Fields and their consequences Flashcards
What is a force field?
a force field is an area in which an object experiences a non-contact force
Are force fields a vector or scalar quantity?
vector
How can force fields be represented? What can the diagrams show
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
How can force fields be formed?
formed during the interaction of masses, static charge or moving charges
What are the two examples of fields and what interaction takes place?
gravitational- formed during the interaction of masses
electric- formed during the interaction of charges
What are some similarities of gravitational and electric fields? (3)
-forces both follow an inverse-square law
-uses field lines to be represented
-both have equipotential surfaces
What are some differences between gravitational and electric fields? (2)
-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
State Newton’s law of gravity
gravity acts on any objects which have mass and is always attractive
What is Newton’s law of gravitation?
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
State the equation for Newton’s law of gravitation
F = (Gm1m2)/r^2
where:
f= force (N)
G= gravitational constant
m1 & m2= masses (kg)
r= distance between centres of masses (m)
What are the two types of gravitational fields?
radial and uniform
Draw a diagram of the two types of gravitational fields. Explain why the field lines are situated in a certain way
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)
How does gravitational field strength differ in both types of gravitational fields?
-constant in uniform field
-varied in radial field
What are the ways we can calculate gravitational field strength? When can one be used?
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)
What is gravitational potential?
the work done per unit mass when moving an object from infinity to that point
State the equation for gravitational potential
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)
Why is gravitational potential a negative value?
-it is defined as zero at infinity
-since gravitational force is attractive, work must be done on a mass to reach infinity
How can gravitational potential be used to find the work done when moving an object in a gravitational field?
ΔW= mΔV
where:
ΔW= change in work done (J)
m= mass (kg)
ΔV= change in gravitational potential (Jkg^-1)
What are equipotentials in gravitational fields?
regions of equal potential
What is something needed to know with equipotentials in general and gravitational potentials?
the gravitational potential difference is zero when moving along the surface so no work is done when moving along an equipotential surface
How can we show equipotential lines on a radial gravitational field diagram? What is something to know about the distances between each equipotential?
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
How can we show equipotential lines on a uniform gravitational field diagram? What is something to know about the distances between each equipotential?
perpendicular lines to field lines, distance between each equipotential is equal
How can we get the gravitational field strength at a certain distance from a gravitational potential-distance graph?
draw a tangent to the curve at that distance and calculate the gradient
How can we work out the gravitational potential difference from a gravitational field strength-distance graph?
finding the area under the curve
What do we mean by orbital period?
the time taken for an object to complete a full orbit of the central body
Derive orbital speed
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
State Kepler’s third law
the square of the orbital period (T) is directly proportional to the cube of the radius (r)
T^2 ∝ r^3
Derive Kepler’s third law
- 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 - cancel 1 of the m’s
ω^2r = (Gm) / r^2 - expand ω
(4π^2 / T^2) x r = (Gm) / r - 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
Give the equation for Kepler’s third law
T^2= (4π^2/ GM) x r^3
where:
T= orbital period (s)
G= gravitational constant
r= radius (m)
M= mass (kg)
How can we find the total energy of a satellite
kinetic energy + potential energy
What do we mean by escape velocity?
the minimum velocity an object must travel at in order to escape the gravitational field at the surface of a mass.
Derive the equation for escape velocity
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
What do we mean by synchronous orbit?
where the orbital period is equal to the rotational period of the object that it is orbiting
What does the ‘geo’ refer to in geosynchronous?
earth
What characteristics do geostationary/geosynchronous orbits have? (5)
-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
What is an example of a geostationary orbit? Why?
geosynchronous satellites
-have an orbital period of 24hrs
-orbit directly above the equator
Why are geosynchronous satellites useful to send signals?
they always orbit above same point on earth so you don’t need to alter the plane of an aerial or transmitter
What does it mean when something has a lower polar orbit?
their altitude is closer to earth’s surface
What is a characteristic of low orbit satellites? What does this mean?
travel much faster than geostationary therefore have a lower orbital period
Why are low orbit satellites useful?
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
State Coulomb’s law
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
State the equation for Coulomb’s law
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)
Define absolute permittivity
a measure of how difficult it is to establish an electric field within a material
How can we work out the absolute permittivity of a material?
ε = ε₀ x εr
where:
ε= absolute permittivity (fm^-1)
ε₀= permittivity of free space
εr= relative permittivity
How can charge affect the force between them?
if the charges have the same sign the force will be repulsive, if they are different the force will be attractive
Define electrical field strength
the force per unit +ve charge
How does the electric field strength differ from a radial or uniform field?
constant in uniform
varied in radial
What are the equations to work out electric field strength? What field can they be used for?
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)
What are the terms to describe the field lines for a radial field that is positive/negative?
positive- source of field lines
negative- sink of field lines
Draw a diagram of the types of electric fields. Explain why they are situated in a certain way
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
What are equipotential lines in an electric field?
regions of equal electric potential
How can we show equipotential lines on a radial gravitational field diagram? What is something to know about the distances between each equipotential?
circular, right angles to field lines, as distance increases the equipotential lines get further apart
How can we show equipotential lines on a uniform gravitational field diagram? What is something to know about the distances between each equipotential?
perpendicular to field lines, equal spaces
How does the electric field strength differ in uniform and radial fields?
constant throughout uniform fields
field strength depends on the distance between charges/density of field lines
Define absolute electrical potential
the work done per unit +ve charge moving it from infinity to a point
State the equation to work out the absolute electrical potential
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)
How can we work out the work done by moving a charged particle between parallel plates of a uniform field?
ΔW= QΔV
where:
ΔW= change in work done
Q= charge that is moving (C)
ΔV= change in potential difference (JC^-1)
What happens to a charged particle when it enters an electric field?
it will experience a force which will cause it to move
What happens to a charged particle if it remains still in a uniform field?
it will move parallel to the electric field lines (along or against depending on its charge)
What happens to a charged particle if it is in motion perpendicular to a uniform field? Include a diagram showing this with both +/- charges
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
How can we find the electric field strength from a potential-distance graph?
find the gradient
How can we work out the electric potential difference from an electric field strength-distance graph?
find the area underneath
What do we mean by capacitance?
the charge stored by a capacitor per unit p.d.
Describe the structure of a capacitor
-two parallel conducting plates e.g. metal film
-separated by an insulator aka dielectric (plastic ceramics, electrolytes)
State two uses of capacitors
-introduce time delaying to a circuit
-temporary storage of charge (electrostatic potential energy store)
How can you work out the capacitance of a capacitor?
C= Q/V
where:
C= capacitance (F)
Q= charge (C)
V= p.d. (V)
What happens when a capacitor is connected to a source of power/charged?
opposite charges build up on the two parallel plates causing a uniform electric field to be formed
What do we mean by permittivity? What material has this property?
-dielectrics
-a measure of the ability to store an electric field in the material
What do we mean by relative permittivity? What else can it be called?
-dielectric constant
-permittivity of a material relative to free space
What is the relative permittivity of air?
1
What does it mean if one material has a higher relative permittivity than another?
that material has a higher opposition of the electric field within it/better at opposing an electric field
How can you calculate the relative permittivity?
εr =ε /ε0
where:
εr= relative permittivity
ε0= permittivity of free space (Fm^-1)
ε= permittivity of a material (Fm^-1)
What factors affect capacitance?
-area of plates
-distance between plates
-relative permittivity of dielectric
How else can we find the capacitance of a capacitor using the plates?
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)
What are dielectrics formed of?
polar molecules
What are polar molecules?
molecules with one end which is positive and the other negative
Describe what the polar molecules of the dielectric are like and how they behave when the capacitor is uncharged
randomly orientated
Describe how the polar molecules behave when the capacitor is charged
-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)
Draw and annotate a diagram showing how polar molecules behave when a capacitor is charged
https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/06/7.6.2-Dielectric-E-Field.png
How can we find the energy stored by a capacitor from a charge-p.d. graph?
find the area
What equations can be used to find the energy of a capacitor?
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)
How can we charge a capacitor? Draw and label the circuit
-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
Describe what happens when the switch for a capacitor is closed
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
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?
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
What values can we find from a current-time graph/charge-time graph?
area under current-time= charge
gradient from charge-time graph= current
What happens when the capacitor connects to a power supply and charges
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
How can we discharge a capacitor? Draw and label the circuit
-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
Draw and annotate the graphs of current, charge and p.d. for when a capacitor discharges.
https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/04/19.2-Discharging-graphs.png
How can we find current/p.d./charge from a charging capacitor graph?
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)
How can we find current/p.d./charge from a discharging capacitor graph?
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)
What are some tips to remembering finding V from discharging/charging capacitor graphs?
swap Q for V using data book
What is RC and what can it also be known as?
-also known as the time constant
-equal to the value of time (s) taken to discharge/charge a capacitor
How can we find the time constant when discharging a capacitor?
find 37% of the max value (approx.)
How can we find the time constant when charging a capacitor?
find 63% of the max value (approx.)
How can you derive the time constant from 1/2 the time taken when discharging? (use current equation in this case)
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
How can we use log graphs to find the time constant?
-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
What are some advantages to using a log graph for discharging
-better averaging process
-rejects anomalies
-more accurate
What happens when we pass current through a wire? When is this true
-induces a magnetic field
-true for any long, straight current-carrying conductor
What is the right hand grip rule? How do we use this for a current carrying wire?
shows the direction of the magnetic field around a current-carrying wire when the thumb points in the direction of the current
What is the structure of field lines around a wire with a magnetic field?
forms concentric rings around the wire, arrows of field lines show the direction of field
What is magnetic flux density? What is its unit and symbol?
measures the strength of a magnetic field
-measured in Tesla
-symbol B
What is magnetic flux density also known as?
magnetic field strength
Is magnetic flux density a scalar or vector value?
vector
-has a magnitude and direction
What can the density of magnetic field lines show?
the strength of the field
higher density = larger strength of field
What is flux known as?
field lines
What happens to a current-carrying wire when places inside a magnetic field with the current perpendicular to the field lines?
a force is exerted
State the Fleming’s left hand rule. Show how it works
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
What should we think about if a question tells us that a ‘current’ is flowing in a certain direction?
use this current as conventional current
What should we think about if a question shows us the flow of electrons?
must treat the electrons as moving opposite to conventional current
Draw field lines on a magnet which has a north and south pole
https://upload.wikimedia.org/wikipedia/commons/thumb/0/0c/VFPt_cylindrical_magnet_thumb.svg/220px-VFPt_cylindrical_magnet_thumb.svg.png?20230924082557
How can we work out the force exerted on a current-carrying wire?
F= BIL
where:
F= force exerted (N)
B= magnetic flux density (T)
I= current (A)
L= length of wire (m)
What happens if the B-field and current-carrying wire are parallel to each other?
no force is exerted
-need to be perpendicular
Why is a force exerted on a current-carrying wire?
electrons from current are moving around which are negatively charged
∴a force is exerted due to charged particles moving in a magnetic field
How can we find the force exerted on an isolating moving charged particle?
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)
What will happen if a charged particle enters a uniform magnetic field? Why?
will travel in a circular path
-the force exerted on it will be perpendicular to its velocity ∴ has a centripetal force
How can we derive the radius of the circular path when a particle enters a uniform magnetic field?
-link centripetal force equation to BQv equation
-mv^2/r = BQv
-rearrange for r
State the equation to find the radius of a particles circular path when entering a uniform B-field
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)
What is an application of the charged particles moving in a circular path when entering a uniform B-field?
particle accelerators called cyclotrons
has many uses e.g.:
-producing ion beams for radiotherapy and radioactive tracers
What do cyclotrons do?
accelerate charged particles from their centre along a spiral path
Describe the structure of a cyclotron
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)
How do cyclotrons function?
-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
What is the purpose of the alternating electric field between the dees?
to accelerate the charged particles to high speeds
Why do we need an alternating p.d.?
needed so the particles can accelerate in both directions (allows the particle to accelerate across gap between opposite electrodes)
Draw and label a cyclotron
https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/06/7.8.5-Cyclotron.png
Why are particles always accelerating in a cyclotron?
experiences a centripetal force due to B-field, changing direction of velocity causes acceleration
How can we derive the time spend of a particle in BOTH dees of a cyclotron (full circle)?
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
Define magnetic flux
the number of flux lines (flux) per square metre
How can we work out magnetic flux?
Φ = BA
where:
Φ = magnetic flux (Wb)
B= magnetic flux density (T)
A= cross-sectional area (m^2)
When can magnetic flux occur?
when the field is perpendicular to the area/coil
Define magnetic flux linkage
measures the amount of flux which has been cut
What do we mean by ‘cutting flux’?
when something is moving through the field lines, like they’re ‘cutting’ it
How can we find out magnetic flux linkage?
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
What happens to the magnetic flux & magnetic flux linkage equation when magnetic field is not fully perpendicular to the coil?
Φ =BAcos θ
NΦ =BANcos θ
where cos θ is the angle between field and normal to plane of coil
What is the magnetic flux if the field is parallel to coil?
0Wb
draw the graph of magnetic flux against the angle to normal
https://cdn.savemyexams.com/cdn-cgi/image/w=960,f=auto/uploads/2021/05/20.2-WE-Calculating-Magnetic-Flux-graph-solution.png
What do we mean by electromagnetic induction? When can this occur?
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
What are some applications of where electromagnetic induction is used?
-in electrical generators which convert mechanical energy to electrical
-transformers which are used in electrical power transmission
What are the two ways which we can control the effects of electromagnetic induction?
using:
faraday’s law
lenz’s law
State Faraday’s law and the equation
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)
State Lenz’s law
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)
What is the equation showing Lenz’s law combined with Faraday’s law? Explain it
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
Explain how Lenz’s law can be verified using a coil and bar magnet. Draw and label the diagram
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
What happens if we leave a bar magnet stationary within a coil?
no emf induced as there isn’t any change in magnetic flux linkage
What are some ways we can find electromagnetic induction? What are some factors to increase emf for each?
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
What happens as we rotate a coil through a uniform magnetic field?
-flux through coil will vary
-emf will constantly change as it rotates (faraday’s law)
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
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
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?
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
What is the size of the emf and flux linkage on a coil if the plane of the coil is perpendicular or parallel?
perpendicular:
emf=0
flux linkage= maximum
parallel:
emf= maximum
flux linkage= 0
What are eddy currents?
-swirling currents induced in conducting materials when exposed to changing magnetic fields
-they create their own magnetic fields
Why does the heat on a metal sheet increase when moving in/out of a magnetic field?
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)
What happens if you cut up a metal sheet then move in/out of a B-field?
disrupt the eddy currents so reduce the effect of the sheet getting very hot
What is a use of eddy currents?
can be used for induction heating
What do we mean by an alternating current?
a current which periodically varies between a positive to a negative value with time
What does it mean when the induced emf formula uses sin instead of cos?
it means the emf is alternating
What do oscilloscopes do?
-used to display, measure and analyse waveforms of electrical circuits
-can be used as an ammeter and voltmeter for both DC and AC supplies
What does the y-gain do on an oscilloscope?
changes the volts per division (y-axis scale)
What does the time-base do on an oscilloscope?
changes the time per division (x-axis scale)
How is an AC voltage shown on an oscilloscope? What things can we get from it?
-represented as a transverse wave
-can determine frequency, time period and peak voltage (Vo)
How is a DC voltage shown on an oscilloscope?
represented as a horizontal line at the relevant voltage
What can we see on an oscilloscope for an AC voltage if the time base is switched on/off?
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
What can we see on an oscilloscope for an AC voltage if the y-gain is switched on/off?
off:
only a horizontal line on x-axis is seen
on:
a transverse wave will appear across whole screen
What can we see on the oscilloscope for a DC voltage when the time-base is switched on/off?
off:
a dot at Vo is shown
on:
a horizontal line at Vo is shown
What can we see on the oscilloscope for a DC voltage when the y-gain is switched on/off?
off:
no reading
on:
horizontal line at Vo/dot at Vo shown (depending if time-base is on/off)
How can we read measurements from an oscilloscope?
-count the number of divisions (adjusting axis to make it easier)
-multiply by volts per div/time-base depending on what you’re measuring
What things can you take away from an oscilloscope reading?
-Vo (peak voltage- max amplitude from equ.)
-peak to peak voltage
(distance from min. to max. point)
-rms voltage
-time period
What do we mean by Irms/Vrms?
the steady direct current/voltage that delivers the same average power in a resistor as the AC current/voltage
How can we work out Irms?
Irms = Io / √ 2
where:
Irms= root mean square current (A)
Io= peak current(A)
How can we work out Vrms?
Vrms= Vo / √ 2
where:
Vrms= root mean square voltage (V)
Vo= peak voltage (V)
What are the AC equivalent current/voltage to Dc values?
DC= AC(rms)
e.g. I rms=I dc
What is an application of AC current and voltage?
national grid:
mains electricity for domestic houses= 230V (rms value)
What is a transformer?
a device that changes high AC voltage at low current to low AC voltage at high current, and vice versa
What are transformers made up of?
-primary coil
-secondary coil
-iron core
draw and label a transformer
https://alevelphysicsnotes.files.wordpress.com/2015/01/transformer.png
What is the iron core of a transformer used for? Why does it need to be soft?
-focuses and directs magnetic field from primary to secondary coil
-soft iron used so it can easily be magnetised and demagnetised
What happens in the primary coil of a transformer?
-an AC current producing an AC voltage is applied
-creates AC magnetic field ∴ a changing NΦ
What happens in the secondary coil of a transformer?
-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
What do we know about the frequency of an output AC voltage from the secondary coil of a transformer?
output voltage has same frequency as input voltage
State the transformer equation
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)
What are the two types of transformers?
step-up
step-down
What do step-up transformers do? What are they used for?
-increase p.d., decrease current
-used between power stations and transmission wires
What do step-down transformers do? What are they used for?
-decrease p.d., increase current
-used between transmission wires and buildings
How can we tell if a transformer is step-up or step-down?
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)
What would happen if a transformer was set up with a direct current connected to a primary coil?
-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)
How can we measure the efficiency of a transformer?
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.
What would equation to work out power be if a transformer was 100% efficient?
100% efficiency= no power loss
power in=power out
IpVp= IsVs
What are the main losses of energy in transformers? How can we reduce these? (4)
-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)
What is the main reason of operating power lines at high voltage (like in national grid)?
reduces current during transmission which reduces heat transfer
I = P/V
How can we find out the power lost from current in a transformer?
P= I^2 x R
where:
P = power (W)
I = current (A)
R = resistance (Ω)
