Topic 6- Magnetism

Question 1

What can magnets do to other magnets and magnetic substances?

Answer 1

Magnets can attract other magnets if their poles are unalike or they can repel them if they are the same poles.

They can also attract magnetic substances.

Question 2

What are the characteristics of magnetically hard materials?

Answer 2

Magnetically hard materials are materials such as steel. They keep their induced magnetism permanently as their domains remain aligned.

Question 3

What are the characteristics of magnetically soft materials?

Answer 3

These are materials such as iron. They do not keep their magnetism permanently as their domains do not always remain aligned.

Question 4

What is a magnetic field?

Answer 4

This is a region where magnetic materials will experience a force.

Question 5

What are magnetic field lines?

Answer 5

These are invisible field lines that show the strength and direction of the magnetic field. The closer the lines are, the stronger the field is.

Question 6

How can magnetism be induced?

Answer 6

By using a permanent magnet, you can induce magnetism on a magnetic material.

You can stroke the permanent magnet onto the magnetic material in one direction.
OR, you can place a magnet close to the magnetic material and it will induce its magnetism.

Question 7

How can you investigate the magnetic field pattern for a bar magnet or between two bar magnets?

Answer 7

Place a bar magnet (or 2) on a piece of paper. Place several plotting compasses around the bar magnet and draw the direction of the compasses’ needle. (Field lines should go from N to S)

Question 8

What happens when you put bar magnets with opposite poles together and what happens to the magnetic field lines?

Answer 8

The magnets will attract.

The filed lines between the opposite poles will be uniform and parallel, as of the two magnets were just one bar magnet.

Question 9

What happens if an electric current is passed through a conductor?

Answer 9

An electric current in a conductor produces a magnetic field around the conductor.

The larger the electric current: the stronger the magnetic field.
The direction of the magnetic field depends on the direction of the current.

Question 10

How are electromagnets made?

Answer 10

Wrap a wire around a soft core (e.g. Iron). Then, pass a current through the coiled-core and an electromagnet is made.

Question 11

What are the magnetic field lines like for a straight wire?

Answer 11

The field around the wire is a simple circle. The direction of the filed lines depends on the direction of the current.
Using the right hand grip rule:
Current direction up: filed lines are anti-clockwise
Current direction down: field lines are clockwise.

Question 12

What is the magnetic field pattern like for a flat circular coil?

Answer 12

The field around the flat coil is like a single wire, but with two circles- one for each direction.

Question 13

What are the magnetic filed lines like for a solenoid?

Answer 13

• The magnetic filed inside the current carrying solenoid is strong and uniform.
• Outside the coil, the field is just like the one around a bar magnet.

Therefore, the ends of a solenoid act like the north and south poles of a magnet.

Question 14

What happens to a charged particle when it moves in a magnetic field?

Answer 14

There is always a force on a charged particle when it moves in a magnetic field as long as the motion in the field is NOT parallel to the field lines.

(The force will be at its strongest if the charged particle is at 90 degrees to the field lines)

Question 15

How do D.C motors work?

Answer 15

1) A d.c current is passed through a coil do wire which is able to move between two opposite magnetic poles.
2) When a current is passed through, the coil has a magnetic field which interrupts with the magnet’s magnetic field, =force= turn.

(Every half turn, the current changes so that the coil doesn’t come to a halt. This is done by using a split ring commutator)

Question 16

How do loudspeakers work?

Answer 16

1) An a.c. current is passed into a coil of wire, which is wrapped around the base of the speaker.
2) The coil of wire is surrounded by permanent magnets, so depending on the direction of the current, a force on the coil is applied, either making it move backwards or forwards.
- these forward and backward movements make the cone vibrate=sound.

Question 17

What is Flemming’s Left Hand rule and what does each finger represent?

Answer 17

Using the rule, we can predict the direction of the resulting force if we know 2 of the 3 directions.
Thumb: force
First finger: magnetic field
Second finger: direction of conventional current

Question 18

How can you increase the force of a current carrying wire in a magnetic field?

Answer 18

You can do this by increasing the current or the strength of the magnetic field.

Question 19

How can a voltage be induced in a magnetic field? How can the size of the voltage be increased?

Answer 19

Move a magnet through a coil of wire.

The size of the induced voltage can be increased by:
-using a stronger magnet
-increase the number of turns on the coil
-moving magnet faster
The direction of the induced voltage can be reversed by putting the other pole of the magnet through.

Question 20

How do generators produce electricity?

Answer 20

Generators rotate a coil in a magnetic field (or a magnetic in a coil) and when either of these spin, lines of fields are cut- creating current.
Every half turn, the current changes direction, and with the help of slip rings and brushes, the contacts do not swap every half term too.

This means the current produced is A.C.

Question 21

How do you increase the amount of current produced from a generator?

Answer 21

• Strength of magnetic field
• Number of coils in a wire
• speed of rotation in field

Question 22

What is the structure of a transformer?

Answer 22

• Primary coil
• Secondary coil
• Iron core (to concentrate field lines= nearly 100% efficiency)

Question 23

What do transformers do?

Answer 23

Transformers change the size of the voltage of an alternating current by having different a different number of turns on each of the input and output sides.

Question 24

What are step up transformers?

Answer 24

These step the voltage up. They do this by having more turns on the secondary coil.

Question 25

What is a step down transformer?

Answer 25

This steps down the voltage. They have more turns on the primary coil than on the secondary.

Question 26

Why are transformers important in the national grid?

Answer 26

• The voltage produced by power stations is too low to be transmitted efficiently. Because P= VI, the low voltage means there will be a high current= resistance= a lot of heat loss.
• A step up transformer is used to increase the voltage (and decrease the current) which increases efficiency.
• At the end of the journey a step down transformer is used so the voltage is decreased and is more useful and safe to use.

Question 27

What is the equation linking: input (primary voltage), output (secondary) voltage, number of turns on primary and number of turns runs on secondary?

Answer 27

Input (p) voltage / output secondary voltage=

Number of turns on primary / number of turns on secondary.

Question 28

What is the equation which is linked to the efficiency of a transformer?

Answer 28

Primary voltage * Primary current = Secondary voltage * Secondary Current

Question 29

How do transformers work?

Answer 29

1) An A.C. supply is passed through the primary coil, creating an alternating magnetic field.
2) The soft iron core concentrates the field through to the secondary coil, inducing a voltage.
3) The ratio of turns change the induced voltage.
* does not work with a D.C supply*

Question 30

What happens when the current in the primary coil of a transformer increases or decreases steadily?

Answer 30

There is a constant voltage produced in the secondary coil.

Question 31

What happens to the voltage of the secondary coil when the voltage of the primary coil is at its highest?

Answer 31

The voltage in the secondary coil will be at its lowest.

Question 32

What is electromagnetic induction?

Answer 32

The creation of a voltage (and sometimes current) in a wire which is experiencing a change in magnetic field.

Question 33

What is the motor effect?

Answer 33

When a current carrying wire is placed in a magnetic field and experiences a force.