Electromagnetic effects

Question 1

What always have magnetic fields around them?

Answer 1

current carrying conductors or moving electrons

Question 2

What is the pattern of a magnetic field in straight wires?

Answer 2

circular and perpendicular to the wire, getting weaker with distance

Question 3

How can you find the direction of a magnetic field in straight wires?

Answer 3

Right hand grip rule n1: your thumb is conventional current, and the rest of your fingers are the magnetic field direction

Question 4

How do you know if the conventional current is going into or out of the paper?

Answer 4

cross: current into the page
dot: current out of the page

Question 5

In a single wire, how can you increase the strength of the magnetic field?

Answer 5

greater current

Question 6

What is the direction/pattern of the magnetic field in a solenoid?

Answer 6

the same as in a bar magnet

Question 7

What is the relationship between current and direction of magnetic field on a solenoid?

Answer 7

the general direction of the current flow indicates the direction of the magnetic field

Question 8

How can you find out the direction of the magnetic field in a solenoid?

Answer 8

right hand grip rule n2: your thumb points to the the north pole, the rest of your fingers are conventional current

Question 9

How can you reverse magnetic field direction in a solenoid?

Answer 9

• reverse current/cell
• wrap wire in opposite direction

Question 10

How can you increase the strength of the magnet in a solenoid?

Answer 10

• more turns of coil
• more current
• wrap wire on an iron core

Question 11

Describe an experiment to identify the pattern/direction of a magnetic field in straight wires and solenoids.

Answer 11

1. use a compass to determine the direction of a field at a single point
2. draw an arrow in the direction of the field
3. move the compass to different points and repeat
4. you will have a diagram of the magnetic field

Question 12

What is a relay?

Answer 12

electromagnetic switch, a circuit with a small current turns on another circuit with a large current

Question 13

How does a relay work?

Answer 13

1. switch is closed
2. small current magnetised the coil
3. iron reeds are attracted and close
4. large current flows

Question 14

What is the energy conversion in a loudspeaker?

Answer 14

it converts electrical energy into sound energy

Question 15

How does a loudspeaker work?

Answer 15

1. alternating current in coil creates an alternating magnetic field in coil
2. this alternating magnetic field will interact with the permanent magnetic field
3. this interaction will make the papercone move in/out producing sound waves

Question 16

If you use high frequency ac in a loudspeaker, what will happen?

Answer 16

high frequency sound waves

Question 17

If you use large voltage ac in a loudspeaker, what will happen?

Answer 17

loud sound waves

Question 18

Why are relays useful?

Answer 18

the turn on larger current circuit by turning on a smaller current circuit: this way, we do not need to use thick, heavy, expensive cables to carry the large current

Question 19

What happens if you supply a solenoid with AC?

Answer 19

the current will change, so the direction of the magnetic field will also change every half cycle

Question 20

What is the variation of magnetic field strength in straight wires?

Answer 20

strongest closest to wire

Question 21

What is the variation of magnetic field strength in solenoids?

Answer 21

uniform magnetic field inside coil

Question 22

What happens if you increase the size of the current in a straight wire?

Answer 22

results in a stronger magnetic field

Question 23

What happens if you increase the size of the current in a solenoid?

Answer 23

results in a stronger magnetic field

Question 24

What happens if you reverse the current in a solenoid?

Answer 24

reverses magnetic field direction

Question 25

What happens if you reverse the current in a straight wire?

Answer 25

reverses magnetic field direction

Question 26

What happens whenever a current is at a right angle to a magnetic field?

Answer 26

it experiences a force, perpendicular to both the current and magnetic field

Question 27

Why is a force experienced when a current carrying conductor is in a magnetic field?

Answer 27

interaction between the current's magnetic field and the permanent magnetic field

Question 28

How can we work out the direction of the force when there is a current carrying conductor in a magnetic field?

Answer 28

Fleming's left hand rule: First finger is magnetic field (N to S) Second finger is conventional current Thumb is the thrust/force

Question 29

How can you reverse the force direction (current carrying conductor in a magnetic field)?

Answer 29

reverse current or reverse magnetic field direction

Question 30

How can you increase the strength of the force (current carrying conductor in a magnetic field)?

Answer 30

stronger magnets, magnets closer together or increase current

Question 31

What happens if you reverse the current in a current-carrying conductor in a magnetic field?

Answer 31

you reverse the force

Question 32

What happens if you reverse the direction of the magnetic field in a current-carrying conductor in a magnetic field?

Answer 32

you reverse the force

Question 33

How do you make a permanent magnet?

Answer 33

1. place steel rod inside coil 2. use high dc current for a time 3. leave steel inside, turn off current

Question 34

How can you demagnetise a magnet?

Answer 34

1. place magnet inside coil 2. use ac current 3. slowly remove magnet with ac current still on

Question 35

Describe an experiment to show the force in a current-carrying conductor in a magnetic field.

Answer 35

1. Place a wire carrying current in between two permanent magnets 2. turn on power supply 3. observe force on wire

Question 36

Will any moving charged particle in a magnetic field experience a force?

Answer 36

yes

Question 37

Determine the direction of the force on charged particles in a magnetic field when the field is into the page.

Answer 37

positive particle: up negative particle: down

Question 38

Determine the direction of the current on charged particles in a magnetic field when the field is into the page.

Answer 38

positive particle: left to right negative particle: right to left

Question 39

What is the basic principle of a d.c motor?

Answer 39

a current + a magnetic field will create a force

Question 40

How does a d.c motor work?

Answer 40

the sides of the coil have a magnetic field, which interacts with permanent magnetic field to produce a force

Question 41

How can we identify the force direction in a d.c motor?

Answer 41

Fleming's left hand rule: first finger is field, second finger is current and thumb is thrust

Question 42

What causes the turning effect in a d.c motor?

Answer 42

a current-carrying coil in a magnetic field

Question 43

Why do we have split-ring commutators on a d.c motor?

Answer 43

to reverse the direction of the current every half turn so that the coil rotates in the same direction

Question 44

What would happen without split-rings?

Answer 44

the coil would flip backwards and forwards (the changing direction of current caused by the coil rotating through 180 would cause this)

Question 45

Why do we need (carbon) brushes in d.c motors? Why are they good for this?

Answer 45

connect commutator to the circuit: low friction, good conductors

Question 46

What do split rings do to the current?

Answer 46

reverse the direction of the current every half turn

Question 47

What do brushes ensure?

Answer 47

that the wires don't get twisted

Question 48

How can the turning effect in a d/c motor be increased?

Answer 48

- increasing number of turns on coil - increasing current - increasing strength of magnetic field

Question 49

What may happen if a conductor moves rapidly across a magnetic field?

Answer 49

creates induced emf or current or p.d

Question 50

What is the basic principle of electromagnetic induction?

Answer 50

movement of a conductor + magnetism -> induced emf/current/pd

Question 51

What would happen if a conductor cutting a magnetic field is not moving?

Answer 51

there will be no induced current

Question 52

When is there a maximum of induced current in electromagnetic induction?

Answer 52

when the conductor is at 90 degrees to magnetic fields

Question 53

What happens when a conductor is moving parallel to a magnetic field?

Answer 53

there will be no induced current

Question 54

What are two situations in electromagnetic induction when there will be no induced current?

Answer 54

1. when there is no movement 2. when the conductor is moving parallel to the magnetic field

Question 55

How can you reverse the current direction in electromagnetic induction?

Answer 55

- reverse wire movement direction - reverse magnetic field direction

Question 56

How can you increase the size of induced current in electromagnetic induction?

Answer 56

- stronger magnetic field - move wire faster - use a coil of wire

Question 57

Describe an experiment to demonstrate electromagnetic induction.

Answer 57

1. connect a wire to an ammeter or a galvanometer 2. place two permanent magnets on either side of the wire 3. move the wire 4. watch what happens to ammeter/galvanometer

Question 58

What is a galvanometer?

Answer 58

a very sensitive ammeter

Question 59

Why does electromagnetic induction work?

Answer 59

the conductor cuts the magnetic field lines, forcing electrons in the wire to move as a current

Question 60

How do we identify the induced current direction in electromagnetic induction?

Answer 60

Fleming's right hand rule: first finger is field, second finger is current and thumb is thrust

Question 61

What are two ways to induce an e.m.f (electromagnetic induction)?

Answer 61

1. moving a magnet so its field lines are cut by a wire 2. moving a wire across a magnetic field so that it cuts it

Question 62

What is Lenz's Law?

Answer 62

the direction of the induced emf (electromagnetic induction) opposes the change that creates it

Question 63

How does the a.c generator work?

Answer 63

1. coil made to rotate 2. sides of coil cut magnetic field lines, inducing emf 3. emf changes direction every half turn, because for half a turn one side moves up and for the other half turn the same side moves down 4. the emf changes size because there is a max. induced emf when the coil is perpendicular

Question 64

What is the simple idea behind ac generators?

Answer 64

movement of coil + magnetic field -> induced emf/current

Question 65

How is the graph of induced emf vs time of an ac motor?

Answer 65

it goes up and down

Question 66

What does a peak in an induced emf vs time graph of an ac motor mean?

Answer 66

long sides of coil are perpendicular to magnetic field, maximum emf is induced

Question 67

What do the zeros in an induced emf vs time graph of an ac motor mean?

Answer 67

top and bottom sides of coil are parallel to magnetic field, so no emf is induced

Question 68

What does a trough in an induced emf vs time graph of an ac motor mean?

Answer 68

long sides of coil move perpendicular to field in opposite direction: induced emf is maximum but in opposite direction

Question 69

What is the function of slip rings in ac generators?

Answer 69

allow direction of induced emf to alternate, ergo causing an alternating current connect coil to brush

Question 70

What are slip rings?

Answer 70

cylindrical conductors that make constant contact with the coil during rotation

Question 71

What is the function of carbon brushes in ac generators?

Answer 71

make electrical connection between rotating coil and circuit, avoiding wires becoming twisted