11 Electromagnetic Induction Flashcards
electromagnetic induction
the process by which an emf is induced when a conductor experiences a changing magnetic field
Electromagnetic induction may involve a
- moving magnet
- moving conductor
- magnetic field around a changing current
Describe the production of an induced emf by a changing magnetic flux and within a uniform magnetic field
moving charges (electrons) in the conductor across a magnetic field → experience magnetic forces →
one side gains -ve charge, the other side loses -ve charge and becomes more +ve → emf (p.d.) is induced
Describe the electromagnetic inductions when D.C. and A.C. batteries are used
- magnetic field due to D.C. current changes from zero to non-zero at the moment when battery is turned on → induces momentary emf
- magnetic field due to A.C. current constantly changes → induces continuous alternating emf
magnetic flux
Φ = BA cosθ
product of normal component of magnetic field strength and area that it links
magnetic flux linkage
NΦ = NBA cosθ
product of number of turns in a coil and the magnetic flux through the coil
magnetic field strength, B
magnetic flux density
(magnetic flux per unit area)
Faraday’s law
ε = -N (dΦ/dt)
the magnitude of induced emf is equal to the rate of change of magnetic flux linkage
Lenz’s law
bonus: what is the logic behind this?
induced emf/current acts in a direction;
to oppose the change that caused it
Lenz’s law is based on the law of conservation of energy. Because any induced current has had energy transferred to it, and this energy must have been taken from the original energy in the system????
Explain why
- There is NO induced emf when a magnet is placed next to a circular coil
- There is induced emf when the magnet moves towards the coil
- When the magnet is simply placed next to the circular coil, there’s NO CHANGE in the magnetic flux linking the coil → no induced emf
- When the magnet moves towards the coil, there’s an INCREASE in the magnetic flux linking the coil → induces emf
The diagram shows a step-up transformer. Explain why the lamp is off when the switch is closed and the lamp illuminates for a SHORT TIME when the switch is suddenly opened.
There is a current in the primary coil when the switch is closed. This current creates a
magnetic flux in the primary coil, which also links the secondary coil due to the soft iron ring. With the switch closed, there is no change in the magnetic flux linkage
at the secondary coil → lamp is not lit. [1]
When the switch is opened, the magnetic flux suddenly decreases/changes to zero → induces an e.m.f. and the lamp illuminates for a short time. [1]
Eventually there is no magnetic flux at either the primary or the secondary coil → no e.m.f. induced → lamp stays off. [1]
A conducting loop enters a region of magnetic field. Describe and explain the SPEED of the loop while it is INSIDE and after it EXITS the magnetic field.
INSIDE: speed decreases as the change in flux induces current and creates magnetic force on the loop, which acts to oppose its motion due to Lenz’s law
OUTSIDE: no more change in flux and magnetic force → constant speed
Explain the operation of an A.C. generator, including changing the generator frequency
Why are the coils in generators are usually wound on iron cores?
increases the strength of the magnetic field
How can you make the output emf of a A.C. generator as large as possible?
- increase the strength of magnetic field (eg. wound on iron core)
- increase number of coils
- increase the area (within the field!)
An electrical supply is rated at 110V ac 50Hz.
What is the maximum value of this voltage?
Vrms = 110 = Vmax/√2 → Vmax=156
Describe how transformers work
- alternating voltage drives alternating current through primary coil
- creates alternating magnetic flux linkage which passes through the iron core to secondary coil
- induces emf in secondary coil
Explain the use of transformers
in A.C. electrical power distribution
Significant amount of energy will be lost (transferred to internal energy) in transmission lines if the currents are too high (P = I²R), so transformers are used to step up voltage to decrease current (P=IV) in transmission lines, thus reducing power loss
How can we increase the efficiency of transformers?
laminate the core to prevent the flow of eddy current, which transfer some of the energy into internal energy
Why is electrical power usually transferred over large distances using alternating currents?
Need transformers to step up/down voltage to reduce current and power loss. DC can’t produce the necessary changing magnetic flux for the operation of a transformer.
rectification
converting ac to dc
What device is used to rectify a current?
diode (only allows current to pass in one direction)
What is capacitor used for?
smooth the voltage of a rectifier so there is less variation
Sketch the V-I characteristic of a diode
RHS: I increases as V increases
LHS: I=0
capacitance
the ability of an object to store electric charge
for a q-V diagram, how do you find
1. capacitance
2. energy stored in the capacitor
GRADIENT = q/V = CAPACITANCE
AREA = qV/2 = CV^2/2 = ENERGY STORED
Describe the exponential discharge of a capacitor
highest rate of discharge at the beginning as there’s the greatest amount of charges on the plate → free electrons experience the greatest force;
the rate of discharge decreases with time, but theoretically never reaches zero
time constant, τ
τ = RC = resistance x capacitance
the time that it takes for the q/V/I to decrease to 1/e × the original value (37%)
