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Chapter 25 Electromagnetic Induction (25.2 (Faraday's law of EM…
Chapter 25
Electromagnetic Induction
25.4
Observing AC
Peak to peak value
- 2x max value
1)
Oscilloscope- displays waveform, increase output PD= taller trace (larger peak value), increase frequency= more cycles on screen
2)
Connect signal generator to LED- make frequency low enough to see brightness vary-
Low freq
- lights up (bright 2x per cycle) and fades repeatedly, if freq raised gradually it flickers faster until too fast to notice
Heating effect of AC
so power is proportional to square of current
Peak current Io
Mean power=
Root mean squares
RMS value of AC current is value of direct current that would supply the same power to a component of resistance R
25.1
EM Induction
An induced EMF in a wire when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines
Increase by
Moving wire faster
Using a stronger magnet
Making wire into a coil and pushing magnet in and out of coil
No emf if magnet is parallel to field lines
Other methods to generate emf
1)
Electric motor in reverse
Falling weight makes motor coil turn between poles of magnet
Emf induced in the coil forces a current round and lamp lights
Faster coil turns, larger lamp
2)
Cycle dynamo
Magnet in a dynamo spins, emf induced in coil
Coil connected to lamp, lamp lights as current forced round circuit
Energy Changes
To keep wire moving in a magnet, work must be done
Work done=energy transferred to components
Combine w=Fscosθ, P=W/t, P=IV and Q=It calculate
Emf= energy transferred from a source to each unit of charge passing through the source
Dynamo Rule
AKA Fleming's Right Hand Rule
Thumb= Motion (F), First Finger= Field (B) and Second Finger= EMF
25.2
Current Carrying Coils
Current carrying wires produce magnetic field when connected to a battery and a current passed through it
Coil a wire, create a
solenoid
with a uniform magnetic field
Solenoid Field Direction
Clockwise (South)
Anticlockwise (North)
Lenz's Law
Solenoid Induction
- pass magnet down centre current generated
Law
- Direction of induced current always such that it opposes the change that causes the current
The environment will resist the change as energy can't be made or destroyed so the induced current wouldn't be in the direction to help the change that causes it
Faraday's law of EM induction
Conducting wire balanced on a metal rail, with magnetic field applied across space covered by the loop and bar
When bar dragged across it cuts through field lines producing an emf, as it moves it experiences a magnetic force in the opposite direction due to the current generated in the loop
To move at constant speed, force applied must = magnetic force
Bar moves at constant speed= s/t, current flowing= I, force opposing motion F=BIL (equal and opposite to applied force), Work done, W=Fs= BILs, Area=Ls
W=BIA
W=QV (V=emf) and Q=It giving
Ɛ=BA/t
Induced emf in a circuit= rate of change of flux linkage through the circuit
Magnetic Flux Φ
Magnetic flux density x area
Units= Weber (Wb)
Magnetic Flux Linkage NΦ
For a coil with N turns
NΦ= NBA
When magnetic field along normal flux linkage= NBA
When coil turned through 180° flux linkage= -BAN
When magnetic field parallel to coil area flux linkage=0
Largest emf produced when flux linkage is big and time is small
If not along normal:
NΦ= NBAcosθ
25.3
The AC Generator
Consists of a rectangular coil that spins in a uniform magnetic field
Flux linkage continuously changes
Steady frequency=
θ=2πft
NΦ=BANcos(2πft)
Emf
Gradient of flux linkage- differentiate above equation to get
Ɛ=BAN2πf sin(2πft)
2πf= w so
Ɛ=BANwsin(wt)
Power stations
Alternator use 3 sets of coils 120° to each other with spinning electromagnet at centre, each coil produce alternating emf 120° out of phase
Three phases supplied to local substations that distribute, if one phase goes down other 2 continue supplying so to same local area
Back emf
Motor runs a current through a coil to make it rotate in a magnetic field
Coil rotating will produce an emf (back emf) itself (Lenz's law tells us this will be in the opposite direction to the emf that drives the current in the first place)
Pd driving motor is V and back emf is Ɛ then
V-Ɛ=IR
Current changes with motor speed
Low speed, high current as induced back emf low
High speed, low current as induced back emf high
When back emf= supply voltage the motor will be at constant speed
DC generator
- introduce split ring, every half cycle coil disconnects and reconnects in reverse so emf always +ve
25.5
Transformers
Step-up
- more turns on secondary coil so voltage is stepped up compared to primary
Step-down
- fewer turns on secondary coil so voltage is stepped down compared to primary
Almost 100% efficient due to
1)
Low resistance windings- reduce power wasted by heating
2)
Laminated core (layers of iron separated by insulator layer)- reduces induced currents (eddy currents) in core itself so magnetic flux as high as possible
3)
Soft iron core- easily magnetised and demagnetised- reduces power wasted through repeated magnetisation and demagnetisation
National Grid
1)
Power Station (25000v)
2)
Step Up Transformer
5)
Heavy Industry (33000v)
6)
Step Down Transformer
4)
Step Down Transformer
3)
Super Grid (400000v)
8)
Step Down Transformers
7)
Light industry (11000V)
9)
Homes (230v)
