The Motor Effect (7.2)

Question 1

What happen when a current flows through a conducting wire?

Answer 1

a magnetic field is produced around the wire

Question 2

2 things

What does the strength of the magnetic field depend on?

Answer 2

the current through the wire and the distance from the wire

Question 3

Shaping a wire to form a solenoid (coils of wire) does what?

Answer 3

increases the strength of the magnetic field created by a current through the wire

Question 4

How would you describe the magnetic field inside a solenoid?

Answer 4

strong and uniform

Question 5

The magnetic field around a solenoid has a similar shape to that of a (…)

Answer 5

bar magnet

Question 6

What is an electromagnet?

Answer 6

a solenoid with an iron core

Question 7

Adding an iron core to a solenoid, creating an electromagnet, does what?

Answer 7

increases the strength of the magnetic field of a solenoid

Question 8

How can the magnetic effect of a current can be demonstrated?

Answer 8

Put a piece of wire that is free to move in a magnetic field, then put a current through it

It will move as current flows through it

Question 9

Draw the magnetic field pattern for a straight wire carrying a current with its direction upwards (showing the direction of the field)

Answer 9

this is the right hand grip rule - learn it

Question 10

Draw the magnetic field pattern for a solenoid (showing the direction of the field)

Answer 10

Question 11

Magnetic fields travel from the (…) pole to (…) pole

Answer 11

north to south

also know what will happen if two like poles are close

Question 12

3 ways

How can you make an electromagnet stronger?

Answer 12

• wrapping the coil around an iron core
• adding more turns to the coil
• increasing the current flowing through the coil

Question 13

The photo shows a relay circuit, describe how it works

Answer 13

When the switch is closed, current is induced and flows and the iron rocker arm is attracted to the electromagnet.

The arm rotates about the central pivot and pushes the contacts together.

Current now flows through circuit 2, which is now switched on and the motor turns

circuit two may have a large current flowing through it - dangerous to person

Question 14

What is the motor effect?

Answer 14

When a conductor carrying a current is placed in a magnetic field, the magnet producing the field and the conductor (which induces its own magentic field) exert a force on each other

fields interact

Question 15

3 things

What does Fleming’s left-hand rule represent?

Answer 15

• the relative orientation of the force
• the current in the conductor
• the magnetic field

Question 16

Present Fleming’s left hand rule. It should look like this…

Answer 16

at right angles to eachother

acronym: FBI

Question 17

In which direction will this wire feel a force?

Answer 17

With forefinger (magnetic field) pointing left to right, and second finger (current) pointing down, your left thumb (force) will point towards you

Question 18

3 things

What are the factors that affect the size of the force on the conductor?

Answer 18

• the current in the wire increases
• the strength of the magnetic field increases
• length of wire in magnetic field

F = BIL

Question 19

A coil of wire carrying a current in a magnetic field tends to (…)

Answer 19

rotate

Question 20

What is a dc motor?

Answer 20

An electrical device that uses direct current to produce a turning effect.

Question 21

What type of motor is an electric motor?

Answer 21

a dc motor

Question 22

4 marks

Describe how an electric motor works?

Answer 22

The sides of the coil (parallel to the magnet) experience a force (in opposite directions)

allow the magnetic fields interact causing the coil to rotate

(each half-turn) the two halves of the (rotating) commutator swap from one brush to the other

(each half-turn) the commutator reverses the current (in the coil)

Question 23

Loudspeakers and headphones do what?

Answer 23

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

Question 24

Describe how moving-coil loudspeaker and headphones work

Answer 24

1. Current creates a magnetic field (around the coil)
2. Which interacts with the permanent magnetic field
3. Producing a (resultant) force (causing the coil/cone to move)
4. When the direction of current reverses, the direction of the (resultant) force reverses to the opposite direction (producing pressure variations in sound wave)

Question 25

What direction is force posting in?

Answer 25

downwards

Magnetic Field (B) pointing from North to South, so pointing to right

Question 26

Why does a force act on a current-carrying wire when it is placed in a magnetic field?

Answer 26

the current creates a magnetic field around the wire, this interacts with the magentic field of the magnets, exerting a force on the wire

Question 27

3 things

How would you increase the force acting on this wire

Answer 27

• increase current
• use stronger magnets
• move magnets closer together

Question 28

What are two ways to reverse direction of a coil in between two opposite magnets?

Answer 28

• swap magnets (change direction of magnetic field)
• reverse direction of current

Question 29

A coil of wire will come to rest where when the forces are removed?

Answer 29

come to rest facing field (at 90^o to it)

Question 30

At what point will no force act on a current carrying conductor in a field?

Answer 30

When the conductor is parallel to the field

Question 30

How would you show that a magnetic field has being produced around the wire?

Answer 30

sprinkle iron filings onto the card
tapping the card will move the filings to show the magnetic field pattern