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Physics > 7: Fields and Their Consequences > Flashcards

7: Fields and Their Consequences Flashcards

1
Q

What type of force is felt in a field?

A

-A non-contact force

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2
Q

Is a force a vector or a scalar?

A

-A vector

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3
Q

What are the similarities between the electrostatic force and the gravitational force?

A

-Both follow inverse square laws
-Both utilise field lines
-Both contain equipotentials

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4
Q

What are the differences between the electrostatic force and the gravitational force?

A

-Gravitational force is always attractive whereas electrostatic can also repel
-The electrostatic force is much stronger(by about 10^20)

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5
Q

Is the gravitational force attractive or repulsive?

A

-Attractive

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6
Q

How is the gravitational force at a point calculated?

A

-Force=(Gravitational constant * mass1 * mass2)/distance^2
-F=(G * M * m)/r^2

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7
Q

What is the definition of gravitational field strength?

A

-The force per unit mass
-g=F/m

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8
Q

How is the gravitational field strength at a point calculated?

A

-Field strength=(gravitational constant * mass)/distance^2
-g=(G * M)/r^2

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9
Q

What is gravitational potential?

A

-The gravitational potential energy per unit mass lost when moving a mass from infinity to a distance r from a body of mass M

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10
Q

What is gravitational potential energy?

A

-The gravitational potential energy lost when moving a mass from infinity to a distance r from a body of mass M

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11
Q

How is the work done in a gravitational field calculated?

A

-Work done=mass * change in potential
-ΔW=m * ΔV
-W=(G * M * m)/r

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12
Q

What is an equipotential surface?

A

-A surface in which no work is done moving an object between points on the equipotential surface

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13
Q

How is the gravitational potential at a point calculated?

A

-V=(G*M)/r

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14
Q

How is the orbital velocity calculated?

A

-Orbital velocity=√ ((Gravitational constant * Mass)/r)
-V=√ ((G * M)/r)

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15
Q

How is escape velocity calculated?

A

-Escape velocity=√ ((2 * Gravitational constant * Mass)/r)
-V=√ ((2 * G * M)/r)

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16
Q

How is the time period and radius of an orbit related?

A

-The time period squared is proportional to the radius cubed
-T^2 ∝ r^3

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17
Q

How do you calculate the electrostatic force at a point?

A

-F=(k *Q *q)/r^2

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18
Q

How do you calculate the electrostatic field strength at a point?

A

-F=(k*Q)/r^2

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19
Q

How do you calculate the electrostatic potential energy at a point?

A

-F=(k *Q *q)/r

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20
Q

How do you calculate the electrostatic potential at a point?

A

-F=(k*Q)/r

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21
Q

Where is the charge found in a charged sphere?

A

-At the centre of the sphere

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22
Q

What is the definition of electric field strength?

A

-The electrostatic force per unit charge

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23
Q

What is the definition of electric potential?

A

-The work done per unit charge moving a charge from infinity to a point

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24
Q

How are the field lines drawn for an electric field?

A

-Drawn in the direction that a positive charge would go

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25
Q

How are the field lines in a gravitational field drawn?

A

-Always towards the body of mass

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26
Q

How does a charged particle behave in a electric field?

A

-It’s motion is parabolic and can undergo circular motion

27
Q

What is the definition of capacitance?

A

-The amount of charge stored for each 1 unit voltage across a set of plates
-C=Q/V

28
Q

How is the capacitance of a dielectric calculated?

A

-C=(A *ε0 *εr)/d
Where:
A=area
ε0=permittivity of free space
εr=relative permittivity of the dielectric
d=distance between plates

29
Q

How is the energy stored in a capacitor calculated?

A

-E=(1/2)QV=(1/2)CV^2=(1/2)*Q^2 /C
Where:
E=energy
Q=charge
V=voltage
C=capacitance

30
Q

What are the equations for charging a capacitor?

A

-I0=Vs/R
-Vs=Vr+Vc

-I=I0 e^(-T/RC)
-Vr=Vs e^(-T/RC)
-Vc=Vs(1-e^(-T/RC))
-Q=Qf(1-e^(-T/RC))

31
Q

What are the equations for discharging a capacitor?

A

-I0=V0/R
-Vr=Vc

-I=I0e^(-T/RC)
-Vr=V0e^(-T/RC)
-Vc=V0e^(-T/RC))
-Q=Q0e^(-T/RC))

32
Q

What is the time constant for a capacitor equal to?

A

-The resistance multiplied by the capacitance
-T=RC

33
Q

How long is the time to halve for a capacitor?

A

-0.69RC

34
Q

What is the time constant for a capcitor?

A

-The time taken for the charge in a capacitor to fall to 37% of its initial value

35
Q

How much charge is stored when charging a capacitor at each time constant?

A

-At 1RC: 63%
-At 3RC: 95%
-At 5RC: 99%

36
Q

How much charge is left when discharging a capacitor at each time constant?

A

-At 1RC: 37% left
-At 3RC: 05% left
-At 5RC: 01% left

37
Q

How does a capacitor charge up?

A

-Electrons move from negative to positive around the 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 potential difference across the plates
-When the p.d across plates = source p.d., the capacitor is fully charged and current stops flowing

38
Q

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

A

-The capacitance of the capacitor
-The resistance of the circuit

39
Q

When does a magnetic field produce a force?

A

-When the field is perpendicular to the flow of current

40
Q

How is the force on a current-carrying wire in a magnetic field calculated?

A

-Force=Flux Density *Current *Length of wire in field
-F=B *I *L

41
Q

How is the force on a charged particle in a magnetic field calculated?

A

-Force=Flux density *charge *velocity
-F=B *Q *v

42
Q

What is the definition of flux density?

A

-The force per unit length per unit current on a current-carrying conductor perpendicular to the field

43
Q

What is the radius of a charged particle in a magnetic field?

A

-Radius=(mass *velocity)/(Flux density *charge)
-r=(mv)/(Bq)

44
Q

What is magnetic flux?

A

-The perpendicular area which cuts a flux density

45
Q

How is magnetic flux calculated?

A

-Magnetic flux=Flux density*area perpendicular
-Φ = BA
-Measured in Weber(Wb)

46
Q

How is flux linkage calculated?

A

-Flux linkage=number of turns*magnetic flux
-ψ=NΦ
-Measured in Weber(Wb)

47
Q

How is flux linkage calculated in a rotating coil?

A

-Flux linkage=Flux density *area *number of turns *angular velocity
-NΦ = BANcosθ = BANcos(ωt)

48
Q

What is Faraday’s law?

A

-The conductor experiences a force due to the current
-This force opposes the motion of the conductor
-The induced emf is directly proportional to the rate of change of flux linkage
-emf=Number of turns*change in flux/change in time
-ε = (N ΔΦ)/Δt

49
Q

What is Lenz’s law?

A

-The direction of the induced current is always such that it opposes the motion that created it

50
Q

How is emf calculated for a rotating coil in a magnetic field?

A

-emf=Flux linkage *angular speed *sin(angular velocity)
-ε = BAN ω sin ωt

51
Q

How is the maximum emf calculated for a rotating coil in a magnetic field?

A

-Max emf=Flux linkage*angular speed
-εMax = BANω

52
Q

How is the root mean square voltage calculated?

A

-Vrms = V0/√2

53
Q

How is the root mean square current calculated?

A

-Irms = I0/√2

54
Q

What is the transformer equation?

A

Ns/Np = Vs/Vp

55
Q

What is the efficiency of a transformer?

A

-Efficiency=(secondary current *secondary voltage)/(primary current *primary voltage)

56
Q

How is power lost in transmission cables?

A

-Lost due to heating via I^2*R

57
Q

What does Fleming’s left hand rule represent?

A

-The direction of the force felt by a current carrying conductor in a field

-Thumb: force
-Index: magnetic field
-Middle: current

58
Q

How are the field lines in a magnetic field drawn?

A

-From north to south

59
Q

How does a transformer work?

A

-A primary coil is wrapped around an iron core with an alternating p.d
-This creates an alternating magnetic field
-This magnetic field induces an EMF in a secondary coil also wrapped around the core.
-This creates a current in the secondary coil.

60
Q

Why are transformers used?

A

-To be able to increase the voltage in transmission cables to minimise energy losses
-To decrease the voltage to ensure safe usage in households

61
Q

What is an eddy current?

A

-The magnetic field of the primary coil induces an emf in the secondary coil
-This induces an emf in the secondary coil and induces min currents
-These currents are the eddy currents

62
Q

How are eddy currents a problem?

A

-The eddy currents produced cause a field opposing the primary coil
-This causes energy loss via resistive heating of the iron core
-This reduce efficiency

63
Q

How can energy losses via eddy currents be reduced?

A

-Use a laminated iron core
-Place an electrical insulator in between thin sheets of iron to reduce the eddy currents circuit

64
Q

Information that can’t be put on flashcards

A

-Graphical representations of g and V against r for gravitational and electric fields
-Derivation of period^2 is proportional to the radius cubed
-Graphs for charge and discharge for a capcitor

Physics (11 decks)

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