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Physics > Section 6 - Electric and Magnetic fields > Flashcards

Section 6 - Electric and Magnetic fields Flashcards

1
Q

How is the build up of static caused by friction?

A
  • When certain insulating materials are rubbed together, - charged electrons will transfer from one to the other
  • The charges electrons are not free to move (due to insulators) so the charge builds up, causing static electricity
  • both materials become electrically charged, with a + static charge on one, and a - static charge on the other
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2
Q

How do electric charges (or things with them) repel/attract?

A

Electrically charges objects exert a force on one another

they can also attract uncharged objects

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3
Q

How does attraction by induction work?

Electrically charged object attracting an uncharged one

A

A negative electrically charged object will repel the negative electrons in the unchraged object. This causes a positive charge on the surface of the uncharged object, which attracts the negatively charged object

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4
Q

How can static cause sparks?

A

As an electric charge builds on an object, potential difference between the object and the earth increases
-> If the p.d. gets large enough, the electrons can jump across the gap between the charged object and the earth (or any earthed conductor) - this is the spark

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5
Q

What are some examples of uses of static electricity?

A

Photocopiers - to copy imapes onto a charged plate before printing them
- To reduce dust and smoke that rises out of industrial chimneys
- Electrostatic sprayers - Painting / insecticide

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6
Q

How do electrostatic sprayers work?

A

The spray gun is charged, which charges up the particles of paint. This makes them repel eachother, creating a fine, dispersed spray. - even coat, no waste
The object to be painted is given the opposite chrage to attract the spray.

Insecticide works like this but by opposite induction not given charge

e.g. Bikes, cars

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7
Q

How can static electricity be dangerous when refuelling cars?

A

As fuel flows out of a filler pipe, e.g. into an aircraft or tanker, static can build up. This can lead to a spark which may cause an explosion in dusty or fumey places e.g. at a petrol station

These are earthed to prevent this

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8
Q

How is the static on aeroplanes dangerous?

A

Friction between the air and the plane causes the plane to become charged.
-> The build up of static can interfere with communication equipment

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9
Q

How is lightning formed?

A

Raindrops and ice bump together inside storm clouds, leaving the top of the cloud positive and the bottom of the cloud negative. This creates a huge voltage and a big spark, which can damage homes or cause fires when it strikes the ground

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10
Q

What is earthing?

A

Connecting a charged object to the ground using a conductor (e.g. a copper wire)

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11
Q

How does earthing prevent dangerous sparks?

A

It provides an easy route for static charges to travel into the ground. this prevents a build up of charge causing a spark

Electrons flow down the conductor to the ground if the charge is negative and flow up the ground through the conductor if the charge is positive

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12
Q

What is an electric field

A

a region around any electrically charged object

the closer you get to the object, the stronger the field

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13
Q

What are the rules for electric field diagrams?

A

Go from + to -
theyre at right angles to the surface
The closer the lines, the stronger the field

With 2 objects:
- If the field lines point in the same direction, the lines join up and the objects are attracted to each other.
- If the field lines are in opposite directions, they push against eachother and the objects repel
- The strength and direction are the same everywhere between 2 plates

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14
Q

What happens when a charged object is placed in an electric field?

A

It feels a force
-> This is caused by the electric fields around the two object interacting

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15
Q

How is sparking explained by electric fields

Hair comb and paper

A

After its run through the hair, its charged and so produces an electric field. This electric field interacts with the peice of paper and so the paper feels a force. This causes it to move towards the comb

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16
Q

How do sparks relate to potential difference

A

Sparks are caused when there is a high enough p.d. between a charged object and the earth. A high p.d. causes a strong electric field between the two. A strong electric field causes electrons in the air to be removed (ionisation). This causes the normally insulatory air to be conductive, allowing a current (spark) to flow through it

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17
Q

What is a magnet and a magnetic field?

A

Magnet - a thing with 2 poles
Magnetic field - a region where other magents or magnetic materials experience a force

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18
Q

What are the rules for drawing magnetic fields?

A

Lines always go from North to South and display which direction a north pole would feel force at each point in the field
Closer lines = stronger field
Further away = weaker field
Magnetic (field and forces) are strongest at the poles

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19
Q

Where do compasses point when not near magnets?

A

The Earth’s north pole (which is actually a magnetic south pole)

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20
Q

What are the main 3 magnetic elements?

A

Iron, Nickel and Cobalt
-> Some of their alloys as well e.g. Steel

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21
Q

What is the force between a magnetic material and a magnet?

Repel/attract

A

Always Attractive

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22
Q

What do permanent magnets do?

A

Produce their own magnetic field all the time

e.g. bar magnets

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23
Q

What do induced/temporary magnets do?

A

Only produce a magnetic field whilst their in another magnetic field

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24
Q

How does magnetic induction work?

A

A permanent magnet placed next to a magnetic material will induce opposite magnetic poles.
N—-S ____ N—-S (induced)
When you take the Permanent magnetic away, the induced magnets return to normal and stop producing a magnetic field

-> This is why the force is always attractive

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25
How is - How quickly an induced magnet loses their magnetism - mesured?
**Magnetically "hard" or "soft"**
26
What are some examples of magnetically "soft" materials | Materials that loose their magnetism very quickly
Pure iron and Nickel-iron alloys
27
What are some examples of magnetically "hard" materials | Materials that loose their magnetism slowly -Permanent magnets come from
Steel
28
How is magnetism used in fridge doors?
There is a permanent magnet strip in the fridge door to keep it closed
29
How is magnetism used in cranes?
These use induced electromagnets to attract and move magnetic materials e.g. moving scrap metal in junk yards
30
How is magnetism used in doorbells?
These use electromagnets which turn on and off rapidly, to repeatedly attract and release an arm which strikes the metal bell to produce a ringing noise
31
How are magnetic seperators used?
Used in recycling plants to seperate items (like cans)
32
How is magnetism used in maglev trains?
These use magnetic repulsion to make trains float slightly above the track ( to reduce losses from friction) and to propel them along
33
How is magnetism used in MRI machines?
These use magnetic fields to create images of the inside of your body without having to use ionising radiation
34
How is magnetism used in media?
In speakers and microphones
35
What does a moving charge cause?
A magnetic field -> when a current flows through a long, straight conductor (e.g. a wire), a magnetic field is created around it
36
How is a magnetic field created by a current directionally proportional to it?
The field is made up of concentric circles perpendicular to the wire, with the wire in the centre | This is shown by the right-hand thumb rule
37
What is the motor effect?
**A current in a magnetic field experiences a force** - When a current-carrying conductor (e.g. a wire) is put between two magnetic poles, the two magnetic fields interact. - this results in a force
38
What angle does the wire have to be to the magnetic field to experience full force during the motor effect?
90° -> if it runs along the magnetic field it wont feel any force, some angles feel a partial force
39
What is flemmings left hand rule?
The directional relationship between components in the motor effect - Thumb - F (force) - 1st Finger - B (Magnetic field/flux dens) - 2nd Finger - I (Current)
40
What is magnetic flux density and length in FBII | FBII only used when force in 90° to the field
MFD = How many field lines there are in a region (shows strength of field) length = Length of conductor thats IN the magnetic field
41
How can the direction of a motor be switched?
swapping the polarity of the d.c. supply (reversing current) or swapping the magnetic poles over (reversing field)
42
How do you increase the magnetic field strength of a loop of wire?
Make more loops - turning it into a solenoid
43
How is a motor kept going the same direction?
Split-ring commutator - swaps contacts every half turn to keep the motor rotating in the same direction
44
How do the field lines in a solenoid work?
The field lines outside the coil overlap and cancel eachother out - so the field is weak apart from at the ends of the solenoid Inside the solenoid, you get lots of field lines pointing in the same direction - the magnetic field is strong and almost uniform
45
What is a solenoid an example of?
An electromagnet
46
How can you increase the strength of a solenoid? | without changing the wiring
Put a block of iron in the centre of the coils - this becomes an induced magnet whenever current is flowing
47
What do transformers use?
Electromagnetic Induction -> Caused by when the magnetic field through an electrical conductor changes (Gets bigger/smaller/reverses)
48
What does a changing magnetic field induce
A potential difference in a conductor
49
What is electromagnetic induction?
The induction of potential difference ( and current if there's a complete circuit) in a wire which is experiencing a magnetic field
50
In what two situations can you get electromagnetic induction?
1) If an electrical conductor (e.g. coil of wire) and a magnetic field move relative to eachother 2) When the magnetic field through an electrical conductor changes (Gets bigger/smaller/reverses)
51
How can you show that electromagenetic induction can be caused by: If an electrical conductor and a magnetic field move relative to eachother
1) **Moving/Rotating either a magnet in a coil of wire** or a conductor in a magnetic field 2) If you move/rotate the magnet/conductor in the opposite direction, the p.d. will be reversed. (Likewise with the magnets polarity) 3) If you move the magnet forwards and backwards, you will produce an A.C.
52
How do you increase the size of induced p.d. in electromagnetic induction?
1) Increasing the strength of the magnetic field 2) Increasing speed of movement/change of field 3) More turns per unit length on the coil of wire
53
What do transformers do?
they use induction to change the size of the potential difference of an alternating current
54
What do transformers consist of?
A primary and secondary coil, joined with an iron core
55
How do transformers work?
1) When an alternating p.d. is applied across the primary coil, it poduces an alternating magnetic field 2) This alternates the magnestisation in the iron core. 3) This changing magnetic field induces a p.d. in the secondary coil
56
What do step-up transformers do?
Increase the p.d. -> They have more turns on the secondary coil than the primary coil
57
What do step-down transformers do?
Decrease the p.d. -> They have less turns on the secondary coil than the primary coil
58
What is the equation for transformer input/output and why does it work?
Input power = Output power Vp x Ip = Vs x Is | Transformers are almost 100% efficient
59
How do generators work?
1) Generators **apply a force to rotate a coil in a magnetic field** (or vice versa) - with oposite construction to motor 2) As the coil (or magnet) spins, a **current is induced** in the coil. This current changes direction every half turn
60
How are dynamos created out of basic generators?
3) A **split ring commutator** is added in **dynamos (d.c. producers)** as it swaps the connection every half turn to keep the current flowing in the same direction
61
How are alternators created out of basic generators?
NO SPLIT RING COMMUTATOR - instead **slip rings and brushes **are added so the contacts dont switch every half turn - producing an **alternating current**
62
What do microphones do?
Generate current** from** sound waves -> Using electromagnetic induction to generate an electrical signal
63
How can microphones convert the pressure variations of a sound wave into variations in current in an electrical circuit?
1) Sound waves hit the flexible diaphram attached to the coil of wire. 2) As the diaphram moves ( and therefore coil), a current is generated within the coil 3) The movement of the coil ( therefore the generatd current) depends on the properties of the sound wave -> Louder sounds make diaphram move further
64
What is in a microphone?
A felxible diaphram attached to a coil of wire surrounding one pole of a magnet, surrounded by the other pole of a magnet.
65
What is the difference between what makes up a microphone and a loudspeaker?
In a loudspeaker, the diaphram is replaced with a paper cone
66
How do loudspeakers work?
1) The coil is wrapped around one pole of a permanent magnet, so the a.c. signal causes a force on the coil -> Which moves the cone 2) When the current is reversed, this force acts in the opposite direction 3) This makes the cone vibrate = the air around the cone vibrates and creates variations in pressure that cause sound waves
67
What does a power station do?
It uses a turbine to turn a huge alternator -> Most of the electricity we use is generated from burning fuels (coal, oil, gas or biomass) in the boilers of big power stations -> The burning fuel is used to heat water and convert it to steam, which turns a turbine
68
How does the turbine in a power station generate electricity?
1) The tubine is connected to a powerfull magnet (usually an electromagnet) inside a generator 2) As the turbine spins, the magnet spins with it, inducing a large p.d. and alternating current in the coils -> Coils are joined in parralel to produce a single output from generator | Similar thing used for other electricity gen.
69
What is the generator in a power station like?
A huge cylinder would with coild of copper wire
70
How do turbines function in Hydroelectric, tidal and windpower?
The turbine is turned directly, without the need for water vapour | Solar does not use turbines
71
What is the national grid?
A network of wires and transformers that connects UK power stations to consumers
72
How does the national grid transfer energy?
At high power: Power = energy / time -> lots of energy is transferred rapidly
73
What does high electrical power need?
Electrical power = current x potential difference -> either a high p.d. or high current
74
What would high current cause in wires?
Waste to thermal stores (heats them up)
75
How is the power lost due to resistive heating found?
Electrical P = current2 x resistance
76
How are transformers used to make the national grid more effecient?
**Created high-voltage, low-resistance cables**
77
How are step-up transformers used at power stations?
To boost the p.d. up really high (400,000V) and keep the current low.
78
How are step-down transformers used in the national grid?
To bring voltage back down to safe, usable levels at the consumers end

Physics (10 decks)

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