P4.2 Uses Of Magnetism

Question 1

Motor effect

Answer 1

2 magnets exerting a force upon each other

When there is a current in the coil, the sides of the coil experiences a force in opposite directions, allowing the current to produce a magnetic field
Split ring commutator ensures that the coil rotates in same direction by reversing the current every half turn
Here the two halves of the commutator swap from one carbon brush to the other
Without a split ring commutator, the coil would change direction every half turn and not rotate in one continuous direction

Question 2

What is generated around a current carrying wire

Answer 2

Magnetic field

Question 3

Gravitational field strength (g)

Answer 3

Measured in newtons per kg (N/kg)
10N/kg on Earth

Question 4

Magnetic field strength / magnetic flux density

Answer 4

Symbol: B
Unit: Tesla (T)

Question 5

Fields, currents, forces all have what

Answer 5

Directions (left right up down)

Question 6

Symbol for something going into the page
(Diagram used in showing direction of current in a wire)

Answer 6

⨂

Question 7

Symbol for something coming out of the page
(Diagram used in showing direction of current in a wire)

Answer 7

⨀

Question 8

Motor effect - size of force equation

Answer 8

F = BIL

F = force on conductor: Newtons
B = magnetic flux density: Tesla
I = current: Amps
L = length of conductor in field: Meters

Question 9

Fleming’s left hand rule

Answer 9

Used to calculate the force on the wire

Finger 1: field : point forward
Finger 2: current: point down
Thumb: movement: point right

Question 10

What happens when fields of 2 wires interact

Answer 10

They both experience a force
Attract if current flow in same direction
Repel if they flow in opposite directions

Question 11

Electric motor (simple)

Answer 11

Uses motor effect to transfer electrical energy to kinetic energy
Consists of a loop/length of wire with a current in a magnetic field
Motor rotates as the current in the loop of wire flows in opposite directions in the opposite sides of coil, forces act in opposite directions on opposite sides of the coil, causing the coil to rotate

Question 12

Electric motor (complex)

Answer 12

North ( \o/ )South
. v ^
. L_ı|⅃
1) The left part of this coil is pushed upwards and the right half downwards. (Check with Fleming’s left-hand rule.)
2) No current, but the coil continues to turn because of its own momentum
3) The direction of the current in the coil and commutator is reversed. Now the blue part is pushed downwards and the red half upwards
Split ring commutator ensures that the motor rotates in the same direction

Question 13

Split ring commutator

Answer 13

Reverses direction of current in the coil of the motor every half turn
Keeps the coil turning the same direction

Split ring commutator diagram:
Coil wire
\ \
|| ||
—▪️( * )▪️— *gaps
\ \
Contact to DC power supply

Question 14

Increase speed of motor

Answer 14

Increase size of force acting F=BIL:

Increase current in coil
Increase strength of magnetic field/magnets
Have more loops/longer wire

Question 15

Catapult field diagram for motor coil

Answer 15

|S N|————>————|S N|
| | ‾‾———>———__ | |
| | ‾‾———>———__ | |
| | \ / ‾ ‾ \ | |
| | \ \ ^ / / ‾‾ | |
| | \ \ | / / | |
| | \ \v(⊙) / / ^(⨂) | |
| | - . ->- . - | | |
| | - . > . - v | |
| | ‾‾———>———__ | |
| | ‾‾———>———__ | |
|S N|————>————|S N|

Question 16

Why some motors would use electromagnets instead of permanent magnets

Answer 16

Produce stronger magnetic fields,
which produces larger force

Question 17

Why bigger motors produce a larger force on the coil

Answer 17

Longer length of coil (loops) in a stronger magnetic field

Question 18

Type of current needed for motors

Answer 18

Alternating current

Question 19

How does an alternating current generator work

Answer 19

For current to be generated, motion, magnets and wires are needed
Magnets provide a magnetic field
Motion is supplied when the coil is manually rotated
Brushes make contact between the split ring commutator and the circuit and ensures that current continuously flows in circuit