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Transcript of Electromagnetic induction Make a coil using wire. The coil should be wide enough to easily move a...
Electromagnetic induction
• Make a coil using wire. The coil should be wide enough to easily move a magnet inside
Electromagnetic induction
• Put your coil in this circuit. The multimeter should be on the μA scale.
μA
Electromagnetic induction
If a magnet is moved inside a coil an electric current is induced (produced)
Generator/dynamo
A generator works in this way by rotating a coil in a magnetic field (or rotating a magnet in a coil)
Motor = generator
If electric energy enters a motor it is changed into kinetic energy, but if kinetic energy is inputted (the motor is turned) electric energy is produced!
The Motor Effect
When a current is placed in a magnetic field it will experience a force (provided the current is not parallel to the field). This is called the motor effect.
Can you copy this sentence into your
books please.
The Motor Effect
The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule.
Centre finger = Conventional Current
First finger = Field direction
Thumb = Motion
The Motor Effect
The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule.
Centre finger = Conventional Current
First finger = Field direction
Thumb = Motion
Can you copy this please?
WITH DIAGRAM!
Sample question
In this example, which way will the wire be pushed? (red is north on the magnets)
Current
Field
Electromagnetic Induction
Imagine a wire moving with velocity v in a magnetic field B out of the page.
vRegion of magnetic field B out of page
Wire moving with velocity v
L
The electrons in the wire feel a force (the motor effect) which pushes the electrons to the right. This creates a potential difference in the wire.
v
L Electrons pushed this way (left hand rule)
The field in the wire that produces this potential difference is given by E = V/L
v
L
e.m.f. (voltage) across the wire in the magnetic field
-+
The force produced by this field E = V/L would push the electrons back again, but this is opposed by the force on the electrons due to the magnetic filed F = Bev
v
L
-+
There exists a balance between the force on the electrons due to the field in the wire and the force due to the field
eE = Bev
v
L
V = vBLThis means that a conducting wire of length L moving with
speed v normally to a magnetic field B will have a e.m.f. of vBL across its ends. This is called a motional e.m.f.
vRegion of magnetic field B out of page
Wire moving with velocity v
L
Magnetic Flux (Ф)
Imagine a loop of (plane) wire in a region where the magnetic filed (B) is constant.
B
The magnetic flux (Ф) is defined as Ф = BAcosθ where A is the area of the loop and θ is the angle between the magnetic field direction and the direction normal (perpendicular) to the plane of the coil.
B
If the loop has N turns, the flux is given by
Ф = NBAcosθ in which case we call this the flux linkage.
B
The unit of flux is the Weber (Wb) (= 1 Tm2)
It can help to imagine the flux as the number of lines of magnetic field going through the area of the coil. We can increase the flux with a larger area, larger field, and keeping the loop perpendicular to the field.
B
Faraday’s law (at last!)
As we seen, an e.m.f. is only induced when the field is changing. The induced e.m.f. is found using Faraday’s law, which uses the idea of flux.
I built the first electric motor and
generator too. I refused all prizes
and awards because that would detract from God’s
glory.
Faraday’s law
The induced e.m.f. is equal to the (negative)
rate of change of magnetic flux,
E = -ΔФ/Δt
Example question
The magnetic field through a single loop of area 0.2 m2 is changing at a rate of 4 t.s-1. What is the induced e.m.f?
“Physics for the IB Diploma” K.A.Tsokos (Cambridge University Press)
Example question
The magnetic field (perpendicular) through a single loop of area 0.2 m2 is changing at a rate of 4 t.s-1. What is the induced e.m.f?
Ф = BAcosθ = BA
E = ΔФ = ΔBA = 4 x 0.2 = 0.8 V
Δt Δt
Another example question!There is a uniform magnetic filed B = 0.40 T out of the page. A rod of length L = 0.20 m is placed on a railing and pushed to the right at a constant speed of v = 0.60
m.s-1. What is the e.m.f. induced in the loop?
L v
The area of the loop is decreasing, so the flux (BAcosθ) must be changing. In time Δt the rod will move a distance vΔt, so the area will decrease by an area of LvΔt
L vLvΔt
E = ΔФ = BΔA = BLvΔt = BLv
Δt Δt Δt
E = 0.40 x 0.20 x 0.60 = 48 mV
L vLvΔt
An important result, you may be asked to do this!
Lenz’s Law
The induced current will be in such a direction as to oppose the change in magnetic flux that created the current
(If you think about it, this has to be so…….)
Slip ring commutator
To use this e.m.f. to produce a current the coil must be connected to an external circuit using a split-ring commutator.
Slip-rings
lamp
Root mean square voltage and current
It is useful to define an “average” current and voltage when talking about an a.c. supply. Unfortunately the average voltage and current is zero!
To help us we use the idea of root mean square voltage and current.
Root mean square voltageFirst we square the voltage to get a quantity that is positive during a whole cycle.
e.m.f.
V
Root mean square voltageWe then find the square root of this quantity.
e.m.f.
V
This value is called the root mean square voltage
An alternating current in the primary coil produces a changing magnetic field in the iron core.
Np turns
Ns turns
Vp Vs
Primary coilSecondary coilIron core
The changing magnetic field in the iron core induces a current in the secondary coil.
Np turns
Ns turns
Vp Vs
Primary coilSecondary coilIron core
It can be shown using Faraday’s law that:
Vp/Vs = Np/Ns and VpIp = VsIs
Np turns
Ns turns
Vp Vs
Primary coilSecondary coilIron core
Power transmission
When current passes through a wire, the power dissipated (lost as heat) is equal to
P = VI across the wire
Since V = IR
Power dissipated = I2R
Power transmission
Power dissipated = I2R
Since the loss of power depends on the square of the current, when transmitting energy over large distances it is important to keep the current as low as possible.
However, to transmit large quantities of energy we therefore must have a very high voltage.
Power transmission
Electricity is thus transmitted at very high voltages using step up transformers and then step down transformers.
Step-up
Step-down
Step-down
15,000 V
250,000 V
15,000 V
220 V
Dangerous?
Current evidence suggests that low-frequency fields do not harm genetic material. This is not fully proven or understood.