Chap 22 EM induction 12 S

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Electromagnetic Induction ‘08 (P)

 Principles of electromagnetic induction_{Principles of electromagnetic induction}

a Deduce from Faraday’s experiments on electromagnetic induction or other appropriate experiments:

i. That a changing magnetic field can induce an e.m.f. in a circuit.

ii. That the direction of the induced e.m.f. opposes the change producing it. iii. The factors affecting the magnitude of the induced e.m.f.

 _{The a.c. generator}_{The a.c. generator}

b. Describe a simple form of a.c. generator (rotating coils or rotating magnet) and the use of slip rings (where needed).

c. Sketch a graph of voltage output against time for a simple a.c. generator.

 The transformer_{The transformer}

d. Describe the structure and principle of operation of a simple iron-cored transformer as used for voltage transformations.

e. Recall and apply the equations V_P / V_S = N_P / N_S and V_PI_P = V_SI_S to new situations or to solve related problems (for an ideal transformer)

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Experiment to Investigate Induced emf in a Coil

 Procedure_Procedure

1.

1. Connect a copper wire coil to a zero center galvanometer.Connect a copper wire coil to a zero center galvanometer.

2.

2. Insert the north pole of a bar magnet into theInsert the north pole of a bar magnet into the coil. A deflection is coil. A deflection is

observed on the galvanometer. Note the direction of the

deflection.

deflection. 3.

3. Hold the magnet stationary inside the coil. Note the deflection on Hold the magnet stationary inside the coil. Note the deflection on

the galvanometer.

the galvanometer. 4.

4. Move the north pole out of the coil. Again, note the deflection.Move the north pole out of the coil. Again, note the deflection.

5.

5. Repeat the above with the south pole. What is the differenceRepeat the above with the south pole. What is the difference??

6.

6. Hold the magnet stationary and move the coil Hold the magnet stationary and move the coil ttowards the owards the

magnet. What do you observe

magnet. What do you observe??

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Electromagnetic Induction experiment

 When the magnet is moving into the coil, the galvanometer register _{When the magnet is moving into the coil, the galvanometer register}

a deflection. This shows that a

a deflection. This shows that a electromotive force and hence a electromotive force and hence a current is induced

current is induced when the magnet is moved into the coil. when the magnet is moved into the coil.

 _{When the magnet is stationary, there is}_{When the magnet is stationary, there is}_{no current}_{no current}_induced._induced.

 When the magnet is moved out of the coil, again there is a _{When the magnet is moved out of the coil, again there is a}

deflection, however the deflection is in the

deflection, however the deflection is in the oppositeopposite direction. direction. What can you infer from this observation?

What can you infer from this observation?

 Moving the coil towards a stationary magnet will also induce a _{Moving the coil towards a stationary magnet will also induce a}

current in the circuit.

 An electromotive force and hence a current is induced whenever _{An electromotive force and hence a current is induced whenever}

the magnetic field inside a coil changes OR

the number of magnetic lines of force passing through a coil

changes

changes..

 _{What happens when the}_{What happens when the}_{other end}_{other end}_{of the bar magnet is moved into}_{of the bar magnet is moved into}

the coil?

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Direction of the Induced emf

 The induced current always flows in a direction that opposes _{The induced current always flows in a direction that opposes}

the change producing it. the change producing it.

 Hence the polarity of the electromagnet will be such that it _{Hence the polarity of the electromagnet will be such that it}

opposes the motion of the bar magnet. opposes the motion of the bar magnet.

• when the N pole of the magnet is _{when the N pole of the magnet is}

moved towards the coil, end A

becomes the N pole.

• When the N pole of the magnet is _{When the N pole of the magnet is}

moved away from the coil, end A

becomes a S pole.

 Lenz’s law:_{Lenz’s law:}

 The direction of the induced current is such that its _{The direction of the induced current is such that its}

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Practices

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Magnitude of Induced EMF



To have a larger current have any of the following:

•

Magnet is moved at a faster speed in or out of the

coil.

•

A stronger magnet is used.

•

The number of turns in the coil is increased.



Faraday’s law of electromagnetic induction:

The magnitude of the induced electromotive force is The magnitude of the induced electromotive force is

proportional to the

proportional to the raterate of change of the magnetic lines of change of the magnetic lines of force linked with the circuit OR

of force linked with the circuit OR

the rate at which the lines of force are cut. the rate at which the lines of force are cut.



A single line of force passing through a single turn

coil constitutes one

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Dynamo Rule



Fleming’s right hand rule (Dynamo rule)



Use to deduce the direction of the induced current,

based on the direction of the magnetic field and the

motion. Vice versa.



F - M - C

motion (or force) magnetic

field

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Practices (TB pg406)

2. Why is it more difficult to move a magnet into a coil which has a larger number of turns?

A coil with a larger number of turns can induce a larger electromotive force or a larger amount of electrical energy. Thus, by the principle of

conservation of energy, more mechanical work needs to be done.

4. Explain why an induced current must flow in such a direction so as to oppose the change producing it.

According to Principle of conservation of energy,

mechanical work has to be done against the opposing force to induce a current. This mechanical energy

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The A.C Generator



A.C generator vs motor

• in generators, mechanical energy is _{in generators, mechanical energy is} used to rotate the coil to obtain

used to rotate the coil to obtain electrical energy.

electrical energy.

• In motors, electrical energy is used to _{In motors, electrical energy is used to} rotate the coil to provide mechanical rotate the coil to provide mechanical

energy. energy.



Components of A.C. generator

• rectangular coil mounted on axle fixed _{rectangular coil mounted on axle fixed} between poles of a magnet.

between poles of a magnet.

• Slip rings attached to end of the coil._{Slip rings attached to end of the coil.}

• Brushes attached to each slip-ring._{Brushes attached to each slip-ring.}

• Electrical load attached to the carbon _{Electrical load attached to the carbon} brushes.

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The A.C Generator

• When coil is rotated, it cuts the magnetic field of _{When coil is rotated, it cuts the magnetic field of}

the magnet.

• This induces a current in the coil._{This induces a current in the coil.}

• Using right hand rule, the induced current flows _{Using right hand rule, the induced current flows}

from A to B, C to D.

• Current flows through the slip rings to the _{Current flows through the slip rings to the}

electrical load, from Q to P.

• When coil rotates through 180° the sides of the _{When coil rotates through 180° the sides of the}

coil will switch place. At this point of time, current

flows from B to A and D to C. (Check using right

hand rule). Current flows from P to Q.

• Current reverse direction_{Current reverse direction} through the electrical through the electrical

load every time the coil pass through the vertical.

Hence the current is alternating.

Hence the current is alternating. _applet

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The A.C Generator

 _{Voltage output}_{Voltage output}

• relates the electromotive force generated in _{relates the electromotive force generated in}

the coil over time.

• Relationship can be seen using a cathode ray _{Relationship can be seen using a cathode ray}

oscilloscope.

• Graph of emf vs time_{Graph of emf vs time}

• When the _{When the}coil is parallel to the magnetic coil is parallel to the magnetic field

field, highest rate of magnetic line of force , highest rate of magnetic line of force being cut by the coil. Hence emf is maximum.

being cut by the coil. Hence emf is maximum.

• Similarly, when _{Similarly, when}coil is perpendicularcoil is perpendicular to the to the magnetic field, the rate is the lowest. This

magnetic field, the rate is the lowest. This

corresponds to the minimum emf.

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The A.C Generator



Factors affecting voltage output

• The speed of rotation of the coil_{The speed of rotation of the coil}

• The number of turns of the coil_{The number of turns of the coil}

• Strength of the magnet_{Strength of the magnet}

• Use of a soft iron core_{Use of a soft iron core}



Describe how each of the above affects the voltage

output

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The A.C Generator



Bicycle dynamo

• Another example of a A.C. generator_{Another example of a A.C. generator}

• the coil does not move. Instead the magnet is moving. _{the coil does not move. Instead the magnet is moving.}

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Eddy Current (Enrichment)

 Changing magnetic fields also induces currents in the iron _{Changing magnetic fields also induces currents in the iron}

core. core.

 The current flows round the core._{The current flows round the core.}

 Current will absorb the power and dissipate it as heat_{Current will absorb the power and dissipate it as heat}

 Eddy current tend to oppose the motion of a solid conductor _{Eddy current tend to oppose the motion of a solid conductor}

in a magnetic field. in a magnetic field.

 Solution_Solution

• Use thin sheets of soft iron_{Use thin sheets of soft iron}

• Insulate each sheet with very thin film of oxide (laminations)_{Insulate each sheet with very thin film of oxide (laminations)}

• Cut slots into iron core. This reduced the amount of current as they _{Cut slots into iron core. This reduced the amount of current as they}

cannot flow across the air gaps formed by the slots.

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How to obtain the following outputs?



Slip rings



Split ring

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Recap

Metallic

conductor Filament lamp diode

Obeys Ohm’s law

(V/I constant) Yes No No

V/I ratio when

current reverse No change No change

Very high when current flow in –ve

to +ve direction

Special characteristics

V/I constant when physical conditions

are constant

Resistance increase when I

increase

Current increases very rapidly when

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Transformer

 Why a transformer?_{Why a transformer?}

• Our local mains supplies 240_{Our local mains supplies 240}VV, but different appliances requires , but different appliances requires different voltages.

different voltages.

• A transformer allows the mains to change to the required voltage _{A transformer allows the mains to change to the required voltage}

(by stepping up or down).

 Faraday’s experiment iron ring experiment._{Faraday’s experiment iron ring experiment.}

• Procedure_Procedure

– When switch is closed, the compass deflects once. _{When switch is closed, the compass deflects once.}

– When the switch is open again, the compass deflects again._{When the switch is open again, the compass deflects again.} – No deflection when a steady current is supplied._{No deflection when a steady current is supplied.}

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Transformer

 How it works_{How it works}

• components:_components: – soft iron core_{soft iron core}

– primary coil, link to the power source_{primary coil, link to the power source} – secondary coil, link to the appliance_{secondary coil, link to the appliance}

• When switch is closed:_{When switch is closed:}

– Current in the coil increases from zero to maximum._{Current in the coil increases from zero to maximum.} – Results in Results in increasing magnetic fieldincreasing magnetic field in primary coil. in primary coil.

– The magnetic field induces a current in the secondary coil.The magnetic field induces a current in the secondary coil.

– Current in secondary coil causes lamp to light up.Current in secondary coil causes lamp to light up.

– Once current in primary coil is steady, there is no more induced Once current in primary coil is steady, there is no more induced current and the lamp will go off.

current and the lamp will go off.

• When switch is open: _{When switch is open:} – – secondary coil lamp primary coil

soft iron core

battery secondary coil lamp primary coil

soft iron core

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Transformer

 Principle of transformers_{Principle of transformers}

• varies the voltage by changing the number of turns in the coil._{varies the voltage by changing the number of turns in the coil.}

 Step up transformer_{Step up transformer}

• more turns in the secondary coil than the primary coil_{more turns in the secondary coil than the primary coil} • increases the voltage_{increases the voltage}

 Step down transformer_{Step down transformer}

• •

 For an ideal transformer_{For an ideal transformer}

coil primary in turns of number coil secondary in turns of number Voltage Input Voltage Output  p s p s N N V V 

 NN_s_s / N / N_p_p is known as the turns ratio is known as the turns ratio

primary

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

Principle of transformers

• Assuming that a transformer is 100% efficient, ie. _{Assuming that a transformer is 100% efficient, ie.} output power = input power output power = input power

• output voltage x output current = input voltage x input current_{output voltage x output current = input voltage x input current}

Transformer

• When voltage is stepped up, the current will be stepped _{When voltage is stepped up, the current will be stepped} down.

down.

• Eg. 22.1, pg. 412Eg. 22.1, pg. 412

V

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Practical Transformer Design



To improve efficiency of the transformer

• a good soft magnetic material_{a good soft magnetic material}

• a laminated core to reduce eddy current. Eddy current is _{a laminated core to reduce eddy current. Eddy current is} induced current in the core.

induced current in the core.

• a special core design such that the magnetic field _{a special core design such that the magnetic field}

produced by the primary coil is link completely with the produced by the primary coil is link completely with the

secondary coil secondary coil..

• low resistance copper wire to reduce losing energy (heat)._{low resistance copper wire to reduce losing energy (heat).}

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Power Transmission

 Power loss during transmissionm (pg. 414)_{Power loss during transmissionm (pg. 414)}

• Electricity transmitted from power station to home via cables._{Electricity transmitted from power station to home via cables.}

• Eg. A power station supplies 10MW at 10kV. The current supplied is:_{Eg. A power station supplies 10MW at 10kV. The current supplied is:}

P P = I V= I V

10 x1010 x1066 _{= I x (10 x 10}_{= I x (10 x 10}33₎₎

II = 1000 A = 1000 A

• Assume resistance of 1_{Assume resistance of 1} per km per km Power loss = I

Power loss = I22R_R

= (1000)

= (1000)22_{x 1}_{x 1}

= 1MW (about 10%)

= 1MW (about 10%) • If 10MW is transmitted at 200kV_{If 10MW is transmitted at 200kV}

I = P / V

I = P / V= (10 x10= (10 x1066_{) / (200 x 10}_{) / (200 x 10}33₎₎ = 50A

= 50A

Power loss = I

Power loss = I22_{R = (50)}_{R = (50)}22_{x 1}_{x 1}

= 2500 W (0.25%)

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Power Transmission



Power loss during transmission

• difficult to have generators with such high voltage output._{difficult to have generators with such high voltage output.}

• Dangerous to have high voltage appliances._{Dangerous to have high voltage appliances.}

• Solution: Using step up transformer before transmission _Solution:Using step up transformer before transmission and using step down transformer and substations before and using step down transformer and substations before

reaching our homes.

reaching our homes. National Grid system National Grid system



Other methods of reducing power loss

• using materials of low resistance to manufacture cables_{using materials of low resistance to manufacture cables}

• increasing the thickness of cable to lower resistance_{increasing the thickness of cable to lower resistance}

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Use of cathode-ray oscilloscope

 describe the use of a cathode-ray oscilloscope

(c.r.o.) to display waveforms and to measure p.d.’s and short intervals of time (detailed circuits,

structure and operation of the c.r.o. are not required)

 interpret c.r.o. displays of waveforms, p.d.’s and

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Cathode Ray Oscilloscope (CRO)

 Instrument to show how voltage across a device varies with _{Instrument to show how voltage across a device varies with}

time. time.

 Deflection of electron beam by an electric field_{Deflection of electron beam by an electric field}

 components:_components:

• Evacuated glass tube _{Evacuated glass tube} • Electron gun_{Electron gun}

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Cathode Ray Oscilloscope (CRO)

 Electron beam moves across screen from left to right_{Electron beam moves across screen from left to right}

 When beam strikes screen, bright dot seen on screen_{When beam strikes screen, bright dot seen on screen}

 Voltage across device changes vertical position of dot_{Voltage across device changes vertical position of dot}

 A voltage time graph, ie how voltage varies with time_{A voltage time graph, ie how voltage varies with time}

 CRO should be calibrated before using for accurate work_{CRO should be calibrated before using for accurate work}

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CRO - Front panel control

• Brightness / intensityBrightness / intensity

– To change the brightness of the trace To change the brightness of the trace on the screen (by changing the

on the screen (by changing the

potential of the grid)

• Focus_Focus

– To make the trace sharper ( similar _{To make the trace sharper ( similar}

to camera/ microscope)

• X-shiftX-shift

– To shift the trace along the X-axis To shift the trace along the X-axis (time axis)

(time axis)

• Y-shift_Y-shift

– To shift the trace along the Y-axis To shift the trace along the Y-axis (voltage)

(voltage)

• Y-gainY-gain

– Amplify the deflection on Y-axisAmplify the deflection on Y-axis

– To allow small voltages drops to be To allow small voltages drops to be seen

seen

• Time baseTime base

– Changes the speed at which the _{Changes the speed at which the}

beam moves across the screen

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CRO - measure Voltage

 Used as a.c. or d.c. voltmeter_{Used as a.c. or d.c. voltmeter}

 Time base off_{Time base off}

 Device to be measured placed between _{Device to be measured placed between}

Y-input terminals Y-input terminals

 Deflection of spot is proportional to _{Deflection of spot is proportional to}

voltage applied. i.e. bigger voltage gets voltage applied. i.e. bigger voltage gets

greater deflection greater deflection

 E.g. _E.g.

Y-gain set at 5 volts per cm (scale), Y-gain set at 5 volts per cm (scale), deflection of 4cm on screen implies deflection of 4cm on screen implies

that the voltage of device is 20V (4cm x that the voltage of device is 20V (4cm x

5V) 5V)

 Constant voltage means spot will _{Constant voltage means spot will}

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CRO - Measure Voltage

 Alternating current results in spot moving up and down along _{Alternating current results in spot moving up and down along}

vertical axis vertical axis

 When frequency is high, spot moves so fast that a vertical _{When frequency is high, spot moves so fast that a vertical}

line is seen. line is seen.

 Spot at top of line when voltage is maximum (+ve), vice _{Spot at top of line when voltage is maximum (+ve), vice}

versa versa

 Length of line gives the peak-to-peak voltage (VLength of line gives the peak-to-peak voltage (V_pp_pp))

 Peak voltage (VPeak voltage (V_p_p) = ) = ½ ½ VV_pp_pp

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CRO - Display waveforms

 To show variation of input voltage with time_{To show variation of input voltage with time}

 Time base on with suitable frequency, a steady _{Time base on with suitable frequency, a steady}

waveform can be seen waveform can be seen

 When connected to microphone (Y-input terminals) _{When connected to microphone (Y-input terminals)}

waveforms of sounds can be seen. waveforms of sounds can be seen.

 Links to chapter on sound_{Links to chapter on sound}

 All 3 displays has the same input, but with different _{All 3 displays has the same input, but with different}

display settings. Given that it has voltage of 5V and display settings. Given that it has voltage of 5V and frequency of 100Hz. State the display settings for gain frequency of 100Hz. State the display settings for gain and time base.

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CRO - measure short time interval

 Set time base to lowest frequency range_{Set time base to lowest frequency range}

 Microphone connected to Y-input_{Microphone connected to Y-input}

 When two sharp sounds are made, two pulses are _{When two sharp sounds are made, two pulses are}

displayed

 Distance between the two pulses is the time interval (factor _{Distance between the two pulses is the time interval (factor}

the screen measurement by the time base)

 E.g.time base is 10ms/ division. There are 2 divisions _{E.g.time base is 10ms/ division. There are 2 divisions}

between the pulses. The time interval would be 20ms.

 Used to find speed of sound using echo method_{Used to find speed of sound using echo method}

• Mic placed few meters from reflecting surface._{Mic placed few meters from reflecting surface.}

• Sharp sound made. CRO register a peak due to sound, _{Sharp sound made. CRO register a peak due to sound,}

another peak seen due to echo.

• Time interval can be measured. With distance between _{Time interval can be measured. With distance between}

mic and wall known, speed of sound can be calculated.

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Physics Insights by Pearson Longman. 08 edition Physics Insights by Pearson Longman. 08 edition

Diode as a rectifier



A rectifier

• Changes current flow from AC to DC_{Changes current flow from AC to DC}

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 Voltage varies from A to B_{Voltage varies from A to B}

 Spot moves uniformly across screen_{Spot moves uniformly across screen}

 When voltage drops from B to C, _{When voltage drops from B to C,}

spot flies back to extreme left. spot flies back to extreme left. During fly-back, spot intensity During fly-back, spot intensity automatically changes to zero. automatically changes to zero.

 Cycle repeats at predetermined frequency using time-_{Cycle repeats at predetermined frequency using}

time-based control. based control.

 Horizontal deflection is a measure of time._{Horizontal deflection is a measure of time.}

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CRO – Advantages (E)

• Nearly infinite resistance and so draws very little current _{Nearly infinite resistance and so draws very little current} (similar to voltmeter, ideal instrument placed in parallel) (similar to voltmeter, ideal instrument placed in parallel)

• Can measure both a.c. and d.c. voltages_{Can measure both a.c. and d.c. voltages}

• Immediate response, compared with conventional _{Immediate response, compared with conventional} voltmeters

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CRO – Television (E)



Television (Cathode ray tube)

• Magnetic coils used to deflect electron beams. They give _{Magnetic coils used to deflect electron beams. They give} a wider angle of deflection

a wider angle of deflection

• Two time base circuits, one moves beam vertically down _{Two time base circuits, one moves beam vertically down} the screen, the other moves it across.

the screen, the other moves it across.

• Colour pictures produced by adding Phosphors to _{Colour pictures produced by adding Phosphors to} fluorescent screen.

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2007 O level Q.11

Fig.7.1 shows a magnet as it

drops through a coil that is connected

to a cathode-ray oscilloscope.

It is difficult to make measurements on

the Trace shown in Fig. 7.1. The controls

Are adjusted to produce the trace shown

in Fig. 7.2.

ai. Explain in detail how the controls shown on the cathode-ray oscilloscope

are adjusted to produce the trace shown in Fig. 7.2.

are adjusted to produce the trace shown in Fig. 7.2. [3][3]

aii. For the trace shown in Fig. 7.2, the Y-gain is set at 5mV/division and the

time base at 0.1s/division. Determine the time between the pulses and

the amplitude of the first pulse.

the amplitude of the first pulse. [2][2] bi. Explain how the first pulse is produced

bi. Explain how the first pulse is produced [3][3]

Bii. On Fig. 7.1, the arrows show the direction of the current as the magnet

enters the coil. Explain how this current opposes the entry of the magnet.

[2]

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 11 or_{11 or}

ai.

ai. The The y-shift controly-shift control was used to was used to move the trace downmove the trace down. . The

The y-gain is increasedy-gain is increased to produce a to produce a larger trace verticallylarger trace vertically. . The

The time base is increasedtime base is increased to give a to give a larger distance between the larger distance between the pulses

pulses on the screen. on the screen. (Description of X and Y plates not required)(Description of X and Y plates not required)

aii.

aii. Time = 7 x 0.1Time = 7 x 0.1 = 0.7s= 0.7s amplitude = 5mV = 3.5

amplitude = 5mV = 3.5 = 17.5 mV= 17.5 mV bi.

bi. The magnet is dropped into the coil. The The magnet is dropped into the coil. The magnetic field of the magnetic field of the magnet cuts the coil

magnet cuts the coil. The . The changing magnetic field in the coilchanging magnetic field in the coil results results in in an

an induced emf induced emf and current flowing in the coil. The induced emf in and current flowing in the coil. The induced emf in the coil is detected by the CRO as shown.

the coil is detected by the CRO as shown.

bii.

bii. Using the Using the right hand grip ruleright hand grip rule, the induced current results in a , the induced current results in a North North pole at the top

pole at the top of the coil. Since the North pole of the magnet is of the coil. Since the North pole of the magnet is facing the facing the North pole of the coil, there is a

North pole of the coil, there is a repulsive force as like repulsive force as like poles repelpoles repel. . (Lenz’s Law is not appropriate here)