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A-Level Physics > Physics 7 - Fields and Their Consequences > Flashcards

Physics 7 - Fields and Their Consequences Flashcards

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1
Q

True or false: “The force felt in a force-field is a non-contact force.”?

A

True.

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2
Q

Is force a vector or scalar quantity?

A

A vector.

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3
Q

What are some similarities between electrostatic and gravitational forces?

A

Inverse square force laws
Potential concept
Equipotential surfaces
Use of field lines

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4
Q

What are some differences between electrostatic and gravitational forces?

A

The gravitational forces from masses always attract, whilst charges may repel or attract.

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5
Q

What is gravity?

A

Gravity is the universal attractive force which acts between all mater.

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6
Q

What is G?

A

The universal gravitational constant. Approximately 6.67×10⁻¹¹m³kg⁻¹s⁻²

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7
Q

What can field lines tell you about a field?

A

The direction of the field and the strength depending on the density of the field lines.

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8
Q

What is g?

A

g is the force per unit area in a uniform field.
In a radial field, the magnitude of g is the proportionality constant at that point between force and mass.
Let g=GM/r²

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9
Q

What is gravitational potential?

A

The potential energy per kilogram at any point in the field. 0 potential is defined at infinity, hence at a point close to a mass the potential of an object would be negative.

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10
Q

What is the work done by moving a mass in a field?

A

mass×change in potential

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11
Q

What is the gravitational potential difference?

A

Gravitational potential difference is the difference in the gravitational potentials of two points in a gravitational field.

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12
Q

What is an equipotential surface?

A

A surface in which every point on the surface has the same potential.

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13
Q

How much work is done when you move 1km in any direction on an equipotential?

A

No work is done when moving across equipotential, as the potential at each point is the same.

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14
Q

Why is gravitational potential a negative value?

A

Work needs to be done to move an object from the inside of the field to outside the field.
Since outside the field’s potential is defined as 0 then the potential inside the field must be negative,

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15
Q

How is the orbital period relating to the radius of a circular orbit?

A

T²∝R³

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16
Q

What equations could one use to find the speed of an orbiting satellite?

A

The orbiting object (mass m) is in circular motion, so we would use F=ma with F=GMm/r² rearranged to a=v²/r=ω²r.
This can be solved to find the speed (v), angular speed (ω), the radius of the orbit (r) or using T=2π/ω its period.

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17
Q

Compare the PE and KE of a lower orbit to a higher one.

A

A lower orbit has less potential energy and more kinetic energy than a higher orbit.

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18
Q

What is the period of a geosynchronous orbit?

A

Geosynchronous have a period of one day.

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19
Q

What symbol represents the permittivity of free space?

A

ε₀

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20
Q

When calculating the force between two particles, what can air be treated as?

A

A vacuum.

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21
Q

For a charged sphere the charge can be assumed to be at what part of the sphere?

A

The centre.

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22
Q

Which is stronger, the gravitational force of subatomic particles or the electrostatic force?

A

The electrostatic force.

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23
Q

What direction do electric field lines go in?

A

Electric field lines always go from positive charge to negative charge.

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24
Q

What is electric field strength?

A

The force per unit charge acting at a point in an electric field.

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25
Q

What is the magnitude of E (electric field strength) in an uniform electric field?

A

V/d
Where:
V=Potential difference between plates (V)
d=Distance

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26
Q

What is the trajectory of a particle entering a uniform field at right angles?

A

It is parabolic.

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27
Q

How is electric potential related to electric field strength?

A

EΔV/Δr

The change in electric potential with respects to to the change in radius length.

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28
Q

How is capacitance calculated?

A 
C=Q/V
Where:
C=Capacitance (F)
Q=Charge in the plates (C)
V=Potential difference across the plates (V)
29
Q

What is the relative permittivity (a.k.a. dielectric constant)?

A

The ratio of the charge stored with the dielectric between the plates to the charge stored when the dielectric is not.
εᵣ=Q/Q₀
The greater the relative permittivity, the greater the capacitance of the capacitor.

30
Q

What does the area under the graph of charge against potential difference represent?

A

The energy stored by the capacitor.

31
Q

Describe the graph of Q against t for the discharging of a capacitor through a resistor.

A

Steep initially negative gradient, curving down from the C-axis becoming closer to parallel to the t-axis as it approaches.

32
Q

Describe the graph of V against t for the discharging of a capacitor through a resistor.

A

Steep initially negative gradient, curving down from the V-axis becoming closer to parallel to the t-axis as it approaches.

33
Q

Describe the graph of I against t for the discharging of a capacitor through a resistor.

A

Steep initially negative gradient, curving down from the I-axis becoming closer to parallel to the t-axis as it approaches.

34
Q

Describe the graph of Q against t for the charging of a capacitor through a fixed resistor.

A

Steep initially positive gradient starting at the origin, curving towards the parallel of the t-axis as it goes further from the v-axis.

35
Q

Describe the graph of V against t for the charging of a capacitor through a fixed resistor.

A

Steep initially positive gradient starting at the origin, curving towards the parallel of the t-axis as it goes further from the V-axis.

36
Q

What is the time constant?

A

The time it takes for the charge in a capacitor to fall to 37% of the initial value given by resistance×capacitance.
A capacitor is considered fully discharged after 5 time constants.

37
Q

How was the 37% derived for the time constant of a capacitor derived?

A

Start with the formula Q=Q₀e⁻ᵗ/ᴿᶜ
When t=RC (after 1 time constant), the formula becomes Q=Q₀e⁻¹
e⁻¹≈0.37, which is where the 37% came from.

38
Q

What is the half time of a capacitor?

A

T½=0.69RC

39
Q

What equations are required for charging a capacitor?

A

Charging up a capacitor produces Q=Q₀(1-e⁻ᵗ/ᴿᶜ) and V=V₀(1-e⁻ᵗ/ᴿᶜ)
Where:
V₀ is the battery PD and Q₀=CV₀.

40
Q

How does a capacitor charge up?

A

Electrons move from negative to positive around a circuit.
The electrons are deposited on plate A making it negatively charged.
Electrons travel from plate B to the positive terminal of the battery, giving the plate a positive charge.
Electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a PD across the plates.
When the PD across the plates=source PD, the capacitor is fully charged and current stops flowing.

41
Q

Describe in terms of the movement of electrons how the PD across a capacitor changes, when it discharges across a resistor.

A

Electrons move in the opposite direction to when the capacitor was charging up.
The charge on plate A decreases as it loses electrons and plate B gains electrons, neutralising them.
The PD decreases exponentially across the plates.

42
Q

State the 3 expressions for the energy stored by a capacitor.

A 
E=½(Q²/C)=½(QV)=½(CV²)
Where:
E=Energy stored by capacitor (J)
Q=Charge on capacitor (C)
C=Capacitance (F)
V=Potential difference (V)
43
Q

What 2 factors affect the time taken for a capacitor to charge or discharge?

A

The capacitance of the capacitor, C. This affects the amount of charge that can be stored by the capacitors at any given potential difference across it.
The resistance of the circuit, R. This affects the current in the circuit and how quickly it flows, hence how quickly the capacitor charges/discharges.

44
Q

When a magnetic field is perpendicular to a current-carrying wire, does the wire feel a force?

A 
Yes
F=BIl
Where
F=Force (N)
B=Magnetic flux density (Wbm⁻²)
I=Current in the wire (A)
l=Length of wire (m)
45
Q

Fleming’s left hand rule for motors represents what properties of what fingers?

A

The thumb represents the thrust/force
The first finger represents the magnetic field
The second finger represents the curret

46
Q

What is magnetic flux density?

A

The flux density, measured in Teslas (T) or Webers per meter squared (Wbm⁻²), is the flux per meter squared/

47
Q

Does a charged particle moving through a field feel a force when it is traveling along the field lines or perpendicular to them?

A

When it is travelling perpendicular to the field.

48
Q

What is the equation for the force felt by a moving charge in a magnetic field?

A 
F=BQv
Where:
F=Force (N)
B=Magnetic flux density (Wbm⁻²)
Q=Charge (C)
v=Velocity of moving charge (ms⁻¹)
49
Q

Describe how force is applied to a charge moving through a magnetic field.

A

Perpendicular to its motion, causing it to move in a circular motion.

50
Q

What fields do cyclotrons use?

A

An electric field and a magnetic field.

51
Q

How does a cyclotron work and what are the electric and magnetic fields’ purposes in a cyclotron?

A

A cyclotron is made up of 2 semicircular electrodes called “Dees” with a magnetic field applied perpendicularly to the Dees and an alternating potential difference between the Dees.
Each Dee is a metal electrode with opposite charges, this creates and electric field in the gap between the two Dees. This is what accelerates the particles.
The magnetic field causes the particles to move in a circular motion, which allows it to gain speed whilst minimising space. As they speed up, the radius of their motion increases, until it breaks free tangential to one of the Dees.

52
Q

What is the formula for magnetic flux?

A 
Φ=BA
Where
Φ=Flux (Wb)
B=Flux density (Wbm⁻²)
A=Area (m²)
53
Q

What is flux linkage?

A

NΦ=the number of turns cutting the flux at on time.

54
Q

What is the flux linkage of a rectangular coil rotating through a magnetic field?

A 
NΦ=BANcos(θ)
Where:
NΦ=Flux linkage (Wb turns)
B=Magnetic flux density (Wbm⁻²)
A=Area (m²)
N=Number of turns
θ=Angle between the flux and the normal to the coil
55
Q

What is Faraday’s Law?

A

The induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage.

56
Q

What is Lenz’s law?

A

The direction of the induced e.m.f. is such as to oppose the change that induces it.

57
Q

What is the formula for induced e.m.f.?

A

ξ=-N(ΔΦ/Δt)

58
Q

What happens when you move a straight conductor through a magnetic field?

A

The electrons experience a force pushing them to one end of the conductor creating an e.m.f. across the conductor. The rod obeys Faraday’s law, it is changing flux as it moves through the field hence an e.m.f. is induced.

59
Q

What would be the e.m.f. produced when rotating a coil at a constant rate in a magnetic field?

A 
ε=BANωsin(ωt)
Where:
ε=E.m.f.
B=Magnetic flux density (Wbm⁻²)
A=Area (m²)
N=Number of turns
ω=Angular velocity of the coil
t=time
60
Q

Describe how one would use an oscilloscope.

A

Oscilloscopes are used to displace AC waves, the x-axis is called the time base and shows how long it takes the wave to move one division and the y-axis shows how much potential difference is needed to move the wave up one division. Using this we find the peak voltage, time period and frequency.

61
Q

How does a transformer work?

A

A primary coil wrapped around an iron core with an alternating potential difference creates an alternating magnetic field, this magnetic field induces an e.m.f. in a secondary coil also wrapped around the core. This creates a current in the secondary coil.

62
Q

What kind of current is produced by a transformer and why?

A

An alternating current.
An e.m.f. is induced by a changing magnetic field, hence the e.m.f. induced is alternating producing an alternating current.

63
Q

Why are transformers used?

A

By changing the number of coils, the transformers can be used to increase the voltage and reduce current when transporting power, with minimal energy loses. The voltage is then dropped again locally to ensure safe usage in households.

64
Q

What equation links the number of coils in a transformer with their voltages?

A

Nₛ/Nₚ=Vₛ/Vₚ
Where:
Nₛ=Number of coils on secondary coil
Nₚ=Number of coils on primary coil
Vₛ=Potential difference across secondary coil (V)
Vₚ=Potential difference across primary coil (V)

65
Q

What is transformer efficiency?

A

The ratio of output power in the transformer to input power.
η=IₛVₛ/IₚVₚ
η=Efficiency (unitless)
Iₛ=Current across secondary coil (A)
Iₚ=Current across primary coil (A)
Vₛ=Potential difference across secondary coil (V)
Vₚ=Potential difference across primary coil (V)
Covert to percentage by multiplying by 100.

66
Q

In a step-up transformer, which has more coils; the primary coil or secondary coil?

A

The secondary coil.
Step-up transformers increase the voltage, hence more coils need to be in the secondary coil for a larger potential difference.

67
Q

What is an eddy current?

A

In a transformer, as the primary coil’s magnetic field induces e.m.f. in the secondary coil it also induces e.m.f. and hence mini currents (eddy currents) within the iron core.

68
Q

Why are eddy currents a problem?

A

By Lenz’s law the e.m.f. created an its field opposes that of the primary coil. This causes energy loss via resistive heating of the iron core by the eddy currents, which reduces efficiency.

69
Q

How can you reduce eddy current losses in a transformer?

A

Use a laminated iron core, thin sheets of iron with an electrical insulator in between, which reduces the eddy currents’ circuit.

A-Level Physics (9 decks)

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