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Physics > 7: Fields and Their Consequences > Flashcards

7: Fields and Their Consequences Flashcards

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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
How are the field lines in a gravitational field drawn?
-Always towards the body of mass
26
How does a charged particle behave in a electric field?
-It's motion is parabolic and can undergo circular motion
27
What is the definition of capacitance?
-The amount of charge stored for each 1 unit voltage across a set of plates -C=Q/V
28
How is the capacitance of a dielectric calculated?
-C=(A *ε0 *εr)/d Where: A=area ε0=permittivity of free space εr=relative permittivity of the dielectric d=distance between plates
29
How is the energy stored in a capacitor calculated?
-E=(1/2)*Q*V=(1/2)*C*V^2=(1/2)*Q^2 /C Where: E=energy Q=charge V=voltage C=capacitance
30
What are the equations for charging a capacitor?
-I0=Vs/R -Vs=Vr+Vc -I=I0 *e^(-T/R*C) -Vr=Vs *e^(-T/R*C) -Vc=Vs(1-e^(-T/R*C)) -Q=Qf(1-e^(-T/R*C))
31
What are the equations for discharging a capacitor?
-I0=V0/R -Vr=Vc -I=I0*e^(-T/R*C) -Vr=V0*e^(-T/R*C) -Vc=V0*e^(-T/R*C)) -Q=Q0*e^(-T/R*C))
32
What is the time constant for a capacitor equal to?
-The resistance multiplied by the capacitance -T=RC
33
How long is the time to halve for a capacitor?
-0.69RC
34
What is the time constant for a capcitor?
-The time taken for the charge in a capacitor to fall to 37% of its initial value
35
How much charge is stored when charging a capacitor at each time constant?
-At 1RC: 63% -At 3RC: 95% -At 5RC: 99%
36
How much charge is left when discharging a capacitor at each time constant?
-At 1RC: 37% left -At 3RC: 05% left -At 5RC: 01% left
37
How does a capacitor charge up?
-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
What factors affect the time taken for a capacitor to charge or discharge?
-The capacitance of the capacitor -The resistance of the circuit
39
When does a magnetic field produce a force?
-When the field is perpendicular to the flow of current
40
How is the force on a current-carrying wire in a magnetic field calculated?
-Force=Flux Density *Current *Length of wire in field -F=B *I *L
41
How is the force on a charged particle in a magnetic field calculated?
-Force=Flux density *charge *velocity -F=B *Q *v
42
What is the definition of flux density?
-The force per unit length per unit current on a current-carrying conductor perpendicular to the field
43
What is the radius of a charged particle in a magnetic field?
-Radius=(mass *velocity)/(Flux density *charge) -r=(mv)/(Bq)
44
What is magnetic flux?
-The perpendicular area which cuts a flux density
45
How is magnetic flux calculated?
-Magnetic flux=Flux density*area perpendicular -Φ = BA -Measured in Weber(Wb)
46
How is flux linkage calculated?
-Flux linkage=number of turns*magnetic flux -ψ=NΦ -Measured in Weber(Wb)
47
How is flux linkage calculated in a rotating coil?
-Flux linkage=Flux density *area *number of turns *angular velocity -NΦ = BANcosθ = BANcos(ωt)
48
What is Faraday's law?
-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
What is Lenz's law?
-The direction of the induced current is always such that it opposes the motion that created it
50
How is emf calculated for a rotating coil in a magnetic field?
-emf=Flux linkage *angular speed *sin(angular velocity) -ε = BAN ω sin ωt
51
How is the maximum emf calculated for a rotating coil in a magnetic field?
-Max emf=Flux linkage*angular speed -εMax = BANω
52
How is the root mean square voltage calculated?
-Vrms = V0/√2
53
How is the root mean square current calculated?
-Irms = I0/√2
54
What is the transformer equation?
Ns/Np = Vs/Vp
55
What is the efficiency of a transformer?
-Efficiency=(secondary current *secondary voltage)/(primary current *primary voltage)
56
How is power lost in transmission cables?
-Lost due to heating via I^2*R
57
What does Fleming's left hand rule represent?
-The direction of the force felt by a current carrying conductor in a field -Thumb: force -Index: magnetic field -Middle: current
58
How are the field lines in a magnetic field drawn?
-From north to south
59
How does a transformer work?
-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
Why are transformers used?
-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
What is an eddy current?
-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
How are eddy currents a problem?
-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
How can energy losses via eddy currents be reduced?
-Use a laminated iron core -Place an electrical insulator in between thin sheets of iron to reduce the eddy currents circuit
64
Information that can't be put on flashcards
-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

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