Topic 6 - Electric and Magnetic Fields

Question 1

What is a build up of static caused by?

Answer 1

Frictional forces

Question 2

how does rubbing an insulator transfer electrons and create a static charge

Answer 2

negatively charged electrons are scraped off one material onto another

• as its an insulator the electrons aren’t free to move
• this builds up static electricity
• the material becomes electrically charged with a positive static.

Question 3

Which charged particles are always moved

Answer 3

negative electrons

Question 4

Electrically charged particles exert a ……on each other

Answer 4

force

Question 5

how do forces of attraction change with distance

Answer 5

further means weaker

Question 6

How can an electrically charged object attract uncharged objects (with example)

Answer 6

Rubbing a balloon on your head transfers electrons to the balloon leaving it negatively charged
-if you hold the balloon against a wall it will stick because the negatively charges on the wall are repelled and positive charges are attracted to negative balloon

Question 7

How does attraction by induction occur

Answer 7

If a negatively charged object is used to charge a neutral object by induction, then the neutral object will create a positive charge.
(The charged object that is brought near will always repel like charges and attract opposite charges.)

Question 8

What is earthing

Answer 8

sending current down to the earth

provides easy route for static charge so none builds up

Question 9

How do spark occur

Answer 9

If a potential difference is large enough, electrons can jump a gap between charged object and earth making a spark
eg)lightning

Question 10

Too much static causes….

Answer 10

sparks

Question 11

Why might you get an electric shock getting out of a car

Answer 11

static charge builds up on car

static charge jumps to earthed conductor nearby being you and you get a shock

Question 12

How do electrostatic sprayers work

Answer 12

• spray gun is charged, so paint droplets become negatively charged
• object being painted has an opposite charge to attract the fine sprays of paint
• gives an even coat as areas with paint become negative so theres no build up
• no paint shadows

Question 13

why is static dangerous in fueling cars/aircraft

Answer 13

as fuel flows out of pipe into tank, static can build up which can lead to sparks which might cause an explosion in dusty places

Question 14

Why is static on planes dangerous and how does it occur

Answer 14

• as a plane flys through the air, friction between the air and plane builds up causing plane to become charged
• this static can interfere with communication equipment

Question 15

How does lighting form

Answer 15

raindrops and ice bump causing friction and leaves top of cloud positively charged and bottom negative which creates huge voltage and spark which jumps across gap to earth as lightning
-damage homes, start fires

Question 16

How can objects be earthed to stop charge building up

Answer 16

by connecting charged object to the ground using conductor through earthing

• provides easy route for static charge
• no build up of static so no sparks

Question 17

How do electrons flow to ground by earthing if ground charge is:
negative
positive

Answer 17

Negative= flow down the conductor

Positive= flow up the conductor

Question 18

What do electric charges do

Answer 18

Create an electric field around any charged object

stronger as you get closer

Question 19

How is the direction of an electric field defined

Answer 19

Field lines go FROM positive TO negative

Question 20

What happens to a charged object is placed in an electric field? and what is it caused by?

Answer 20

it feels a force caused by two charged objects interacting

Question 21

what happens field lines between charged objects point in opposite directions

Answer 21

They repel

Question 22

How do you get a uniform field

Answer 22

between two positively charged parallel plates

Question 23

How can sparking be explained with electric fields (and therefore static)

Answer 23

• When an object is statically charged it creates its own electric field
• interactions in this field cause sparking
  eg) plastic comb becomes charged and produces an electric field which can interact with paper without touching them

Question 24

What is ionisation in a electric field

Answer 24

Electrons in the air particles to be removed

Question 25

When is air not an insulator

Answer 25

when its ionised enough that it becomes more conductive and sparks are made

Question 26

What do magnets produce

Answer 26

magnetic fields

Question 27

Where do field lines go to in a magent

Answer 27

north to south

Question 28

the closer the field lines the .....the magnetic field

Answer 28

stronger

Question 29

where is the magnetic field strongest at

Answer 29

the poles

Question 30

What are the 3 main magnetic elements

Answer 30

iron, nickel, cobalt some alloys containing these will also be magnetic like steel

Question 31

What is a permanent magnet

Answer 31

produces its own magentic field

Question 32

What is an induced magnet

Answer 32

only produces a field when in another magnetic field putting any magnetic material into a magnetic field it becomes an induced magnet

Question 33

What is magnetic induction

Answer 33

force between magnet and magnetic material is always attractive

Question 34

What does magnetically soft mean and give examples

Answer 34

Lose their magnetism very quickly pure iron and nickel iron alloys

Question 35

what does magentically hard mean and give examples

Answer 35

lose there magnetism more slowly permanent magnets are made from magnetically hard materials

Question 36

Name some uses of magnetic fields

Answer 36

Doorbells- electromagnets that turn on and off rapidly to repeatedly attract and release an arm which strikes a metal bell MRI machines- magnetic fields to creat images inside your body without having to use ionsing radiation

Question 37

A moving charge creates a

Answer 37

magnetic field

Question 38

explain the right hand rule

Answer 38

thumb represents current curled up fingers represent the direction of the magnetic field at right angles to the current

Question 39

in order to experience full force in a motor the wire needs to be at......

Answer 39

90 degrees to the magnetic field | if it runs along field it wont experience any force

Question 40

What is flemmings left hand rule

Answer 40

Used to explain the motor effect Thumb = motion first finger = field second finger = current

Question 41

How can you find the size of a force

Answer 41

Force = magnetic flux density x current x length (of conductor)

Question 42

Describe how a motor works

Answer 42

- forces (which act on current in magnetic field) act on two arms of a current carrying coil - they act in opposite directions so coil rotates - the split ring commutator swaps the contacts every half turn to keep motor rotating in same direction

Question 43

how can the direction of a motor be reversed

Answer 43

by swapping the polarity of the D.C supply or swapping the magnetic poles over

Question 44

What is a solenoid | describe the field lines:

Answer 44

a long coil of current carrying wire | - the field lines around each separate coil of wire line up in same direction so the field is strong and uniform

Question 45

why is a solenoid an electromagnet

Answer 45

-its an electromagnet as field can be turned off using electric current

Question 46

how can you increase strength of solenoid

Answer 46

by putting a block of iron in the center of coil which becomes an induced magnet

Question 47

what is electromagnetic induction in terms of Potential difference

Answer 47

induction of potential difference in a wire which is experiencing a change in magnetic field

Question 48

what two situations can you get electromagnetic induction

Answer 48

when electrical conductor (coil of wire) and magnetic field are relative to each other -this can be done by rotating/ moving/ changing polarity of magnet when magnetic field through conductor changes (like in a transformer) -this can be done by increasing STRENGTH of magnetic field, increasing SPEED, increasing TURNS ON COIL

Question 49

How does a transformer work

Answer 49

-use electromagnetic induction to change size of Potential difference of an alternating current by: increasing number of coils on secondary coil than primary coil in step down

Question 50

How do dynamos work

Answer 50

- generate a direct current by applying a force to rotate coil in magentic field (like in a motor) - current is induced in coil which changes direction every half turn due to split ring communicator to keep current flowing in same direction

Question 51

How do Alternators work

Answer 51

- similar way to dynamos, force applied to rotate coil in magnetic field with an induced current in coil - but instead of split ring has slip rings and brushes so the contacts dont swap every half turn which produces and alternating current

Question 52

How do microphones work

Answer 52

- using electromagnetic induction - sound waves hit diaphragm which is attached to coil of wire which makes both move creating a current in the coil -

Question 53

How do loudspeaker work

Answer 53

-coil wrapped around pole of permanent magnet so a.c signal causes a force on the coil -when current reversed the force acts i different direction -this moves the paper cone as it vibrates it as pressure causes sound waves

Question 54

Describe how a power station generates electricity for the national grid

Answer 54

- burning fuel heats water which creates steam - steam powers turbine - turbine connected to powerful magnet inside generator - the magnet spins, inducing a large voltage and a.c current and single output generated - generator joined to national grid

Question 55

How are transformers used

Answer 55

The high current heats up wires so energy can be wasted so to reduce this transformers are used and high voltage cables. - Step up make Voltage really high at 400,000 V and keep current low (usually in less comercial areas like across country - step down decrease voltage to safe usable levels for consumers like in towns and homes