6.1.1 - 6.1.10 - Magnetism and Electromagnetism

Question 1

What are the poles of Magnets?

Answer 1

The ends of a magnet are called poles
Magnets have two poles: a north and a south

Question 2

What is the law of magnetism?

Answer 2

When two magnets are held close together, there will be an attractive or repulsive force between the magnets depending on how they are arranged:The Law of Magnetism states that:
Two like poles (S and S or N and N) repel each other
Two unlike poles (S and N) attract each other
The attraction or repulsion between two magnetic poles is an example of a non-contact force

Question 3

What are the different magnetic materials and their features?

Answer 3

Magnetic materials can be soft or hard
Magnetically soft materials (e.g. iron):
Are easy to magnetise
Easily lose their magnetism (temporarily magnetised)
Magnetically hard materials (e.g. steel):
Are difficult to magnetise
Do not easily lose their magnetism (permanently magnetised)
Permanent magnets are made out of magnetically hard materials
Electromagnets are made out of magnetically soft materials
This means that electromagnets can be made magnetic or non-magnetic as an when required

Question 4

What is a magnetic field?

Answer 4

The region around a magnet where a force acts on another magnet or on a magnetic material (such as iron, steel, cobalt and nickel)

All magnets are surrounded by a magnetic field

Question 5

What are magnetic lines?

Answer 5

Magnetic field lines are used to represent the strength and direction of a magnetic field
The direction of the magnetic field is shown using arrows
The strength of the magnetic field is shown by the spacing of the magnetic field lines
If the magnetic field lines are close together then the magnetic field will be strong
If the magnetic field lines are far apart then the magnetic field will be weak

Question 6

What are the rules for drawing Magnetic field lines?

Answer 6

Always go from north to south (indicated by an arrow midway along the line)
Must never touch or cross other field lines

Question 7

How is a magnetic field around a bar magnet?

Answer 7

The magnetic field is strongest at the poles
This is where the magnetic field lines are closest together
The magnetic field becomes weaker as the distance from the magnet increases
This is because the magnetic field lines are getting further apart

Question 8

How do the bar magnets look when attracting and reflecting each other?

Answer 8

Question 9

What do the Magnetic fields look like around different configurations of two bar magnets?

Answer 9

Question 10

What is a uniform magnetic field?

Answer 10

Two bar magnets can be used to produce a uniform magnetic field
Point opposite poles (north and south) of the two magnets a few centimetres apart
A uniform magnetic field will be produced in the gaps between opposite poles

Question 11

What are the features of Uniform Magnetic lines?

Answer 11

A uniform magnetic field is one that has the same strength and direction at all points
To show that the magnetic field has the same strength at all points there must be equal spacing between all magnetic field lines
To show that the magnetic field is acting in the same direction at all points there must be an arrow on each magnetic field line going from the north pole to the South Pole
This field can be determined by using plotting compasses that will point from north to south or by using iron filings

Question 12

What are the magnetic metals?

Answer 12

Iron
Cobalt
Nickel
Steel is an alloy which contains iron, so it is also magnetic
Magnetic materials (which are not magnets) will always be attracted to the magnet, regardless of which pole is held close to it

Question 13

How to test if a material is magnetic?

Answer 13

If it can be repelled by the known magnet then the material itself is a magnet
If it can only be attracted and not repelled then it is a magnetic material

Question 14

What are permanent magnets?

Answer 14

Permanent magnets are made out of permanent magnetic materials, for example steel
A permanent magnet will produce its own magnetic field
It will not lose its magnetism

Question 15

What are Induced Magnets?

Answer 15

When a magnetic material is placed in a magnetic field, the material can temporarily be turned into a magnet.
This is called induced magnetism
When magnetism is induced on a material:
One end of the material will become a north pole
The other end will become a south pole

Question 16

What is Core practical investigating Magnetic fields?

Answer 16

To investigate the magnetic field pattern for a permanent bar magnet and between two bar magnets

Step 1:

Place the magnet on top of a piece of paper
Draw a dot at one end of the magnet (near its corner)

Step 2:

Place a plotting compass next to the dot, so that one end of the needle of the compass points away from the dot
Use a pencil to draw a new dot at the other side of the compass needle

Step 3:

Move the compass so that it points away from the new dot, and repeat the process above

Step 4:

Keep repeating the previous process until there is a chain of dots going from one end of the magnet to the other
Then remove the compass, and link the dots using a smooth curve – this will be the magnetic field line

Step 5:

Repeat the whole process several times to create several other magnetic field lines

Step 6:

Repeat the whole process for two bar magnets placed 5 cm apart first facing the same pole then facing opposite poles

Question 17

What are the results of this experiment

Answer 17

Question 18

How should the Magnetic field pattern for the 2 bar magnets look?

Answer 18

Question 19

How is an magnetic field formed

Answer 19

When a current flows through a conducting wire a magnetic field is produced around the wire
The shape and direction of the magnetic field can be investigated using plotting compasses

Question 20

What is a magnetic field features?

Answer 20

The magnetic field is made up of concentric circles
A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles
As the distance from the wire increases the circles get further apart
This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases
The right-hand thumb rule can be used to work out the direction of the magnetic field

Question 21

What are the factors affecting field strength?

Answer 21

The strength of the magnetic fields field depends on:
The size of the current
The distance from the long straight conductor (such as a wire)
A larger current will produce a larger magnetic field and vice versa
The greater the distance from the conductor, the weaker the magnetic field and vice versa

Question 22

What is the motor effect?

Answer 22

A wire with current flowing through it is placed in a magnetic field and experiences a force

Question 23

What causes the motor effect?

Answer 23

This effect is a result of two interacting magnetic fields
One is produced around the wire due to the current flowing through it
The second is the magnetic field into which the wire is placed, for example, between two magnets
As a result of the interactions of the two magnetic fields, the wire will experience a force

Question 24

What is a D.C Motor?

Answer 24

The motor effect can be used to create a simple d.c. electric motor
The force on a current-carrying coil is used to make it rotate in a single direction
The simple d.c. motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field
The coil of wire, when horizontal, forms a complete circuit with a cell
The coil is attached to a split ring (a circular tube of metal split in two)
This split ring is connected in a circuit with the cell via contact with conducting carbon brushes

Current flowing through the coil produces a magnetic field
This magnetic field interacts with the uniform external field, so a force is exerted on the wire
Forces act in opposite directions on each side of the coil, causing it to rotate:
On the blue side of the coil, current travels towards the cell so the force acts upwards (using Fleming’s left-hand rule)
On the black side, current flows away from the cell so the force acts downwards
Once the coil has rotated 90°, the split ring is no longer in contact with the brushes
No current flows through the coil so no forces act

Question 25

What are the factors affecting the D.C Motor?

Answer 25

The speed at which the coil rotates can be increased by: Increasing the current Increasing the strength of the magnetic field The direction of rotation of coil in the d.c motor can be changed by: Reversing the direction of the current Reversing the direction of the magnetic field by reversing the poles of the magnet The force supplied by the motor can be increased by: Increasing the current in the coil Increasing the strength of the magnetic field Adding more turns to the coil

Question 26

How do loudspeakers work to the motor effect?

Answer 26

The speed at which the coil rotates can be increased by: Increasing the current Increasing the strength of the magnetic field The direction of rotation of coil in the d.c motor can be changed by: Reversing the direction of the current Reversing the direction of the magnetic field by reversing the poles of the magnet The force supplied by the motor can be increased by: Increasing the current in the coil Increasing the strength of the magnetic field Adding more turns to the coil An alternating current passes through the coil of the loudspeaker This creates a changing magnetic field around the coil As the current is constantly changing direction, the direction of the magnetic field will be constantly changing The magnetic field produced around the coil interacts with the field from the permanent magnet The interacting magnetic fields will exert a force on the coil The direction of the force at any instant can be determined using Fleming’s left-hand rule As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction This makes the coil oscillate The oscillating coil causes the speaker cone to oscillate This makes the air oscillate, creating sound waves

Question 27

What are the factors affecting the magnetic force?

Answer 27

Magnetic forces are due to interactions between magnetic fields Stronger magnetic fields produce stronger forces and vice versa For a current carrying conductor, the size of the force exerted by the magnetic fields can be increased by: Increasing the amount of current flowing through the wire This will increase the magnetic field around the wire Using stronger magnets This will increase the magnetic field between the poles of the magnet Placing the wire at 90o to the direction of the magnetic field lines between the poles of the magnet This will result in the maximum interaction between the two magnetic fields

Question 28

What is Flemings left hand rule?

Answer 28

The direction of the force (aka the thrust) on a current carrying wire depends on the direction of the current and the direction of the magnetic field All three will be perpendicular to each other This means that sometimes the force could be into and out of the page (in 3D) The direction of the force (or thrust) can be worked out by using Fleming's left-hand rule:

Question 29

How to do Flemings left hand rule?

Answer 29

Step 1: Determine the direction of the magnetic field Start by pointing your First Finger in the direction of the (magnetic) Field. Step 2: Determine the direction of the current Now rotate your hand around the first finger so that the seCond finger points in the direction of the Current Step 3: Determine the direction of the force The THumb will now be pointing in the direction of the THrust (the force) Therefore, this will be the direction in which the wire will move

Question 30

What are electromagnetics?

Answer 30

When an electric current flows in a wire it creates a magnetic field around the wire By winding the wire into a coil we can strengthen the magnetic field by concentrating the field lines If this wire is wound around a soft magnet, such as an iron, then an electromagnet is made The electromagnet is magnetic only when current flows through the wire

Question 31

How can the strengths of an electromagnetic field be increased?

Answer 31

Increasing the current in the coil Adding more turns to the coil The magnetic field around an electromagnet has the same shape as the one around a bar magnet The field can be reversed by reversing the direction of the current However, bar magnets are always magnetic, unlike electromagnets

Question 32

How are magnetic field slightly different in a straight wire?

Answer 32

When a current flows through a conducting wire a magnetic field is produced around the wire The shape and direction of the magnetic field can be investigated using plotting compasses The magnetic field is made up of concentric circles A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles As the distance from the wire increases the circles get further apart This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases The right-hand thumb rule can be used to work out the direction of the magnetic field Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field If there is no current flowing through the conductor there will be no magnetic field Increasing the amount of current flowing through the wire will increase the strength of the magnetic field This means the field lines will become closer together

Question 33

How are magnetic fields different in a flat circular coil?

Answer 33

When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it To increase the strength of the magnetic field around the wire it should be coiled to form a solenoid The magnetic field around the solenoid is similar to that of a bar magnet

Question 34

What is a Magnetic field different in a solenoid?

Answer 34

The magnetic field inside the solenoid is strong and uniform Inside a solenoid (an example of an electromagnet) the fields from individual coils Add together to form a very strong almost uniform field along the centre of the solenoid Cancel to give a weaker field outside the solenoid One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole To work out the polarity of each end of the solenoid it needs to be viewed from the end If the current is travelling around in a clockwise direction then it is the south pole If the current is travelling around in an anticlockwise direction then it is the north pole If the current changes direction then the north and south poles will be reversed If there is no current flowing through the wire then there will be no magnetic field produced around or through the solenoid

Question 35

How can the strength of a magnetic field produced around a solenoid be increased?

Answer 35

Increasing the size of the current which is flowing through the wire Increasing the number of coils Adding an iron core through the centre of the coils The iron core will become an induced magnet when current is flowing through the coils The magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall

Question 36

How does a magnetic force affect a charge?

Answer 36

When a current-carrying wire is placed in a magnetic field, it will experience a force if the wire is perpendicular This is because the magnetic field exerts a force on each individual electron flowing through the wire Therefore, when a charged particle passes through a magnetic field, the field can exert a force on the particle, causing it to deflect The force is always at 90 degrees to both the direction of travel and the magnetic field lines The direction can be worked out by using Fleming's left-hand rule If the particle is travelling perpendicular to the field lines: It will experience the maximum force If the particle is travelling parallel to the field lines: It will experience no force If the particle is travelling at an angle to the field lines: It will experience a small force