Electromagnetic Induction Flashcards
Induced Electromotive Force
occurs when a conductor moves relative to a magnetic field, the e- in the conductor have a force exerted on them and they gain electrical energy
IMPORTANT - emf is only induced if the magnetic field lines are “cut” by the conductor (i.e. moving parallel to field lines will not induce emf) Max emf is induced when the conductor is moving perpendicular to the magnetic field lines
Deriving induced emf formula
if the e in the middle of the conductor is in equilibrium then:
magnetic force due to the motion (UP) = electrical force due to the emf (DOWN)
Bqv = Eq
but E=V/d = V/l so Bqv = (Vq)/l
V = Bvl
as no current is flowing, emf = V = Bvl
Magnetic flux definition
ø = BAcosθ
where B = strength of magnetic field
A = area the field covers
and θ = angle between field lines and the normal to the plane of area
units = Tm2 or Wb (Weber)
Faraday’s Law
gives SIZE of induced emf
the magnitude of an indced emf is proportional to the rate of change of flux linkage
(flux linkage = product of the number of turns in a coil and the flux through the coil)
emf = -N(deltaø/deltat)
Lenz’s Law
gives DIRECTION of induced emf
the direction of the induced emf is such that if an induced current were to flow, it would oppose the change which it caused
emf = -N(deltaø/deltat)
explains - at front of formula
How is AC current generated?
if a coil of wire is spun in a magnetic field at a constant speed and is connected to a circuit outside the magnetic field then the induced emf will be sinusodial and a AC current will flow
when the coil is vertical:
- induced emf = 0 as no cutting of flux lines occurs and rate of change of flux is minimum
- max flux occurs
when coil is horizontal:
- induced emf = max as coil cuts flux lines at 90 degrees and rate of change of flux is greatest
- flux = 0
i.e. ø is the -ve gradient of emf
direction of induced voltage changes every half cycle and as the induced current flows in a direction to create a magnetic field that opposes the rotation of the coil, work must be done to keep the coil rotating
How can size of induced voltage be increased?
- having more loops of wire
- having a stronger magnetic field
- rotating the coil at a faster rate
What affect will increasing the speed of rotation of the coil have?
- reduce the period of emf oscillation
- increase the amplitude of emf (increases rate of change of flux linkage)
e.g. doubling speed of rotation doubles amplitude and hlaves period
AC rms def.
the AC rms value of current or voltage is that which would produce the same amount of [pwer as the equivalent DC value
What is a transformer?
used to increase or decrease voltages
the primary coil where AC current is input creates alternating flux in the iron core which induces alternating emf in a secondary coil
varying the ratio of turns of wire between the two cold will determine by how much the voltage is stepped up or down
they are very efficient
Step - up transformer
secondar voltage > primary voltage
secondary coil has greater number of turns than primary
Step - down transformer
primary voltage > secondary voltage
primary coil has a greater number of turns than secondary
How are energy losses reduced by a transformer?
- having an iron core to ensure that a strong magnetic field between the primary and secondary windings. This ensure that very little magnetic flux leaks and does not pass through the secondary coil
- iron core is made of soft iron so it can be rapidly magnetised and demagnetised
- the core is laminated (has alternating layers of iron and insulating material) to stop the formation of eddy currents in the core. Eddy currents are currents induced by the core itself by the øand causes energy losses by heating up the core
- using low resistance wire for the coils
Power transmission
- transmitted at very high voltages to reduce energy losses
- power lines have some R so heat up
- the power lost is prop. to I^2, so transmitting at high V and low I, power losses are reduced
however high V are dangerous
Rectification
process by which AC is converted to DC
involves the use of diodes (electrical devices that only allow current through in one direction)
2 types:
- half -wave
- full wave
Half - wave rectification
a diode is connected in series with the AC supply
current through load is not steady but is DC
Full - wave recification
uses a diode bridge
Capacitors
a device that can store electrical charge
a parallel plate capacitor is one formed from 2 parallel metal plates with an insulating material (dielectric) between to separate charges
Capacitance
ability of a capacitor to store charge
defined as: the ratio of charge stored to potential difference between the plates
How do dielectrics work in capacitors?
- dielectric materials have polarised molecules (+ve and -ve ends)
- when placed in an electric field, the molecules align themselves with the field
- electric field strength between the plates is reduced
- E = V/d, d is fixed so V between plates is reduced
- as C = q/V, if V is reduced and q stays the same, C is increased
- more charge can be stored for a lower V
Charging of capacitors
when being charged the V across the capacitor and I in the circuit will have an exponential relationship with time
usually to charge a capacitor, it is connected in series with a resistor to a DC power supply
during charging:
- the V across the capacitor grows to its max value (= emf of supply)
- the current of the circuit falls to zero
time constant for charging = time taken for the voltage to reach 63% of its maximum value, or the current to fall to 37% of its maximum value
Discharging of capacitors
during discharging:
- the V across capacitor falls to zero
- the current in the (discharging) circuit falls to zero
time constant for discharging = time taken for the voltage or current to fall to 37% of its maximum value (decrease by 67%)
Capacitors in Series
the +ve plate of one capacitor will be connected to the -ve plate of the next capacitor
capacitors in series will all have the same charge (but different V)
Capacitors in Parallel
the +ve plate of one capacitor is connected to the +ve plate of the next capacitor
capacitors in parallel have the same V (but store a different charge)
Capacitors and Rectification
adding a capacitor in parallel to the load will smooth the current through the load
the capacitor charges when current flows and discharges when current drops
a large time constant will provide good smoothing but the disadvantage is that the capacitor will draw a large current whilst charging
