P7: Magnetism

Question 1

only a few materials are magnetic. What elements are magnetic?

Answer 1

• Iron
• Nickel
• Cobalt
• Magnadur

Question 2

what are dipoles?

Answer 2

atoms that behave like magnets having a north pole and a south pole

Question 3

what is magnetic induction?

Answer 3

transfer of magnetic properties from one body to another without physical contact

Question 4

what shows the north pole in a magnetic field?

Answer 4

the arrow heads

Question 5

what is the only true test for polarity of a magnet and why?

Answer 5

the only true test is REPULSION. For repulsion to occur the object must already be a magnet whose nearby pole is similar to that of the other magnet.

Question 6

what are the 2 type of magnets?

Answer 6

Temporary magnet (soft magnets) Permanent magnets (hard magnets)

Question 7

true are false: all metals can be used to make magnets

Answer 7

false. only a few

Question 8

list some materials used to make hard magnets

Answer 8

magnadur

alnico

ticonal

alcomax

Question 9

list some materials used to make soft magnets

Answer 9

iron, mumental and stalloy

Question 10

important features of magnetic field lines

Answer 10

-they never cross -the field is stronger where the lines are more concentrated and weaker where they are further apart -there is lateral repulsion between the lines -the field is uniform where the lines are parallel -they run from a north (N) pole to a south (S) pole -there is longitudinal tension within each line

Question 11

dropping or hitting magnet will cause it to_____

Answer 11

loose some of its magnetism

Question 12

uses of permanent magnets

Answer 12

-Loudspeakers -Directional compasses

Question 13

simple way to make a permanent magnet.

Answer 13

rub a known magnet along the lenght of a steel bar

Question 14

Uses of soft magnets

Answer 14

-electromagnets which are used for lifting metal objects, electric bells and electromagnetic circuit breakers

Question 15

Attraction between magnets

Answer 15

Question 16

repulsion between magnets

Answer 16

Question 17

Attraction between magnetic bars that are side by side

Answer 17

Question 18

How can the magnetic field stenght of a solenoid be increased:

Answer 18

• using more loops of wire
• increasing the current throught the wire
• placing an iron core inside the solenoid

Question 19

Repulsion between magnetic bars that are side by side

Answer 19

Question 20

what is the direction of the field lines in a magnet?

Answer 20

north pole of the magnet to the south pole of the magnet

Question 21

Two ways of mapping the the magnetic field of a bar magnet

Answer 21

• using iron filings
• using a plotting compass

Question 22

Why does the earth have a magnetic field?

Answer 22

Its core is composed of iron and nickel which are magnetic metals.

Question 23

A directional compass always points north. Why is this so?

Answer 23

The directional compass hasa magnetic needle that will point in the direction of the earth’s magnetic field, which is from north to south, where the south corresponds the the geographical north.

Question 24

why does a directional compass point to the north pole of the earth?

Answer 24

The magnetic field lines of the earth point towards the north pole of the earth. The magnetised needle of the directional compass always points in the direction of the magnetic field lines so it will point towards the north pole of the earth.

NB the earth’s magnetic south pole core is directed towards its geographic north pole, and the magnetic north pole of the earth is directed toward its geographic south pole.

Question 25

How to identify the poles of a magnet using the earth's magnetic field

Question 26

what is an electromagnet?

Answer 26

it is a temporary magnet produced when an electrical coil is placed around a soft iron core and current is passing throught the coil. Once the current stops the magnetic property is lost (temporary magnet)

Question 27

Right had grip rules for direction of magnetic field produced by electric currents

Answer 27

1. For a wire: imagine gripping the wire with the right hand with the thumb in the direction of the current. The magnetic field will follow the direction of the other fingers from north to south 2. For a coil: imagine gripping the coil with the right hand with the fingers in the direction of the current. The magnetic field will follow the direction of the thumb from north to south

Question 28

what is the relationship between the earths geographic poles and its magnetic poles?

Answer 28

They are the opposite of each other ie. the geographic north pole corresponds to the magnetic south pole and vice versa

Question 29

what is a solenoid?

Answer 29

a cylindrical coil of wire acting as a magnet when carrying electric current.

Question 30

Explain the motor effect?

Answer 30

a wire carrying an electric current will experience a magnetic force when placed in a magnetic field from a permanent magnet

Question 31

what are the factors affecting the force felt by a current-carrying wire in a magnetic field?

Answer 31

1. length of the wire 2. electric current in the wire 3. strength of the permanent magnet

Question 32

How to find the direction of the force acting on a wire in a magnetic field?

Answer 32

Fleming's left hand rule

Question 33

what is a d/c motor?

Answer 33

An electrical machine that converts electrical to mechanical energy

Question 34

How does the DC motor work?

Answer 34

* coil of wire is mounted on an axle and placed between a pair of magnets where N and S poles face each other * The free ends of the coil is connected to a circuit by a split ring called a commutator via graphite brush contacts. * current is passed through the coils * the side near the north pole will be pushed upwards while the side near the south pole will be pushed downward according to Fleming's rule * these two forces will cause the coil to rotate * when the coil has rotated half-turn the current will reverse making sure that the direction of the forces remains the same which keeps the coil rotating in the same direction * the turning moment of the coil can be used to do work

Question 35

Uses of the D/C motor

Answer 35

* fans * starters *

Question 36

The D/C motor

Answer 36

Question 37

A/C Generator

Answer 37

Question 38

Label the diagram. What device is this?

Answer 38

A D/C Motor ## Footnote A - graphite brush; B - splt-ring commutator; C - direction of force on the coil; D - D/C power source; E - magnet; F - wire coil

Question 39

Label the diagram. What device is this?

Answer 39

A/C generator A - graphite brush; B - slip rings ; C - rotation of the coil; D - voltmeter measuring pd; E - magnet; F - wire coil

Question 40

what is a transformer?

Answer 40

A device that changes the voltage of an alternationg current

Question 41

A step up transformer has (1)\_\_\_\_\_\_\_\_\_ turns on the (2)\_\_\_\_\_\_\_\_\_\_\_ coil than on the (3)\_\_\_\_\_\_\_ coil and as a result the (4)\_\_\_\_\_\_\_\_\_\_\_ is increased while the (5)\_\_\_\_\_\_\_ is ___________ by the same (6)\_\_\_\_\_\_\_\_\_\_ that the voltage was decreased.

Answer 41

1. more 2. secondary 3. primary 4. voltage 5. current 6. factor

Question 42

A step down transformer has (1)\_\_\_\_\_\_\_\_\_ turns on the (2)\_\_\_\_\_\_\_\_\_\_\_ coil than on the (3)\_\_\_\_\_\_\_ coil and as a result the (4)\_\_\_\_\_\_\_\_\_\_\_ is decreased while the (5)\_\_\_\_\_\_\_ is ___________ by the same (6)\_\_\_\_\_\_\_\_\_\_ that the voltage was decreased.

Answer 42

1. less 2. secondary 3. primary 4. voltage 5. increased 6. factor

Question 43

what happens to the voltage in a step-up transformer

Answer 43

it increases

Question 44

what happens to the current in a step-up transformer?

Answer 44

it decreases by the same factor that the voltage was stepped up

Question 45

what happens to the voltage in a step-down transformer?

Answer 45

it decreases (is stepped down)

Question 46

what happens to the current in a step-down transformer?

Answer 46

it decreases by the same factor that the voltage was stepped down

Question 47

Is this a step-up or step down transformer?

Answer 47

step-down

Question 48

Is this a step-up or step down transformer?

Question 49

what is the transformer equation?

Answer 49

Question 50

true or false: the input power in a tranformer is the same as the output power

Answer 50

true

Question 51

Equation for input and output power of a transformer

Answer 51

Power = VI VI_out = VI_in

Question 52

Advantages of using AC for transferring electrical energy through long distances

Answer 52

* the volatge of AC can be easily changed to match the voltage rating of consumer electronics by using a transformer * AC voltage and current can be stepped up or down with little loss of energy * The AC can be from the power station at low current to reduce energy loss as heat * Thinner transfer cables can be used as the current can be stepped down, therefore reducing material cost.

Question 53

what is the main practical application of transformers?

Answer 53

Power companies use step-up tranformers to increased the voltage and decrease the current of the ac so it can be carried at larger distances with minimum resistance. Before the electricity reaches the consumer, step-down transformers decrease the voltage of the AC to appropriate values, according to the type of consumer (factories, hospitals, homes etc)