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Physics > 17 Motor Effect & Electromagnetic Induction > Flashcards

17 Motor Effect & Electromagnetic Induction Flashcards

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1
Q
  1. In a speaker, there is a current carrying coil
  2. 90º to a magnetic field
  3. Due to LHR there is a force on the coil
  4. The coil is attached to the cone - therefore the cone moves in one direction

If the current in a speaker is a.c, describe the movement of the coil and cone and how this creates a sound wave.

A
  1. The alternating current means the current in the coil changes direction continuously.
  2. This means the force on the coil/cone changes direction continuously - cone moves in and out (vibrates)
  3. This creates a sound wave (compression wave) in front of the cone at the same frequency as the alernating current.
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2
Q

Describe the left hand rule (LHR)

A
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3
Q

When a magnet is moved in and out of a coil, the magnetic field lines of the magnet cut the coil at 90º , which induces an a.c. current.

State three way in which you could increase the current induced?

A
  1. More turns in the coil
  2. Stronger magnet
  3. Move the magnet faster
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4
Q

Explain why a current is induced in the wire.

A
  1. Wire is moved downward (thuMb- MOVEMENT)
  2. 90º to a magnetic field (First finger- FIELD)
  3. Therefore a current is induced in the wire (seCond finger- CURRENT)
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5
Q

The current flows into the left brush making contact with the left of the split ring commutator, this means the current always travels into the page on the left side of the coil. This is 90º to the field and due to LHR the force on the left side of the coil is upward.

The coil rotates clockwise.

What would happen if the battery was reversed?

A

The direction of the current is reversed AND the coil will rotate anticlockwise

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6
Q

Draw the magnetic field around a solenoid.

A
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7
Q

Use the LHR to explain why the wire moves to the left.

A
  1. Current carrying wire (seCond finger- CURRENT)
  2. 90º to magnetic field (First finger field)
  3. Using LHR this creates a force to the left- wire moves left (thuMb movement)
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8
Q

How do you use the right hand grip rule to work out which end of a solenoid is the north end?

A

Fingers- wrap your fingers in the direction of the current

Thumb- will point to the North end of the solenoid

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9
Q

Explain why a current is induced in the coil

A
  1. Magnet is moved into the solenoid (thuMb- MOVEMENT)
  2. Magnetic field lines around magnet cut the coil at 90º (First finger- FIELD)
  3. Therefore a current is induced in the coil (seCond finger- CURRENT)
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10
Q

Using the transformer equation below. Describe what would happen to the voltage across a transformer if the number fo turns in the secondary is three times larger than the number of turns in the primary.

A

If no. of turns triples, the voltage triples.

Voltage in the seconday coil will be three times greater than the voltage in the primary

Step-up transformer

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11
Q

How do we use dot and crosses to represent the direction of a mangetic field or current?

A

dot means out of the page

cross means into the page

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12
Q

When a magnet is moved in and our of a coil, a current is induced in the coil. What is this phenomenon called?

A

Electromagnetic induction

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13
Q

Complete the explanation of why the comutator allows the motor to rotate

  1. There is a current carrying coil
  2. 90º to a magnetic field
  3. Using LHR- left side of coil experiences an upward force.
  4. Using LHR- current is travelling the opposite direction on the right side of coil- it experiences a downward force
  5. Coil rotates clockwise
A
  1. When coil is at 90º, the current in the coil reverses due to the commutator and brushes.
  2. Current continues to travel IN on the left and OUT on the right

8 Using LHR- left side of coil still experiences an upward force and the right hand side still experiences a downward force

  1. Coil continues to rotate clockwise
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14
Q

When the magnet is moved into the coil, a current is induced and the needle on the ammeter moves to the left.

Describe what happens when the magnet is pulled out of the coil.

A

The needle with move to the right

Reversing the direction of the magnet, reverses the induced current

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15
Q

What is a step-down transformer?

A

no. of primary turns > no. of seconday turns

primary voltage > secondary voltage

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16
Q

How would you demonstrate the direction of the magnetic field around a single wire?

A

Place compasses around the wire

The compass will point in the direction of the magnetic field

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17
Q

Why is voltage stepped up using transformers before transmitting long distances?

A

If current is reduced then the power loss along the transmission lines is reduced.

Less heat loss along the wire

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18
Q

Describe the force on the particle below

A

No force - as particle is moving parallel to the field (it must move 90º to the field)

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19
Q

What can be done to increase the magnetic field around a solenoid?

A

1- more turns

2- increase the current

3- add an iron core

20
Q

When a wire is moved downward through a magnetic field, the wire cuts the magnetic field lines at 90º and induces a current in the wire.

What would happen if the wire is moved more quickly?

A

A larger current would be induced

21
Q

Draw the shape of a magnetic field around a coil.

A
22
Q

How do you determine the direction of a magnetic field around a wire?

A

Use the right hand grip rule

Thumb- direction of current

Fingers- wrap in direction of magnetic field (clock wise or anti-clockwise)

23
Q

Explain why a current is induced in a wire when it it is moved downward through a magnetic field

A
  1. Wire is moved downward (thuMb - MOVEMENT)
  2. The wire cuts the magnetic field lines at 90º - First finger- FIELD)
  3. This induces a current in the wire- (seCond finger- CURRENT)
24
Q

Electromagets are used in electric bells.

  • When the button is pushed, a current flows in the electromagnet -This creates a magnetic field around the electromagnet
  • The electromagnet attracts the soft iron armature which moves to the right
  • This causes the hammer to hit the bell.

What happens next?

A

The moment the iron armature moves to the right this creates a _break in the circuit- t_he circuit is no longer complete.

The current drops to zero and the magnetic field around the electromagnet disappears.

The iron armature is no longer attracted and springs back to the left

The armature completes the circuit again and the whole cycle repeats alowing the bell to repeatedly sound.

25
Q

Why is iron a better core to use in an electromagnet than steel?

A

Iron is magnetically soft- the domains in iron line up easily - it can magnetise and demagentise easily

Steel is magnetically hard- the domains do not line up easily- it cannot magnetise easily and it can be used to make permananet magnets becaouse it also does not easily demagnetise

26
Q

When a wire is moved downward through a magnetic field, the wire cuts the magnetic field lines at 90º and induces a current in the wire.

What would happen if the strength of the magnets were increased?

A

A large current would be induced

27
Q

How is the primary voltage, seconday voltage, no. of primary turns, no. of secondary turns related?

A
28
Q

Explain why a current is induced in the coil when the magnet is pushed into the coil

A
  1. The magnet is moving (thuMb- MOVEMENT)
  2. The magnetic field lines of the magnet are cutting the wires of the coil at 90º - (First finger- FIELD)
  3. This induces a current in the coil - (seCond finger- CURRENT)
29
Q

What happens to the magnetic field around a wire if more wires are added side by side- How can you tell from the image that the magnetic field is stronger?

A

More wire- stronger field

The closer the magnetic field lines the stronger the field.

iron filing will line up more easily and more closely together in a stronger magnetic field.

30
Q

Explain why a current is induced in the wire.

A
  1. Wire is moved downward (thuMb- MOVEMENT)
  2. 90º to a magnetic field (First finger- FIELD)
  3. Due to LHR a current is induced in the wire (seCond finger- CURRENT)
31
Q

The current flows into the left brush making contact with the left of the split ring commutator, this means the current always travels into the page on the left side of the coil. This is 90º to the field and due to LHR the force on the left side of the coil is upward.

Why is the force on the right side of the coil downward?

A

The direction of the current is reversed or out of the page on the right side of the coil.

Therefore the direction of the force is in the opposite direction OR downward

The coil rotates clockwise

32
Q

What happens to the direction of the magnetic field around a magnet if the direction of the current it reversed?

A

The direction of the magnetic field is reversed.

33
Q

When a wire is moved downward through a magnetic field, the wire cuts the magnetic field lines at 90º and induces a current in the wire.

What would happen if the number of turns in the wire was increased?

A

A large current would be induced

34
Q

What happens to the magnetic field around a solenoid if you add an iron core (magnetically soft)

A

Stronger magnetic field

35
Q

When a wire is moved down through a magnetic field, the wires cut the magnetic field lines at 90º and this induces a current in the wire which moves the ammeter needle to the left.

What happens when the wire is pulled up and out of the magnetic field?

A

The needle will move to the right

Reversing the direction of the movement of the wire will reverse the direction of the induced current.

36
Q

How does an a.c current in the primary coil induce a current/voltage in the secondary coil of a transformer?

A
  1. The alternating current in the primary current creates a changing magnetic field around the primary coil
  2. This continuously cuts through the secondary coil
  3. inducing an alternating current in the secondary coil
37
Q

If a vertical wire has the current upward in the wire, work out the direction of the magnetic field using your Right Hand Grip Rule

A

Thumb- up and in direction of current

Finger wrap in direction of magnetic field.

38
Q

Describe the parts of a transformer

A
  1. laminated iron core
  2. primary coil N1
  3. secondary coil N2
  4. primary alternating voltage V1
  5. secondary alternaing voltage V2
39
Q

Describe the shape of the magnetic field around a wire

A

Circular field shape

field lines getting futher apart- field getting weaker as you move away from the wire

40
Q

Describe why the iron core must be laminated in a transformer.

A

A laminated iron core reduces the eddy currents in the iron core and makes the transformer more efficient - less heat loss in the core.

41
Q

How would you construct a simple electromagnet?

A

1) wrap a wire around an iron nail
2) connect a cell across the wire

42
Q

What is a step-up transformer?

A

no. of secondary turns > no. of primary turns

secondary voltage > primary voltage

43
Q

The current flows into the left brush making contact with the left of the split ring commutator, this means the current always travels into the page on the left side of the coil. This is 90º to the field and due to LHR the force on the left side of the coil is upward.

The coil rotates clockwise.

What would happen if the magnetic poles were reversed

A

The direction of the field reverses, and therefore the direction of the force reverses- the coil rotates anticlockwise

44
Q
A
  1. No force at all

Note: Particle is moving parallel to the field- no force.

45
Q

Transformers are nearly 100% efficient. What does this mean?

A

Power in = power out

IpVp=IsPs

46
Q

What sort of transformer must be used after electricity has been transmitted across the country and before it is used in homes and factories?

A

step-down

The voltages transmitted through the power lines are around 400 000V! this must be stepped down to a safe level (240V) for homes.

Physics (28 decks)

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