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OCR A-Level Physics A > Chapter 23: Magnetic Fields > Flashcards

Chapter 23: Magnetic Fields Flashcards

1
Q

What is a magnetic field?

A

A region of space in which moving charged particles are subject to a magnetic force

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2
Q

What generates a circular magnetic field around a current-carrying wire?

A

The flowing electrons

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3
Q

What is a permanent magnet?

A

An object made from a magnetized material that creates its own persistent magnetic field

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4
Q

What demonstrates the magnetic field produced by a permanent magnet?

A

Iron filings

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5
Q

How do iron filings react in a magnetic field?

A

Their atoms align with the field, creating an induced magnetic field

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6
Q

What is the difference between magnetically hard and soft materials?

A

Soft materials can be magnetized but do not stay magnetized; hard materials do stay magnetized

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7
Q

What is the Earth’s magnetic field primarily due to?

A

The movement of charged particles within its molten iron core

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8
Q

True or False: The magnetic north of the Earth is actually in the northern hemisphere.

A

False

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9
Q

What do magnetic field lines represent?

A

The direction and strength of the magnetic field

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10
Q

What is the right-hand grip rule used for?

A

To determine the direction of the magnetic field around a current-carrying wire

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11
Q

What is Fleming’s Left-hand Rule used for?

A

To determine the direction of force on a current-carrying wire in a magnetic field

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12
Q

Fill in the blank: The force on a current-carrying wire in a magnetic field is greatest when the current is ______ to the magnetic field.

A

perpendicular

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13
Q

What is the unit of magnetic flux density?

A

Tesla (T)

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14
Q

What does the equation F = BIL sin θ represent?

A

The force on a current-carrying wire in a magnetic field

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15
Q

How can the magnetic flux density be determined experimentally?

A

Using the equation B = F/IL when the current is at right angles to the magnetic field

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16
Q

What happens to a charged particle moving in a uniform magnetic field?

A

It experiences a force perpendicular to its velocity

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17
Q

What is the result of the force acting on a free particle in a magnetic field?

A

The particle follows a curved trajectory

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18
Q

What does the term ‘centripetal acceleration’ refer to?

A

Acceleration towards the center of a circular path

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19
Q

What is the relationship between magnetic flux and magnetic flux density?

A

Magnetic flux density is the amount of magnetic flux per unit area

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20
Q

True or False: The speed of a charged particle changes when moving in a magnetic field.

A

False

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21
Q

What is the effect of a magnetic field on iron filings?

A

They align with the magnetic field

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22
Q

What is a solenoid?

A

A coil of wire that acts as an electromagnet when an electric current passes through it

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23
Q

Fill in the blank: The magnetic field lines for a solenoid are similar to that of a ______.

A

bar magnet

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24
Q

What does the density of magnetic field lines indicate?

A

The strength of the magnetic field

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25
Q

What is the effect of increasing the number of turns in a coil on the magnetic field?

A

It causes a greater magnetic field

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26
Q

What is the purpose of using a soft core in an electromagnet?

A

To enhance the magnetic field produced by the coil

27
Q

What occurs when a large magnetic field is applied in the reverse direction to a permanent magnet?

A

It demagnetizes the permanent magnet

28
Q

What phenomenon causes the aurora borealis and aurora australis?

A

The interaction between the Earth’s magnetic field and solar wind

29
Q

What does the term ‘magnetic flux’ refer to?

A

The total magnetic field passing through a given area

30
Q

What is the formula for the force on a single charge in a magnetic field?

A

F = BQv

31
Q

How does the magnetic field of the Earth protect us?

A

It deflects harmful solar wind particles

32
Q

What is the relationship between speed (v), radius (r), and force (F) in circular motion?

A

F = mv²/r

This formula describes the centripetal force required for circular motion.

33
Q

How does the radius of the circular motion depend on the properties of the particle?

A

r = m / B

Here, m is the mass of the particle, and B is the magnetic field strength.

34
Q

What is the significance of the momentum to charge ratio (p/Q) in particle motion?

A

Particles with a particular p/Q will undergo spiraling motion with a specific radius.

35
Q

What apparatus is designed to visualize the tracks of ionizing particles?

A

Bubble chamber

A bubble chamber makes the tracks of charged particles visible as bubbles in a liquid.

36
Q

What is the function of a velocity selector?

A

Isolate particles with a specific velocity

It uses a combination of electric and magnetic fields.

37
Q

What is the formula for the electric force in a velocity selector?

A

F = EsQ

Where Es is the electric field strength and Q is the charge.

38
Q

What is the formula for the magnetic force in a velocity selector?

A

F = BsQv

Where Bs is the magnetic field strength and Q is the charge.

39
Q

In mass spectrometry, what happens to molecules before they are analyzed?

A

They are broken down into constituent atoms, ionized, and accelerated.

40
Q

What is magnetic flux density (B)?

A

Magnetic flux per unit area.

41
Q

How is magnetic flux (Φ) calculated for a uniform magnetic field?

A

Φ = BAcos θ

Where θ is the angle between the magnetic field and the area.

42
Q

What does electromagnetic induction refer to?

A

Induction of current due to a change in magnetic flux linkage.

43
Q

What does Faraday’s law of electromagnetic induction state?

A

ε ∝ Δ(N) / Δt

Where ε is the induced e.m.f.

44
Q

What is Lenz’s law?

A

The induced e.m.f. opposes the change that produced it.

45
Q

What happens to the induced current when a wire is stationary in a magnetic field?

A

The induced current falls to zero.

46
Q

What is the purpose of a search coil?

A

Determine the strength of a magnetic field from induced current.

47
Q

What is the primary function of a generator?

A

Convert kinetic energy into electrical energy.

48
Q

What is the main principle behind the operation of an electric motor?

A

Produces a force on a coil when current passes through it.

49
Q

What is the function of a split ring commutator in a DC motor?

A

Reverses the current direction to maintain a constant turning effect.

50
Q

What is a transformer used for?

A

Raise or lower the voltage of alternating current sources.

51
Q

What determines if a transformer is step-up or step-down?

A

The number of turns in the primary vs. secondary coil (Ns/Np).

52
Q

How does a step-up transformer reduce power losses?

A

Increases voltage and decreases current for long-distance transmission.

53
Q

What is the equation for the power loss in transmission lines?

A

Ploss = I²R

Where R is the resistance of the transmission lines.

54
Q

What is the role of transformers in mobile phone chargers?

A

Step down the voltage from mains supply to a safer level.

55
Q

What is the purpose of a step-down transformer in a charger?

A

To lower the voltage supplied to the phone

56
Q

Which device is preferred for measuring changing voltage in transformers?

A

Oscilloscope

57
Q

What equation is used to estimate the efficiency of a transformer?

A

η = (Pₒ / Pᵢ) × 100%

58
Q

What does the variable resistor (rheostat) do in transformer experiments?

A

Vary the current at a constant voltage

59
Q

List three methods to make transformers more efficient.

A
  • Using lower resistance wires in coils
  • Employing laminated iron in the core
  • Using soft iron cores
60
Q

True or False: Eddy currents in the core of a transformer improve efficiency.

A

False

61
Q

What is the effect of using laminated iron in a transformer core?

A

Channels magnetic flux more efficiently by reducing eddy currents

62
Q

Fill in the blank: The efficiency of a transformer can be calculated as η = _______.

A

IₚVₚ / IₛVₛ × 100%

63
Q

What type of wires can be used in transformer coils to reduce power loss?

A

Lower resistance wires

64
Q

Why are soft iron cores preferred in transformers?

A

Easier to magnetize and demagnetize

OCR A-Level Physics A (42 decks)

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