A2 Level physics > Chapter 15 > Flashcards

Chapter 15 Flashcards

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1
Q

What type of current/ magnetic flux must there be in order to produce electricity?

A

-Alternating magnetic flux.

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2
Q

What is the convention of the direction of flux?

A

-Direction of flux taken so that that flux emerges at N pole and enters at S pole.

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3
Q

Attractive forces and flux:

A
  • Attractive forces between poles act to make paths shorter
  • These forces also make the paths straighter (also shorter)
  • Forces between poles are such as to tend to mae flux paths shorter and straighter. The flux behaves like an elastic string.
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4
Q

Faraday’s law of induction: description

A

-emf is proportional to the rate of change of flux linkage
E ∝ Nd Φ/ dt
-The e.m.f is large when the rate of change of flux is large. The N turns are in series so the e.m.fs each add up in turn.

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5
Q

Lenz’s law: description

A

-The induced e.m.f opposes the change of flux producing it

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6
Q

Lenz’s law: equation

A

E = -Nd Φ/ dt

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7
Q

How do you find the emf in a graph of flux against time?

A

emf is proportional to the slope of the graph

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8
Q

How do you find the flux in a graph of emf against time?

A

flux is proprtional to the area under the graph

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9
Q

How to draw flux lines around a coil of current carrying wire:

A
  • Evenly spaced lines
  • Perpendicular to coil
  • Right hand rule (direction of lines)
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10
Q

Transformer

A

A transformer changes the peak voltage of an alternating potential difference
A transformer has two electric circuits, with different numbers of turns, wound over a common
magnetic circuit, generally a closed iron core.

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11
Q

Explain why an alternating current in the primary coil induces an emf in the secondary coil.

A

-An alternating current in the primary generates a magnetic field, which is constantly alternating direction. This is directed by the iron core to the secondary coil, where an alternating magnetic flux linkage is produced.
=> E = Nd Φ/ dt

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12
Q

Difference between magnetic flux, magnetic flux density, magnetic flux linkage:

A

Magnetic flux = flow of e.m.f
Magnetic flux linkage = flow of e.m.f in N number of coils
Magnetic flux density = field strength

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13
Q

Ratio of turns and voltage

A

Vp/Vs = Np/Ns

The ratio of alternating voltages across the two coils is approximately equal to the ratio if their numbers of turns.

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14
Q

The transformer ratio rule relates…

A

relates the peak p.d.s across the primary and secondary coils Vp and Vs to the number of turns of the primary coil Np and of the secondary coil Ns. The rule is an idealisation, assuming that all the flux in the magnetic circuit passes through both coils, and that there are negligible drops in p.d. across the resistances of the two coils.

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15
Q

Efficiency of transformer

A

The efficiency of a typical transformer can be quite close to 100% so the current ratio Is / Ip is equal to Vp / Vs. If the potential difference is stepped up, the current is stepped down and vice versa.

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16
Q

Magnetic field

A

-Magnetic fields can be represented by field lines. Field lines go from north to south. The strength of a magnetic field is represented by how tightly packed the lines are; closer together = stronger the field.

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17
Q

Magnetic field around a wire carrying electric current

A
  • There is a magnetic field around a wire carrying electric current.
  • The direction of the magnetic field around a current-carrying wire can be worked out using the right hand rule:
  • Stick your thumb up and curl your fingers round
  • The thumb represents the current and the curled fingers represent the magnetic field.
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18
Q

Current carrying wires experiencing a force:

A
  • A wire carrying a current in a magnetic field will experience a force
  • If a current-carrying wire is put in a magnetic field, the field around the wire and the field from the magnetics interact.
  • This causes a force on the wire
  • If the current is parallel to the flux lines then no force acts
  • The direction of the force is always perpendicular to both the current and mag field.
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19
Q

Fleming’s left hand rule

A
  • The direction of the force is given by Fleming’s left-hand rule
  • Thumb = Motion (force)
  • First finger = Magnetic field (field lines)
  • Second finger = Current
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20
Q

Equation for the force on a current carrying wire:

A

F= BIL
Where B = magnetic field strength
I = current
L = Length of wire

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21
Q

What is magnetic field strength measured in?

A

-The magnetic field strength, otherwise known as flux density, it measured in teslas, T (vector quantity)

1 Tesla = Wb/ m^2 (Webers per unit area)

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22
Q

Motor

A
  • If a current-carrying loop is placed in a magnetic field, the forces (left hand rule) cause the loop to rotate
  • A split-ring commutator can reverse each time the loop is vertical
  • Power from electrical supply power to mechanical motion
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23
Q

Electric circuit: Conductance

A

Conductance = I/ V
Conductance = σA/ L
Units of conductance = Siemens, S or AV^-1

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24
Q

Magnetic circuit: Permeance

A

Permeance = Φ/ NI
Permeance = μA/ L
Units of permeance = Wb A^-1

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25
Permeance
- Permeance is larger for large cross sections of iron and smaller for long lengths of iron - Increased permeance for a short and fat core
26
Flux density equation
B = Φ/ A | flux/ area
27
What does flux density mean
-B, Indicates the strength of the magnetic field
28
How do you increase flux?
- Increase the area of the core - greater permeability - Shorter and fatter core
29
What should you avoid in magnetic circuits?
-Avoid air gaps because it reduces the permeance of the whole magnetic circuit
30
How do you find the flux density if you are given the permeability of the circuit?
``` -B = μnI (n= turns per metre) ```
31
Electromagnetic induction
Electromagnetic induction is the generation of an emf due to changing magnetic flux in a circuit. The magnetic flux φ through a surface of area A which is perpendicular to the lines of a uniform magnetic field is B A, where B is the magnetic flux density.
32
Why does Lenz's law have a negative sign?
The induced emf acts in a direction so as to oppose the change. This is Lenz's law and is a consequence of the conservation of energy.
33
Ratio equation involving N turns and current:
Is/ Ip = Np/ Ns = Vp/ Vs
34
Problem with transformers
-A big engineering problem with transformers is because iron conducts electric currents very well, there are conducting paths within the iron itself. These are called eddy currents; as a result it is wasteful current flow and just makes the iron hot.
35
How is this problem with transformers overcome?
-One solution is to cut the paths with thin sheets of laminations of iron, insulated from the other with a varnish. Laminated iron core = loss of energy by eddy currents is much reduced.
36
Generators
-Generators work similarly to transformers, accept a section is cut out and able to rotate. This electromagnet is rotated mechanically (e.g from water in a dam) which then spins the rotor and flux alternates in the stator core inducing an alternating emf in the stator winding. -A generator produces an emf as a result of relative motion between a magnetic flux and an electrical conductor.
37
Example of how electricity is generated in a dynamo
A very simple model dynamo in which a rectangular coil spins at a constant rate in a uniform magnetic field. An alternating emf is generated by the spinning coil because the magnetic flux through the coil changes sinusoidally. A graphite brush presses on each slip ring to maintain continuous contact so that the coil is part of a complete circuit when a load is connected to the brushes.
38
Cylindrical rotor
- The rotor and stator are slotted with coil windings in the slots. - As the rotor spins, the magnetic flux pattern turns with it. Thus flux through the stator coils is continually rising and falling. This induces an alternating emf in the stator coils. - The rotor coils are arranged so that flux density varies sinusoidally around the surface of the rotor.
39
Three-phase generators
- You can get a larger output by using a three-phase generator - It has three sets of coils arranged at 120 degree angles around rotor. So instead of have just one sinusoidal output that at points will have no emf, this provides a constant power output. - Transmits the same power along four wires instead of six.
40
A rotating field motor
- These are rotors used in small/ low power appliances; two or more alternating fields at different angles with different phases can make a rotating field. - There are three phase pairs of coils and a rotating magnet. - The poles of a permanent magnet rotor are pulled round by the poles of the rotating field. The rotor turns are the frequency of rotation of the field
41
The squirrel cage motor
- A rotating flux in the motor - Force between opposite poles drags rotor round - The squirrel cage rotor turns more slowly than the magnetic flux. Because of the difference in speed large currents are induced in the good conductors of the rotor. These currents produce poles on the surface of the rotor. The poles on the stator attract the poles on the rotor and drag the rotor.
42
How does the force act on a straight length of current-carrying conductor
-At right angles both to the conductors and to the field producing the force.
43
Moving wires cut flux so...
moving flux cuts wires
44
Rotating coils: Plane of coil parallel to flux
- Zero flux linking coil - Maximum rate of cutting flux - Maximum rate of change of flux linking coil
45
Cutting flux =
Change of flux linked
46
Lines cut =
Change in lines linking
47
Rotating coils: Plane of coil perpendicular to flux
- Maximum flux linking coil - Zero rate of cutting flux - Zero rate of change of flux linking coil
48
Generator
- Power from mechanical motion power to electrical supply - Rotation forces supplied externally - Turned clockwise against forces, F - E.m.f generated - Current driven against emf - Mechanical torque supplied
49
Equation for e.m.f if the speed of the conductor is v:
E = vLB
50
'back e.m.f
-The voltage is induced as the power must push the current against the voltage.
51
The universal a.c/d.c commutator motor
- A commutator is a changeover switch. it reserves the current in the rotor every half turn, so as to keep the rotor turning in the same direction - The universal a.c/ d.c motor provides large torque at low speed. It can run at high speed.
52
Magnetic flux is produced by current turns: equation
Φ = NIΛ | where Λ = permeance
53
Motors
- Induction motors work by rotating flux inducing currents in, and thus producing forces on, a conducting motor - A d.c motor works by providing direct current to the rotor, reversing the direction of the current twice per rotation using a commutator - A motor is also a generator: a 'back emf' opposing the supply is generated as the motor turns.
54
Magnetic flux
-Measures the strength of the magnetic field per unit area Φ= BA -When you move a coil it causes NΦ = flux linkage NΦ = NBA
55
Conducting rod moving through a magnetic field:
- Charges accumulate on a conductor moving through a magnetic field - If a conducting rod moves through a magnetic field it will experience a force, so charges accumulate at one end of the rod - This induces an e.m.f across the ends of the rod exactly as a battery would
56
Changes in flux induce a electromotive force
- An electromotive force is induced whenever there is relative motion between a conductor and a magnet - The conductor can move and the magnetic field stay still or the other way round - An emf is produced whenever lines of force (flux) are cut - Flux cutting always induces emf but will only induce a current if the circuit is complete.
57
Changing flux at an angle
At the instant when the coil plane has turned through angle θ from the position of maximum flux linkage, the flux linkage: Nφ = BAN cos θ where A is the area of the coil. This may be written as Nφ = BAN cosωt where ω is the angular speed of the coil and t is the time taken to reach this position after passing through the position of maximum flux linkage.
58
Other equation for emf using ω
ε = BAN ω
59
Alternator
An alternator is a generator which produces an alternating emf by rotating a magnetised rotor inside coils wound on a stationary stator. The coils are at rest, and the magnetic flux rotates.
60
Torque in generators
When a generator is operating the induced current in the coils has a magnetic flux associated with it, causing a motor effect. The motor effect will oppose the spinning of the generator, by Lenz's law. A noticeable increase in torque is needed to keep a generator spinning at constant frequency when a current is drawn from it.
61
Electric motor
A motor consists of an electric circuit and a magnetic circuit. A simple moving coil motor has a rectangular coil on a spindle between opposite magnetic poles. Two fixed brushes provide continuous electrical contact with the coil via a split-ring commutator. When a direct current is passed through the coil, forces are exerted on the coil due to the interaction between the current-carrying wires and the magnetic field. These forces cause the coil to turn about the spindle
62
How the split ring commutator works
The split-ring commutator reverses the direction of the current round the coil each time the coil rotates through 180°, so that the forces on the coil continue to make the coil turn in the same direction.
63
Induction motors
Most motors used in industrial applications are induction motors. In an induction motor, the currents in the rotor are not fed directly from the supply but are induced by an alternating magnetic flux through the rotor. The rotor is in the form of a 'squirrel cage' of copper conductors embedded in an iron cylinder. The flux through the rotor is made to rotate by creating it from coils around the rotor which carry currents with a phase difference between them. In large industrial motors the phase differences derive from the three phases of the national grid supply.
64
SI units of magnetic flux
The SI unit of magnetic flux is the weber (Wb). The SI unit of magnetic flux density is the tesla (T), equal to 1 weber per square metre (Wb m^-2).
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