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(A) Physics > P4 > Flashcards

P4 Flashcards

1
Q

What are magnetic field lines

A

Lines that model a magnetic field - denser (more) lines means a stronger magnetic field

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2
Q

What is a magnetic field

A

An area in which a magnet can apply a force

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3
Q

What is an induced magnet

A

A magnet that is created when a magnetic material is placed in a magnetic field , which may or may not stay when the field is removed

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4
Q

What is a permanent magnet

A

A magnet which stays magnetic when other magnets or an electrical current is released

(E.g when removed from a magnetic field it is still magnetic)

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5
Q

What is a domain

A

A small magnetic region in a magnetic metal (may be activated or not)

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6
Q

How is magnetism induced

A

When the domains line up with the magnetic field lines (when in a magnetic field) causing them to be attracted towards the magnet.

Sometimes when a magnet is removed the induced magnet may stay magnetic

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7
Q

In permanent magnets what are the domains like

A

They all line up with each other pointing in one direction

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8
Q

What are the domains like in unmagnatised metals

A

They point in random directions

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9
Q

How can the magnetic field lines be shown in an experiment

A

When placing iron filings between two attracting magnets, the filings line up clearly on the field lines

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10
Q

Why does the North of the compass point to the ‘North pole’

A

A compass needle lines up with the magnetic field lines of the Earth.
This means the North side will point to the magnetic south pole of the Earth.
However this point is actually the geographical north pole (so the north of a compass points to the north pole)

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11
Q

What direction do field lines travel

A

From North to South

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12
Q

What happens when current passes through a wire

A

A magnetic field is created

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13
Q

How can we tell what direction a magnetic field is when current passes through a wire.

A

Using the right hand grip rule

Point your thumb in the direction the current is traveling and close in the rest of your fingers. The direction your fingers point is the direction the current is moving
(Clockwise or anticlockwise)

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14
Q

What is a solenoid

A

A coil of wire
Each coil will have its own magnetic field

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15
Q

What does the magnetic field strength around a wire depend on

A

Strength of the field (magnetic flux density / magnetic field strength) depends on

Magnitude of current : bigger current = stronger field
Distance from the wire : nearer the wire = a stronger field

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16
Q

What is a magnetic field strength measured in

A

Teslas

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17
Q

How can you calculate magnetic flux density

A

Force (N) = magnetic flux density (T) × current (A) × length (m)

So
Magnetic flux density = Force ÷ (current × length)

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18
Q

How do solenoids increase the magnetic field strength

A

Each loop of wire will have its own field strength

By having more loops of wires these field lines will accumulate (but dont cross each other) creating a greater strength field - as when there is a greater magnetic flux density there is a greater force

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19
Q

How do you model the direction of current (into or out of the page)

A

X - current is going into the page
. - current is going out of the page

(Imagine it like you are throwing a dart / one is being thrown at you. If you see the x at the back its going away but if you see the point at the front it is coming towards you

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20
Q

how to draw magnetic field lines using compass

A

Place the magnet in the middle of a page (you can trace around it)
Place your compass next to the magnet and mark the direction the needle points
Move the compass so the back of the needle lines up with the dot
Repeat this until you reach the end of the magnet (and draw the dots together and draw an arrow to show the direction the needle was pointing.)

This can be repeated on the other side and from different starting positions closer or further away (to draw more field lines)

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21
Q

What happens when you combined two magnetic fields

A

A force is produced (repulsion or attraction)

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22
Q

How can we tell the direction of this force using the direction of current and direction the the field lines.

A

By using Flemings left hand rule

Our thumb represents the direction of force
Our first finger represents the direction of field lines
Our second finger shows the direction of current (into or out of the page)

When using this rule ensure all your fingers are perpendicular

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23
Q

What is a motor

A

A component which produces rotation from the combination of the fields due to magnets and current carrying wires

Electrical energy to kinetic

24
Q

How can you get a coil of wire to spin.

A

Combining magnetic fields produces a force
When we place a loop in a magnetic field and connect it to a battery it spins, one side will go upwards and one side downwards

However once it reaches a vertical position it will spin back the other way and stay at 90

25
Q

What are split ring commutators

A

A component of direct current motor or generator. In a motor it ensures that the coil always spins in the same direction. In a generator it means that a potential difference is induced

26
Q

How do electric motors work

A

Electrical energy is used to generate kinetic energy

Placing a wire with current in a magnetic field causes a force to be applied and the wire begins to spin

When reaching 90° the coil stops rotating, and the current must be reversed

The wires are connected to split ring commutator which touch brushes, allowing electrical current to flow.

As the split ring commutator rotates with with the wire there is a brief moment when electrical current is not flowing and the coil rotates due to its own momentum

The brushes then touch the commutators again, but now the current is reversed to movement in the same direction continues

This process repeats

27
Q

What does the split ring commutator do

A

It makes sure the force on the left is always upwards and on the right is always downwards
(Ensure continuous motion)

28
Q

How can you change the speed of motors

A

Changing the current strength
Changing the magnetic field stregth
Changing the number of coild in a wire
Changing the coil length

29
Q

What is electromagnetic induction

A

The process of producing an induced potential difference in a conductor due to the conductor cutting magnetic fields

Conductor e.g (closed wire)

30
Q

What is induced potential difference

A

The potential difference produced across a conductor that is cutting across field lines

31
Q

How do you increase the induced potential difference

A

Moving the wire through coils faster - more field lines are cut
Using a stronger magnetic field
Using more wire / more loops - p.d induced in each loop (as magnetism is in each loop) so total p.d is greater

32
Q

What is alternating potential difference

A

A potential difference that changes direction, usually described as positive or negative

33
Q

What is an alternator

A

A generator which produces alternating potential difference and a.c

34
Q

What are slip rings

A

The component of an alternator that connects the coil to an external circuit so that an alternating potential difference is induced

35
Q

What is a dynamo

A

A generator which produces a direct potential difference that changes magnitude but not direction

36
Q

How does an alternator work

A

A simple circuit is placed in a magnetic field, and is connected to two slip rings which allow current to flow around the coil

P.d is induced when the wire is placed in a magnetic field

The slip rings allow the current to change direction, so the rotation of the wire is changin direction (making it complete full loops)

The spinning in an alternator is the same as moving a magnet in and out of an electric field

P.d is at 0 when the wire is vertical but increases when moving one direction and decreases when moving in the other

It produces an alternating p.d and a.c

37
Q

How does a dynamo work

A

A coil of wire connected to a split ring commutators is placed in a magnetic field

Due to the generator effect p.d is induced, and as the wires make a full circuit current is induced

This causes the wire (and each commutator to spin)

As the wire is connected to the same commutator p.d and current are not reversed
(Uses direct current)

The wire then flips direction (instead of there being a continuous motion)

38
Q

The output in dynamos and current can be increased by :

A

Using a stronger magnetic field
Using more turns on the coil
Spinning the coil faster

39
Q

What is a transformer

A

A circuit component that uses two coils and a core (usually iron) to which changes potential difference

40
Q

How do transformers work

A

When current flows through a coil it produces a magnetic field
Using an a.c current (flows in one direction then changes to the other) the field lines constantly change

(Above is extra)
As the magnetic field is ‘trapped’

The alternating p.d across the primary coil induces an a.c in the primary coil
An a.c in the primary coil produces a magnetic field in the iron core that is always changing
So a changing p.d is induced in the secondary coil

The amount p.d induced in the second coil will depend on the number of loops

41
Q

What is a step up transformer

A

A transformer with more coils in the secondary coil than primary

The voltage will increase as the magnetic field is greater in the secondary coil

42
Q

What is a step down transformer

A

A transformer with more turns in the primary coil than the secondary

More voltage comes in at the primary coil than is released at the secondary coil

43
Q

In transformers what is the ratio of turns equal to

A

The ratio of p.d

P.d across primary ÷ P.d across Secondary = turns in primary ÷ turns in secondary

44
Q

When are transformers used

A

To transport enough energy across to homes a step up transformer is used - the energy must be high in power lines as some heat is lost to the surroundings.

Then to use safely in your home a step down transformer brings the voltage back down

45
Q

How does a dynamic microphone work

A

It is similar to a GENERATOR (uses the generator effect?

A diaphragm is connected to a coil of wire
Thee wire is also looped around a permanent magnet

A sound wave is a pressure wave
When the wave hits the diaphragm of a microphone area of high pressure (compressions) push the diaphragm in and areas of low pressure (rarefactions) pull it out

As the diaphragm moves in and out so does the coil, (generator effect)
A p.d is induced across the ends of the wire. + Current
This is the electrical signal that a microphone produces

This signal then can transfer the sound information to an amplifier and then a loudspeaker

46
Q

How do loudspeakers work

A

It uses the motor effect

A cone is attached to an AC supply and a permanent magnet

When the current passes through the wire a magnetic field is generated
This magnetic field interacts with the permanent magnet, and depending on the direction of current is repelled or attracted
As there is AC the direction is constantly changing, the coil and magnet are constantly pushing and pulling
The change in direction if force produces a soundwave

47
Q

What is a diaphragm

A

The part of a microphone or loud speaker which moves (when a sound wave hits it or to produce a sound wave)

48
Q

What are compressions

A

A part of a sound wave where pressure is high

49
Q

What are rarefactions

A

Part of a sound wave where the pressure is low

50
Q

Simple loudspeaker using a paper cone

A

Make a paper cone and attach a paper cylinder to it
Wrap a pair of wire around the cylinder and a pair of magnets.
Connect the wire ends to a changing p.d and you will hear sound

51
Q

How is potential difference induced by magnetism

A

In a CLOSED circuit placing a magnet by the wire, causes potential differences to be induced.

52
Q

What is the generator effect

A

Moving a wire through a magnetic field produces p.d at both ends.

Cutting the wire through the field induces p.d but moving it along the field does not

If your using a complete circuit then current is induced

53
Q

What is the motor effect

A

Placing a wire in a magnetic field produces a force

Force produced = mfd × current × length

Mfd - magnetic field density

It only applies to wires 90° to the field

54
Q

What is a radial magnetic field

A

A magnetic field where the strength depends on the distance from the field

55
Q

What is a uniform magnetic field

A

A magnetic field where energy is equal throughout the field

(A) Physics (10 decks)

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