Electromagnetic Induction Flashcards
Define electromagnetic induction
The generation of an emf when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines
For a magnet moved near a wire to induce an emf, how can this emf be increased?
- Moving the magnet faster
- Using a stronger magnet
- Making the wire into a coil and pushing the magnet in and out of the coil
For a source of emf, state the equation showing the rate of energy transfer to the component
Rate of Energy Transfer = Induced emf x the current
Describe electromagnetic induction for the movement of a metal rod through a magnetic field
When electrons move across a magnetic field, they experience a force at right angles to their direction of motion
Since a metal rod contains free electrons, when it is moved across a magnetic field, the electrons feel a force and are moved to one end, thus creating a charge at either end.
State the dynamo rule
The dynamo rule (AKA Fleming’s right-hand rule) shows that the direction of the induced current is opposite to the direction of flow of electrons
What does each finger in Fleming’s right-hand rule represent?
ThuMb - Motion of conductor
First finger - Field
SeCond finger - Current
In which direction does current flow round the North pole end of a solenoid?
Anticlockwise
Define solenoid
A cylindrical coil of wire acting as a magnet when carrying electric current
State Lenz’s law
Lenz’s law states that the direction of the induced current is always such as to oppose the change that causes the current
Explain Lenz’s law in terms of the conservation of energy
Since energy is always conserved, the induced current could never be in a direction to aid the change that caused it; that would mean producing electrical energy from nowhere
Define magnetic flux
Magnetic flux is the product of the average magnetic field (magnetic flux density, B) times the perpendicular area that it penetrates (A)
Φ = BA
Give the unit for magnetic flux
Weber (Wb) = 1 Tm²
State Faraday’s law of electromagnetic induction
The induced emf in a circuit is equal to the rate of change of flux linkage through the circuit
ε = - NΔΦ / Δt
Define magnetic flux linkage
The magnetic flux through a coil of N turns
Flux linkage = NΦ = NBA
where B is the magnetic flux density perpendicular to area A
Give the equation for the magnetic flux linkage when:
i) The magnetic field is along the normal to the coil face
ii) The coil is turned through 180°
iii) The magnetic field is parallel to the coil area
i) = NΦ = BAN
ii) = -NΦ = -BAN
iii) = 0, since no field lines pass through the coil area
Give the equation for the emf induced for a moving conductor of length L through a magnetic field
Induced emf, ε = BLv
Give the equation for the emf of a alternating current generator where ε₀ is the peak emf and the coil rotates at a steady frequency
ε = ε₀ sin2πft
where t is the time after θ = 0
therefore: ε = ε₀ sinωt
since the angular velocity of the coi, ω = 2πf
For a coil spinning at a steady frequency f, give the equation for the angle from the normal, θ, after time t
θ = 2πft
Describe how a spinning coil can be used as a DC generator
By replacing the 2 slip rings of an AC generator with a split-ring. The emf does not reverse its polarity because the connections between the split-ring and the brushes reverse every half cycle
For a rotating coil, when is the rate of change of flux the greatest?
When the flux through it is 0
Define back emf
Where an emf is induced in the spinning coil of an electric motor because the flux linkage through the coil changes. The emf induced acts against the pd applied to the motor in accordance to Lenz’s law
At any instant:
V - ε = IR
where ε is the back emf, V is the pd applied to the motor, I is the current through the motor coil and R is the circuit resistance
Give the relationship between the induced emf (back emf) and the speed of rotation of the motor and thus the current also
The induced emf is proportional to the speed of rotation of the motor, therefore the current changes as the motor speed changes
State the equation showing the electrical power for the back emf of a motor
Electrical power supplied by source = electrical power transferred to mechanical power + electrical power wasted due to circuit resistance
IV = Iε + I²R
Define transformer
An apparatus for reducing or increasing the peak voltage of an alternating current
Describe a transformer and how it works
Any transformer consists of 2 coils: primary and secondary
When the primary coil is connected to a source of alternating pd, an alternating magnetic field is produced in the core. This field passes through the secondary coil so an alternating emf is induced in the secondary coil by the changing magnetic field
Describe a step-up transformer
A transformer with more turns on the secondary coil than on the primary coil. So the secondary voltage is stepped up compared with the primary voltage
(Ns > Np so Vs > Vp)
Describe a step-down transformer
A transformer with fewer turns on the secondary coil than on the primary coil. So the secondary voltage is stepped down compared with the primary voltage
(Ns
Give the transformer rule
Vs / Vp = Ns / Np
State and explain 3 reasons why transformers are almost 100% effective
1) Low-resistance windings to reduce power wasted due to the heating effect of the current
2) A laminated core which consists of layers of iron separated by layers of insulator prevents eddy currents (induced currents in the core) so that the magnetic flux is as high as possible
3) A core of ‘soft iron’ which is easily magnetised and demagnetised, which reduces power wasted through repeated magnetisation and demagnetisation of the core
Give the equation for the efficiency of a transformer
Efficiency = Power delivered by secondary coil / power supplied to primary coil Efficiency = IsVs / IpVp x 100%
Give the equation for the electrical power supplied to the primary coil for an almost 100% efficient transformer
Power supplied to 1° coil = Power supplied by 2° coil
Explain why electricity is carried round the UK in a grid system
Because the transmission of electrical power over long distances is much more efficient at high voltage
