4. (b) Magnetism

Question 1

State the factors that can affect the magnitude of induced an e.m.f

Answer 1

Speed in which the magnet moves
Strength of the magnet
The number of coils of the solenoid

Question 2

State Lenzes Law

Answer 2

The direction of an induced current opposes the change causing it

Question 3

Describe the simple construction of an A.C generator

Answer 3

Consists of:
+ rectangular coil which is forced to spin in a magnetic field
+ the coil is connected to an ammeter via metal brushes that press on two metal slip rings

Question 4

Function of brushes and slip ring in an a.c generator

Answer 4

Provide a continuous connection between the coil and the ammeter

Question 5

Graph of voltage output against time for a simple a.c generator

Answer 5

Page 214

Question 6

Describe an experiment that shows that a changing magnetic field can induce an e.m.f in a circuit

Answer 6

Connect a solenoid with an ammeter
Move the magnet in and out of the solenoid
Observe the ammeter reading - a current flowing in both direction can be observe -> there is e.m.f induced
Move the magnet in 1 direction -> ammeter shows positive value
Move the magnet in reverse direction -> negative value
Magnet remains still: no voltage induced
To have an induced e.m.f, a magnet must cut through the magnetic field line

Question 7

State the properties of magnets

Answer 7

A magnet has a magnetic field around it
The field is strongest at its poles
Field movement : N -> S
Can be created with an induced electric current
Magnetic field can be detected when they produce force

Question 8

How to induce magnetism in a bar of ferrous material

Answer 8

Place a magnet near it

When the magnet is removed, the magnetism induced is lost

Question 9

What are ferrous materials?

Answer 9

Metals which contains iron

Any ferrous materials can be magnetized or demagnetized

Question 10

What are non-ferrous materials?

Answer 10

Metals which do not contain any iron
Only a few can be magnetized or demagnetized
Eg. Cobalt, Nickel

Question 11

How to magnetise a metal?

Answer 11

Moving one end of a bar magnet along the material repeatedly in the same direction.
The head of the metal will become the same pole of the end of the magnet as it tries to repel the magnet as it approaches.
The tip of the metal will become the opposite pole of the end of the magnet as it tries to attract the magnet as it moves away
Or
Place the metal in a long coil of wire and pass a large DC current through the coil

Question 12

How to demagnetise a metal?

Answer 12

Smash it with a hammer, dropping, etc
Heating to a high temperature
Place the metal in a coil of wire with AC current passing through then gradually removing it from the coil

Question 13

Distinguish between the magnetic properties of iron and steel

Answer 13

Steel is a hard magnetic material, it is hard to magnetise but keeps it magnetism
Iron is a soft magnetic material, it is easily magnetised but easily loses its magnetism

Question 14

Describe the elctromagnets

Answer 14

Also called temporary magnet
Created by an iron core with insulated wire coiled around
Made from soft ferromagnetic materials
When an electric current pass through the wire, the core becomes magnetised
When the current is switched off, the core loses most of its magnetism
Eg of uses: lift car in a rubbish dumb, circuit breaker

Question 15

Describe the permanent magnets

Answer 15

Made from hard ferromagnetic materials eg. Steel
Made in different shapes including bars, flat rectangle, discs, cylinders,…
Eg of uses. Fridge magnet, horseshoe magnet, loudspeaker magnet

Question 16

Advantages of temporary magnet over permanent ones

Answer 16

Can be switch on and off
Strength of magnetic field can be change, by changing the current
Can easily be made into a variety of shapes and are less expensive to make

Question 17

Describe how an a.c generator works

Answer 17

Coil is rotated -> cuts magnetic field line -> emf generated -> current flow
Each side of the coil travels upward then downward then upward, etc
-> the current flows backward and forward -> alternating current

Question 18

How can the emf induced by an ac generator can be increased

Answer 18

Increase the area of the coil
Using stronger magnet
Rotate the coil faster

Question 19

Describe a transformer

Answer 19

Used for voltage transformation
Consists of two coil of insulated wire, both wound around the same iron core
ac current applied to the primary coil -> produces a changing magnetic field in the core
The line of the alternating magnetic field pass through the second coil and induce an ac current in the second coil

Question 20

Why can’t a dc current be used for a transformer

Answer 20

The voltage applied to the primary coil must be ac because constant current cannot cause electromagnet induction

Question 21

Two equations for transformer

Answer 21

Vp / Vs = Np / Ns - p= primary; s= secondary; N= number of turns

Vp / Ip = Vs / Is

Question 22

Describe the use of the transformer in high voltage transmission of electricity

Answer 22

When a current flows through a wire, some energy is lost as heat
The higher the current, the more heat is lost
-> need to transmit electricity at a low current to reduce theses losses -> requires high voltage ( P= IV)
Step up transformer is used at power station -> produce very high voltage
But too high voltage are dangerous for the use in home -> step down transformer is used to reduce the voltage to a lower level before it is used

Question 23

Advantages of high voltage transmission

Answer 23

Less power lost
Thinner, light and cheaper cables can be used since current is reduced
Save money

Question 24

Why energy losses in cables are lower when the voltage is high

Answer 24

Due to the flow of electrons in the wire, there is heat loss while transferring electrical energy
The larger the current, the more energy is lost as heat to the surrounding
Base on the equation P = IV, when a generator supplies an amount of power, high voltage -> smaller current -> less energy is lost -> more efficient at supplying energy

Question 25

Describe the pattern of magnetic field due to current in straight wires

Answer 25

All current have a magnetic field around them
A straight wire has a circular magnetic field around it
Move further away from the wire, the magnetic field gets weaker
Reverse the current, the magnetic field also reverse

Question 26

describe an experiment to show that a force acts on a current carrying conductor in a magnetic field

Answer 26

Place a wire connected to a power pack between two magnets
Turn on the power
Notice that the wire moves
Reverse the current -> opposite effect
Reverse the direction of the field -> opposite effect
Revere both current and field -> nothing changes

Question 27

What happen if a magnetic field is apply at right angle to an electron beam

Answer 27

The electron beam is pushed sideway

Question 28

Left hand rule

Answer 28

Thumb - motion
First finger - field
Second finger - current

Question 29

Describe the effect of a current carry coil in a magnetic field and how to increase the effect

Answer 29

A current carrying coil in a magnetic field experience a turning effect
The turning effect can be increased by
- increasing the current
- use a stronger magnet
- increase the number of turns on the coil

Question 30

Describe a simple electric motor

Answer 30

Consists of a rectangular coil of wire that spins in a magnetic field when a current passes through the coil
Designed to use the motor effect
When a current pass through, it spins because:
- a force acts on each side of the coil due to the motor effect
- the force on one side is in the opposite direction to the force on the other side
The turning direction can be reversed by reversing the current or the magnetic field

Question 31

Describe the role of split ring commutator in a dc motor

Answer 31

Reverse the current round the coil every half turn of the coil so that the force generated by the current keeps turning the coil in the same direction

Question 32

Describe the cathode ray

Answer 32

Consisted of a stream of electrons emitted from a heated cathode (a filament)
The anode attracts the electrons from the cathode
The cathode is heated to increase the average energy of the electron so they can jump to the anode
If there is a hole in the anode, some of the electrons pass through the hole and emerge a narrow beam
There must be a vacuum environment otherwise gases atoms would stop electrons from reaching the anode

Question 33

What is the process of thermionic emission?

Answer 33

Emitting electrons from a heated cathode

Increase the p.d between the anode and cathode increase the speed of the electrons moving to the anode

Question 34

Describe the deflection of cathode rays by an electric field

Answer 34

The beam of electrons is attracted towards the positive plate and repelled by the negative plate
The bigger the p.d between the plate, the larger the deflection

Question 35

Describe the cathode ray oscilloscope

Answer 35

Use to display electrical signals on a screen
An electron beam passes between 2 pairs pf deflecting plates and directed towards a fluorescent screen at the end of the tube
Where the beam heats the screen, a dot appears

Question 36

Describe the 2 pairs of plate in a cathode ray oscilloscope

Answer 36

The varying voltages is applied to the Y plates, causing the spot to move up and down by different amount according to the voltage
A steadily increasing voltage is applied to the X plats so the the spot on the screen moves steadily from left to right ( this is the time axis)