Download Electric and Magnetic Fields Revision File

Unit G485: Fields, Particles and
Frontiers of Physics Revision
Unit Structure
1. Electric and magnetic fields
2. Capacitors and exponential decay
3. Nuclear physics
4. Medical imaging
5. Modelling the universe
Electric and Magnetic Fields
Describe the magnetic field patterns of a long straight
current-carrying conductor and a long solenoid
Electric and Magnetic Fields
Define Magnetic Flux Density
A measure of the strength of a magnetic
field at a given point, expressed by the
force per unit length on a conductor
carrying unit current at that point.
B = F / IL
Electric and Magnetic Fields
Define Magnetic flux
Magnetic flux = magnetic flux density x
cross-sectional area perpendicular to
field direction (B x A normal to B).
Electric and Magnetic Fields
Define Magnetic flux
Electric and Magnetic Fields
Define Magnetic flux linkage
Magnetic flux linkage =NBA
Electric and Magnetic Fields
State and use Fleming’s left-hand rule.
If the first two fingers and thumb
of the left hands are placed at right
angles then the first finger is in the
direction of the field, the second in
the direction of the current and the
thumb in the direction of motion .
Electric and Magnetic Fields
State and use Faradays Law of electromagnetic induction
Induced e.m.f is proportional to the rate of
change of (magnetic) flux
Electric and Magnetic Fields
Electric and Magnetic Fields
Describe Lenz’s Law
The direction of the emf induced in a conductor is
such as to oppose the change producing it.
Electric and Magnetic Fields
Applying Lenz’s Law
Which magnet will
reach the bottom of
the tube first ?
The non magnetised. The magnetised steel will induce an
emf in the copper which in turn will create eddy currents in
the copper. These eddy currents will produce a magnetic
field which will oppose the change producing them – they
will slow down the steel.
Electric and Magnetic Fields
State and use Faradays Law of electromagnetic induction
- The Graph Question
Graphical analysis often involves a graph of flux linkage .
Note that to work out the total flux linkage you will have to
multiply by the number of turns.
To work out the rate of change you will have to work out the
gradient.
Electric and Magnetic Fields
State and use Faradays Law of electromagnetic induction
- The ac Generator Question
Electric and Magnetic Fields
Describe the function of a simple ac generator
An electric generator converts mechanical
energy in the form of the rotation energy of a
coil of wire into electrical energy .
Electric and Magnetic Fields
State and use Faradays Law of electromagnetic induction
– The Aircraft Question
Emf generated = Blv ( where v is the velocity NOT
voltage)
Electric and Magnetic Fields
Analyse the circular orbits of charged particles in
magnetic fields
You will have to equate BQv and mv2 /r
The particle does not speed up because the force is
perpendicular to its velocity so no work is done in the
direction of its velocity.
Electric and Magnetic Fields
Electric and Magnetic Fields
Describe how electric field lines represent an
electric field
The direction of the electric field is defined as
the direction in which a positive charge would
move if it were free to do so. So the lines of
force can be drawn with arrows that go from
positive to negative .
Electric and Magnetic Fields
State what is meant by an electric field
An electric field is a region of space where
a stationary charge experiences a force
due to its charge .
Electric and Magnetic Fields
How do we work out the force between two
charges ?
Similar to Newton’s Law of Gravitation
Both fields are radial and 1/r2
But electric can be repulsive as well as attractive.
Electric and Magnetic Fields
Define Electric Field Strength
Electric field strength at a point in space
is the force per unit positive charge .
Electric and Magnetic Fields
Electric Field Strength - Numerically
Units Vm-1 or NC-1
Electric and Magnetic Fields
Explain the effect of a uniform electric field on the
motion of a charged particle
If E is uniform, then the acceleration of the
charged particle is constant. If the particle has a
positive charge, then its acceleration is in the
direction of the electric field. If the particle has
negative charge, then its acceleration is in the
direction opposite the electric field .
Electric and Magnetic Fields
Explain the effect of a uniform electric field on the
motion of a charged particle
You may have to use the equations of motion once you
have worked out an acceleration ( F = eE and F = ma )
Remember there is no horizontal acceleration.
Electric and Magnetic Fields
Explain the effect of a uniform electric field on the
motion of a charged particle- The Transfer Equation
eV = ½ m v
2
The most forgotten equation !
Electric and Magnetic Fields
Explain the use of deflection of charged particles
in the magnetic and electric fields of a mass
spectrometer
Electric and Magnetic Fields
Explain the use of deflection of charged particles in the
magnetic and electric fields of a mass spectrometer
The question is nearly always about the radius !
Electric and Magnetic Fields
Describe the function of a simple transformer .
A simple transformer is two coil of insulated wire wound
on to a laminated soft iron core. And alternating e.m.f is
applied across the primary coil. The current produced
creates an alternating magnetic field which links the
secondary coil and an e.m.f is induced in the secondary
Electric and Magnetic Fields
Describe the function of a simple transformer .
Numerically
Electric and Magnetic Fields
Define The Tesla
This is the magnetic flux density if a wire
of length 1m carrying a current of 1 A
has a force of 1 N exerted on it in a
direction perpendicular to both the flux
and the current.
Electric and Magnetic Fields
Define The Weber
The Weber is the magnetic flux which, linking
a circuit of one turn, would produce in it an
electromotive force of 1 volt if it were
reduced to zero at a uniform rate in 1 second .
(From Faradays Law)
One Weber is equal to one Tesla metre²