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Topic 12: Electromagnetic induction. Topic 12: Electromagnetic induction. Topic 12: Electromagnetic induction. Describe the inducing of an emf by relative motion between a conductor and a magnetic field. Electromagnetic Induction. N. The direction of the induced current is reversed if… - PowerPoint PPT Presentation
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Topic 12: Electromagnetic induction
Topic 12: Electromagnetic induction
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Topic 12: Electromagnetic induction
Describe the inducing of an emf by relative motion between a conductor and a magnetic field.
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Electromagnetic InductionElectromagnetic Induction
N
The direction of the induced current is reversed if…1) The wire is moved in the opposite direction
2) The field is reversed
The size of the induced current can be increased by:1) Increasing the speed of movement
2) Increasing the magnet strength
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Electromagnetic Electromagnetic inductioninduction
The direction of the induced current is reversed if…
1) The magnet is moved in the opposite direction
2) The other pole is inserted first
The size of the induced current can be increased by:
1) Increasing the speed of movement
2) Increasing the magnet strength
3) Increasing the number of turns on the coil
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The north pole of a permanent bar magnet is pushed along the axis of a coil as shown below.
V
N Sax is o f co il
The pointer of the sensitive voltmeter connected to the coil moves to the right and gives a maximum reading of 8 units. The experiment is repeated but on this occasion, the south pole of the magnet enters the coil at twice the previous speed.Which of the following gives the maximum deflection of the pointer of the voltmeter?A. 8 units to the rightB. 8 units to the leftC. 16 units to the rightD. 16 units to the left (1)
Faraday’s LawFaraday’s Law24/04/23
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Derivation of EmfDerivation of Emf
By conservation of energy
Derive the formula for the emf induced in a straight conductor moving in a magnetic field. Students should be able to derive the expressioninduced emf = Blv without using Faraday’s law.
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Induced E.m.f.Induced E.m.f.
Magnetic fluxMagnetic flux24/04/23
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Define magnetic flux and magnetic flux linkage.
Magnetic fluxMagnetic flux24/04/23
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Magnetic FluxMagnetic FluxAs we said, magnetic field strength is also called magnetic flux density. The “magnetic flux” is the amount of flux that passes through a given area:
Magnetic flux = magnetic flux density x area Φ = BA (in Weber, Wb) in T in m2
For a coil of N turns the total magnetic flux is NΦ
ΦFaraday (1791-1867)
Faraday’s law:
The induced EMF for a magnet in a coil is directly proportional to the rate of
change of flux linkage:EMF = NΔΦ ΔT
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Magnetic fluxMagnetic flux
Define magnetic flux and magnetic flux linkage.
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Induced e.m.f Induced e.m.f
NΦ is known as the magnetic flux linkage
Φ the magnetic flux through one coil
Faraday (1791-1867)
Describe the production of an induced emf by a time-changing magnetic flux.
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Cutting Magnetic FieldsCutting Magnetic FieldsConsider a conductor of length l moving at speed v through a magnetic field at 900:
From Faraday’s Law: Not on syllabusInduced emf = -N ΔΦ
ΔtBut Φ=BA and B is constant, so:Induced emf = -NB A tThis is a single wire, so N=1, and A = lvt, therefore:
Induced emf = -Blv
Calculating the EMFCalculating the EMF24/04/23
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Lenz’s LawLenz’s Law24/04/23
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Lenz’s LawLenz’s LawConsider a magnet in a solenoid:
N
S
The current induced by the magnet induces a north pole that repels the magnet again.
Lenz’s Law:
Lenz (1804-1865)
Any current driven by an induced emf opposes the change that caused it. In other words,
emf = -NΦ/t
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Lenz’s LawLenz’s LawClick to play
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Right hand ruleRight hand rule
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Lenz’s law questionLenz’s law question
• Explain why it is harder to turn a bicycle dynamo when it is connected to a light bulb than when it is not connected to anything.
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When a simple d.c. electric motor is connected to a battery, the current which flows during the first few seconds varies (approximately) as shown by the graph below.
Explain the shape of this graph.
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QuestionsQuestionsHamper page 211 Q’s 39,40,41IB review pack Q’s 1,2,4,5
12.2 Alternating current12.2 Alternating current24/04/23
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GeneratorGenerator
Hyperlink
Describe the emf induced in a coil rotating within a uniform magnetic field.
Explain the operation of a basic alternating current (ac) generator.
Students should understand, without any derivation, that the induced emf is sinusoidal if the rotation is at constant speed.
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Effect of changing the speed of rotation of Effect of changing the speed of rotation of the coil on the induced emfthe coil on the induced emf
Slow rotation Faster rotation
Describe the effect on the induced emf of changing the generator frequency.
Students will be expected to compare the output from generators operating at different frequencies by sketching appropriate graphs.
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Generator questionGenerator question
• A rectangular coil, 2cm by 3cm, rotates in a uniform magnetic field of flux density 0.15T. The axis around which the coil rotates is at 90° to the flux lines.The coil has 250 turns and its rotational frequency is 50s-
1.a)Calculate the maximum emf induced in the coil. What is the position of the coil (relative to the flux lines) when the induced emf has its maximum value?b)What is the magnitude of the induced emf at a time 5×10-3s after it has passed through its maximum value.c)Calculate the magnitude of the induced emf at a time 2.5×10-3s after it has passed through its maximum value.
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Revision of DC and ACRevision of DC and AC
DC stands for “Direct Current” – the current only flows in one direction:
AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz)
1/50th s
240V
V
V
Time
T
Discuss what is meant by the rootmean squared (rms) value of analternating current or voltage.
Students should know that the rms value of analternating current (or voltage) is that value of the direct current (or voltage) that dissipates power in a resistor at the same rate. The rms value is also known as the rating.
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Rms and peak valuesRms and peak valuesWhat value do you use for an a.c. current?
Discuss what is meant by the root mean squared (rms) value of an alternating current or voltage.
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Rms calculationsRms calculationsThis question is about rms currents and voltages.
For the circuit shown above calculate
a the rms value of the current, I
b) the maximum value of the current
c) the mean power dissipated in R1
d) the rms value of the voltage across R2.
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Coupled Inductors 12.1.4Coupled Inductors 12.1.4
Describe the production of aninduced emf by a time-changingmagnetic flux.
The national gridThe national gridExplain the use of high-voltage step-up and step-down transformers in the transmission of electrical power.
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TransformersTransformers
Time
Time
Time
Current through primary
Magnetic flux through core
EMF induced in secondary
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Transformer.exe
Hyperlink
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TransformersTransformersTransformers are used to _____ __ or step down _______. They only work on AC because an ________ current in the primary coil causes a constantly alternating _______ ______. This will “_____” an alternating current in the secondary coil.
Words – alternating, magnetic field, induce, step up, voltage
We can work out how much a transformer will step up or step down a voltage:
Voltage across primary (Vp)
No. of turns on secondary (Ns)Voltage across secondary (Vs)
No. of turns on primary (Np)
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Describe the operation of an idealtransformer.
Solve problems on the operation ofideal transformers.
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Transformer questionsTransformer questions• a)Explain the operation of a step-up transformer.• b)A transformer has 250 turns on its primary coil and 4000 turns
on its secondary coil. It is connected to a 220V supply. Under normal operating conditions the current flowing through the secondary coil is 25mA and the transformer is 90% efficient (assume that the main source of inefficiency is the resistance of the secondary coil).
• Calculate • i)the secondary voltage when on open circuit (that is, when the
secondary coil is not connected to anything). • ii)the current flowing through the primary coil when the current
flowing through the secondary coil is 25mA.
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Some transformer questionsSome transformer questions
Primary voltage
Vp
Secondary voltage
Vs
No. of turns on primary Np
No. of turns on secondary Ns
Step up or step down?
12V 24V 100 ? ?
400V 200V 20 ? ?
25,000V 50,000V 1,000 ? ?
23V 230V 150 ? ?
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Energy losses in a transformer Energy losses in a transformer • Energy can be lost as:• (a) heat in the coils because of the resistance of
the wire;• (b) incomplete transfer of magnetic field;• (c) heating of the core due to induced currents in
it. This is reduced by making the core out of insulated soft iron in laminated strips. If this were not done the cores of large transformers would get so hot that they would melt.
Outline the reasons for power losses in transmission lines and real transformers.
Transmission of electricityTransmission of electricity24/04/23
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The transmission of electricity The transmission of electricity
To minimise losses through heating, then the current should be as low as possible i.e. make the voltage as high as possible.
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Example of power lossExample of power loss
Explain the use of high-voltage stepup and step-down transformers in the transmission of electrical power.
Students should be aware that, for economic reasons, there is no ideal value of voltage for electrical transmission.
QuestionsQuestions
Hamper page 218 Q’s 46,47.IB pack Q’s 7,8,9.
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Discuss some of the possible risks involved in living and working near high-voltage power lines.
Students should be aware that current experimentalevidence suggests that low frequency fields do not‑harm genetic material. Students should appreciate that the risks attached to the inducing of current in the body are not fully understood. These risks are likely to be dependent on current (density), frequency and length ofexposure.
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Health risksHealth risks
• Power cables carry a.c. currents• These produce e-m fields• These can induce currents in the body• These might do harm• Risk increases with • Current• Frequency• Time of exposure.
Suggest how extra-low-frequency electromagnetic fields, such as those created by electrical appliances and power lines, induce currents within a human body.
