Topic 9 Flashcards Preview

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Flashcards in Topic 9 Deck (31)
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1
Q

What are the rules for drawing magnetic field lines?

A

1) Lines start at the North Pole and end at the South Pole (or form closed loops for point fields).
2) Lines cannot cross.
3) The closer together the lines, the stronger the field.

2
Q

Definition: Magnetic flux density

A

The strength of magnetic field at a point in space, which is represented by the closeness of field lines.

(T)

3
Q

Definition: Magnetic flux

A

The amount of magnetic field passing perpendicularly through a defined area.

(Wb)

4
Q

Equation: Magnetic flux

A

Φ = BAsinθ

Φ = magnetic flux (Wb)

B = magnetic flux density (T)

A = area (m²)

sinθ = angle between magnetic field and area

NOTE: if the field is perpendicular to the area,

θ = 90°

⇒ sinθ = 1

so Φ = BA

5
Q

Definition: Magnetic flux linkage

A

The total magnetic flux inside a coil of N turns.

6
Q

Equation: Magnetic flux linkage

A

MFL = NΦ

MFL = magnetic flux linkage (Wb turns)

N = number of turns

Φ = magnetic flux (Wb)

7
Q

Fleming’s left-hand rule

A

Used on positive charges to predict the direction of magnetic force acting upon it.

Thumb = direction of force

First finger = direction of field (N -> S)

Second finger = direction of motion of particle

8
Q

Fleming’s right-hand rule

A

Used on negative charges to predict the direction of magnetic force acting upon it.

Thumb = direction of force

First finger = direction of field (N -> S)

Second finger = direction of motion of particle (or current)

9
Q

Equation: Magnetic force (on charged particle) *

A

F = Bqvsinθ

F = magnetic force (N)

B = strength of magnetic field (T)

q = charge (C)

v = velocity (ms⁻¹)

θ = angle between direction of movement and field

10
Q

Equation: Magnetic force (on current-carrying conductor) *

A

F = BIlsinθ

F = magnetic force (N)

B = strength of magnetic field (T)

I = current (A)

l = length of current-carrying conductor inside the field (m)

θ = angle between flow of current and field

11
Q

What is the motion of a charged particle in a field?

A

1) The magnetic force is always perpendicular to the direction of motion, so it acts as a centripetal force on the charged particle.
2) Therefore, the charged particle accelerates centripetally, so moves in a circular path.

12
Q

What are the features of induced emf?

A
  • If there is no motion, there is no induced emf.
  • When the cool and magnet move in relation to each other, an emf is induced.
  • If the direction of motion changes, so does the sign of emf.
  • The faster the motion, the greater the induced emf.
  • The effect is the same whether the magnet or coil are moved.
13
Q

Definition: Electromagnetic induction

A

1) Relative motion between a coil and a magnet occurs (or) the current through a wire periodicaly changes direction due to an A.C. power supply (or) current is frequently switched on and off.
2) This causes the flux linkage to change.
3) Therefore an emf is induced onto the coil.
4) The size of induced emf is directly proportional to the rate of change of flux linkage.

14
Q

How does a spinning coil generator work?

A

1) As the coil rotates, the flux through the coil changes continuously, inducing an emf at the same frequency as the rotation.
2) Increasing the frequency of the rotation increases the amplitude and frequency of the induced alternating emf.

15
Q

How does a rotating magnet generator work?

A

1) The induced emf alternates with the same frequency as the rotating magnet.
2) Increasing the rate of rotation increases the frequency and amplitude of alternating emf.
3) Therefore, the alternating current is increased.

16
Q

How does a transformer work?

A

1) An alternating current is applied to the primary coil, causing the magnetic flux to continuously change.
2) This induces an emf in the secondary coil.

17
Q

Why do transformers not work with a D.C. power supply?

A

Because there is no change in magnetic flux linkage and so an emf isn’t induced.

18
Q

How does flux linkage affect emf?

A
  • The faster the rate of change of magnetic flux, the greater the induced emf. Therefore the larger the gradient, the greater the induced emf.
  • The positive peak of induced emf relates to the negative gradient of magnetic flux.
19
Q

Definition: Faraday’s Law

A

The magnitude of induced emf is equal to the rate of change of magnetic flux linkage.

20
Q

Definition: Lenz’s Law

A

The direction of an induced emf is such as to oppose the change creating it.

21
Q

Equation: Faraday’s and Lenz’s Law *

A

ε = -d(NΦ) / dt

ε = emf (V)

  • = emf opposes change

NΦ = magnetic flux linkage (Wb turns)

d / dt = rate of change

22
Q

What is the difference between D.C. and A.C.?

A

For D.C. supplies, current only flows in one direction, of equal magnitude at all times.

For A.C supplies, the direction of current flow changes periodically, with differing magnitudes.

23
Q

What is a root mean square value?

A

The D.C. equivalent of an A.C. Power supply.

24
Q

Equation: Root-mean-square values *

A

Vrms = Vo / √2

Irms = Io / √2

Vrms = pd of D.C. supply

Vo = peak pd of A.C. supply

25
Q

How do you work out the direction of magnetic field around a point charge?

A

Use the right hand fist rule with the thumb showing the direction of the flow of current.

26
Q

How does mass affect radius of path of particle in magnetic field?

A

The larger the mass, the larger the radius.

27
Q

How are fields indicated when drawn on a page?

A

X = into the page

O = out of the page

28
Q

How can you work out the radius of a charged particle in a magnetic field?

A

F = Bqv

F = mv2 / r

⇒ Bqv = mv2 / r

⇒ r = mv2 / Bqv

⇒ r = mv / Bq

29
Q

What is the relationship between centripetal and magnetic force?

A

Bqv = mv2 / r

30
Q

What is an assumption made when using F = Bqv?

A

That current is perpendicular to the magnetic field so that sinθ = 1

31
Q

What are the three ways in which emf can be enduced?

A
  • Alternating current.
  • Flick the current on and off.
  • Move the wire and coil relative to each other.

These all cause a change in the magnetic flux linkage.