Electromagnetic Effects

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टीचर्स के लिए: Electromagnetic Effects (Physics, CIE) के लिए इस्तेमाल के लिए तैयार लेसन स्लाइड्स, रिवीज़न नोट्स — इन्हें अपने लेसन में इस्तेमाल करें, या टॉपिक को एक इंटरैक्टिव क्लास एक्टिविटी की तरह चलाएं जिसे आपके स्टूडेंट्स लाइव गेम की तरह खेलें।

लेसन नोट्स

Electromagnetic Induction

An e.m.f. is induced in a conductor when there is relative movement between the conductor and a magnetic field (conductor moves in stationary field OR conductor stationary in changing field).
If the conductor is part of a complete circuit, an induced current flows.
Lenz's law (Extended): The direction of the induced e.m.f. always opposes the change causing it (e.g., induced magnetic field repels approaching magnet).
Right-hand dynamo rule (Extended): First finger = Field, thuMb = Motion, seCond = Current (direction of induced e.m.f.).
Factors increasing induced e.m.f.: faster motion, more turns on coil, stronger magnet, larger coil area.

Demonstrating Induction

Experiment 1: Move a bar magnet in/out of a coil connected to a sensitive voltmeter. Stationary → zero reading; moving in → deflection; moving out → opposite deflection.
Experiment 2: Move a wire between magnetic poles, connected to a voltmeter. Stationary → zero; moving → deflection; reverse direction → opposite deflection.
Induced e.m.f. only occurs while there is relative motion (cutting field lines).
Factors increasing induced e.m.f.: faster movement, more turns, stronger magnet, longer wire (for wire experiment).

Setup for demonstrating electromagnetic induction with a coil, magnet, and sensitive voltmeter.

The A.C. Generator

An a.c. generator converts mechanical energy to electrical energy using a rotating coil in a magnetic field.
Components: permanent magnet, rotating coil, slip rings and carbon brushes to connect to external circuit.
As the coil rotates, it cuts field lines, inducing an alternating e.m.f. (and current).
Maximum e.m.f. when coil is parallel to field (moving perpendicular); zero e.m.f. when coil is perpendicular to field (moving parallel).
Output graph is a sine/cosine wave; frequency equals rotation frequency.
To increase maximum e.m.f.: increase rotation speed, more turns, stronger magnet, add soft iron core.

The relationship between the rotation of a coil in a magnetic field and the induced e.m.f. in an AC generator

Magnetic Effect of a Current

A current-carrying wire produces a magnetic field of concentric circles around the wire.
Right-hand grip rule: Thumb points in current direction; curled fingers show field direction.
A solenoid (coiled wire) produces a field similar to a bar magnet (one end north, other south).
Field strength increases with larger current, more turns, and inserting a soft iron core (electromagnet).
Applications: relay circuits (electromagnet switches a second circuit), electric bells, loudspeakers (motor effect).

Magnetic field around a current-carrying wire, shown with the right-hand grip rule.

Force on a Current-Carrying Conductor

A current-carrying conductor in a magnetic field experiences a force (motor effect) if current is perpendicular to field.
Fleming's left-hand rule: Thumb = Force (Thrust), First finger = Field, Second finger = Current.
Reversing current or magnetic field reverses the force direction.
Charged particles (e.g., electrons) in a magnetic field also experience a force, deflecting perpendicular to both velocity and field.

A copper rod experiencing a force when current is switched on in a magnetic field.

Electric Motors

A d.c. motor uses the motor effect to rotate a coil continuously.
Components: coil in magnetic field, split-ring commutator and carbon brushes connected to a d.c. supply.
The split-ring commutator reverses current every half turn, so the coil rotates in the same direction.
Forces on opposite sides of the coil are opposite, causing rotation.
To increase speed/turning effect: increase current, stronger magnet, more turns on coil.
To reverse rotation: reverse current direction or magnetic field.

Structure of a simple d.c. motor, showing the coil, split-ring commutator, carbon brushes and magnetic field.

Transformers

A transformer changes the size of an alternating voltage using electromagnetic induction.
Structure: primary coil, secondary coil, and a soft iron core (easily magnetised).
Step-up transformer: more turns on secondary than primary → increases voltage (Vs > Vp).
Step-down transformer: fewer turns on secondary than primary → decreases voltage (Vs < Vp).
Transformer equation: Vp / Vs = Np / Ns (where N = number of turns).
High-voltage transmission reduces energy loss (lower current for same power, less heating in wires).

Structure of a simple transformer, showing the primary coil, secondary coil, soft iron core and the magnetic field linking them.

स्लाइड्स

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साइन अप करें — फ्री लॉग इन

प्रैक्टिस सवाल

फ्री प्रीव्यू — 45 में से 8 सवाल। सभी देखने के लिए साइन अप करें।

1.Which of the following metals can be permanently magnetised?
Easy
- AAluminium
- BCopper
- CSteel
- DTungsten
2.What is the name of the component in a transformer that is made of soft iron?
Easy
- APrimary coil
- BSecondary coil
- CCore
- DInsulation
3.In a simple a.c. generator, what is the function of the slip rings?
Medium
- ATo reverse the current every half turn
- BTo allow the coil to rotate continuously
- CTo provide a uniform magnetic field
- DTo connect the coil to the external circuit
4.A transformer has 100 turns on the primary coil and 500 turns on the secondary coil. The input voltage is 20 V. What is the output voltage?
Medium
- A4 V
- B100 V
- C500 V
- D1000 V
5.Which change would increase the strength of an electromagnet?
Easy
- ADecreasing the current in the coil
- BRemoving the iron core
- CIncreasing the number of turns on the coil
- DUsing a shorter wire
6.A coil is connected to a sensitive voltmeter. A bar magnet is pushed into the coil and then held stationary inside it. What does the voltmeter show?
Medium
- AA constant deflection while the magnet moves, then zero
- BA deflection only while the magnet is moving
- CNo deflection at any time
- DA constant deflection while the magnet is stationary inside
7.In a d.c. motor, what is the purpose of the split-ring commutator?
Medium
- ATo increase the magnetic field strength
- BTo reverse the direction of the current in the coil every half turn
- CTo connect the coil to the power supply continuously
- DTo reduce friction
8.A wire carrying a current is placed in a magnetic field. The direction of the force on the wire can be found using:
Easy
- AFleming's right-hand rule
- BFleming's left-hand rule
- CThe right-hand grip rule
- DLenz's law

और सवाल

9.Which of the following is a correct statement about the magnetic field around a straight current-carrying wire?
Easy
- AThe field lines are straight lines parallel to the wire
- BThe field lines are concentric circles centred on the wire
- CThe field lines are radial lines pointing away from the wire
- DThe field lines are uniform and parallel
10.A student uses a step-up transformer. Which statement is correct?
Medium
- AThe primary coil has more turns than the secondary coil
- BThe output voltage is less than the input voltage
- CThe output voltage is greater than the input voltage
- DThe transformer works with a d.c. supply
11.In an a.c. generator, the induced e.m.f. is maximum when the coil is:
Medium
- AParallel to the magnetic field
- BPerpendicular to the magnetic field
- CAt 45° to the magnetic field
- DStationary
12.What is the name of the effect where a current-carrying conductor experiences a force in a magnetic field?
Easy
- AElectromagnetic induction
- BMotor effect
- CGenerator effect
- DMagnetic effect of a current
13.A transformer has 500 turns on the primary coil and 50 turns on the secondary coil. The input voltage is 230 V. What is the output voltage?
Medium
- A23 V
- B2300 V
- C115 V
- D46 V
14.A student investigates electromagnetic induction using a coil connected to a sensitive voltmeter. She pushes a bar magnet into the coil and then holds it stationary inside. Which of the following best describes the voltmeter reading during this experiment? (Assume positive deflection when magnet enters.)
Medium
- AA positive spike then zero
- BA positive spike then constant positive
- CA constant positive reading
- DZero throughout
15.Which of the following will increase the strength of an electromagnet?
Easy
- AIncreasing the current in the coil
- BUsing a plastic core
- CDecreasing the number of turns on the coil
- DUsing a shorter wire
16.A step-up transformer is used in the transmission of electricity. Which statement is correct?
Medium
- AIt increases voltage and decreases current
- BIt increases voltage and increases current
- CIt decreases voltage and decreases current
- DIt decreases voltage and increases current
17.A transformer has a primary coil with 100 turns and a secondary coil with 200 turns. The primary voltage is 12 V a.c. What is the secondary voltage?
Medium
- A24 V
- B6 V
- C12 V
- D48 V
18.In an a.c. generator, if the frequency of rotation is doubled, what happens to the period of the output voltage?
Medium
- AIt halves
- BIt doubles
- CIt stays the same
- DIt quadruples
19.Which metal is commonly used for the core of a transformer?
Easy
- ASoft iron
- BSteel
- CCopper
- DAluminium
20.A student sets up a coil connected to a sensitive voltmeter. She pushes a bar magnet into the coil and observes a deflection. She then pulls the magnet out of the coil. What will she observe on the voltmeter?
Medium
- AA deflection in the opposite direction to when the magnet was pushed in
- BA deflection in the same direction as when the magnet was pushed in
- CNo deflection
- DA constant deflection, larger than before
21.In a simple a.c. generator, the coil is rotated at a constant speed. The graph of induced e.m.f. against time is a sine wave. If the number of turns on the coil is doubled and the speed of rotation is halved, what happens to the peak e.m.f. and the frequency?
Hard
- APeak e.m.f. remains the same, frequency halves
- BPeak e.m.f. doubles, frequency halves
- CPeak e.m.f. halves, frequency remains the same
- DPeak e.m.f. remains the same, frequency doubles
22.A student uses a plotting compass to map the magnetic field around a current-carrying straight wire. Which of the following best describes the pattern she observes?
Medium
- AConcentric circles around the wire, with the compass needles tangential to the circles
- BRadial lines pointing outwards from the wire
- CParallel lines along the wire
- DSpiral lines moving away from the wire
23.A transformer has 200 turns on the primary coil and 50 turns on the secondary coil. The primary coil is connected to a 230 V a.c. supply. What is the output voltage?
Medium
- A57.5 V
- B920 V
- C1150 V
- D46 V
24.In a d.c. motor, the split-ring commutator reverses the current in the coil every half turn. Why is this necessary?
Medium
- ATo ensure the coil rotates continuously in the same direction
- BTo increase the speed of rotation
- CTo reduce the current in the coil
- DTo change the direction of the magnetic field
25.A copper rod is placed between the poles of a strong magnet and connected to a battery so that a current flows through it. The rod experiences a force. Which change would reverse the direction of the force?
Easy
- AReversing the direction of the current only
- BIncreasing the current
- CUsing a stronger magnet
- DReversing both the current and the magnetic field
26.A step-up transformer is used to transmit electrical power over long distances. Which statement explains why high voltage transmission is advantageous?
Medium
- AFor the same power, higher voltage means lower current, reducing energy losses in the cables
- BHigher voltage increases the current, making the system more efficient
- CHigher voltage reduces the need for insulation
- DHigher voltage allows the use of thinner cables without any loss
27.In an experiment to demonstrate electromagnetic induction, a coil is connected to a sensitive voltmeter. A bar magnet is moved towards the coil. Which of the following would increase the induced e.m.f.?
Medium
- AMoving the magnet faster
- BUsing a weaker magnet
- CMoving the magnet away from the coil
- DUsing a coil with fewer turns
28.A student sets up a simple d.c. motor. The coil rotates slowly. Which two changes could be made to make the coil rotate faster?
Medium
- AIncrease the current and use a stronger magnet
- BDecrease the current and use a weaker magnet
- CIncrease the number of turns on the coil and use a weaker magnet
- DDecrease the number of turns and increase the current
29.What is the name of the effect where an electromotive force (e.m.f.) is induced in a conductor due to relative movement between the conductor and a magnetic field?
Easy
- AElectromagnetic induction
- BElectrostatic induction
- CMagnetic effect of a current
- DMotor effect
30.Which component in a simple a.c. generator allows the alternating current to flow between the rotating coil and the external circuit?
Easy
- ASlip rings
- BSplit-ring commutator
- CCarbon brushes
- DPermanent magnet
31.Which metal is commonly used for the core of an electromagnet because it is easily magnetised and demagnetised?
Easy
- ASteel
- BSoft iron
- CCopper
- DAluminium
32.In a transformer, what is the function of the soft iron core?
Easy
- ATo conduct electricity between the coils
- BTo provide a magnetic field
- CTo concentrate the magnetic field and link the primary and secondary coils
- DTo insulate the coils from each other
33.A student uses a plotting compass to map the magnetic field around a current-carrying solenoid. Which of the following best describes the pattern she observes?
Medium
- AConcentric circles around the solenoid
- BStraight lines parallel to the solenoid
- CA pattern similar to that of a bar magnet, with field lines emerging from one end and entering the other
- DNo field lines are detected outside the solenoid
34.Which of the following will NOT increase the strength of an electromagnet?
Medium
- AIncreasing the current in the coil
- BIncreasing the number of turns on the coil
- CUsing a longer wire with the same number of turns
- DInserting a soft iron core into the coil
35.An e.m.f. is induced in a conductor only when the conductor cuts magnetic field lines.
Medium
True or false?
36.Which of the following factors does NOT affect the magnitude of the induced e.m.f. in a coil?
Easy
- AThe speed at which the magnet is moved
- BThe number of turns on the coil
- CThe colour of the insulation on the wire
- DThe strength of the magnet
37.Which of the following will increase the strength of an electromagnet? (Select all that apply)
Medium
- AIncreasing the current in the coil
- BIncreasing the number of turns on the coil
- CUsing a plastic core instead of iron
- DInserting a soft iron core into the coil
38.In a step-up transformer, the secondary coil has more turns than the primary coil.
Easy
True or false?
39.Match each component of a simple a.c. generator to its function.
Medium
- Permanent magnet
- Slip rings
- Carbon brushes
- Provide a uniform magnetic field
- Allow alternating current to flow between coil and external circuit
- Maintain electrical contact with the slip rings
40.Arrange the following steps in the correct order to describe the operation of a d.c. motor from the start.
Medium
- Current flows through the coil, producing a magnetic field.
- The coil rotates due to opposite forces on each side.
- The split ring reconnects after the coil passes the vertical position.
- The coil's momentum carries it past the vertical position where no current flows.
41.Which of the following changes would increase the strength of an electromagnet? (Select all that apply)
Medium
- AIncreasing the current in the coil
- BDecreasing the number of turns on the coil
- CInserting a soft iron core into the coil
- DUsing a longer wire with the same number of turns
42.Complete the sentence about the A.C. generator.
Easy
In a simple a.c. generator, the coil is connected to the external circuit using ____ and carbon brushes.
43.A transformer has 200 turns on the primary coil and 50 turns on the secondary coil. The primary voltage is 230 V a.c. Calculate the secondary voltage.
Medium
44.State Lenz's law.
Easy
45.Explain why a step-up transformer is used in the transmission of electrical power over long distances.
Medium

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