physics topic 7

Question 1

what are the poles of a magnet

Answer 1

Where magnetic forces are strongest.

Question 2

what are permanent magnets

Answer 2

they produce their own magnetic fields. Can attract and repel.

Question 3

what are induced magnets

Answer 3

Material that becomes magnetic when placed in a magnetic field.
Induced magnets can only attract.

Question 4

give examples of metals that are magnetic

Answer 4

Iron, steel, nickel and cobalt

Question 5

what is a magnetic field

Answer 5

The region around a magnet where a force acts on another magnet (or magnetic material).

Question 6

describe the direction of a magnetic field

Answer 6

The direction of a magnetic field line is always from north pole to south pole.

Question 7

how can the magnetic field direction of a magnet be seen

Answer 7

Placing a magnetic compass in the magnetic field of a bar magnet causes the compass to point from north to south

Question 8

how can the magnetic field shape of a magnet be seen

Answer 8

Place the bar magnet under a piece of paper and sprinkle iron filings over
the paper. Tapping the paper will produce the magnetic field pattern

Question 9

how does a magnetic compass work

Answer 9

A magnetic compass contains a small
bar magnet which points in the direction of the Earth’s magnetic field.

Question 10

what does the magnetic field pattern produced by compasses lead us to believe

Answer 10

Earth’s core is magnetic.

Question 11

what is thought to be the origin of the earths magnetic core

Answer 11

movement of molten iron in the core.

Question 12

what happens when a current flows through a conducting wire

Answer 12

a magnetic field is produced around the wire.

Question 13

what does increasing the current flowing through an electromagnet do

Answer 13

increases the strength of the magnetic field.

Question 14

what does Increasing the distance from the wire in an electromagnet do

Answer 14

decreases the strength of the magnetic field.

Question 15

what is the right hand rule

Answer 15

Thumb points to direction of current
(+ to -)

Fingers curl to show direction of magnetic field

Question 16

what is electromagnetism

Answer 16

When a current flows through a conducting wire a magnetic
field is produced around the wire.

Question 17

what is a solenoid

Answer 17

a coil of wire

Question 18

how can you increase the strength of an electromagnetic field

Answer 18

Have more turns on the COIL

Increasing the CURRENT

Have an IRON CORE

Question 19

what is an electromagnet

Answer 19

A solenoid with an iron core is an electromagnet.

Question 20

what does a relay switch do

Answer 20

It uses a solenoid to attract an iron
armature

Question 21

why are relay switches used

Answer 21

Relays are used so that a
small current can turn on a
larger current in an isolated
circuit.

This reduces the amount of
thicker, more expensive,
wires needed.

Question 22

Explain how an ignition circuit
works.

Answer 22

• closing the switch causes a current to pass through the electromagnet
• the iron core (of the electromagnet) becomes magnetised
• the electromagnet / iron core attracts the iron arm
• the iron arm pushes the (starter motor) contacts together
• the starter motor circuit is complete
• a current flows through the starter motor (which then turns)

Question 23

describe how an electric bell works

Answer 23

Close Switch

Current flows through circuit

Making an electromagnet

Attracts iron strip (armature)

Causing hammer to hit gong

Circuit breaks

Electromagnet deactivates

Iron strip (armature) returns to original position

Question 24

describe what happens when you put a wire carrying a current in a magnetic field
(force on a conductor)

Answer 24

A wire carrying a current

Has a magnetic field around it (right hand thumb rule)

If you put this wire into ANOTHER magnetic field

The magnetic fields will interact with each other (attract and repel)

Causing the wire to MOVE

Question 25

what factors effect a force on a conductor

Answer 25

Magnetic Flux Density (B) in tesla Current (I) in amperes Length of Conductor (L) in metres

Question 26

how are the factors that effects a force on a conductor linked in an equation

Answer 26

Force (N) = Magnetic flux density (T) x Current (A) x Length(m)

Question 27

when does the maximum force on a wire occur

Answer 27

Occurs when wire is perpendicular to the magnetic field Because it cuts through more of the magnetic field

Question 28

describe the motor effect

Answer 28

When a conductor carrying a current is placed in a magnet field the magnet producing the field and the conductor exert a force on each other.

Question 29

describe Flemings left hand rule

Answer 29

THUMB = Direction of Force (motion) INDEX FINGER = Magnetic field (N to S) SECOND/MIDDLE FINGER = Current (+ to -)

Question 30

how do headphones and loudspeakers use the motor effect

Answer 30

Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.

Question 31

how do speakers work

Answer 31

Alternating current creates a magnetic field around the coil which interacts with the permanent magnet field producing a force causing the coil/cone to move Leading to vibrations which produces sound when the direction of the current reverses, the direction of the force reverses

Question 32

what is a generator

Answer 32

If you move the coil of wire in a magnetic field it generates an induced current (electricity)

Question 33

How to increase the size of the induced potential difference in a generator

Answer 33

Rotate coil faster Stronger magnetic field Increase number of turns of wire on coil

Question 34

what is the Direction of Induced Potential difference in a generator dependant on

Answer 34

dependent on the direction of rotation of the coil and the orientation of the magnetic field.

Question 35

what does Reversing the direction of rotation of the coil in a generator do

Answer 35

will reverse the direction of the induced potential difference/induced current.

Question 36

what does Swapping the polarity of both magnets do in a generator

Answer 36

will reverse the direction of the induced potential difference/induced current.

Question 37

describe how Generators/Alternators work

Answer 37

Alternators rotate a coil through a magnetic field This induces a potential difference As there is a complete circuit, a current is induced Generators have a slip-ring commutator – allows coil to always remain in contact to the circuit This induces an alternating current in the coil Every half turn, the current and potential difference reverses direction (A.C)

Question 38

describe how Dynamos work

Answer 38

Dynamos rotate a coil through a magnetic field This induces a potential difference As there is a complete circuit, a current is induced Dynamos have a split-ring commutator –the two ends of the coil swap from one brush to the other so the direction of the induced current does not reverse every half rotation (D.C)

Question 39

what angle should wire be to get max voltage in alternator and dynamo outputs

Answer 39

90 degrees

Question 40

how do microphones work

Answer 40

Pressure from sound waves Causes the diaphragm to vibrate Which causes the coil to vibrate Coil moves through the magnetic field Which induces a potential difference and current

Question 41

why is the core of an electromagnet iron

Answer 41

because iron is quickly magnetised

Question 42

describe step up transformers

Answer 42

Increase the potential difference so Vs > Vp More turns on the secondary coil

Question 43

describe step down transformers

Answer 43

Decrease the potential difference so Vs < Vp More turns on the primary coil

Question 44

how do transformers work

Answer 44

insulation prevents electricity/current flowing through the iron core alternating current (a.c) in the primary coil produces a changing magnetic field in the iron core Which produces magnetic field in the secondary coil induces an alternating potential difference across the secondary coil and therefore an alternating current (a.c) in the secondary (coil)

Question 45

how are transformer used in the national grid

Answer 45

to make the transmission of electricity more efficient.

Question 46

what does increasing the potential difference do to overhead power lines

Answer 46

decrease the current in the overhead powerlines.

Question 47

Potential difference and current are what

Answer 47

inversely proportional