6) Magnetism & Electromagnetism

Question 1

Poles of a magnet

Answer 1

-north
-south

Question 2

Law of magnetism

Answer 2

-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

Permanent magnet

Answer 3

-magnet that can produce its own magnetic field and will repel and attract other magnets when it is in contact with them
-made from magnetic material
-cannot be demagnetised

Question 4

Soft magnetic materials

Answer 4

-get magnetised when an external magnetic field is applied
-demagnetised when the external field is removed
-electromagnets are made of soft magnetic materials

Question 5

Hard magnetic materials

Answer 5

-magnetised when an external magnetic field is applied
-stays magnetised permanently when the external field is removed

Question 6

Magnetic material examples

Answer 6

-iron
-cobalt
-nikel
-steel (iron + carbon)

Question 7

Magnetic field

Answer 7

The region around a magnet where a force acts on another magnet or on a magnetic material
-strongest at the poles where field lines are closest
-weaker as distance from the magnet increases, field lines further apart

Question 8

Magnetic field lines

Answer 8

-represents strength and direction of magnetic field
-lines close together: strong
-lines far apart: weak
-always go from N to S
-must never touch/ cross other field lines

Question 9

Uniform magnetic field

Answer 9

Produced in the gaps between opposite poles
-same strength, direction at all points
-equal spacing between field lines
-arrow on each field line going from N to S

Question 10

Induced magnets

Answer 10

When a magnetic material is placed in a magnetic field, the material can temporarily be turned into a magnet.

Question 11

Investigate magnetic field pattern - bar magnet

Answer 11

1. Place a compass on a piece of paper at the end of the North pole of a magnet
2. Draw a point where compass points
3. Place the compass next to the point so the needle points away from the dot
4. Draw a new dot where the compass points
5. repeat until there is a chain of points from one end of the magnet to the other
6. Link the points using a smooth curve
7. Repeat to create several other magnetic field lines

Question 12

Electromagnetism

Answer 12

-When a current flows through a conducting wire a magnetic field is produced around the wire
-circular field pattern - no poles
-distance from wire increases, circles get further apart

Question 13

Right hand thumbs (up) rule

Answer 13

Thumb - direction of current
Other fingers - direction of field
-reverse direction of current, reverse direction of magnetic field

Question 14

Factors affecting electromagnetic field strength of current carrying wire

Answer 14

-size of current
-distance from long straight conductor (wire)

Question 15

Motor effect

Answer 15

-A current carrying wire is produces a magnetic field
-this field interacts with other magnetic field (that it is placed in) so a force is applied on the wire
-force pushes the wire so it rotates

Question 16

Fleming’s left hand rule

Answer 16

-thumb - direction of force
-pointer- direction of magnetic field
-middle - direction of current

Question 17

Split ring commutator

Answer 17

-swaps the contacts of the coil
-reverses the direction current is flowing
-reverse the direction of force
-coil will continue to rotate

Question 18

Motor effect - Speed which coil rotates can be increased by

Answer 18

-increase current
-increase strength of magnets

Question 19

Motor effect - force supplied by the motor can be increased by

Answer 19

-increase current in the coil
-increase strength of magnet
-adding more turns to the coil

Question 20

Magnetic field patterns - solenoid

Answer 20

-inside a solenoid, the fields from individual coils add to form a very strong almost uniform field along the centre of the solenoid
-one end acts like N pole - current travelling anticlockwise
other end acts like S pole - current travelling clockwise
-current enters: S/ current leaves: N

Question 21

Magnetic force on a charge

Answer 21

When a charged particle in a parallel wire passes through a magnetic field, the field can exert a force on the particle, causing it to deflect
-parallel to field lines - no force
-at angle - small force

Question 22

Motor effect - Loudspeakers

Answer 22

1. a current in the coil creates a magnetic field which interacts with the permanent magnet generating a force, pushes the cone outwards
2. the current is made to flow in the opposite direction
3. the direction of the magnetic field reverses
4. the force on the cone now pulls it back in
5. repeatedly alternating the current direction makes the cone vibrate in and out
6. the cone vibrations cause pressure variations in the air - which are sound waves

Question 23

Increase strength of electromagnet’s magnetic field

Answer 23

-increase current in the coil
-add more turns to the coil

Question 24

Increase strength of magnetic field around a solenoid

Answer 24

-increase size of current
-increase number of coils
-add an iron coil through the centre of the coils - iron will become an induced magnet

Question 25

Electromagnetic induction

Answer 25

A voltage is induced in a conductor or a coil when it moves through a magnetic field or when a magnetic field changes through it

Question 26

Moving the wire into a fixed magnetic field

Answer 26

-wire cuts through the field lines -induces a potential difference in the wire

Question 27

Moving a magnet into a fixed conductor (coil)

Answer 27

-As the magnet moved through the coil, the field lines cut through the turns on the coil -This induces a potential difference in the coil

Question 28

Electromagnetic induction - Factors affecting induced pd

Answer 28

1. The speed at which the wire, coil or magnet is moved 2. The number of turns on the coils of wire 3. The size of the coils 4. The strength of the magnetic field

Question 29

Alternator

Answer 29

-type of generator that converts mechanical energy to electrical energy in the form of alternating current -slip rings and brushes provide a continuous connection between the coil and the meter -induced pd in the circuit repeatedly changes direction as coil spins -current deflects in both negative and positive

Question 30

Dynamos

Answer 30

-direct-current generator -same as alternator except has a split ring commutator -changes the coil connections every half turn as induced pd is about to change direction -current in external circuit always flows in the same direction -varies from 0 to max value twice each cycle of rotation, never changes polarity

Question 31

Transformer

Answer 31

An electrical device that can be used to increase or decrease the potential difference of an alternating current Consists of: -primary coil -secondary coil -iron core - easily magnetised

Question 32

How transformers work

Answer 32

1. An alternating pd is applied to the primary coil 2. generating a changing mf around a coil 3. mf induces an alternating mf in the iron core 4. Iron core's mf induces a pd across the secondary coil 5. causes a current to flow along secondary coil

Question 33

Step up transformer

Answer 33

-increases pd of a power source -more turns on secondary coil than primary coil -for electricity transmitted over large distances, current in wire heats them, energy loss -transferred at high voltages from power stations

Question 34

Step down transformer

Answer 34

-decreases pd of a power source -fewer turns on secondary coil than primary coil -for domestic uses

Question 35

Relationship between output, input pd, number of turns

Answer 35

Pd across primary coil/ pd across secondary = no. of turns on primary coil/ number of turns on secondary