CORE 7: FIELDS & THEIR CONSEQUENCES
|7.6||ALTERNATING CURRENTS AND TRANSFORMERS|
# a force field is a region where a body experiences a non-contact force
# 3 types of force field exist: gravitational, electric, magnetic
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2. GRAVITATIONAL FIELDS
# Newton’s Law: the force between 2 point masses is proportional to the product of
the masses and inversely proportional to the square of their separation.
F = Gm1m2 / r2
# G is called the gravitational constant, units N m2 kg -2
# Gravitational field strength g, at a point in a field, is the force that would act
on 1 kg placed at that point.
g = F / m , units are N / kg
# Field strength is a vector: get neutral points between two fields
# variation of g for a radial field (eg away from the centre of the Earth): g = GM / r2
# Equivalence of g as F/m and as acceleration of free fall.
# Gravitational potential, at a point in a field, is the work done in bringing
1 kg up from ¥ to that point.
V = W / Q , units are J / C.
# potential at any point in a radial field, V = − GM / r
# Gravitational potential energy = mass x gravitational potential.
Gpe = − mGM / r
# Work Done, W, in moving a mass in a field: W = m DV
# Equipotential surfaces. No work is done when moving along an equipotential surface.
# Graph showing how gravitational potential V varies with distance r away (in a radial field):
# Field strength = − (potential gradient), g = − DV / Dr so, field strength at any point
on the graph = the gradient at that point.
# DV = area under the g against r graph.
# Geostationary satellites : 24 hours, equatorial, about 42 000km orbit
radius (equivalent to an orbit height of 36 000km), west to east.
# Exploration satellites : orbits much closer in, shorter period eg few hours,
Can be polar orbit then all of globe can be scanned by single satellite.
# T2 ∝ r3 , applied to orbits (Keplers 3rd law)
# use of logarithmic plots to show relation between T and r
you need to be able to show how Newton's equations agree with Keplers 3rd law:
# kinetic energy, Ek = ½ m v2 = ½ m G M / r
total energy = potential energy + kinetic energy
ETotal = Ep + Ek = –G M m / r + ½ m G M / r = − ½ m G M / r
# Escape velocity (ve) is the minimum velocity of projection for an object to escape the Earth’s gravitational field
using the KE to GPE principle, ½ m v2 = (–) G M m / R , ve = (2 G M / R)1/2
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3. ELECTRIC FIELDS
# Coulomb’s Law: the force between 2 point charges is proportional to the product
of the charges and inversely proportional to the square of their separation,
F = Q1 Q2 / 4πε0 r2
where ε0 is called the permittivity of free space, having units F / m
# Electric field strength E, at a point in an electric field, is the force that would
act on 1C placed at that point.
E = F / Q units are N / C
# Variation of E away from the centre
of a charged sphere (radial field):
E = Q / 4πε0 r2
# E between parallel plates (uniform field):
E = V / d (other unit for E is V / m)
# Electric potential, at a point in a field, is the work done in bringing
+1 C up from ¥ to that point.
V = W / Q , units J / C or Volts V
# Electric potential in a uniform field, V = Q / 4πε0 r.
Graph showing how electric potential varies with distance away (in a radial field):
# Electric Potential Energy = charge x electric potential,
Epe = QV, Epe = qQ / 4πε0r for a charge q in a radial field.
# Work Done, W, in moving a charge q in a field: W = qDV
E = V / d derived from work done, F x d = q DV
# Field strength = ±(potential gradient) , (NB electric fields can be attractive or repulsive)
E = ± DV / Dr units V / m
# Charged particles moving at right angles to an electric field follow a parabolic
trajectory, if the field is uniform the particles experience a constant force.
Equations for uniform acceleration apply here: SUVATS!
Charged particles moving parallel to the E field experience no force.
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# Capacitance of a capacitor = charge on 1 plate divided by p.d. between plates
# dielectric equation, C = A εr ε0 / d
where εr is called the relative permittivity or dielectric constant
# when a dielectric is inserted it becomes polarised, reducing the resultant electric field between the plates
which in turn increases the capacitance of the capacitor
# time constant = R x C (units seconds)
# Charging capacitor: time constant = time taken for a capacitor to charge to 63%
of final (max) charge or voltage
# Discharging capacitor: time constant = time taken for a capacitor to discharge
to 37% of initial charge or voltage
# Note that the half charge time (or half discharge time) is 0.69 RC
# capacitor charge equation use, Q = Q0 ( 1 − e - t / RC ), capacitor discharge equation use, Q = Q0 e - t / RC
# Energy stored by a charged capacitor: E = ½ QV from area under the Q / V graph.
Also, E = ½ C V2 and E = ½ Q2 / C.
# Graph work: you need to know a whole range of graphs for both charging and discharging: V / t , I / t, Q / t, E / t
Also, watch out for the log graphs: ln Q / t , ln V / t
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5. MAGNETIC FIELDS
# Fleming’s left hand rule : thumb = force direction, 1st finger = field direction, 2nd finger = current direction:
F = B I L
# Magnetic flux density of a field = the force that acts on a wire of length 1 m
carrying a current of 1 A whilst lying at right angles to the field.
1 Tesla = 1 N A-1 m-1
# A charged particle moving in a B field follows a circular trajectory because
a force continually acts at right angles to its direction – the condition for
circular motion. This is a centripetal force.
F = B Q v, B Q v = mv2 / r
# Magnetic flux F threading a surface is the flux density normal to that surface
x the area of that surface: F = B A
1 Weber = 1 Tesla x 1 m2
# Magnetic flux linkage is the flux density normal to a coil x area of coil x
number of turns on coil: N F = B A N
# Faraday’s Laws (combined statement) the emf induced in a conductor cutting
through field lines = the rate of flux cutting. For a coil of N turns this is written:
E = DNF / Dt
# Lenz’s Law -the direction of the induced emf/current is always so as to oppose
the effect that caused it. (this is a statement of the law of conservation of energy)
NB. In the diagram above, the coil becomes an induced electromagnet (Faraday)
whose upper pole is a North (Lenz).
Induced currents are often called eddy currents.
# AC generators: in a coil rotating at ω rad/s in a uniform B field:
In Figure1 above, the flux linking the coil = BAN cosθ ,
where θ = angle between the normal to the coil and the magnetic field.
also, θ = ωt , and when Faraday's E = DNF / Dt is applied we get:
# EMF induced in the coil, E = BANω sinωt
NB. In figure 2 flux linking the coil is a minimum but E is a maximum in this position.
6. ALTERNATING CURRENTS
# SINUSOIDAL VOLTAGE: one for which a graph of pd against time is a sine curve
# INSTANTANEOUS values vary continuously throughout 1 cycle.
PEAK (maximum) values are more useful
# FREQUENCY = 1 / T
# ROOT MEAN SQUARE VALUE (RMS) : The rms value of an alternating current is the value of the
DIRECT current which would produce the same power.
ADVANTAGE : dc theory may be used on rms values
# RMS AND PEAK values are related:
VRMS = V0 IRMS = I0
# Oscilloscopes can be used to measure peak values directly from which rms values can be calculated,
using the equation above.
Peak voltage = height of trace x voltage gain(in V/div)
Oscilloscopes can also be used to measure frequencies,
by first measuring the TIME PERIOD T of the signal, then using f = 1 / T.
Time period = horizontal length of 1cycle x timebase setting
# TRANSFORMERS: turns ratio equation , Vs / Vp = Ns / Np ,
for an ideal transformer, Power out = Power in, Is Vs = Ip Vp
in practice, Power out < Power in , so Is Vs = ε Ip Vp
where ε = efficiency of the transformer (ideal transformer ε = 1)
# Be familiar with the causes of transformer inefficiency.
# Be familiar with causes of power losses in transmission lines.
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