Magnetic Fields

Question 1

Magnetic field facts

Answer 1

All magnets produce magnetic fields
Always North to South
Closer lines = stronger field
Further from magnet = weaker field
Magnetic field is strongest at poles

Question 2

Magnetic field definition

Answer 2

Region where other magnets or magnetic materials experience a force

Question 3

What happens when 2 unlike poles are placed together

Answer 3

Attraction
Uniform field in between (same strength everywhere between the poles)
Draw at least 3 field lines with the same distance between each line

Question 4

What happens when 2 like poles are placed together

Answer 4

Repulsion

Field lines curve away from each other

Question 5

All magnetic materials

Answer 5

Anything with:

• Iron (e.g. steel)
• Nickel
• Cobalt

Question 6

Permanent magnets definition

Answer 6

Produces magnetic field all the time

Question 7

Induced magnets definition

Answer 7

Only produces magnetic field when in another magnetic field

Question 8

Type of force between magnet and magnetic material

Answer 8

Always attractive because poles of magnetic induces unlike poles in magnetic material

Question 9

How quickly induced magnets lose their magnetism

Answer 9

Magnetically “soft” e.g. pure iron, nickel-iron alloys = quickly
Magnetically “hard” e.g. steel = slowly
Permanent magnets are made from magnetically hard material

Question 10

How magnets are used in fridge doors

Answer 10

Permanent magnetic strip to keep it closed

Question 11

How magnets are used in cranes

Answer 11

Induced electromagnets to attract and move magnetic materials e.g. scrap metal

Question 12

How magnets are used in doorbells

Answer 12

Electromagnet is turned on and off rapidly
Arm is attracted and released repeatedly
Arm strikes bell to produce ringing noise

Question 13

How magnets are used in magnetic separators

Answer 13

Magnet attracts scrap metal in recycling plants

Question 14

How magnets are used in Maglev trains

Answer 14

Magnetic repulsion used to make train float above track (reduce friction)
Also used to propel train along

Question 15

How magnets are used in MRI scans

Answer 15

Magnetic fields used to create images inside of body

No ionising radiation required (less hazardous)

Question 16

Magnetic fields and current

Answer 16

When current flows, a magnetic field is made
The field is made up of concentric circles perpendicular to the wire
Changing direction of current changes direction of field

Question 17

Factors of magnetic fields made by current

Answer 17

Larger the current = stronger field

Closer to wire = stronger field

Question 18

Right hand thumb rule

Answer 18

Thumb = direction of current
Fingers = direction of magnetic field

Question 19

Motor effect

Answer 19

When a current-carrying conductor is put between magnetic poles, the magnetic fields interact and create a force
Wire has to be at 90° to experience full force
Force acts the same relative to direction of magnetic field and current

Question 20

Fleming’s left hand rule

Answer 20

thuMb = motion
First finger = field
seCond finer = current

Question 21

Force action on a conductor formula

Answer 21

Force (N)= magnetic flux density (T)x current(A) x length(m)
Current has to be 90° to magnetic field for formula to be accurate

Question 22

How motors work

Answer 22

Current flows through current in a magnetic field
Forces act up or down on either side of coil, causing it to turn
Current is then temporarily cut off due to split ring commutator
Momentum causes coil to keep turning and current flows but in the opposite direction
Forces up and down are oppositely re-established so coil keeps turning in the same direction

Question 23

How direction of rotation in a motor can change

Answer 23

Swapping polarity of d.c. current (reversing current)

Swapping magnetic poles over (reversing field)

Question 24

How to work out direction of rotation in a motor

Answer 24

Current always goes positive to negative
Use Fleming’s left hand rule to work out direction of force
Work out direction of turning

Question 25

Solenoid definition

Answer 25

Long coil with lots of loops | Type of electromagnet

Question 26

Solenoid magnetic field facts

Answer 26

Inside of coil, lots of lines are pointing in same direction Outside of coil, overlapping field lines cancel each other our so field is weak part from at the ends of solenoid Field strength can be increased using an iron core inside the solenoid, as it is induced

Question 27

Electromagnetic induction definition

Answer 27

Creating potential difference in a wire experiencing a change in magnetic field

Question 28

How to cause electromagnetic induction

Answer 28

Rotating / moving a magnetic in a coil of wire Rotating / moving a conductor in a magnetic field Magnetic field through an electrical conductor changes Induced p.d. / current always opposes the change that made it

Question 29

How to increase size of induced potential difference

Answer 29

Increasing strength of magnetic field Increasing speed of movement / change of field Increasing turns per length on coil of wire

Question 30

Purpose of transformers

Answer 30

Uses induction to change the potential difference in an alternating current

Question 31

How transformers work

Answer 31

Two coils of wire are joined with an iron core Alternating p.d. is applied across primary coil Produced alternating magnetic field alternates magnetisation in iron core Changing magnetic field induces p.d. in secondary field

Question 32

Coils in step-up transformers

Answer 32

More coils on secondary than on primary | Increases p.d.

Question 33

Coils in step-down transformers

Answer 33

More turns on primary than on secondary | Decreases p.d.

Question 34

Power formula for transformers

Answer 34

potential difference across primary coil x current through primary coil = potential difference across primary coil x current through secondary coil

Question 35

How most power stations generate energy

Answer 35

Fuels burned to turn heat into steam, which turns a turbine Turbine spins powerful magnet inside of generator (cylinder wrapper with coils of copper wire) Large p.d. and alternating current induced in the coils Coils joined together in parallel to create a single output from generator

Question 36

Why electricity is transferred with high voltage

Answer 36

Power = voltage x current High voltage or high current needed for high power High current means more energy lost through heat stores so high voltage is used to transfer lots of power

Question 37

How transformers are used in the national grid

Answer 37

Step-up transformers boost p.d. (400,000 V) at power stations Step-down transformer lower p.d. to safer levels at consumers’ end

Question 38

Ratio between voltage and number of turns in transformers

Answer 38

input voltage / output voltage = number of turns on primary coil / number of turns on secondary coil

Question 39

How dynamos work

Answer 39

Force rotates a coil in a magnetic field As coil spins, current is induced in the coil Current changes direction every half-turn Split-ring commutator swan connection every half-turn so current keeps flowing in the same direction

Question 40

How alternators work

Answer 40

Force is applied to rotate a coil in a magnetic field As coil spins, current is induced in the coil Alternators have slip rings and brushes so contacts don’t swap every half-turn Alternating potential difference is induced, as well as a.c. current if in a complete circuit

Question 41

How microphones work

Answer 41

Sound waves hit flexible diaphragm attached to coil of wire Coil of wire surrounds one pole of permanent magnet and is surrounded by other pole As diaphragm moves, current is generated in the coil Movement of coil depends on vibration of diaphragm

Question 42

How loudspeakers work

Answer 42

coil is attached to paper cone Coil is wrapped around one pole of a permanent magnet so a.c. signal causes force on coil, moving cone When current is reversed, force acts in opposite direction Movements make come vibrate, causing variations in air pressure and creating a sound wave