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BMAT > Magnetism > Flashcards

Magnetism Flashcards

1
Q

What is a magnet?

A

A magnet is an object that produces a magnetic field

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2
Q

Types of magnet

A

Permanent magnet

Temporary magnet

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3
Q

How many poles does a permanent magnet have?

A

2 poles. North and South

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4
Q

Like poles

A

Repel each other

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5
Q

Unlike poles

A

Attract each other

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6
Q

What happens when two magnets are pulled closer to each other?

A

The strength of the attraction or repulsion increases.

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7
Q

How do you test for a permanent magnet

A

It must be able to repel another permanent magnet (attraction only proves its a magnetic material)

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8
Q

What is known as the north seeking pole?

A

The North Pole on a bar magnet

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9
Q

What is known as the south seeking pole?

A

The South Pole of the bar magnet

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10
Q

What is a magnetic field?

A

This is the area around a magnet upon which magnetic forces act on other magnets or magnetic materials.

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11
Q

What are magnetic field lines?

A

These are lines used to represent magnetic field.

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12
Q

State the characteristics of magnetic field lines

A
  • must start on North Pole and end on South Pole
  • form closed loops
  • cannot start and end in space
  • cannot cross one another
  • point in the direction of the force thay would be exerted on a free North pole
  • are close together where the field is stronger
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13
Q

How do you trace magnetic field lines

A

Using a magnetic compass. The needle will point in the direction of the field at each point that it is placed on

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14
Q

What are magnetic materials?

A

These are materials that become affected when placed on a magnetic field

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15
Q

What are soft magnetic materials?

A

These are materials that are easily magnetised but also are easily demagnetised e.g iron

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16
Q

What are hard magnetic materials?

A

These are materials that are difficult to magnetise but once magnetised, are difficult to demagnetise e.g steel

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17
Q

What are electromagnets?

A

Consists of a soft iron core made into a magnet by the passing an electric current through a coil surrounding it.

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18
Q

Why is iron commonly used?

A

This is because it is a soft magnetic material

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19
Q

What happens when current is passed through the coils?

A

When current passes through the coil, the core magnetises creating a combined magnetic effect from the current and the strongly magnetised core.

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20
Q

What happens when the current is removed ?

A

The coil quickly demagnetises causing the electromagnet to get turned off.

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21
Q

Why should you not use hard mgnetic materials for electromagnets?

A

The electromagnet would take longer to turn on and off.

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22
Q

What happens when you place a pole of bar magnet near an unmagnetised material and why?

A

There is always attraction.
The bar magnet induces an opposite pole in the magnetic material.

i.e. holding a north pole near it will induce a south pole in it.

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23
Q

How is the induced magnetism used?

A

Induced magnetism is used to make permanent magnets by placing a hard magnetic field in a strong magnetic field such as a solenoid.

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24
Q

What happens when current flows in a wire?

A

It creates a magnetic field in the surrounding area.

*The magnetic field is due to the the beam of charged particles such as electrons or ions, rather than the wire itself. If they moved through a vacuum a magnetic field will also be created.

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25
Q

What happens to the magnetic field if the current is reversed?

A

Reverses the magnetic field.

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26
Q

What happens to the magnetic field if the current is increased?

A

Magnetic field strength increases.

27
Q

Describe (and draw) the magnetic field patterns aroun current-carrying wires.

A

Consists of concentric circles
Circles get further apart (field strength decreases) with increasing distance from the wire.

(This can be modelled using the right hand grip rule)

28
Q

What happens when a straight current carrying wire is wound into a tight coil?

A

The magnetic field created by each part of the coil combines to form a strong field running through the cores centre.

29
Q

Draw the pattern formed from the coiling a current-carrying wire.

A

Look at notes.

30
Q

What is a solenoid?

A

A long coil which consists of many narrow coils wound together.

31
Q

What would the resultant magnetic field produced by a solenoid look like?

Find a picture of this for better visualisation.

A

The resultant magnetic field would be very uniform through the centre of the solenoid.
Comparable to a bar magnet as it has poles at each end of the solenoid.
Weak field at the sides of the soleonids and opposite in direction to the field inside the solenoid.
Field strength and direction is controlled by current.

32
Q

How can you identify the poles of solenoid?

A

By looking at the direction the coils are wound in. If the end of the coils:

  • turn clockwise = north pole.
  • turn anticlockwise = south pole.
33
Q

So what is the strength of a magnetic field around a wire dependant on?

A

1) The current flowing through the wire
2) The distance from the current-carrying wire
3) The surroundings of the wire - i.e. magnetic material, like a soft iron core, surrounding the wire can increase field strength.
4) - Increasing the number of turns per unit length if a solenoid is used.

34
Q

What are ferromagnetic materials?

A

Ferromagnetic materials are those substances which exhibit strong magnetism in the same direction of the field when a magnetic field is applied to it.
Iron is an example of this because each iron atom creates a magnetic dipole (so has a north and south pole). So when iron is subjected to an external magnetic field, the atomic dipoles line up with this field to produce a stronger magnetic field.

35
Q

Due to the ferromagnetic propoerties of iron, it has applications in:

A

Electromagnetic devices.

36
Q

What do electromagnets make use of and why?

A

Solenoids are used to create a strong magnetic field.

37
Q

Key similarity between electromagnets and permanent magnets?

A

Field shape

38
Q

Key differences between electromagnets (EMs) and permanent magnets (PMs)?

A
  • EMs can be turned on and off whereas PMs are always on.
  • EMs’ field strength can be varied whereas PMs’ field strength is largely constant.
  • The polarity of an EM can be reversed by reversing the current whereas a PM has a constant dipole.
39
Q

How is a permanent magnets made?

A

By placing a HARD magnetic material (iron alloy or neodymium) in a strong external magnetic field. These magnets retain their strength for a long time.

40
Q

How is the magnetism of permanent magnets weakened?

A

By impact

By heating above it above its curie temperature.

41
Q

Electromagnets can have different field strengths by changing the ​current​ or the number of coils. State a problem with increasing the current and how to overcome it.

A

Current can only be increased so much before the wire becomes too hot. This problem can be eliminated by usin superconducting coils, which have zero resistance and require low temperatures to function.

42
Q

What is the motor effect?

A

When a current carrying wire is placed in a magnetic field it experiences a force. This principle is called the motor effect.

1) The force is perpendicular to direction of the current and the direction of the magnetic field.
2) if the current carrying wire is parallel to the magnetic field,no force is experiences by the wire.
3) If the current carrying wire is perpendicular to the magnetic field, then the force experienced is at a maximum,
4) The force occurs as the magnetic field created by the current carrying wire interacts with the magnetic field of the permanent magnet creating a force both on the wire and the magnet.

*Once again it is the property of moving charge that creates a magnetic field than the actual wire.

43
Q

The direction of the force experienced can be predicted using Fleming’s Left Hand Rule. What does the middle finger, index figure and thumb indicate?

A 
Middle = Direction of conventional current flow in wire
Index = direction of magnetic field from north pole to the south pole.
Thumb = direction of the motor force.
44
Q

How does reversing the current only affect the motor force?

A

Reversing current = reverses the direction of the motor force

45
Q

How does reversing the magnetic field only affect the motor force?

A

Reversing magnetic field = reverses the direction of the motor force

46
Q

Reversing both the current and magnetic field?

A

No change in the direction of the motor fore

47
Q

If the current and the magnetic field are not perpendicular, you then use the component of the magnetic field that is perpendicular to the current.

A

What does this mean? How do you use this in the equation?

48
Q

What does the strength of the motor force depend on?

A

Current:​ increasing the current increases the force.
Magnetic field​: increasing the field strength increases the force
Length​ of wire in the field: increasing the length of wire in the field increases the force.
Angle ​between current and magnetic field: greatest at 90 degrees and zero when parallel.

49
Q

The factors (which the motor force depend on) can be combined to create the following equation to calc the magnitude of the motor force. What is the equation?

A

F = BIL

F = the motor effect force in Newtons (N)
B = the magnetic field strength in Tesla (T) - 1T = 1N/m/A
I = the current in Amperes (A)
L = the length of wire perpendicular to the field in Metres (M)
50
Q

How does a DC motor work?

A
  • A current carrying rectangular coil is placed in a ​permanent​ magnetic field.
  • The motor effect forces on either side of the coil can produce a turning effect
  • The maximum force occurs when the coil is in the plane of the field and the current is perpendicular​ to the field.
  • The force is zero when the coil is perpendicular to the field and the current ​parallel ​to the field
  • In order for the coil to continue rotating in the same direction, the current must be reversed every time the coil passes the vertical.
  • This is done by using a ​split ring commutator ​which rotates with the coil and connects to the DC power supply by two conducting brushes and acts as a rotating switch.
51
Q

Example of DC power supply?

A

Battery

52
Q

What is the conducting brush made of and why? Why do they need to be replaced often?

A

Conducting brushes are usually made of graphite or copper or brass. This is because they have low friction and have good electrical conductivity.
Conducting brushes need to be replaced often as they can wear out.

53
Q

The turning effect of the coil is determined by:

A
  1. Current​ in the coil: greater current = greater turning effect.
  2. Size​ of the coil: the larger the area of the coil = greater turning effect.
  3. Number of turns​ on the coil: the larger the number of turns = greater turning effect.
    Angle ​of coil in field: maximum turning effect when the coil is in the plane of the field and zero when perpendicular to the field.
  4. Winding the coil onto a ​soft iron core​: this increases the magnetic field strength and increases the turning effect.
  5. Magnetic field strength​: a stronger magnetic field creates a greater turning effect.
54
Q

NEW SUB TOPIC: Electromagnetic Induction

A constant pd can be induced in a conductor by:

A
  • Constant movement of a conductor in stationary magnetic field or constant movement of magnetic field across a stationary conductor. This cuts the magnetic field lines.
  • Using an electromagnet connected to an ​alternating current​ (AC) supply.
  • There is only an induced voltage when a change occurs (cutting across field lines or changing the field)
  • For point 1, if either the conductor or the magnetic field, stops moving then pd and current stop. For 2nd point, if AC supply is on, then constant pd and current.
55
Q

Electromagnetic induction (constant movement of conductor in stationary magnetic field or constant movement of magnetic field across a stationary conductor) always results in ______ ______, but it only induces a ________ if there is a _______ _______.

A
  • Induced voltage
  • Current
  • Closed circuit
56
Q

When will we not get an induced potential difference or current even if the current carrying wire is moved in the magnetic field?

A

If the wire is moved along the magnetic field, a potential difference nor a current will be induced. Has to cross the magnetic field.

57
Q

The magnitude of the induced voltage is directly proportional to:

A
  • The rate at which a wire cuts magnetic field lines
  • The rate at which the magnetic field through a conductor (e.g. a coil) changes.
  • The stronger the magnet used equates to more field lines being cut per unit of time so using a stronger magnet​ also​ results in a greater voltage being induced. (shaping into a coil)
58
Q

If an electromagnet (magnetic field formed by an electric current) is used to generate a magnetic field, the induced voltage can be increased by:

A
  • Increasing the ​frequency​ of the AC supply.
  • Increasing the ​amplitude​ of the AC supply.
  • Increasing the ​number of coils​ on the electromagnet. Produces a stronger magnet
59
Q

An induced voltage always opposes?

A

The motion that causes it. This is to obey the lawa of conservation of energy,

60
Q

Why does an induced voltage always oppose the motion that causes it?

A

This is because when a closed loop of wire cuts through magnetic field lines, once a voltage is induced, a current is induced. This induced current creates its own magnetic field that opposes to the original magnetic field, and thus the movement of the magnet.
For example, when the north pole of a magnetic field is moved into a coild of a closed circuit. That point of the coil also becomes the north pole. This repels the magnet opposing its motion to enter. When we try to pull the north pole out, the end of the coil becomes a south pole. This attracts the magnet, opposing its motion to leave.

61
Q

The induced voltage direction can be reversed by:

A
  • Reversing the ​direction ​of cutting the magnetic field lines.
  • Decreasing ​magnetic field strength instead of increasing it - HOW THO?
62
Q

How does an AC generator work?

A

A simple AC generator consists of a coil rotating in a magnetic field causing the coil to continuously​ cut the magnetic field and induce a voltage. The voltage being induced ​alternates​ as the coil rotates and cuts the magnetic field in theopposite direction. Therefore this generator produces an AC output.

63
Q

The ​amplitude ​of the output AC can be increased by:

A
  • Rotating the coil more ​rapidly​ - this also determines the output ​frequency.
  • Increasing the ​magnetic field strength.
  • Increasing the ​area​ of the coil in the magnetic field.
  • Increasing the ​number of turns​ of coil in the magnetic field.
    All of these factors increase the number of ​field lines​ being cut per unit time.
64
Q

What is electromagnetic induction used for?

A

They are used in generators to transfer mechanical energy (rotation of coil) into electrical energy.

*The source of mechanical energy can also be from chemical energy: burnign of fossil fuels to ehat water, to produce steam that turns turbines.
Can also be from kinetic energy: water/wind flows through turbines turning them.

Transformers that use it to can convert AC at one voltage to AC of a different voltage.

BMAT (3 decks)

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