Magnetic or ferromagnetic materials are materials that are attracted by magnets.

Front 3

Give 3 examples of magnetic or ferromagnetic materials.

Back 3

examples of magnetic or ferromagnetic materials:

1) cobalt

2) nickel

3) steel

Front 4

What is a permanent magnet?

Back 4

A material that retains its magnetism for a long time is called a permanent magnet.

Front 5

With what materials are permanent magnets usually made up of? (3)

Back 5

Permanent magnets are usually made up of:

1) steel (an alloy of iron iron and carbon)

2) alloys like alnico (aluminium, nickel and cobalt)

3) alcomax (iron, nickel, copper, cobalt, aluminium)

Front 6

What is the permanent magnet, ceramic magnet made up of?

Back 6

Ceramic magnet, a permanent magnet is made from ferrites-iron oxide compounds that contain other elements like boron, barium or nickel.

Front 7

What are the properties of magnets like ceramic magnet?

Back 7

Magnets like the ceramic magnet are often hard and brittle.

Front 8

What element are the strongest permanent magnet made up of?

Back 8

Magnets made from the rare element neodymium are among the strongest permanent magnets.

Front 9

Where are Neodymium magnets used? (3)

Back 9

Neodymium magnets are used in:

1) earphones

2) loudspeakers

3) hard disk drives

Front 10

State the 4 properties of magnets.

Back 10

Properties of magnets;

1) attract magnetic materials.

2) magnets have magnetic poles.

3) When we suspend a bar magnet freely, it will come to rest in the North-South direction.

north-seeking pole (north pole or N pole)

The end of the magnet that points to the northern end of the Earth is called the north-seeking pole (north pole or N pole) The end of the magnet that points to the southern end of the Earth is called the south-seeking pole (south pole or S pole) .4) Law of magnetism: Like poles repel, unlike poles attract.

The end of the magnet that points to the southern end of the Earth is called the south-seeking pole (south pole or S pole) .

4) Law of magnetism:

Like poles repel, unlike poles attract.

, unlike poles attract.

Front 11

State the use of magnets due to the rest in the North-South direction when suspended freely.

Back 11

Magnets are used in compasses for navigation purposes due to the rest in the North-South direction when suspended freely.

Front 12

How is a magnet identified?

Back 12

If we move one end of the object towards the N pole of a suspended bar magnet, and repulsion occurs, we can conclude that the object is a magnet.

Front 13

Why when attraction occurs, we cannot conclude that the object is a magnet?

Back 13

When attraction occurs, we cannot conclude that the object is a magnet because the object may either be a magnetic material (not a magnet) that is attracted by the suspended magnet,

or a magnet itself with the opposing pole on the approaching end (To be certain, we have to test the other end with the N pole to see if repulsion occurs) .

Front 14

What is the only test to confirm that an object is a magnet?

Back 14

Repulsion is the only test to confirm that an object is a magnet.

Front 15

What is magnetic induction?

Back 15

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

Front 16

Describe how magnetic induction can occur without any contact with the magnet.

Back 16

A permanent magnet is brought near (but does not touch) an iron bar. The iron bar becomes an induced magnet. The N pole of the permanent magnet induces an S pole in the nearer end of the iron bar, while the further end of the iron bar becomes an N pole.

Front 17

State 2 ways of making magnets.

Back 17

Ways of making magnets :

1) Stroking method ;

2) Electrical method using a direct current.

Front 18

Define the stroking method to make a magnet.

Back 18

Stroking method:

An unmagnetised steel bar is stroked several times with the same pole (e.g N pole) of a permanent magnet from one end to the other end in one direction.

The stroking magnet is lifted sufficiently high above the steel bar between successive strokes. The steel bar will become a permanent magnet due to magnetic induction.

The pole produced at the end of the steel bar where the strokes finish is opposite to the stroking pole used.

Front 19

Describe the electrical method by using a direct current to make a magnet.

Back 19

Electrical method by using a direct current ;

The steel bar to be magnetised is placed in a solenoid (a cylindrical coil of insulated copper)

The solenoid used should have several hundred turns of insulated copper wire.

When a large direct current (d.c) is passed through the solenoid, the unmagnetised steel bar will become magnetised after a while.

This is because when an electric current flows through the solenoid, it produces a strong magnetic field which magnetises the steel bar.

Front 20

What is a solenoid?

Back 20

A solenoid is a cylindrical coil of insulated copper wires.

Front 21

State the method where the poles of the magnet can be determined.

Back 21

The poles of the magnet can be determined by the right-hand grip rule.

Front 22

State the 3 ways to demagnetise magnets.

Back 22

Ways of demagnetising magnets;

1) Heating

2) Hammering

3) Electrical method using an alternating current.

Front 23

How can heating be used to demagnetise magnets?

Back 23

If we heat a magnet strongly with a Bunsen flame, the magnet will lose its magnetism very quickly.

Front 24

How can hammering be used to demagnetise magnets?

Back 24

Hammering alters the alignment of the magnetic domains, causing the magnet to lose its magnetism.

Front 25

What is an alternating current?

Back 25

An alternating current is an electric current which varies its direction many times per second.

Front 26

What is a magnetic field?

Back 26

A magnetic field is a region in which a magnetic object, placed within the influence of the field, experiences a magnetic force.

Front 27

Describe how we can find the pattern of the magnetic field around a bar magnet by iron filings.

Back 27

To reveal the pattern of the magnetic field around a bar magnet, we can place it on a piece of paper and sprinkle iron filings lightly around it, while tapping the paper gently.

Front 28

Describe the experiment to find the direction of the magnetic field lines with a plotting compass.

Back 28

Objective: to plot magnetic field lines with a compass

Apparatus: bar magnet, plotting compass, plain paper, pencil

Procedure:

1. Place the bar magnet at the centre of the paper so that its N pole faces North and its S pole faces South.

2. Starting near one pole of the magnet, the positions of the ends, S and N, of the compass needle are marked by pencil dots X and Y respectively.

The compass is then moved so that S end is at Y and the new position of the N end is marked with a third dot Z.

3. Repeat the process of marking the dots. Join the dots and this will give the plot of the magnetic field.

Precautions:

1. Check that the plotting compass is in good working order.

2. Ensure that there is no strong magnetic field (other than the Earth's magnetic field) near the plotting compass.

Front 29

State the 3 principles that are applied when drawing magnetic field lines.

Back 29

Principles that are applied when drawing magnetic field lines;

1) Magnetic field lines are directed outward from the N pole and towards the S pole.

2) Magnetic field lines do not cross or intersect one another.

3) To represent strong magnetic fields , the fields line are drawn closer together. Field lines drawn further apart represents a weak magnetic field.

Front 30

Draw the magnetic field lines between an N pole and an S pole.

Back 30

Front 31

Draw the magnetic field lines between two N poles.

Back 31

Front 32

What is X called and why?

Back 32

X is called a neutral point because the fields from both magnets cancel out each other.

Front 33

What is magnetic shielding?

Back 33

Magnetic shielding prevents surrounding magnetic fields from reaching sensitive areas of a piece of equipment whose operation may be affected by the fields.

Front 34

Give 2 examples of materials that can be used for magnetic shielding.

Back 34

Thin sheets of soft magnetic materials (e.g iron and MuMetal) can be used for magnetic shielding.

Front 35

Describe an experiment to investigate the magnetic properties of iron and steel.

Back 35

Experiment to investigate the magnetic properties of iron and steel:

Apparatus ;

Bar magnet, iron and steel bars of equal dimensions - 20 cm in length, iron paper clips.

Procedure:

1) Let the N pole of the bar magnet attract one end of the iron bar. The other end of the iron bar is dipped into a tray of paper clips.

Record the maximum number of paper clips that are attracted to the iron bar.

2) Pull the magnet away from the iron bar. Observe what happens and record the number of paper clips still attracted to the iron bar.

3) Now repeat step 1 using the steel bar in place of the iron bar.

Record the maximum number of paper clips attracted to the steel bar.

4) Pull the magnet away from the steel bar. Note and record the number of paper clips still attracted to the steel bar.

(Picture in hint).

Observation;

Iron is a stronger induced magnet and was thus able to pick up more paper clips.

Iron, however does not retain its magnetism.

Steel is a weaker induced magnet and was thus able to pick up fewer paper clips. Steel, however retain its magnetism as the paper clips were still attracted to the steel bar even though the magnet was removed.

Hint 35

Front 36

What are soft magnetic materials?

Back 36

Soft magnetic materials are magnetic materials such as iron which are easily magnetised but do not retain their magnetism.

Front 37

What are hard magnetic materials?

Back 37

Hard magnetic materials are magnetic materials such as steel, which are harder to magnetise but retain their magnetism.

Front 38

What are hard magnetic materials used for?

Back 38

Hard magnetic materials are used to make permanent magnets.

Front 39

What are soft magnetic materials such as iron used for?(2)

Back 39

Soft magnetic materials such as iron are used as

1) temporary magnets in electromagnets in the cores of transformers and

in the cores of transformers and2) in magnetic shielding.

2) in magnetic shielding.

Front 40

Describe 4 uses of permanent magnets

Back 40

A moving-coil ammeter is a device used to measure current. The ammeter consists of a coil suspended in the magnetic field of a permanent magnet. When a current flows into and out of a coil, a turning effect is produced on the coil. A pointer attached to the coil will then move to show a value on the scale proportional to the size of the current.

2) Magnetic door catch

1) Moving- coil ammeter A moving-coil ammeter is a device used to measure current. The ammeter consists of a coil suspended in the magnetic field of a permanent magnet. When a current flows into and out of a coil, a turning effect is produced on the coil. A pointer attached to the coil will then move to show a value on the scale proportional to the size of the current. 2) Magnetic door catch Magnetic strips are fitted to the doors of freezers and refrigerators to keep the doors closed. 3) Peranent magnets are essential in the operation of numerous electrical machines such as d.c. motors and a.c. generators4) Permanent magnets (usually neodymium magnets ) are used in moving-coil loudspeakers. At the core of the loudspeaker, there is a very strong permanent magnet.

Magnetic strips are fitted to the doors of freezers and refrigerators to keep the doors closed.

3) Peranent magnets are essential in the operation of numerous electrical machines such as d.c. motors and a.c. generators

4) Permanent magnets (usually neodymium magnets ) are used in moving-coil loudspeakers. At the core of the loudspeaker, there is a very strong permanent magnet.

Front 41

Describe how electrical method using an alternating current demagnetise magnets.

Back 41

Place a magnet inside a solenoid connected to an alternating current (a.c.) supply.

The magnet is then slowly withdrawn in the East-West direction with the alternating current still flowing in the solenoid.