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AQA A-level Physics > Fields and their consequences > Flashcards

Fields and their consequences Flashcards

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1
Q

What is a force field?

A

a force field is an area in which an object experiences a non-contact force

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2
Q

Are force fields a vector or scalar quantity?

A

vector

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3
Q

How can force fields be represented? What can the diagrams show

A

represented as diagrams containing field lines

the distance between/density of field lines represents the strength the force exerted by the field in that region

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4
Q

How can force fields be formed?

A

formed during the interaction of masses, static charge or moving charges

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5
Q

What are the two examples of fields and what interaction takes place?

A

gravitational- formed during the interaction of masses
electric- formed during the interaction of charges

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6
Q

What are some similarities of gravitational and electric fields? (3)

A

-forces both follow an inverse-square law
-uses field lines to be represented
-both have equipotential surfaces

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7
Q

What are some differences between gravitational and electric fields? (2)

A

-in gravitational fields, the force exerted it always attractive, while in electric fields the force can either be repulsive or attractive
-electric force acts on charge, while gravitational force acts on mass

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8
Q

State Newton’s law of gravity

A

gravity acts on any objects which have mass and is always attractive

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9
Q

What is Newton’s law of gravitation?

A

the magnitude of the gravitational force between two masses is directly proportional to the product of the masses and is inversely proportional to the square of the distance between them (where distance is measured between the two centre of the masses

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10
Q

State the equation for Newton’s law of gravitation

A

F = (Gm1m2)/r^2

where:
f= force (N)
G= gravitational constant
m1 & m2= masses (kg)
r= distance between centres of masses (m)

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11
Q

What are the two types of gravitational fields?

A

radial and uniform

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12
Q

Draw a diagram of the two types of gravitational fields. Explain why the field lines are situated in a certain way

A

https://cdn.savemyexams.com/cdn-cgi/image/w=640,f=auto/uploads/2020/10/13.1.1.2-Gravitational-field-lines.png

-a uniform field exerts the same gravitational force on a mass everywhere in the field ie. why the field lines are parallel and equal

-in a radial field, the force exerted depends on the position of the object in the field (as the object moves further away from the field the magnitude of the force would decrease as the density of the field lines would increase)

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13
Q

How does gravitational field strength differ in both types of gravitational fields?

A

-constant in uniform field
-varied in radial field

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14
Q

What are the ways we can calculate gravitational field strength? When can one be used?

A

g= f/m (using w=mg)

g= GM/r^2 (for radial fields only)

where:

g= gravitational field strength (Nkg^-1)
G= gravitational constant
M= mass of body producing field (kg)
r= distance from mass where you are calculating field strength (m)

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15
Q

What is gravitational potential?

A

the work done per unit mass when moving an object from infinity to that point

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16
Q

State the equation for gravitational potential

A

V= -GM/r

where:

V= gravitational potential (Jkg^-1)
G= gravitational potential
M= mass of object causing the field (kg)
r= the distance between the centres of the objects (m)

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17
Q

Why is gravitational potential a negative value?

A

-it is defined as zero at infinity
-since gravitational force is attractive, work must be done on a mass to reach infinity

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18
Q

How can gravitational potential be used to find the work done when moving an object in a gravitational field?

A

ΔW= mΔV

where:

ΔW= change in work done (J)
m= mass (kg)
ΔV= change in gravitational potential (Jkg^-1)

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19
Q

What are equipotentials in gravitational fields?

A

regions of equal potential

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20
Q

What is something needed to know with equipotentials in general and gravitational potentials?

A

the gravitational potential difference is zero when moving along the surface so no work is done when moving along an equipotential surface

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21
Q

How can we show equipotential lines on a radial gravitational field diagram? What is something to know about the distances between each equipotential?

A

drawn in circles around the object, right angles to the field lines

the gravitational potential surfaces get further apart as one moves away from the planet

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22
Q

How can we show equipotential lines on a uniform gravitational field diagram? What is something to know about the distances between each equipotential?

A

perpendicular lines to field lines, distance between each equipotential is equal

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23
Q

How can we get the gravitational field strength at a certain distance from a gravitational potential-distance graph?

A

draw a tangent to the curve at that distance and calculate the gradient

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24
Q

How can we work out the gravitational potential difference from a gravitational field strength-distance graph?

A

finding the area under the curve

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25
Q

What do we mean by orbital period?

A

the time taken for an object to complete a full orbit of the central body

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26
Q

Derive orbital speed

A

as there is a body in orbit/circular motion, the centripetal force is equal to the gravitational force

(Gm1m2)/r^2 = mv^2/r

v= √GM/r

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27
Q

State Kepler’s third law

A

the square of the orbital period (T) is directly proportional to the cube of the radius (r)

T^2 ∝ r^3

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28
Q

Derive Kepler’s third law

A
  1. when an object orbits a mass, it experiences a gravitational force which also acts as the centripetal force due to the circular motion. We can then equate the centripetal and gravitational force
    mω^2r = (Gm1m2)/r^2
  2. cancel 1 of the m’s
    ω^2r = (Gm) / r^2
  3. expand ω
    (4π^2 / T^2) x r = (Gm) / r
  4. rearrange to make T the subject
    T^2= (4π^2/ GM) x r^3

As 4π^2/GM is a constant, it shows T^2 ∝ r^3

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29
Q

Give the equation for Kepler’s third law

A

T^2= (4π^2/ GM) x r^3

where:

T= orbital period (s)
G= gravitational constant
r= radius (m)
M= mass (kg)

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30
Q

How can we find the total energy of a satellite

A

kinetic energy + potential energy

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31
Q

What do we mean by escape velocity?

A

the minimum velocity an object must travel at in order to escape the gravitational field at the surface of a mass.

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32
Q

Derive the equation for escape velocity

A

This is the velocity at which the object’s kinetic energy is equal to the magnitude of its gravitational potential energy

Ek= Ep
1/2mv^2= (Gm1m2)/r

as gravitational potential energy= mΔV, v= √(2GM)/r

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33
Q

What do we mean by synchronous orbit?

A

where the orbital period is equal to the rotational period of the object that it is orbiting

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34
Q

What does the ‘geo’ refer to in geosynchronous?

A

earth

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35
Q

What characteristics do geostationary/geosynchronous orbits have? (5)

A

-remain directly above the equator
-is in plane of the equator
-always orbits at the same point above the earths surface
-moves from west to east (same direction as earth spins)
-has an orbital time period equal to earths rotational period of 24hrs

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36
Q

What is an example of a geostationary orbit? Why?

A

geosynchronous satellites

-have an orbital period of 24hrs
-orbit directly above the equator

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37
Q

Why are geosynchronous satellites useful to send signals?

A

they always orbit above same point on earth so you don’t need to alter the plane of an aerial or transmitter

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38
Q

What does it mean when something has a lower polar orbit?

A

their altitude is closer to earth’s surface

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39
Q

What is a characteristic of low orbit satellites? What does this mean?

A

travel much faster than geostationary therefore have a lower orbital period

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40
Q

Why are low orbit satellites useful?

A

travel faster so require less powerful transmitters and can potentially orbit across the entire earth’s surface making them useful for:

-monitoring the weather
-making scientific observations about places which are unreachable
-military applications

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41
Q

State Coulomb’s law

A

the magnitude of the force between two point charges in a vacuum is directly proportional to the produce of their charges, and inversely proportional to the square of the distance between the charges

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42
Q

State the equation for Coulomb’s law

A

F= 1/(4πε₀) x Q1Q2/r^2

where:

F= force (N)
ε₀= permittivity of free space
Q1 & Q2= charges (C)
r= distance between centres of each charge (m)

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43
Q

Define absolute permittivity

A

a measure of how difficult it is to establish an electric field within a material

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44
Q

How can we work out the absolute permittivity of a material?

A

ε = ε₀ x εr

where:
ε= absolute permittivity (fm^-1)
ε₀= permittivity of free space
εr= relative permittivity

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45
Q

How can charge affect the force between them?

A

if the charges have the same sign the force will be repulsive, if they are different the force will be attractive

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46
Q

Define electrical field strength

A

the force per unit +ve charge

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47
Q

How does the electric field strength differ from a radial or uniform field?

A

constant in uniform

varied in radial

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48
Q

What are the equations to work out electric field strength? What field can they be used for?

A

E= f/Q
E= ΔV / Δr (for both uniform and radial fields)

E= V/d (uniform fields formed by parallel plates)

E= 1/(4πε₀) x Q/r^2 (radial fields)

where:
E= electric field strength (NC^-1) or (Vm^-1)
f= force (N)
Q= charge (C)
ε₀= permittivity of free space
r= distance between centres of charges (m)

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49
Q

What are the terms to describe the field lines for a radial field that is positive/negative?

A

positive- source of field lines
negative- sink of field lines

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50
Q

Draw a diagram of the types of electric fields. Explain why they are situated in a certain way

A

https://cdn.savemyexams.com/cdn-cgi/image/w=1280,f=auto/uploads/2021/04/18.1-Radial-E-field-lines.png
https://cdn.savemyexams.com/cdn-cgi/image/w=1280,f=auto/uploads/2021/05/7.4.4-Parallel-Plates-Work-Done.png

the direction of the arrows shows the direction of the force on a +ve test charge

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51
Q

What are equipotential lines in an electric field?

A

regions of equal electric potential

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52
Q

How can we show equipotential lines on a radial gravitational field diagram? What is something to know about the distances between each equipotential?

A

circular, right angles to field lines, as distance increases the equipotential lines get further apart

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53
Q

How can we show equipotential lines on a uniform gravitational field diagram? What is something to know about the distances between each equipotential?

A

perpendicular to field lines, equal spaces

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54
Q

How does the electric field strength differ in uniform and radial fields?

A

constant throughout uniform fields

field strength depends on the distance between charges/density of field lines

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55
Q

Define absolute electrical potential

A

the work done per unit +ve charge moving it from infinity to a point

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56
Q

State the equation to work out the absolute electrical potential

A

V= 1/(4πε₀) x Q/r

where:
V= electrical potential (V)
ε₀= permittivity of free space
Q= charge of object (C)
r= distance from centre of charge (m)

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57
Q

How can we work out the work done by moving a charged particle between parallel plates of a uniform field?

A

ΔW= QΔV

where:
ΔW= change in work done
Q= charge that is moving (C)
ΔV= change in potential difference (JC^-1)

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58
Q

What happens to a charged particle when it enters an electric field?

A

it will experience a force which will cause it to move

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59
Q

What happens to a charged particle if it remains still in a uniform field?

A

it will move parallel to the electric field lines (along or against depending on its charge)

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60
Q

What happens to a charged particle if it is in motion perpendicular to a uniform field? Include a diagram showing this with both +/- charges

A

it will experience a constant electric force and travel in a parabolic trajectory

https://cdn.savemyexams.com/cdn-cgi/image/w=1280,f=auto/uploads/2021/04/18.1-Parabolic-trajectory.png

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61
Q

How can we find the electric field strength from a potential-distance graph?

A

find the gradient

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62
Q

How can we work out the electric potential difference from an electric field strength-distance graph?

A

find the area underneath

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63
Q

What do we mean by capacitance?

A

the charge stored by a capacitor per unit p.d.

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64
Q

Describe the structure of a capacitor

A

-two parallel conducting plates e.g. metal film
-separated by an insulator aka dielectric (plastic ceramics, electrolytes)

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65
Q

State two uses of capacitors

A

-introduce time delaying to a circuit
-temporary storage of charge (electrostatic potential energy store)

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66
Q

How can you work out the capacitance of a capacitor?

A

C= Q/V

where:
C= capacitance (F)
Q= charge (C)
V= p.d. (V)

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67
Q

What happens when a capacitor is connected to a source of power/charged?

A

opposite charges build up on the two parallel plates causing a uniform electric field to be formed

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68
Q

What do we mean by permittivity? What material has this property?

A

-dielectrics
-a measure of the ability to store an electric field in the material

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69
Q

What do we mean by relative permittivity? What else can it be called?

A

-dielectric constant
-permittivity of a material relative to free space

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70
Q

What is the relative permittivity of air?

A

1

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71
Q

What does it mean if one material has a higher relative permittivity than another?

A

that material has a higher opposition of the electric field within it/better at opposing an electric field

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72
Q

How can you calculate the relative permittivity?

A

εr =ε /ε0

where:
εr= relative permittivity
ε0= permittivity of free space (Fm^-1)
ε= permittivity of a material (Fm^-1)

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73
Q

What factors affect capacitance?

A

-area of plates
-distance between plates
-relative permittivity of dielectric

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74
Q

How else can we find the capacitance of a capacitor using the plates?

A

https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/05/7.6.2-Capacitance-and-Dielectric-Equation.png

where:
C= capacitance (F)
A= cross-sectional area of plates (m^2)
d= separation of the plates (m)
εr= relative permittivity of dielectric
ε0= permittivity of free space (Fm^-1)

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75
Q

What are dielectrics formed of?

A

polar molecules

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76
Q

What are polar molecules?

A

molecules with one end which is positive and the other negative

77
Q

Describe what the polar molecules of the dielectric are like and how they behave when the capacitor is uncharged

A

randomly orientated

78
Q

Describe how the polar molecules behave when the capacitor is charged

A

-polar molecules rotate to align antiparallel with the applied electric field
-their own inherent electric field opposes that of the capacitor

—>decreases overall E-field (increase charge, increase capacitance)

79
Q

Draw and annotate a diagram showing how polar molecules behave when a capacitor is charged

A

https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/06/7.6.2-Dielectric-E-Field.png

80
Q

How can we find the energy stored by a capacitor from a charge-p.d. graph?

A

find the area

81
Q

What equations can be used to find the energy of a capacitor?

A

E=1/2 x QV
E= 1/2 x CV^2
E= Q^2 / (2C)

where:
E= energy (J)
Q= charge (C)
V= p.d. (V)
C= capacitance (F)

82
Q

How can we charge a capacitor? Draw and label the circuit

A

-connect to a circuit with a switch, power supply and resistor
-measure p.d. and current with a data logger
https://cdn.savemyexams.com/cdn-cgi/image/f=auto,width=1920,w=1920/uploads/2023/07/19-2-charging-and-discharging-circuit-new.png

83
Q

Describe what happens when the switch for a capacitor is closed

A

initially on capacitor, Q=0 ∴ Vc=0 as Q=CV
as capacitor charges ↑Vc, ↓V(R) which represents a reduces current as I= V(R)/R decays from initial value

84
Q

Draw and annotate the graphs of current, charge and p.d. for when a capacitor charges. What notation is used for the max value of a quantity?

A

https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/06/7.7.1-Charging-Graphs.png

max current= Io
max p.d.= Vo
max charge= Qo

85
Q

What values can we find from a current-time graph/charge-time graph?

A

area under current-time= charge

gradient from charge-time graph= current

86
Q

What happens when the capacitor connects to a power supply and charges

A

when a capacitor is connected to a power supply, current flows and negative charge builds on one plate. Electrons are repelled to the positive terminal, creating equal but opposite charges on each plate, causing a potential difference. As charge increases, potential difference increases, but electron flow decreases due to electrostatic repulsion, eventually leading to zero current

87
Q

How can we discharge a capacitor? Draw and label the circuit

A

-connect to a closed circuit with just a resistor
-use data logger to measure p.d. and current
https://cdn.savemyexams.com/cdn-cgi/image/f=auto,width=1920,w=1920/uploads/2023/07/19-2-charging-and-discharging-circuit-new.png

88
Q

Draw and annotate the graphs of current, charge and p.d. for when a capacitor discharges.

A

https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/04/19.2-Discharging-graphs.png

89
Q

How can we find current/p.d./charge from a charging capacitor graph?

A

I = Io x e^(-t/RC)
V = Vo( 1 - e^(-t/RC) )
Q = Qo( 1- e^(-t/RC) )

where:
Io/Vo/Qo= max current/p.d./charge
I/V/Q= current/p.d./charge on capacitor when charging
e= exponential function
t= time (s)
R= resistance (Ω)
C= capacitance (F)

90
Q

How can we find current/p.d./charge from a discharging capacitor graph?

A

I = Io x e^(-t/RC)
V= Vo x e^(-t/RC)
Q= Qo x e^(-t/RC)

where:
Io/Vo/Qo= max current/p.d./charge
I/V/Q= current/p.d./charge on capacitor when discharging
e= exponential function
t= time (s)
R= resistance (Ω)
C= capacitance (F)

91
Q

What are some tips to remembering finding V from discharging/charging capacitor graphs?

A

swap Q for V using data book

92
Q

What is RC and what can it also be known as?

A

-also known as the time constant
-equal to the value of time (s) taken to discharge/charge a capacitor

93
Q

How can we find the time constant when discharging a capacitor?

A

find 37% of the max value (approx.)

94
Q

How can we find the time constant when charging a capacitor?

A

find 63% of the max value (approx.)

95
Q

How can you derive the time constant from 1/2 the time taken when discharging? (use current equation in this case)

A

0.5 x Io = Io x e^(-t/RC)
0.5 = e^(-t/RC)
ln 0.5 = -t/RC
t = -ln(0.5) x RC

-ln(0.5) is approx 0.69
∴ time taken to discharge to 1/2 of inital value:
t = 0.69RC

96
Q

How can we use log graphs to find the time constant?

A

-take natural log of both sides of a I/V/Q discharging equation
-use log rules to simplify
-plot graph of ln(I/V/Q) against t
-gradient = -1/RC
-rearrange to find RC

97
Q

What are some advantages to using a log graph for discharging

A

-better averaging process
-rejects anomalies
-more accurate

98
Q

What happens when we pass current through a wire? When is this true

A

-induces a magnetic field
-true for any long, straight current-carrying conductor

99
Q

What is the right hand grip rule? How do we use this for a current carrying wire?

A

shows the direction of the magnetic field around a current-carrying wire when the thumb points in the direction of the current

100
Q

What is the structure of field lines around a wire with a magnetic field?

A

forms concentric rings around the wire, arrows of field lines show the direction of field

101
Q

What is magnetic flux density? What is its unit and symbol?

A

measures the strength of a magnetic field
-measured in Tesla
-symbol B

102
Q

What is magnetic flux density also known as?

A

magnetic field strength

103
Q

Is magnetic flux density a scalar or vector value?

A

vector
-has a magnitude and direction

104
Q

What can the density of magnetic field lines show?

A

the strength of the field
higher density = larger strength of field

105
Q

What is flux known as?

A

field lines

106
Q

What happens to a current-carrying wire when places inside a magnetic field with the current perpendicular to the field lines?

A

a force is exerted

107
Q

State the Fleming’s left hand rule. Show how it works

A

Helps to find direction of force exerted on a current-carrying wire

using left hand:
thumb= force
index finger= B field
middle finger= conventional current

https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/05/20.1-Flemings-left-hand-rule_1.png

108
Q

What should we think about if a question tells us that a ‘current’ is flowing in a certain direction?

A

use this current as conventional current

109
Q

What should we think about if a question shows us the flow of electrons?

A

must treat the electrons as moving opposite to conventional current

110
Q

Draw field lines on a magnet which has a north and south pole

A

https://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/VFPt_cylindrical_magnet_thumb.svg/220px-VFPt_cylindrical_magnet_thumb.svg.png?20230924082557

111
Q

How can we work out the force exerted on a current-carrying wire?

A

F= BIL

where:
F= force exerted (N)
B= magnetic flux density (T)
I= current (A)
L= length of wire (m)

112
Q

What happens if the B-field and current-carrying wire are parallel to each other?

A

no force is exerted
-need to be perpendicular

113
Q

Why is a force exerted on a current-carrying wire?

A

electrons from current are moving around which are negatively charged

∴a force is exerted due to charged particles moving in a magnetic field

114
Q

How can we find the force exerted on an isolating moving charged particle?

A

F= BQv

where:
F= magnetic force on particle (N)
B= magnetic flux density (T)
Q= charge on particle (C)
v= speed of particle (ms^-1)

115
Q

What will happen if a charged particle enters a uniform magnetic field? Why?

A

will travel in a circular path
-the force exerted on it will be perpendicular to its velocity ∴ has a centripetal force

116
Q

How can we derive the radius of the circular path when a particle enters a uniform magnetic field?

A

-link centripetal force equation to BQv equation

-mv^2/r = BQv

-rearrange for r

117
Q

State the equation to find the radius of a particles circular path when entering a uniform B-field

A

r= mv/BQ

where:
r= radius of path (m)
m= mass of particle (kg)
v= particle’s velocity (ms^-1)
B= magnetic flux density (T)
Q= charge of particle (C)

118
Q

What is an application of the charged particles moving in a circular path when entering a uniform B-field?

A

particle accelerators called cyclotrons

has many uses e.g.:
-producing ion beams for radiotherapy and radioactive tracers

119
Q

What do cyclotrons do?

A

accelerate charged particles from their centre along a spiral path

120
Q

Describe the structure of a cyclotron

A

made up of:
-two hollow semi-circular electrodes called ‘dees’
-a uniform magnetic field applied perpendicular to electrodes
-an alternating p.d. applied to electrodes (creating an electric field between them)

121
Q

How do cyclotrons function?

A

-charged particles start at the cyclotron centre, fired into one of the electrodes
-then move in semi-circles guided by a magnetic field towards an electrode
-accelerated by an electric field, they spiral outwards (radius increases), switching direction (alternating p.d.) until they exit the cyclotron

122
Q

What is the purpose of the alternating electric field between the dees?

A

to accelerate the charged particles to high speeds

123
Q

Why do we need an alternating p.d.?

A

needed so the particles can accelerate in both directions (allows the particle to accelerate across gap between opposite electrodes)

124
Q

Draw and label a cyclotron

A

https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/06/7.8.5-Cyclotron.png

125
Q

Why are particles always accelerating in a cyclotron?

A

experiences a centripetal force due to B-field, changing direction of velocity causes acceleration

126
Q

How can we derive the time spend of a particle in BOTH dees of a cyclotron (full circle)?

A

centripetal force=force exerted on particle
mrω^2 = BQv
mrω^2 = BQ(rω)

mω=BQ
ω=BQ/m
2π/T = BQ/m

T= m/BQ x 2π

127
Q

Define magnetic flux

A

the number of flux lines (flux) per square metre

128
Q

How can we work out magnetic flux?

A

Φ = BA

where:
Φ = magnetic flux (Wb)
B= magnetic flux density (T)
A= cross-sectional area (m^2)

129
Q

When can magnetic flux occur?

A

when the field is perpendicular to the area/coil

130
Q

Define magnetic flux linkage

A

measures the amount of flux which has been cut

131
Q

What do we mean by ‘cutting flux’?

A

when something is moving through the field lines, like they’re ‘cutting’ it

132
Q

How can we find out magnetic flux linkage?

A

NΦ =BAN

where:
NΦ= magnetic flux linkage (Wb turns)
B= magnetic field density (T)
A= cross-sectional area (m^2)
N= number of turns on coil

133
Q

What happens to the magnetic flux & magnetic flux linkage equation when magnetic field is not fully perpendicular to the coil?

A

Φ =BAcos θ
NΦ =BANcos θ

where cos θ is the angle between field and normal to plane of coil

134
Q

What is the magnetic flux if the field is parallel to coil?

A

0Wb

135
Q

draw the graph of magnetic flux against the angle to normal

A

https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/05/20.2-WE-Calculating-Magnetic-Flux-graph-solution.png

136
Q

What do we mean by electromagnetic induction? When can this occur?

A

the process in which an emf is induced in a closed circuit due to changes in magnetic flux

can occur either when:
-a conductor cuts through a magnetic field
-the direction of a magnetic field through a coil changes

137
Q

What are some applications of where electromagnetic induction is used?

A

-in electrical generators which convert mechanical energy to electrical
-transformers which are used in electrical power transmission

138
Q

What are the two ways which we can control the effects of electromagnetic induction?

A

using:
faraday’s law
lenz’s law

139
Q

State Faraday’s law and the equation

A

the magnitude of induced emf is equal to the rate of change of flux linkage

ϵ = ΔNΦ / Δt

where:
ϵ = emf (V)
NΦ= magnetic flux linkage (Wb turns)
t= time taken (s)

140
Q

State Lenz’s law

A

the direction of induced current is such as to oppose the motion causing it (tries to counteract the change in the magnetic field that caused it)

141
Q

What is the equation showing Lenz’s law combined with Faraday’s law? Explain it

A

ϵ = -N(ΔΦ / Δt)

shows:
-when a bar magnet moves through a coil, it creates an emf due to a change in magnetic flux
-also induces a current in the coil, generating its own magnetic field which opposes the bar magnet’s field
-coil magnetic field acts in opposite direction to B-field of bar magnet ∴ -ve sign

142
Q

Explain how Lenz’s law can be verified using a coil and bar magnet. Draw and label the diagram

A

confirming Lenz’s law:
-use a bar magnet, a coil, and a sensitive ammeter
-push a magnet pole into the coil, noting current direction on the ammeter using the right-hand grip rule
-reversing the magnet shows opposite deflection on meter, as the induced field repels magnet

∴ showing Lenz’s law (as direction of induced current opposes change)

https://cdn.savemyexams.com/cdn-cgi/image/f=auto,width=1920,w=960/uploads/2022/07/20-2-lenzs-law-experiment-1.png

143
Q

What happens if we leave a bar magnet stationary within a coil?

A

no emf induced as there isn’t any change in magnetic flux linkage

144
Q

What are some ways we can find electromagnetic induction? What are some factors to increase emf for each?

A

moving bar magnet through a coil connected to a voltmeter
-move magnet faster
-add more turns to coil
-increase strength of bar magnet

moving a wire through a magnetic field
-increasing length of wire
-moving wire between magnets faster
-increasing strength of magnets

145
Q

What happens as we rotate a coil through a uniform magnetic field?

A

-flux through coil will vary
-emf will constantly change as it rotates (faraday’s law)

146
Q

When will we have maximum emf when we have a rotating coil in a uniform B-field? Show this using a diagram and label it

A

when the coil cuts through the most field lines
https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/06/7.9.5-Coil-Turning-E.m.f.png

147
Q

How can we find the induced emf from a rotating coil using a graph of emf against time? What is the max emf equal to?

A

ε = BANω sin(ωt)

where:
ε = emf induced in the coil (V)
B = magnetic flux density (T)
A = cross-sectional area of the coil (m^2)
ω = angular speed of the coil (rad s-1)
t = time (s)

εo= BANω

148
Q

What is the size of the emf and flux linkage on a coil if the plane of the coil is perpendicular or parallel?

A

perpendicular:
emf=0
flux linkage= maximum

parallel:
emf= maximum
flux linkage= 0

149
Q

What are eddy currents?

A

-swirling currents induced in conducting materials when exposed to changing magnetic fields
-they create their own magnetic fields

150
Q

Why does the heat on a metal sheet increase when moving in/out of a magnetic field?

A

due to eddy currents in sheet

eddy currents circulating in the plane of the metal generate current loops that oppose the effect that caused them (Lenz’s law occurs)

151
Q

What happens if you cut up a metal sheet then move in/out of a B-field?

A

disrupt the eddy currents so reduce the effect of the sheet getting very hot

152
Q

What is a use of eddy currents?

A

can be used for induction heating

153
Q

What do we mean by an alternating current?

A

a current which periodically varies between a positive to a negative value with time

154
Q

What does it mean when the induced emf formula uses sin instead of cos?

A

it means the emf is alternating

155
Q

What do oscilloscopes do?

A

-used to display, measure and analyse waveforms of electrical circuits
-can be used as an ammeter and voltmeter for both DC and AC supplies

156
Q

What does the y-gain do on an oscilloscope?

A

changes the volts per division (y-axis scale)

157
Q

What does the time-base do on an oscilloscope?

A

changes the time per division (x-axis scale)

158
Q

How is an AC voltage shown on an oscilloscope? What things can we get from it?

A

-represented as a transverse wave
-can determine frequency, time period and peak voltage (Vo)

159
Q

How is a DC voltage shown on an oscilloscope?

A

represented as a horizontal line at the relevant voltage

160
Q

What can we see on an oscilloscope for an AC voltage if the time base is switched on/off?

A

off:
vertical line is shown on the y-axis, its total length being 2Vo and peak voltage at Vo

on:
a transverse wave will appear across whole screen

161
Q

What can we see on an oscilloscope for an AC voltage if the y-gain is switched on/off?

A

off:
only a horizontal line on x-axis is seen

on:
a transverse wave will appear across whole screen

162
Q

What can we see on the oscilloscope for a DC voltage when the time-base is switched on/off?

A

off:
a dot at Vo is shown

on:
a horizontal line at Vo is shown

163
Q

What can we see on the oscilloscope for a DC voltage when the y-gain is switched on/off?

A

off:
no reading

on:
horizontal line at Vo/dot at Vo shown (depending if time-base is on/off)

164
Q

How can we read measurements from an oscilloscope?

A

-count the number of divisions (adjusting axis to make it easier)
-multiply by volts per div/time-base depending on what you’re measuring

165
Q

What things can you take away from an oscilloscope reading?

A

-Vo (peak voltage- max amplitude from equ.)

-peak to peak voltage
(distance from min. to max. point)

-rms voltage

-time period

166
Q

What do we mean by Irms/Vrms?

A

the steady direct current/voltage that delivers the same average power in a resistor as the AC current/voltage

167
Q

How can we work out Irms?

A

Irms = Io / √ 2

where:

Irms= root mean square current (A)
Io= peak current(A)

168
Q

How can we work out Vrms?

A

Vrms= Vo / √ 2

where:

Vrms= root mean square voltage (V)
Vo= peak voltage (V)

169
Q

What are the AC equivalent current/voltage to Dc values?

A

DC= AC(rms)
e.g. I rms=I dc

170
Q

What is an application of AC current and voltage?

A

national grid:
mains electricity for domestic houses= 230V (rms value)

171
Q

What is a transformer?

A

a device that changes high AC voltage at low current to low AC voltage at high current, and vice versa

172
Q

What are transformers made up of?

A

-primary coil
-secondary coil
-iron core

173
Q

draw and label a transformer

A

https://alevelphysicsnotes.files.wordpress.com/2015/01/transformer.png

174
Q

What is the iron core of a transformer used for? Why does it need to be soft?

A

-focuses and directs magnetic field from primary to secondary coil
-soft iron used so it can easily be magnetised and demagnetised

175
Q

What happens in the primary coil of a transformer?

A

-an AC current producing an AC voltage is applied
-creates AC magnetic field ∴ a changing NΦ

176
Q

What happens in the secondary coil of a transformer?

A

-changing magnetic field passed to secondary coil through iron core
-causes changing NΦ ∴ emf is induced (Faraday’s law) and produces an AC output voltage

177
Q

What do we know about the frequency of an output AC voltage from the secondary coil of a transformer?

A

output voltage has same frequency as input voltage

178
Q

State the transformer equation

A

Ns/Np = Vs/Vp

where:
Ns= number of turns in secondary coil
Np= number of turns in primary coil
Vs= output voltage in secondary coil (V)
Vp= input voltage in primary coil (V)

179
Q

What are the two types of transformers?

A

step-up
step-down

180
Q

What do step-up transformers do? What are they used for?

A

-increase p.d., decrease current
-used between power stations and transmission wires

181
Q

What do step-down transformers do? What are they used for?

A

-decrease p.d., increase current
-used between transmission wires and buildings

182
Q

How can we tell if a transformer is step-up or step-down?

A

using faraday’s law

-step-up transformers have more turns on secondary coil
(as more induced emf is outputted)

-step-down transformers have more turns on primary coil
(as less induced emf is outputted)

183
Q

What would happen if a transformer was set up with a direct current connected to a primary coil?

A

-no emf induced on secondary coil
-DC current generates static B field
∴ Φ is consistent so no rate of flux linkage on secondary winding, so no induced emf (faraday’s law)

184
Q

How can we measure the efficiency of a transformer?

A

efficiency= useful power output/ power input

using P=IV:
efficiency= IsVs/ IpVp

where:
Is and Vs= secondary current/p.d.
Ip and Vp= primary current/p.d.

185
Q

What would equation to work out power be if a transformer was 100% efficient?

A

100% efficiency= no power loss

power in=power out
IpVp= IsVs

186
Q

What are the main losses of energy in transformers? How can we reduce these? (4)

A

-loss of flux linkage in coils
(wind coils tight to reduce leakage)

-eddy currents in core
(laminate core with insulator, increase resistance of core)

-energy lost as heat in wires
(decrease resistance of wire, use thick wires)

-when core cannot easily be magnetised (use a soft core)

187
Q

What is the main reason of operating power lines at high voltage (like in national grid)?

A

reduces current during transmission which reduces heat transfer

I = P/V

188
Q

How can we find out the power lost from current in a transformer?

A

P= I^2 x R

where:
P = power (W)
I = current (A)
R = resistance (Ω)

AQA A-level Physics (11 decks)

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