EDEXCEL Physics unit 6 notes

 Unit 6: Magnetism and Electromagnetism.

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UNIT 6: Magnetism and Electromagnetism

When a bar of magnet is hung from a thin thread and allowed to swing, it stops swinging and the pole of magnet facing the north side is known as the north pole and south-seeking pole is known as south pole.

Metals such as iron, cobalt and nickel are magnetic which means a magnet will attract this metal. For example, if there are iron pins fallen then can be quickly collected with the help of a magnet.

Two like pole (N and N pole, S and S Pole) will repel each other. But unlike poles will attract each other.

Practical 1: Magnetic Fields 

Method 1:

a) Place a compass on a piece of paper, with pencil plot a dot on the north side of compass needle.

b) Shift the compass to next position in contact with the dot and mark the position of second needle.

c) Repeating the process so that a field line is traced from the north to the south pole in magnet with a greater number of compasses.

Method 2:

a) Over the magnet place a piece of magnet.

b) Take iron fillings and sprinkle over the top of paper and then lightly tap the paper.

c) Then the pattern of iron filled is sketched- this shows the shape of magnetic field

Same poles will attract each other. There is an area in the field which hare equally spaced out and pointing the same direction is called as uniform field.

Unlike poles of two magnets will repel each other. The magnetic field lines of two north poles are shows clearly that the fields repel and a zone X is created which is called a “neutral point”.

These forces of attraction and repulsion are examples of non-contact forces. And arrows always point from north to the south pole.

Magnetic fields:

Area around the magnet where force can act on another magnet or other magnetic material. The strength of field is proportional to the force exerted by a magnetic object. If there is less magnetic field, then there is less force. Magnetic field is represented by Magnetic field lines. Each line starts from north pole to south pole. When these lines are drawn closer to each other they are stronger in force and when line are far apar they have less force.

Combining magnetic fields:

The magnetic field lines of two or more magnets would never intersect as the compass needle would have to point in two directions at once which is not possible. Although magnetic fields might cross for two different magnets but not the magnetic field lines.

Magnetic field of earth:

The compass needle that pointing towards the north pole is actually pointing at the south pole, as opposite poles attract. (Not the geographical north pole)

Permanent magnet: a piece of metal that is magnetised, it becomes a permanent magnet. Example: Steel

Induced magnet: A piece of magnet is temporary magnet that only remains magnetised until it is placed in a magnetic field. This magnetic field can be provided by a coil of wire carrying current which produces magnetic field

Hard material: A hard magnetic material turns into permanent magnet after magnetisation.

Soft material: Will become an induced or temporary magnet.

Domains: Parts of magnet which act like small magnet.

In a bar of iron, the domains are unaligned and jumbled up and when a magnet is near, the domains become lined up. So, the bar gets attracted either a north or south pole of a permanent magnet.

In steel bar once the domain has lined up, they are in one direction.

 

Ways of Magnetising

Stroking: Repeatedly stroking a unmagnetized stee bar with a permanent magnet turns the steel bar into permanent magnet. Stroking lines up all the domains.

Solenoid: Steel bar is placed in a very large but short pulse of current through the solenoid produces a strong magnetic field which magnetises the bar. If an iron bar is used, it is called an electromagnet. The iron bar remains magnet until there is electricity flowing through the bar but once the current stops it changes to non-magnetic.

 

Magnetic field near a straight wire:

When an electric current passes through a conductor it produces a magnetic field around it.

When current passing through the wire is small, field is weak.

When the current is large, magnetic field near to the bar stays strong and further away from the wire gets weaker.

The direction of magnetic field could be found using a compass and if current direction is reversed, the direction of magnetic field also reverses.

The direction of current into the paper is shown using ⊗, field lines are always clockwise.

The current out of the paper is shown using ⊙, the field lines are always anti-clockwise. 

Right hand grip rule:

The thumb of the right hand along a wire in the direction of current. Fingers point towards the direction of magnetic field.

PRACTICAL:

Using the right-hand rule to check that the fields go around the wire in anticlockwise direction. 

Magnetic field induced by coil of wire:

Current carried in a single loop of wire produces the magnetic field around it. 

In diagram above, it can be known by the right-hand grip rule that A has anti-clockwise magnetic fields and B has clockwise magnetic field. In the middle part, the magnetic fields combine to produce a field from right to left. The loop of wire behaves like a very short bar magnet. The field lines always come out of the north (on left) towards the south (on right).

Magnetic field created from a solenoid or long coil of wire is shown in diagram on the left side. The magnetic field created from one adds onto next, which causes a resultant magnetic field similar to that of long bar magnet’s field.

Large magnetic field production:

The magnetic field strength is increased by: 

a) Using larger current.

b) More number of turns of wire.

c) Iron in middle of solenoid.

Caution I to be taken as too much of current can make solenoid to hot and melt the wire. So larger solenoids must be cooled by water. There is also limit to number of turns that can be added into space.

A long solenoid and bar magnet produce the same shape of magnetic field.

Superconducting magnets: strongest magnetic field. At very low temperatures (4 K), materials like niobium and lead become superconductors. These materials have no electrical resistance and so no heating takes place. So large magnetic field can be produced by huge current (5000 A), passing through solenoid, kept in liquid helium.

The motor effect:

A current flowing through wire induces a magnetic field is placed between two permanent magnets. The two magnetic fields cut each other and cause a deflection in the field. A current through the wire causes it to be pushed down and away from the pole of magnet. Reversing the current makes wire move upwards, away from the poles of magnet. It is called motor effect.

The coil of wire carrying current induces a circular magnetic field. This circular magnetic field squashes the field lines between poles of magnet. The squashing of wire causes the movement of wire.

The force acting on foil is proportional to:

a) Strength of magnetic field between two poles 

b) Current 

c) Length of wire between two poles 

The change of wire in a magnetic field is at right angles to current direction and magnetic field. This can be known using left hand rule

The left-hand rule:

Predicting the movement of conductor placed in magnetic field:

Spread out the index, second finger and thumb out so that each are at right angles to each other.

First finger: Field of magnets.

Second finger: Current.

Thumb: Point in the direction of movement of wire.

This rule works when:

Current is at right angle.

Field and current are parallel to each other.

Wire stays where it is.

Ammeters:

A loop of wire is pivoted on axle between the poles of a magnet. When current is switched Onn, the left-hand side of the loop moves downwards and right-hand side moves upwards. (Left hand rule). When nothing stops the loop of wire, it turns the DC side to the top and AB is at bottom.

When a spring is attached to the loop, it is restricted in movement and when a larger current is passed through the loop, there is more force and so the spring is stretched more, so the loop is allowed to turn more.

Ammeter: sensitive if the loop turns a long way with small current. The sensitivity will be increased when:

a)  More number of turns in coil.

b)  Strong magnets are used.

c)   Weak springs.

Charged particles and Deflection:

Current: Flow of charged particles (usually electrons), These charged particles can be deflected by magnetic field by the force exerted by field.

Force depends on:

a)  Strength of magnetic field.

b)  Speed of particles.

c)   Charge of particles.

Deflecting force direction on particles can be assumed using the left-hand rule. Particles that move parallel to the magnetic field do not get deflected.