M4, 6 Electromagnetism Flashcards
Electric field
- goes from positive to negative.
- Electric field lines leave and enter a surface at right angles.
E = q / epsilon0 x A
E = q / 4 x pi x epsilon0 x r2
Electric field strength
E = -V/d = F/q
Work in an electric field
W = qV = qEd W = K, therefore, qV = mv2/2.
Direction of charges in an electric field
Negative charges experience a force in the opposite direction to the electric field. Positive charge goes in the same direction.
Coulombs Law
F = q1q2/4 x pi x epsilon0 x r2
If force is + then charges are repulsive (same sign). If force is - then charges are attractive (opposite signs).
Right hand push rule
Fingers - electric field direction, palm - force direction, thumb - positive particle direction velocity (negative is opposite).
Magnetic force
F = qvBsin(theta)
B in Teslas.
Goes from north to south.
Centripetal force
Fc = FB Fc = mv2/r ac = v2/r T = 2pi x m/qB r = mv/qB
Discovery of the electron - JJ Thomson: Paddle Wheel Experiment
cathode ray tubes.
Proved electrons have a mass as they pushed the wheel, charge was negative as they were repelled from the negative terminal and attracted to the positive terminal.
Maltese cross experiment
cathode ray tube. Casts a sharp shadow. Therefore, at the time the cathode rays (electrons) were thought to be electromagnetic (like light).
Ohm’s law
V = IR
Kirchoff’s current law
Total current entering a junction is equal to total current leaving junction.
Total Resistance
1/RTotal = 1/R1 + 1/R2 + 1/R3 + …
Current in a wire
I = q/t
Average speed of electrons in a wire
v = L/t
Force on electrons
F = LIBsin(theta) B = mu(0) x I / 2pi x r
Force per unit length between wires
F/L = mu(0)I1I2 / 2pi x r
Solenoid magnetic field
B = mu(0)NI/L
N is number of turns.
Lorentz force
The force acting on a charged particle moving through a magnetic field.
Motor effect
If the charged particles are constrained to move within a conductor, then the charged particles will exert a force on the conductor. (wires, right hand grip rule, AS OR rule).
Magnetic flux
‘flow’, in Webers (Wb). Φ = BAcos(theta)
Change in magnetic flux
change in Φ = Φfinal - Φinitial
Faraday’s Law of Electromagnetic Induction
Emf (induced voltage) = epsilon = -N x change in Φ/ change in t
N is flux linkage, number of turns.
Lenz’s Law
The induced change in flux in a metal pipe/solenoid opposes the original change in flux of a moving magnet or magnetic field through the pipe/solenoid.
Induced emf of wire
emf = BvL
Magnetic flux density
- same as magnetic field strength = B
Magnetic flux linkage
= NΦ
N is the number of loops. Each loop produces its own emf, and the emfs from each loop add to the total emf. Emf is only produced when flux is changing.
Transformers
Primary current must be AC to create a change in electromagnetic flux.
Step-down transformer
secondary coil has less coils than primary coil. Emf is lower.
Step-up transformer
secondary coil has more coils than primary coil. Emf is higher.
Eddy current
of electrons due to changing magnetic field causing loss of energy as heat.
Laminated iron core
means smaller eddy currents, therefore less energy lost. ‘Soft’ iron core: does not keep its magnetic field, takes on the magnetic field of whatever it is in.
Transformer equations
assuming ideal transformer:
V(P) / V(S) = N(P) / N(S)
V(P) x I(P) = V(S) x I(S)
Power loss equation
P = I2R.
Methods to minimise energy loss in transformers
- Circular eddy currents flow at right angles to direction of change in magnetic flux. Eddy currents reduced by layering the iron core, insulated with lacquer (laminating). 2. The coil wires are thicker on the high current side of the transformer. Thicker wires have less resistance, so this minimises resistance heating the coils. 3. Large transformers may have a cooling oil circulating through a heat exchanger, much like a radiator system of a car engine.
Right hand rule for solenoids
Remember thumb points to the north end of the solenoid. Whatever pole of a bar magnet is being moved towards a solenoid the end of the solenoid closest will have the same pole as the bar magnet closest to it. If move away then opposite pole.
Emf equation
When flux=0, emf=max.
epsilon = -NΦ/t = -NBAsin(theta)
AC Induction motors
are brushless.
Use electromagnetic induction to rotate.
Have slip rings.
The flux and emf have the same frequency.
Theta = 2pi x ft.
emf = 2pi x fNBAcos(2pi x ft).
f in rev/sec.
No current is supplied directly to the rotating coils, but a current is induced in them by a changing magnetic field, produced by an AC current.
I = (emf supplied - back emf) / R.
Rms voltage
root means square.
v(rms) = v(peak)/sqrt2
DC motor
uses a current carrying coil in a magnetic field. The coil experiences a torque due to the interaction of the field with the current. This makes the coil rotate. The net result is the conversion of electrical potential energy into kinetic energy. T = Frsin(theta)
Stator
the part of the motor that doesn’t move. Includes the casing of the motor and the magnets. The input wires and brushes are usually attached to the stator.
Rotor
the rotating part. Consists of the armature, which holds the coils, the coils themselves, and the commutators.
Commutators
change the electrical contacts on the wires as the coil’s momentum carries it past its balance point. Acts as a switch, changes the direction of the current every half rotation (split-ring commutator).
Brushes
made of graphite or carbon blocks usually provide the sliding contact.
Back emf
the induced emf in a motor.
Generators
converts the kinetic energy used to spin a coil in a magnetic field into electrical potential energy that creates a current in a coil.
AC generator
Generator using alternating current. Has a slip ring.
DC generator
provides an output emf that is always positive. Uses split ring commutator.
