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Physics GCSE > Electromagnetism > Flashcards

Electromagnetism Flashcards

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1
Q

What do all magnets produce? (3)

A
  • All magnets produce a magnetic field- a region where other magnets or magnetic materials eg (iron, nickel, steel and cobalt) experience a force and this is a non-contact force.
  • Line always gone from north to south and they show which direction the force acts.
  • The closer together the lines are, the stronger the magnetic field and the further away the magnet gets, the weaker the field is.
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2
Q

Where is the magnetic field the strongest?
What is the force like between a magnet and a magnetic material?
What happens when two magnets are placed near each other?

A
  • the magnetic field is the strongest at the poles of a magnet
  • The force between a magnet and a magnetic material is always attractive no matter what the pole.
  • When 2 magnets are placed near each other, they will exert a force on each other which can either be attractive or repulsive- like poles repel and unlike poles attract
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3
Q

List 2 types of magnets

-What happens with the magnet is induced and what happens when it is permanent? (3)

A
  • Induced or permanent.
    1) Permanent magnets produce their own magnetic fiels and steel is used instead of iron for a permanent magnet because steel does not lose its magnetism easily but iron does.
    2) Induced magnetism are a magnetic material that turned into a magnet when put into a magnetic field.
    3) The forces between a permanent and induced magnet are always attractive and when you take away the magnetic field, induced magnets quickly lose their magnetism and stop producing a magnetic field.
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4
Q

What does a moving charge create?(2)

What does the strength of the magnetic field depend on?

A
  • When a current flows through a wire, a magnetic field is created around the wire.
  • The field is made up on concentric circuits perpendicular to the wire with the wire in the centre.

-The strength of the magnetic field depends on the current through the wire and the distance from the wire. The larger the current through the wire or the closer to the wire you are, the stronger the field.

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5
Q

How to differ a magnetic material from a magnet?

A
  • Take a N pole ad approach a material- it will attract and switch the pole and the material will still attract therefore it is a magnetic material.
  • The magnet will only have 1 side that will attract and the other will repel.
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6
Q

Define the magnetic field?

What is the direction of the magnetic field?

Describe a magnetic compass?

A

-The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field.

-The direction of the magnetic eld at any point is given by the direction of the force that would act on another north pole placed
at that point. The direction of a magnetic eld line is from the north (seeking) pole of a magnet to the south(seeking) pole of the magnet.

-A magnetic compass contains a small bar magnet. The Earth has a magnetic eld. The compass needle points in the direction of the Earth’s magnetic eld.

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7
Q

How can you use a magnetic compass to detect the magnetic fields of a magnet?(4)

A

1) Place the bar magnet in the middle of the paper. Trace the outline of the magnet – that way you can put it back in the exact same spot if it gets bumped.
2) Place the compass at one pole of the magnet and make a dot next to it showing the direction the compass arrow points.

3) Move the compass so that the base of the arrow is at the dot you’ve just made. Now make a new mark where the tip of the arrow is pointing this time.
Keep doing this until you reach the other end of the magnet.
Connect the dots.

4) Go back and begin again, starting at a different spot than you did the first time. Repeat the above steps.
Repeat the process until you have drawn as many lines as you can for both ends of the magnet. You will now have an accurate representation of magnetic field lines.

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8
Q

Explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic?

A
  • When the compass is not near a magnet, compasses always point north- this is because the Earth generates its own magnetic field which shows the core of the Earth must be magnetic.
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9
Q

What happens when you change the direction of the current? What can you use to help with the direction of the current.

A
  • The magnetic field’s direction also changes with changing current and you can use the right hand thumb rule to work out which way it goes.
  • Your thumb points to the direction of the current and curling your fingers- that points to the direction of the magnetic field.
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10
Q

How can you increase the strength of the magnetic field that a wire produces? (1)
What is a solenoid? (1)

Reference to the mf of the solenoid- what is it similar to? How can you increase the strength of the solenoid?(1)

How can a solenoid become an electromagnet?(1)

A

1) Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire. The magnetic field inside a solenoid is strong and uniform( same strength and direction) because lots of fields are pointing in the same direction that are very close to each other and the closer together the fields are, the stronger the field is.
2) Solenoid- A long coil of insulated wire and solenoids are used in lots of devices where strong magnetic fields need to be produced.
3) The magnetic field around a solenoid has a similar shape to that of a bar magnet. Adding an iron core increases the strength of the magnetic field of a solenoid and the iron core becomes an induced magnet whenever current is flowing.
4) A solenoid with an iron core( a magnet whose magnetic fields can be turned on and off with an electric current) is an electromagnet.

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11
Q

What is an electromagnet?

A

A electromagnet is a solenoid in which the insulated wire is wrapped around an iron bar and when a current passes along a wire, a magnetic field is created around the wire and because of this, the magnetic field of the wire magnetises the iron bar- when the current is switched off, the iron bar loses most of its magnetism

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12
Q

What can electromagnets be used for? (4)

A

1) Electromagnets are used in some cranes to attract and pick up things made from magnetic materials like iron and steel eg. in scrap yard. Using an electromagnet means that magnet can be switched on when needed to picked objects up and switched off when not needed.
2) A circuit breaker- a switch in series with an electromagnet and the switch is held closed by a spring. When the current is too large, the switch is pulled open by the electromagnet and it stays open until it resets manually.
3) The electric bell
4) Electromagnets can be used within other circuits to act as a switch eg in electric motors:
- when the switch in the circuit one is closed, it turns into an electromagnet which attracts the iron contact on the rocker.
- the rocker pivots and closes the contacts, completing the circuit two and turning on the motor.

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13
Q

Describe in detail how an electric bell works.

A

When the electric bell is connected to a battery, the current will flow when the circuit is switched on and the coil becomes magnetised as it produces a magnetic field due to having a soft iron core and becomes an electromagnet. The iron will be attracted as it is a magnetic material that will cause the hammer to hit the gong and echo a sound. One the soft iron moves forward, the circuit breaks and the electromagnet is switched off and therefore becomes demagnetised as the iron cannot retain its electromagnetism- because of this, the armature springs back and the make-and-break switch closes again so whole cycle repeats

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14
Q

Give 3 ways an electromagnet can be made stronger?

A

1) Increase the turns of the number of coils.
2) Increase current.
3) Place a soft iron core inside the solenoid.

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15
Q

Describe the motor effect?

How can the size of the force be increased?

Describe the direction of the force?

A

1) When a current carrying conductor is placed between magnetic poles, the magnetic field around the wire interacts with the magnetic field that it has been placed in and this causes the magnet and the conductor to exert a force on each other- this is called the motor effect and causes the wire to move.
2) The size of the force can be increased when`:
- the wire is perp to the mf, if the wire runs parallel to the magnetic field, it won’t experience any force at all.

3) The direction of the force is always at right angles to the magnetic field of the magnets and the direction of the current in the wore and the direction of the force will be reversed if the direction of the current or the mf is reversed.

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16
Q

How can the size of the force be increased in the motor effect?(2)

How to find the direction of the current, force and field?

A
  • increasing the current.
  • using a stronger magnet.

Fleming’s left hand:

1) Thumb- direction of the force(motion)
2) First finger- field.
3) SECond- current.

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17
Q

Equation to find the size of the force? (GIVE UNITS) What’s the condition?

A

When the currrent is at 90 degrees to the magnetic field, the force acting on it can be found:

F= BIL
F=N
B= tesla
I= amps.
L= m
18
Q

Describe the bases of an electric motor(3)

Why is graphite used?(1)

How can the direction of the motor be reversed?(2)

How can you increase the speed of the motor?(5)

A
  • A coil of wire carrying a current in a magnetic field tends to rotate. This is the basis of an electric motor.
  • The coil is on a spindle and the forces act on either side of the coil due to the motor effect and the force on one side is in the opposite direction to the force on the other side.
  • The coil is connected to the battery by two metal or graphite brushed and the brushes press onto the metal split ring commutator swap the contacts every half turn to keep te motor rotating in the same direction.
  • Graphite is used because it is a form of cabron and conduct electricity and is v slippery and grahite causes very little friction when in contact with the rotating commutator.
  • Direction can be reversed either by:

1) swapping the polarity of the dc supply so reversing the current or
2) swapping the magnetic poles over so reversing the field.

Increase the speed:

  • increase current.
  • increase length.
  • increase mf
  • increase the nu of turns in the coil.
  • wrap a soft iron core.
19
Q

How do loudspeakers work?(4)

A

Loudspeakers and headphones both use electromagnets.

  • An ac current sent through coil of wire attatched t the base of a paper cone. The coil surrounds one pole of a permanent magnet and surrounded by the other pole, so the current moves a force on the coil which causes the cone to move.
  • When the current reverses, the force acts in the opoosite direction, which causes the cone to move in the opposite direction too.
  • Loudspeakers and headphones use variations in current to make the cone vibrate which makes the air around the cone vibrate and create the variation in pressure that creats the sound wave.
  • The freq of the sw is the same as the freq of the ac so by controlling the freq of ac, you can alter the sound wave produced.
20
Q

Briefly electromagnetic induction and how can a generator effect be created? (2)

Define the generator effect?(2)

A

If a wire experience a change in magnetic flux, a pd will be induced across it. The faster the change(the greater the rate of change), the greater the pd across it and this process is called electromagnetic induction.
-if the conductor is part of a complete circuit, the induced pd makes the electric current pass through the circuit and this is known as the generator effect.

Generator effect:
If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor. If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect and the direction of the current will switch when the direction of the movement switches.
-We only see the generator effect if the wire passes through the magnetic field and experiences a change in magnetic flux, it has to move backwards and forwards and as the pd keeps swapping direction, an alternating current is produced.

21
Q

How to differ the motor effect grom the generator effect?

A
  • The motor effect is the effect whereby a wire or coil with a current is placed in a magnetic field and it experiences a force.
  • Electromagnetic induction (the generator or dynamo effect) is the effect whereby a wire or coil moving in a magnetic field generates an emf.

Generator effect makes electricity from motion whereas motor effect makes motion from electricity

22
Q

How does the induced pd and current in the generator get larger? (3)

A
  • Stronger magnetic field.
  • Increase the speed of the wire and move it more rapidly.
  • If we shape the wire into a coil or a solenoid( the greater the number of turns on the coil, then the greater the pd and current)
23
Q

How are other ways an induced current produced in the generator effect? (3)

A
  • By moving the magnet in and out of the coil will produce an induced current and the direction of the current will change as the direction of movement changes. The direction of the current can also be reversed if the polarity of the magnet is reversed.
  • A current will be induced and this induced current will create its own mf and this magnetic field will oppose the movement of the magnet. Eg if a North Pole is inserted into the coil, the coil’s end will become North Pole and will repel the coil, making it harder to be pushed in however when that end of the coil is pulled out, the end of the magnet will become south pole and attract the magnet, making it harder to pull it out- because the induced curred makes it harder to move the magnet, this means we are doing work. We are transferring energy from the movement of the magnet to the movement of the coil and therefore electricity is generated by the work done by the person
  • Therefore an INDUCED CURRENT ALWAYS OPPOSES THE CHANGE THAT MADE IT.
24
Q

How can ac be created in the generator effect?(2)

A
  • turn the magnet in the coil, the mf of the coil changes this change in magnetic field induced a pd which makes the current flow in the wire.
  • when magnet turns, the direction of the field is reversed and hence pd and current flows in opposite direction around the coil of wire and if you keep turning the magnet in the same direction, then an alternating current will be produced.
25
Q

How is the generator effect used? Describe an alternator,

When is the value the peak value?

The faster the coil rotates?(3)

What can you use to see the generated pd?

A

The generator effect is used in an alternator to generate ac and in a dynamo to generate dc.

Alternators:

  • A simple ac generator is made up of a coil that spins in a uniform magnetic field.
  • The current in the circuit repeatedly changes its direction so the induced pd and the current alternates
  • Instead of split ring commutator, they have slip rings and brushes so the contacts don’t swap every half turn.
  • The side of the pd is greatest when the plane of the coil is parallel to the diection of the f and the coil cross directly through the magnetic field lines so induced pd is peak value.
  • when it’s perpendicular, ts sides of te cpil move parallel to the field lines and do not cross them.

The faster the coil rotates:

  • the bigger the frequency.
  • the bigger the peak value of ac.
  • the bigger the pd.

-Use an oscilliscope to see how pd generated in a coil changes over time

26
Q

How can you increase the peak value of an ac current in an alternator?(3)

A
  • using a manet with stronger mf.
  • coil with a bigger area
  • more number of turns in a coil.
  • increase speed of the movement so it cuts more magnetic field line
27
Q

What is a dynamo?(3)

A
  • generate direct current.
  • they have split ring commutators instead of slip rings and this swaps the connection every half turn to keep the current flowing in the same direction
  • induced pd does not reverse its direction as it does in the alternator bevause as the coil spins, the split ring commutator reconnects the coil the opposite way around in the circuit every half turn and this happens when the coil is perpendicular to the magnetic field lines
28
Q

Describe how a microphones work?(2)

A

Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits

-Sound waves hit the flexible diaphragm that is attatched to the coil of wire wrapped around a magnet and the pressure variations of the sound wave on the diaphragm make it vibrate so the coil vibrates in the magnetic field and this alternating pd induced in the coil has the same frequency as the sound waves

29
Q

How is an alternating pd and current induced in an AC generator? (3)

A
  • The coil moves in the field and therefore experiences a change in magnetic flux through the area of the coil.
  • An apd is induced in the coil and the pd is alternating as the coil’s orientation in the field swaps direction as it rotates
  • if it’s part of a complete circuit, an ac current will flow.
30
Q

What.’s the equation that relates voltage and current in a transformer ( LOOK AT TRANSFORMERS IN ELECTRICITY FC)

A

VpxIp= Vsx Is

31
Q

What do transformers do in regards to pd and current? How do transformers work?(3)

A

Transformers change an alternating pd( and hence the current)

1) They work through electromagnetic induction so an alternating pd is applied to primary coil, the soft iron core magnetises which acts as a solenoid and will have a changing magnetic field.
2) The soft iron core is a magnetic material that ensures the secondary coil experience is linked to the primary’s changing magnetic flux. As a consequence, an alternating pd is induced in the secondary coil.
3) If the secondary coil forms a complete circuit, an alternating current will flow in the secondary coi

32
Q

What’s the equation that relates to transformers?

How will you know which is a step up transformer or step down transformer from this eq?

A

Ns/ Np = Vs/ Vp

Ns- number of coils in the secondary coil.
Vs- Output voltage in the secondary coil.

If Ns is greater than Np- it is a step up transformer.
If Np is greater than Ns, than our pd of vs is less hence it is a a step down transformer.

33
Q

An 18V battery could not be used as the input of a transformer. Suggest why

A
  • A battery provides a DC when we need an Ac current.
34
Q

What’s the problem with high current and so what do we as a result in the national grid?

A
  • High current results in loads of energy being lost as the wires heat up and energy is transferred to the thermal energy store of surroundings- much cheaper to boost the pd to 400 000V and keep the current as low as possible.
  • for a given power, increasing the pd decreases the current which decreases the energy lost.
35
Q

How is electricity distributed?

A

-Electricity is distributed via the National Grid

36
Q

Describe the National Grid?

A

It is a giant system of cables and transformers that covers the UK and connects power stations to consumers. It transfers electrical power from power stations(power plants) and to where its demand is needed eg homes and industry.

37
Q

When does demand increase?

How do power stations manage this demand?

A
  • Demand for electricity alters throughout the day and demand increases when people get up in the morning, come home from school or work or when it starts to get dark or cold outside- popular events being showcased.
  • Power stations often run well below their maximum power output so there’s spare capacity to cope with a high demand and lots of power stations that can start up quickly are also kept in standby just in case.
38
Q

What does the National Grid use to transfer a huge amount of power? What’s the problem?

What’s the cost-effective way that makes National Grid efficient?

A
  • To transfer large amounts of current, they have high PD or a high current however having high current has the prob that you lose lots energy as the wires heat up and energy is transferred to the thermal energy store of te surroundings
  • It’s cheaper to increase pd and keep the current as low as possible as by increasing the pd, the current is decreased which decreases the power lost by the wires due to the resistance heating which further reduces energy lost to the surroundings and hence makes a national grid efficient.
39
Q

What is the potential difference changed by? How do they change potential difference?

WHY ARE THE TURNS IN EACH COIL OF A TRANSFORMER INSULATED FROM EACH OTHER?

A
  • Transformers
  • Step up transformers is when the pd is increased for efficient transmission from the power station to the National Grid and step down transformers are used to decrease the pd from the transmission of electricity from the National grid for domestic uses hence pd needs to be safe, usable levels.

TO AVOID CONDUCTION WHEN WIRES ARE SHORT OR TO PREVENT SHORTENING.

40
Q

Explain why transformers do not work with direct current? (3)

A

1) Direct current flows in one direction only.
1) If there is no change in current, there is no changing in magnetic field.
1) So no induction of pd in secondary coil

41
Q

Explain the difference between the alternator and the dynamo and the currents they both generate.

A

2.1) As the coil turns, there is a change in magnetic flux therefore this causes a potential difference to be induced across the ends of the wire and if the wire is part of a complete circuit, current flows through the coil.
An alternator has slip rings which allows the end of the rotating coil to stay connected to the same ends of the circuit at all times.
As the coil turns, the direction of the current induces changes direction as the diection of motion on each arm reverses every half term.
The current produced is alternative.

The dynamo has a split ring which swaps the connection between the rotating coil and the rest of the circuit, every hald term, keeping the current flowing in the one direction and the current produced is dc.

42
Q

Why do we use high pd and low current (4 marks)

A

1) To be effective, the national grid needs to transmit a large amount of power.
2) P= VI hence we need either a large pd or a large current.
3) To be efficient, the national grid needs to minimise energy losses
1) And a high current leads to energy loses as the wire heats up and trnsfers energy to the thermal energy store of the suroundings so the electricity need to be transmitted t a low current.

Physics GCSE (8 decks)

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