Magnetism and electromagnetism

Question 1

Where are the magnetic forces strongest for a magnet

Answer 1

the poles

Question 2

A permanent magnet produces its own _______

Answer 2

magnetic field

Question 3

What is an induced magnet?

Answer 3

a material that becomes a magnet when it’s placed in a magnetic field

it loses magnetism when removed from the magnetic field

Question 4

Induced magnetism always causes a ________

Answer 4

force of attraction

Question 5

Magnetic field definition

Answer 5

the region around a magnet where a force acts on another magnet or/and magnetic material

Question 6

The force between a magnetic and a magnetic material is _________

Answer 6

always one of attraction

Question 7

What does the force of the magnetic field depend on?

Answer 7

the distance of the field from the magnet

Question 8

What is the direction of the magnetic field lines?

Answer 8

North to south pole

Question 9

How do we know about the Earth magnetic field?

Answer 9

A magnetic compass contains a small bar magnet.

The compass needle points in the direction of theEarth’s magnetic field.

Question 10

What happens when current flows through a conducting wire?

Answer 10

a magnetic field is produced around the write

Question 11

How to increase the strength of a magnetic field?

Answer 11

• increase the amount of current through the wire
• the decrease the distance from the wire
• shape the wire into a solenoid
• add an iron core

Question 12

Describe the magnetic field lines inside a solenoid?

Answer 12

strong and uniform

Question 13

How can you experiment the effects of “the size of current” and the “distance” from a wire

Answer 13

by placing magnetic compass at different points along the wire and turning the power supply on and off

Question 14

Why does a solenoid increase the magnetic field strength?

Answer 14

• it concentrates a longer piece of wire into a smaller area

- the lopped shape means that the magnetic field lines around the wire are all in the same direction

Question 15

What is the motor effect?

Answer 15

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

Question 16

What is the motor effect caused by?

Answer 16

the field created by the current interacting with the magnetic field

Question 17

How can the force in the motor effect be increased?

Answer 17

increasing :

• the size of the current
• the length of conductor in the magnetic field
• the flux density (density of the field lines around a magnet)

Question 18

What is the formula to find the force of something when you have flux density?

Answer 18

F= BIL

Force = magnetic flux density x current x length (of wire within the field)

Question 19

What would happen to the direction of the force, if the direction of the current or magnetic field is reversed?

Answer 19

it will reverse

Question 20

How can the direction of the force on the conductor be found?

Answer 20

with Flemings left-hand rule

Question 21

What is an electric motor?

Answer 21

A coil of wire carrying a current in a magnetic field and rotates

Question 22

Why do electric motors rotate?

Answer 22

because the current going up one side of the coil is in the opposite direction to the current coming back down the other side, so one side moves up and the other moves down

Question 23

What do the carbon brushes in electric circuits do?

Answer 23

allow the electric current to pass onto the split-ring commutator

Question 24

Why does a coil stop when it reaches 90 degrees (vertical position) if it didn’t have a split-ring commutator?

Answer 24

because the current would continue to flow through each side of the coil in the same direction (opposite directions), so the direction of the forces exerted on both sides wouldn’t change.

this means the tip od the coil will be pulled upwards and the bottom downwards so the coil will stop rotating

Question 25

What is a split-ring commutator?

Answer 25

a device used to reverse the direction of flow of an electric current in motors and generators

Question 26

How is electric current passed onto the split-ring commutator?

Answer 26

the spilt ring is connected to conducting brushes

Question 27

How do coils rotate continuously?

Answer 27

each time the coil passes the vertical position, the direction in which current flows around the coil reverses

this reverses the direction of the force exerted on each side of the coil which in turn cause continued rotation in the clockwise rotation

Question 28

How do you make the coil rotate faster?

How do you make the coil turn in the opposite direction?

Answer 28

increasing the current or the magnetic field

reversing the current or the magnetic field

Question 29

What do headphones and loudspeakers use the motor effect for?

Answer 29

to convert variations in current in electrical circuits to the pressure variations in sound waves

it converts electrical energy into kinetic and sound energy

Question 30

How do you find the direction of field lines?

Answer 30

right-hand grip

Question 31

How do you find the north pole of a solenoid

Answer 31

right-hand grip:

• fingers represent the direction of current flow
• thumb will point to the north pole of the solenoid

Question 32

How do Loudspeakers work?

Answer 32

1) when a current flows through the coil, both the coil and the speaker core experience a force due to the magnetic field. The speaker core moves
2) Because the current is alternating when the direction of the current through the coil reverses, so too does the direction of the force exerted on it and the speaker core
3) this causes the speaker core to vibrate back and forth, producing a sound wave
4) the air around the core also vibrates creating variations in pressure that cause a sound wave

Question 33

How do scrapyards work?

Answer 33

Scrap vehicles are lifted in a scrap yard using powerful electromagnets attached to cranes

when the current is turned on, the electromagnet attracts magnetic material below it

when the current is turned off, there is no more magnetic force

Question 34

How do circuit breakers work?

Answer 34

this is a switch in series with an electromagnet, held by a spring

when the current is too large, the switch is pulled open by the electromagnet and it stays open until it is reset manually

Question 35

How do electric bells work?

Answer 35

1) When the switch is pushed, the electromagnet is magnetised
2) The electromagnet attracts the armature
3) The hammer strikes the gong and breaks the circuit
4) the armature springs back, completing the circuit again and remagnetising the electromagnet
5) the cycle repeats for as long the button remains pushed

Question 36

How do relays work?

Answer 36

Relays are used to switch an electrical machine on or off

1) a small current through the coil of the electromagnet magnetises the iron core
2) this pulls the armature onto the electromagnet, whose other end pushes the contacts together, completing the circuit.
3) When the current is switched off, the contacts open again, switching the circuit off.