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Magnetic Fields Flashcards

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1
Q

What does a current carrying wire produce?

A

It’s own magnetic field

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2
Q

How does a current carrying wire experience a force?

A

The write interacts with an external field

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3
Q

A current carrying wire will experience maximum magnetic force if…

A

If the current through is perpendicular to the direction of magnetic field lines

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4
Q

What is the equation for Force

A

F = BILSinΘ

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5
Q

What is the equation for the Magnetic Force on a moving charge?

A

F = BQv

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6
Q

What is a cyclotron

A

A type of particle accelerator that accelerated charge particles from their centre along a spiral path

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7
Q

What is the process of accelerating particles in Cyclotron?

A
  1. The source of charged particle is placed at the centre of the cyclotron and they are fired into one of the electrodes
  2. The magnetic field in the electrode makes them follow a semi circular path - since its perpendicular to their motion, until it eventually leave the electrode
  3. The pd applied between the electrodes accelerate the particle across the gap to the next electrode - since there is an electric field in the gap.
  4. Since the speed of the particle is now higher they will follow a circular path with a larger radius(r ∝ v) before leaving the electrode again.
  5. The pd is then reversed so the particle accelerates towards the opposite electrode.
  6. This process is repeated as the particle spirals outwards and eventually have a speed large enough to exit cyclotron
  7. The alternating pd is needed to accelerate the particles across the gap between the opposite electrodes - otherwise the particles will only accelerate in one direction.
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8
Q

Magnetic Flux Density (Magnetic field strength) Definition

A

The Force per unit current per unit length on a current-carrying conductor placed perpendicular to the magnetic field

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9
Q

What is current?

A

The rate of flow of positive charge

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10
Q

Why does a particle in uniform magnetic field which is perpendicular to its direction of motion travel in circular path?

A

The magnetic force is always perpendicular to its velocity, therefore the charge travels in circular motion.

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11
Q

How to find the radius of the orbit of a charged particle in a magnetic field?

A

F = mv^2 / r = the centripetal force.

Equate this to the magnetic force on a moving charged particle:
mv^2 / r = Bqv

rearrange for r:
R = mv/Bq

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12
Q

What is electromagnetic induction?

A

When emf is induced in a closed circuit conductor due to changes in magnetic flux.

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13
Q

How does Electromagnetic induction happen?

A

When a conductor cuts through magnetic field lines.
This causes a change in magnetic flux.
Which causes work to be done.
This work is transformed into electrical energy.
In a complete circuit, a current will be induced.

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14
Q

What is the equation for magnetic flux

A

ɸ = BA

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15
Q

What is the equation for magnetic flux linkage?

A

ɸN = BAN

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16
Q

What is the Magnetic Flux equation when the magnetic field in not perpendicular to area?

A

ɸ = BA cosӨ

Ө = the angle between the magnetic field lines and the line perpendicular to the plane of the area.

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17
Q

When is magnetic flux maximum?

A

CosӨ = 0 degrees - the magnetic lines are perpendicular to the plane of the area

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18
Q

When is magnetic field minimum?

A

CosӨ = 90 degrees - the magnetic field lines are parallel to the place of area

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19
Q

When is emf induced?

A

When there is a change in magnetic flux density
When the is a changing cross-sectional area.
When there is a change in angle.

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20
Q

Definition of Faraday’s Law:

A

The magnitude of the induced emf is directly proportional to the rate of change in magnetic flux linkage.

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21
Q

The definition of Len’s Law:

A

The induced emf acts in such a direction to produce effects that oppose the change causing it.

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22
Q

The equation for Faraday’s Law:

A

ε=N (change in ɸ / change in t)

23
Q

The equation for Len’s Law:

A

ε = -N (change in ɸ / change in t)

24
Q

What is the equation for the emf in the conductor as it moves through the magnetic field?

A

Distance travelled by conductor:
s = vt

Area of magnetic field it cuts:
A = Lvt

Total flux the conductor cuts through:
ΔΦ=BA =BLvΔt

Faraday’s Law:
ε=N (change in ɸ / change in t)

Substitute the change in magnetic flux equation into emf
ε = BLvΔt/ Δt

Change in t cancels:
ε = BLv

25
Q

What is the Flux Linkage equations that includes angular speed?

A

NΦ=BAN cos(ωt)

26
Q

What is the the emf equations that includes angular speed?

A

ε =BANωsin(ωt)

27
Q

What does this equations show:
ε =BANωsin(ωt)

A

The equation shows that the emf varies sinusoidally and it is 90°out of phase with the flux
linkage

28
Q

What is an alternating current?

A

A current which periodically varies between a positive and negative value with time

29
Q

What is definition for peak voltage/current?

Peak volatage or current can be determined from the amplitude of current-time OR voltage-time graph

A

The maximum value of the alternating voltage/current.

30
Q

What is the equation for peak voltage?

A

peak voltage V =peak-to-peak voltage ÷ 2

31
Q

What is the Root Means Square(rms) of AC definition?

A

The square root of the mean of the squares of all the values of the current in one cycle
OR
The equivalent direct current that produces the same power

32
Q

What is the equation for rms of Current?

A

Irms= I peak / √2

33
Q

What is the definition of the rms of voltage?

A

The square root of the mean of the squares of all the values of the voltage in one cycle
OR
The equivalent dc voltage that produces the same power

34
Q

The equation for the rms of Voltage?

A

Vrms = V peak / √2

35
Q

What is the equation for the average power?

A

Average power = I rms * V rms

36
Q

What is an AC voltage on an oscilloscope represented as?

A

Transverse Wave

37
Q

What is a dc Voltage on an oscilloscope represented as?

A

Horizontal Line

38
Q

What is a transformer?

A

A device that changes high alternating voltage at low current to low alternating voltage and high current, and vice versa.

39
Q

Why is a soft iron core used?

A

Focuses and directs the magnetic field from the primary to the secondary coil.

Easily magnetised and demagnetised.

40
Q

IF in the primary coil and A.C produces an alternating voltage, what happens?

A

Creates an alternating magnetic field inside the iron core and therefore a changing magnetic flux linkage.

The changing magnetic field is passed to the secondary coil, resulting in a changing magnetic flux linkage in the secondary coil.

From Faraday’s Law, an emf is induced.

Emf produces and alternating output voltage from the secondary coil - at the same frequency as the input voltage.

41
Q

Transformer Equation that links number of turns and voltage:

A

Ns / Np = Vs / Vp

42
Q

What are the two types of transformers?

A

Step-up = Ns>Np - Increases the voltage

Step down = Np>Ns - decreases the voltage

43
Q

When is a step up transformer used?

A

Between the power stations and transmission wires

44
Q

When is a step down transformer used?

A

Between the transmission wires and buildings

45
Q

Power in = Power out

A

Ip * Vp = Is * Vs

46
Q

Transformer Efficiency equation:

Is the ration of the power out to the power in.

A

(Is * Vs) / (Ip * Vp)

47
Q

What are Eddy Currents?

A

Key sources of energy loss in a transformer - make the transformer unlikely to be 100% efficient.

48
Q

How do Eddy Current arise:

A

A changing magnetic field from the alternating current / voltage, creates a changing magnetic field in the core THAT ACTS AGAINST THE FIELD THAT INDUCED THEM.

And emf is therefore induced.

A current flows, as the core is made of a conducing material.

The current also dissipates energy by generating heat

49
Q

How to reduced Eddy Currents?

A

Laminating the iron core with layers of insulation

Having a core made from a high resistivity metal

50
Q

How is the transformer inefficient?

A

The coil has resistance - this causes heat energy to be lost from the current flowing through the coil - LARGER the current, GREATER the amount of heat energy lost.

The core may have induced Eddy Currents, the reversal of magnetism, poor insulation between the primary and secondary coil.

51
Q

What are the ways to reduce energy loss in a transformer?

A

Making the core from soft iron / iron alloys to allow easy magnetisation and demagnetisation (REDUCE HYSTERESIS LOSS)

Laminating the core

Using thick wires - especially in the secondary coil of a step down transformer

Using a core that allows the flux so that the primary coil to be linked to the secondary coil.

52
Q

Why are transformers used to step up the voltage in National Grid?

A

Since power transmitted is I*V - increased voltage reduces the current.

Smaller currents have smaller heating effect on wires.

This reduces energy loss to the surroundings of the wires.

Therefore, this improves the efficiency of the electrical energy and power transmitted through the wires.

53
Q

Equation that links Power, Current and Resistance:

A

P = (I^2) * R

P = (I^2) therefore doubiling the current produces four times the power loss

Physics (4 decks)

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