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[GCSE] physics > P7: Magnetism and electromagnetism > Flashcards

P7: Magnetism and electromagnetism Flashcards

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1
Q

At which part of a magnet are the magnetic forces strongest

A
  • the poles of the magnet
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2
Q

What happens when two magnets are brought close to each other?

A

they exert a force on each other

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3
Q

What does the law of magnetism state?

A

Two like poles repel each other
Two unlike poles attract each other

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4
Q

What type of force is exerted between two magnetic poles?

A
  • a non - contact force
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5
Q

Give two differences between a permanent magnet and an induced magnet

A

permanent magnet: produces its own magnetic field and can either attract or repel
induced magnet - only becomes magnet when placed in a magnetic field and can only attract

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6
Q

What happens when an induced magnet is removed from a magnetic field?

A
  • the induced magnet loses most/all of its magnetism
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7
Q

What is a magnetic field

A
  • the region surrounding a magnet where another magnet (or magnetic material) experiences a non-contact force
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8
Q

give four examples of magnetic materials

A
  • iron
  • steel
  • cobalt
  • nickel
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9
Q

How does the strength of a magnetic field alter as you move further away from the magnet producing it ?

A
  • the magnetic field strength decreases the further you move away
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10
Q

In what direction does a magnetic field point?

A
  • from the north pole to the south pole
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11
Q

What does a magnetic compass contain?

A
  • a small bar magnet that points in the direction of the Earth’s magnetic field
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12
Q

PRACTICAL: Describe how to investigate the shape and direction of a magnetic field

A
  1. Place the bar magnet on top a piece of a paper and draw a dot at one end of the magnet
  2. mark the direction the compass needle points
  3. move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
  4. join the points to show the field lines
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13
Q

Explain how the behavior of a magnetic compass is related to evidence that the core of the earth is magnetic.

A
  • On Earth, the north arrow on a magnetic compass will point towards the geographic North Pole
  • This is because the geographic North Pole is a magnetic south pole
  • The north pole of the magnetic compass is attracted to the Earth’s magnetic south pole
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14
Q

What is produced when current flows through a conducting wire?

A
  • a magnetic field is produced around the wire
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15
Q

What two things determines the strength of the magnetic field around a current-carrying wire

A
  • the magnitude (size) of the current flowing through the wire
  • the distance of the magnetic field from the wire
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16
Q

What is a solenoid?

A
  • a coil of wire which when current passes through creates a strong magnetic field
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17
Q

Describe the magnetic field around a current-carrying wire

A

The magnetic field is made up of concentric circles

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18
Q

How can the right-hand thumb rule be used to work out the direction of the magnetic field?

A
  • thumb point is along the direction of the current
  • the other fingers give the direction of the field
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19
Q

What happens when current is reversed?

A
  • the magnetic field direction will also be reversed
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20
Q

Give three ways to increase the strength of a magnetic field

A
  • increasing the size of the current
  • increasing the number of coils
  • adding an iron core through the centre
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21
Q

Describe the magnetic field found inside a solenoid

A
  • strong and uniform
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22
Q

What is an electromagnet and what does it do with regards to the magnetic field
?

A
  • a solenoid with an added iron core
  • adding the iron core increases the strength of the magnetic field
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23
Q

How does an electric bell work

A
  • A current passes through the electromagnet creating a magnetic field
  • This attracted the iron armature, causing the hammer to strike the bell
  • The movement of the armature breaks the circuit at.
  • This stops the current, destroying the magnetic field and so the armature returns to its previous position
  • This re-establishes the circuit, and the whole process starts again
24
Q

What is a circuit breaker and how does it work?

A

It breaks a circuit when the current gets too high
- it acts as a resettable fuse
- when current gets too high
- magnetic field strength of electromagnet increases
- switch is made from magnetic material and attracts to the EM.
- circuit breaks

25
Q

What is the motor effect?

A
  • A current carrying wire creates a magnetic field .
  • This can interact with another magnetic field, causing a force that pushes the wire at right angles.
26
Q

What rule is used to determine the force experienced due to the motor effect?

A

Fleming’s left hand rule

27
Q

Describe how the Fleming’s left-hand rule is used

A
  • the thumb, index finger and second finger are held mutually at right angles
  • index finger shows the direction of the magnetic field from North to South
  • second finger shows the direction of the current from positive to negative terminal
  • the thumb then shows the direction of the force experienced
28
Q

give three ways to increase the size of the force exerted by a magnetic field and a current carrying wire

A
  • increasing the current
  • using stronger magnets
  • placing the wire at 90 degrees to the direction of the magnetic field lines between the poles
29
Q

what is the equation that links force, magnetic flux density, current and length and units for them

A

F= BIL
F = force acting on current-carrying wire in Newtons (N)
B = magnetic flux density (which is the strength of the magnetic field) in Tesla (T)
I = current flowing through the conductor in Amps (A)
L = length of the conductor that is in the magnetic field in metres (m)

30
Q

what criteria must be met for the equation that links force, magnetic flux density, current and length to hold

A
  • the conductor must be at right angles to the magnetic field it is placed in
31
Q

How does an electric motor work?

A
  • a coil (loop) of wire carrying a current, is placed in a magnetic field
  • the wire experiences a force in opposite directions
  • this causes a rotational effect until it reaches a vertical position
  • Here, the split ring commutator swaps the contacts of the coil, reversing the direction of current
  • thus, reversing direction of the force, as a result the coil will continue to rotate
  • This reverses the direction in which the current is flowing
32
Q

How do loudspeakers make use of the motor effect?

A
  • the motor effect is used to convert variations in the current into pressure variations
  • which produce audible sound
33
Q

Explain how a loudspeaker works

A
  • a current in the coil creates a magnetic field
  • the magnetic field interacts with the permanent magnet generating a force which pushes the cone out
  • the current is made to flow in the opposite direction
  • the direction of the magnetic field on the cone now pulls it back in
  • repeatedly alternating the current direction makes the cone vibrate in and out at the same frequency as the electrical signal
  • the cone vibrations cause pressure variations in the air which are sound waves
34
Q

How is pitch changed in a loudspeaker

A
  • the higher the frequency of the electrical signal , the higher the pitch of sound
35
Q

Describe the generator effect (4)

A
  • when a wire experiences a changing magnetic field
  • the magnetic field cuts through the wire
  • A potential difference is induced at either end of the wire
  • If the wire is part of a complete circuit a current is also induced
36
Q

Give three ways to increase the induced potential difference /current?

A
  • use a magnet with a stronger magnetic field
  • increase the number of turns in coil
  • increase the speed of the magnet / coil
37
Q

Give two ways that the generator effect is used to generate different types of current?

A
  • in an alternator to produce alternating current
  • in a dynamo to produce direct current
38
Q

Describe how an alternator works

A
  • a coil spins in a magnetic field this could be due to someone turning it or water/steam
  • as the coil spins, it experiences a change in the magnetic field
  • a potential difference is induced
  • the induced pd is continuously changing directions
  • as there is a closed circuit, a current is induced
  • induced current continuously changes direction as the pd changes direction every half turn so the current also does
39
Q

Describe how an AC trace graph looks like

A
  • the peaks on the trace = the rectangular coil is parallel to the direction of the magnetic field so there is the greatest change in magnetic field
  • when perpendicular to the magnetic field, the induced pd is zero
40
Q

Explain why an alternator has slip rings

A
  • it prevents the wires from getting twisted together
  • as it maintains contact with coil and brushes
41
Q

Describe how a dynamo works

A
  • As the coil rotates, it cuts through the field lines
  • This induces a potential difference between the end of the coil
  • The split ring commutator changes the connections between the coil and the brushes every half turn
  • This keeps the current leaving the dynamo in the same direction
42
Q

State the energy transfer occurring in a generator

A

from kinetic energy to electrical energy

43
Q

What electromagnetic effect does a microphone take advantage of and how

A
  • the generator effects
  • it converts the pressure variations in sound waves into alternating current in a circuit
44
Q

Describe how a microphone works

A
  • pressure waves travel towards microphone
  • these hit the diaphragm and make it vibrate
  • the cone+ coil attached to the diaphragm vibrates
  • permanent magnet’s magnetic field cuts through vibrating coil causing it to experience an alternating magnetic field
  • this induces an alternating potential difference
  • a complete loop is formed, producing
    an alternating current
45
Q

Describe the energy transfer for a motor

A

from electrical energy to kinetic energy

46
Q

Compare a loudspeaker with a microphone

A

Both the loudspeaker and microphone involve alternating current
- they both have a coil and a cone which vibrates
- the loudspeaker starts with an alternating current whereas the microphone has an induced one
- the microphone uses the generator effect, but the loudspeaker uses the motor effect
- loudspeaker transfers electrical energy to sound (kinetic) energy whereas microphone transfers sound to electrical

47
Q

Describe the makeup of a basic transformer

A
  • a primary coil and a secondary coil of wire wrapped around an iron core
48
Q

why is iron used as the core for a transformer

A

it is easily magnetised

49
Q

why is iron used as the core for a transformer

A

it is easily magnetized

50
Q

Explain how an alternating current in one coil is used to induce a current in a second coil in transformers

A
  • An alternating current creates an alternating magnetic field in the primary coil
  • the iron core strengthens the magnetic field making it an electromagnet with an alternating magnetic field
  • the alternating magnetic field is not present in the secondary coil
  • the changing magnetic field induces an alternating pd in the secondary coil
  • if this is connected to a closed circuit, an alternating current will flow
51
Q

How can one calculate the output potential difference (voltage) of a transformer

A

potential difference across primary coil / potential difference across secondary coil = number of turns on primary coil/ number of turns on secondary coil
Vp/Vs = Np/Ns

52
Q

What does a step-up transformer do and its makeup

A
  • increases the potential difference of a power source
  • has more turns on the secondary coil than on the primary coil
53
Q

What does a step-down transformer do and its makeup

A

decreases the potential difference of a power source

has fewer turns on the secondary coil than on the primary coil

54
Q

Give the equation to calculate current of a transformer

A

Vp × Ip = Vs × Is
potential difference across primary coil x current across primary coil = pd across secondary coil x current across primary coil

55
Q

Give three roles of a transformer

A
  • increase the potential difference of electricity before it is transmitted across the national grid
  • lower the high voltage electricity used in power lines to the lower voltages used in houses
  • used in adapters to lower mains voltage to the lower voltages used by many electronic devices
56
Q

Explain the advantage of transmitting electricity at a very high potential difference

A
  • This will result in a smaller current being transmitted through the power lines
  • This is because P = IV, so if V increases, I must decrease to transmit the same power
  • A smaller current flowing through the power lines results in less heat being produced in the wire
    -This will reduce the thermal energy loss in the power lines
57
Q

What can be said about the electrical power input and output of a 100% efficient transformer

A
  • the electrical power input is equal to the electrical power output

[GCSE] physics (9 decks)

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