Electromagnetism - 100-Question Answer-Sheet Review

How to revise: Attempt each question first, then open its model answer. Green = correct, amber = partially correct, and red = incorrect or unattempted.
67
Correct
20
Partially correct
7
Incorrect
6
Unattempted

Class 8 Electromagnetism: Complete Answer-Sheet Review

100 textbook questions · student grading · corrected model answers · explanations

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Jump to: Test Your Concepts 1-50 · Concept Application 1-50 · Visual reference

Test Your Concepts - Questions 1-50

Question 1: Is the magnetic field formed in a solenoid uniform or non-uniform?

Status: Correct
Show model answer and explanation
It is nearly uniform inside a long solenoid. The field lines inside are straight, parallel and almost equally spaced; near the ends and outside, the field becomes non-uniform.

Question 2: What is electromagnetic induction?

Status: Correct
Show model answer and explanation
Electromagnetic induction is the production of an induced EMF, and a current if the circuit is closed, when the magnetic flux linked with a conductor changes. The change may be produced by moving a magnet or coil, or by changing current in a nearby coil.

Question 3: Mention two uses or applications of a solenoid.

Status: Partially correct
Show model answer and explanation
A solenoid is used to make electromagnets by inserting a soft-iron core and to produce a strong, nearly uniform magnetic field. Applications include electric bells, relays, lifting magnets and electromagnetic valves.

Question 4: A solenoid behaves like a ______ magnet.

Status: Correct
Show model answer and explanation
A current-carrying solenoid behaves like a bar magnet, having identifiable north and south poles. With a soft-iron core it becomes a strong temporary electromagnet.

Question 5: Is electromagnetic field a vector or a scalar?

Status: Correct
Show model answer and explanation
A magnetic field is a vector because it has both magnitude and direction at every point.

Question 6: Mention two applications of electromagnetic induction.

Status: Correct
Show model answer and explanation
Two important applications are the electric generator/dynamo and the transformer. Induction is also used in induction cooktops, microphones and wireless charging.

Question 7: State (a) Ampere's swimming rule and (b) right-hand grip rule.

Status: Partially correct
Show model answer and explanation
Ampere's swimming rule: imagine a swimmer moving with the current and facing the magnetic needle; the north pole deflects toward the swimmer's stretched left hand. Right-hand grip rule: grip the conductor with the right hand; thumb points with current and curled fingers show the magnetic-field direction.

Question 8: In an electric bell, ______ is attracted towards an electromagnet.

Status: Correct
Show model answer and explanation
The soft-iron armature is attracted toward the energized electromagnet. The attached hammer then strikes the gong.

Question 9: What is Maxwell's right-handed corkscrew rule?

Status: Correct
Show model answer and explanation
If a right-handed screw advances in the direction of current, the direction in which its head rotates gives the direction of the circular magnetic field around the conductor.

Question 10: What energy transformation takes place in an electric generator?

Status: Correct
Show model answer and explanation
A generator converts mechanical energy into electrical energy by rotating a coil in a magnetic field and inducing an EMF.

Question 11: Electromagnets are used ______.

Status: Correct
Show model answer and explanation
Electromagnets are used wherever a controllable temporary magnet is required, for example in electric bells, relays, cranes for lifting iron, motors, speakers and medical equipment.

Question 12: What is (a) a solenoid and (b) an electromagnet?

Status: Correct
Show model answer and explanation
A solenoid is a long cylindrical coil of many closely wound insulated turns. An electromagnet is a temporary magnet made by passing current through a coil, usually wound around a soft-iron core.

Question 13: If 25 J of work is done and conversion efficiency is 60%, find the electrical energy generated.

Status: Correct
Show model answer and explanation
Efficiency = useful energy/input energy. Electrical energy = 0.60 × 25 J = 15 J. The remaining 10 J is lost mainly as heat and other forms.

Question 14: If current in a circular loop increases, does its magnetic field increase?

Status: Correct
Show model answer and explanation
Yes. For the same loop, magnetic-field strength is directly proportional to current, so increasing current makes the field stronger.

Question 15: What are the different types of electric power stations?

Status: Correct
Show model answer and explanation
The chapter identifies hydroelectric, thermal and nuclear power stations. Each ultimately uses a generator; they differ in the source used to rotate the turbine.

Question 16: If current in a coil is anticlockwise as observed, that face becomes which pole?

Status: Unanswered
Show model answer and explanation
The face becomes a north pole. The clock-face rule is: anticlockwise current → north face; clockwise current → south face.

Question 17: If current in a circular loop is clockwise facing you, which pole is observed?

Status: Partially correct
Show model answer and explanation
The face toward you is a south pole. Use the clock-face rule: clockwise current makes the observed face south.

Question 18: What is the frequency of the AC supplied to houses?

Status: Correct
Show model answer and explanation
In the context of this textbook, household AC frequency is 50 Hz, meaning 50 complete cycles each second.

Question 19: What is an electric generator?

Status: Incorrect
Show model answer and explanation
An electric generator is a device that converts mechanical energy into electrical energy using electromagnetic induction. The written answer reversed this conversion.

Question 20: Current goes into the page through a straight conductor. What is the field direction?

Status: Correct
Show model answer and explanation
The magnetic field is clockwise as viewed from the front. Point the right thumb into the page; the curled fingers turn clockwise.

Question 21: Electric power stations generate ______ current.

Status: Correct
Show model answer and explanation
They generate alternating current (AC), whose direction reverses periodically.

Question 22: What happens to the field at a solenoid's centre when current increases?

Status: Correct
Show model answer and explanation
The magnetic field increases. For a long solenoid, B is proportional to current I and to turns per unit length.

Question 23: What is the shape of field lines around a straight current-carrying conductor?

Status: Correct
Show model answer and explanation
They are concentric circles centred on the conductor, lying in planes perpendicular to it.

Question 24: Which field is stronger: a solenoid on wood or on soft iron?

Status: Unanswered
Show model answer and explanation
The solenoid wound on a soft-iron core has the stronger field because soft iron becomes magnetized and concentrates the magnetic flux.

Question 25: Define (a) alternating current and (b) direct current.

Status: Correct
Show model answer and explanation
AC periodically reverses direction and usually changes magnitude. DC flows in one direction; a cell or battery supplies DC.

Question 26: State the use of (a) a galvanoscope and (b) a telegraph.

Status: Correct
Show model answer and explanation
A galvanoscope detects the presence and direction of a small current. A telegraph sends coded messages over wires, traditionally using Morse code.

Question 27: Detection of small currents can be done by ______.

Status: Incorrect
Show model answer and explanation
The expected answer is a galvanometer (a galvanoscope detects presence, while a galvanometer can measure small current).

Question 28: What principle does an electric bell use?

Status: Correct
Show model answer and explanation
It works on the magnetic effect of current/electromagnetism: current magnetizes the coil, which attracts the armature and repeatedly makes and breaks the circuit.

Question 29: What is a transformer?

Status: Partially correct
Show model answer and explanation
A transformer changes the voltage of alternating current using mutual induction. It has primary and secondary coils on a laminated soft-iron core; it may step voltage up or down.

Question 30: What is an electric motor?

Status: Correct
Show model answer and explanation
An electric motor converts electrical energy into mechanical energy. A current-carrying coil in a magnetic field experiences a turning effect.

Question 31: Explain the working of a galvanoscope.

Status: Partially correct
Show model answer and explanation
A galvanoscope contains a magnetic needle inside a coil. When current flows, the coil's magnetic field deflects the needle; reversing current reverses deflection. It detects current but is not designed for precise measurement.

Question 32: Transformer: Np=100, Ns=200 and Vp=200 V. Identify type and find Vs.

Status: Correct
Show model answer and explanation
Ns>Np, so it is step-up. Using Vs/Vp = Ns/Np: Vs/200 = 200/100 = 2, hence Vs = 400 V.

Question 33: State properties of field lines around a straight current-carrying conductor.

Status: Partially correct
Show model answer and explanation
They are concentric circles centred on the wire, in planes perpendicular to it. Direction follows the right-hand grip rule. Field is strongest near the wire, increases with current and decreases with distance.

Question 34: Write a short note on a dynamo.

Status: Correct
Show model answer and explanation
A dynamo is a small generator, commonly used on a bicycle. Rotation of a magnet or coil changes magnetic flux and induces electricity, converting wheel motion into electrical energy for the lamp.

Question 35: Np:Ns=3:2, total turns=300, Vs=200 V. Find turns and Vp.

Status: Correct
Show model answer and explanation
Let Np=3x and Ns=2x. Then 5x=300, so x=60: Np=180, Ns=120. Vs/Vp=Ns/Np=2/3, so Vp=300 V.

Question 36: Explain Maxwell's right-handed corkscrew rule.

Status: Correct
Show model answer and explanation
Turn a right-handed screw so that it advances along the current. The rotation direction of the screw head gives the magnetic-field direction around the conductor. Reversing current reverses the field.

Question 37: What is a transformer? How many types are there?

Status: Correct
Show model answer and explanation
A transformer changes AC voltage by mutual induction. There are two basic types: step-up (Ns>Np, raises voltage) and step-down (Ns

Question 38: Vp=250 V, Vs=5 V, Np=1000. Find Ns and identify transformer.

Status: Correct
Show model answer and explanation
Ns/Np=Vs/Vp=5/250=1/50. Therefore Ns=1000/50=20 turns. Since Vsstep-down transformer.

Question 39: State properties of field lines due to a circular current-carrying loop.

Status: Incorrect
Show model answer and explanation
Each small part produces circular field lines. Near the centre, fields combine in the same direction, becoming nearly straight, parallel and strong. Field strength increases with current and number of turns; the loop has north and south faces.

Question 40: Illustrate the field around a straight current-carrying conductor.

Status: Partially correct
Show model answer and explanation
Draw concentric circles around the wire and mark direction using the right-hand grip rule. A dot means current out of the page and an anticlockwise field; a cross means current into the page and a clockwise field.

Question 41: Illustrate the field produced by a circular current-carrying loop.

Status: Correct
Show model answer and explanation
The loop's field resembles a bar magnet. Lines pass through the centre nearly parallel and return outside. Mark one face north and the other south using the clock-face rule.

Question 42: Write the properties of field lines formed by a solenoid.

Status: Partially correct
Show model answer and explanation
Inside a long solenoid, lines are straight, parallel and equally spaced, showing a strong uniform field. Outside they curve from north to south like a bar magnet; field is strongest inside and depends on current, turns per unit length and core material.

Question 43: Draw a neat labelled diagram of a transformer.

Status: Partially correct
Show model answer and explanation
A complete diagram should label the AC input, primary coil Np, laminated soft-iron core, secondary coil Ns and output/load. Both coils are electrically separate but magnetically linked by changing flux in the core.

Question 44: In electromagnetic induction, what supplies the electrical energy of induced current?

Status: Correct
Show model answer and explanation
It comes from the mechanical work/kinetic energy used to move the magnet or coil against the opposing electromagnetic force. Energy is converted, not created.

Question 45: Mention applications of electromagnets.

Status: Partially correct
Show model answer and explanation
Applications include electric bells, relays, motors, generators, loudspeakers, magnetic cranes, separation of iron, MRI and other medical devices, and making strong permanent magnets.

Question 46: Explain alternating and direct current.

Status: Correct
Show model answer and explanation
DC flows in one direction and is supplied by cells/batteries. AC reverses direction periodically; at 50 Hz it completes 50 cycles per second. AC is easily transformed for efficient transmission.

Question 47: Explain the construction and working of a DC motor.

Status: Partially correct
Show model answer and explanation
A rectangular coil lies between magnetic poles and connects to a battery through brushes and split rings. Opposite forces on its sides form a couple and rotate it. Every half-turn the split rings reverse coil current, keeping torque in the same rotational direction.

Question 48: Explain the construction and working of an electric bell.

Status: Partially correct
Show model answer and explanation
Main parts: electromagnet, armature, spring, contact screw, hammer, gong, battery and switch. Current magnetizes the coil and pulls the armature; the hammer strikes the gong and breaks contact. The coil demagnetizes, the spring restores contact, and the rapid cycle repeats.

Question 49: Explain the construction and working of an electric generator.

Status: Partially correct
Show model answer and explanation
A coil rotates between magnetic poles; slip rings and brushes connect it to the external circuit. Rotation changes magnetic flux and induces alternating EMF. The current reverses every half-turn, converting mechanical to electrical energy.

Question 50: Write a note on telegraph.

Status: Partially correct
Show model answer and explanation
A telegraph sends messages as current pulses through a long circuit. Pressing a Morse key energizes a distant electromagnet, moving an armature to produce dots and dashes. The earth may serve as the return path, reducing the wire requirement.

Concept Application - Questions 1-50

Question 1: Each loop of a solenoid acts like a tiny magnet with north and south poles. True or false?

Status: Correct
Show model answer and explanation
True. Every current loop has a magnetic dipole moment. In a solenoid, the fields of all turns reinforce one another.

Question 2: In a step-down transformer, primary turns are fewer than secondary turns. True or false?

Status: Correct
Show model answer and explanation
False. A step-down transformer has Np>Ns, so Vs

Question 3: Field lines around a straight current-carrying conductor are circular. True or false?

Status: Correct
Show model answer and explanation
True. They form concentric circles centred on the wire.

Question 4: Motor angular speed is independent of the number of coil turns. True or false?

Status: Unanswered
Show model answer and explanation
False. More turns increase magnetic torque for the same current and field, so motor behaviour is not independent of turn count.

Question 5: The magnetic effect of current gives rise to mechanical forces. True or false?

Status: Correct
Show model answer and explanation
True. A current-carrying conductor in a magnetic field experiences force; in a motor, forces produce torque.

Question 6: An electric bell works on electromagnetic induction. True or false?

Status: Correct
Show model answer and explanation
False. It works on the magnetic effect of current and attraction by an electromagnet, not induction.

Question 7: Electricity and magnetism are unrelated branches. True or false?

Status: Correct
Show model answer and explanation
False. Current produces magnetic fields and changing magnetic fields induce EMF; together they form electromagnetism.

Question 8: Ns:Np=10:3. What type of transformer is it?

Status: Correct
Show model answer and explanation
Ns>Np, so it is a step-up transformer.

Question 9: Clockwise current as observed makes that coil face which pole?

Status: Correct
Show model answer and explanation
It becomes a south pole. Anticlockwise would make it north.

Question 10: Which right-hand rule gives magnetic-needle deflection near a conductor?

Status: Correct
Show model answer and explanation
Ampere's swimming rule gives the stated needle-deflection direction. The grip rule directly gives magnetic-field direction.

Question 11: In a telegraph, what connects to the positive terminal and what to the negative terminal?

Status: Incorrect
Show model answer and explanation
The Morse key connects to the positive terminal; the negative terminal is connected to earth to complete the return path.

Question 12: How can the ringing frequency of an electric bell be altered?

Status: Correct
Show model answer and explanation
By adjusting the contact screw and armature spring, changing the make-break rate.

Question 13: Concave cylindrical magnets in a DC motor produce what field?

Status: Correct
Show model answer and explanation
They produce a radial magnetic field, helping maintain torque over rotation.

Question 14: When is EMF induced between a magnet and conductor?

Status: Correct
Show model answer and explanation
When there is relative motion that changes the magnetic flux linked with the conductor.

Question 15: Match A-J with a-j.

Status: Correct
Show model answer and explanation
Correct matching: A-d, B-j, C-a, D-f, E-c, F-i, G-g, H-b, I-e, J-h.

Question 16: Transformer Np=100, Ns=150: choose the correct statement.

Status: Correct
Show model answer and explanation
Ns>Np, so statement A (step-up) is true and statement B is false. Correct option: (a), only A.

Question 17: Transformer Np=250, Ns=150: identify it.

Status: Correct
Show model answer and explanation
Nsstep-down transformer; correct option: only statement B.

Question 18: What voltage is induced by a rotating generator coil?

Status: Correct
Show model answer and explanation
An alternating voltage, because the induced polarity reverses every half-turn.

Question 19: Current flows south to north; compass is below the wire. Which way does it deflect?

Status: Correct
Show model answer and explanation
Using Ampere's rule/right-hand rule, it deflects toward the west for the arrangement stated.

Question 20: How are magnetic field lines arranged inside a solenoid?

Status: Correct
Show model answer and explanation
They are parallel to one another and nearly equally spaced, indicating a uniform field.

Question 21: A rotating generator coil induces which kind of voltage?

Status: Correct
Show model answer and explanation
Alternating voltage. This repeats Question 18; rotation reverses induced polarity each half-turn.

Question 22: A steel bar placed in a solenoid changes into what?

Status: Incorrect
Show model answer and explanation
It becomes an electromagnet while current flows. Steel may retain more magnetism than soft iron, but the intended option is electromagnet.

Question 23: 50 J work, 45% converted: find electric energy.

Status: Correct
Show model answer and explanation
Electrical energy = 0.45×50 = 22.5 J; correct option (c).

Question 24: Faraday observations: stronger magnet increases deflection and induced current is alternating. Which are true?

Status: Correct
Show model answer and explanation
Both A and B are true. Stronger flux change gives larger induced EMF, and reversing relative motion reverses current.

Question 25: At fixed distance, field around a straight conductor increases with what?

Status: Correct
Show model answer and explanation
It increases with current strength; correct option (c).

Question 26: What form do magnetic lines around a straight conductor take?

Status: Correct
Show model answer and explanation
Concentric circles; correct option (b).

Question 27: In induction, which energy converts to which?

Status: Correct
Show model answer and explanation
Kinetic/mechanical energy converts to electrical energy; correct option (d).

Question 28: On what does induced voltage depend: area, speed, turns, or all?

Status: Unanswered
Show model answer and explanation
All of the above. Induced EMF grows with a faster flux change, more turns and greater effective coil area.

Question 29: Which solenoid statement is true?

Status: Correct
Show model answer and explanation
Only B is true: a soft-iron core makes it a temporary magnet. Field strength does depend on core material.

Question 30: What common use do galvanometer and galvanoscope have?

Status: Correct
Show model answer and explanation
Both can detect small currents; correct option (d).

Question 31: For which magnet-coil motions is EMF induced?

Status: Correct
Show model answer and explanation
There must be relative motion: cases (a), (b) and (d). If magnet and coil move together at equal velocity, flux linkage is unchanged.

Question 32: Why transmit electrical energy at high voltage and low current?

Status: Incorrect
Show model answer and explanation
For fixed power P=VI, raising voltage lowers current. Wire loss is P_loss=I²R, so lower current greatly reduces heating and energy loss.

Question 33: Can Ampere's right-hand rule determine magnetic-needle deflection?

Status: Correct
Show model answer and explanation
Not directly. It gives magnetic-field direction around the conductor; needle deflection is found by combining that field with the needle's orientation or using Ampere's swimming rule.

Question 34: Does a straight current-carrying conductor have N/S poles? What shape gives poles?

Status: Correct
Show model answer and explanation
A straight wire's circular field has no separate north and south poles. Bend it into a circular coil or solenoid; its two faces/ends act as poles.

Question 35: Np=250, Ns=500, Vp=200 V, Ip=5 mA. Find Vs and Is.

Status: Correct
Show model answer and explanation
Vs/Vp=Ns/Np=2, so Vs=400 V. For an ideal transformer VpIp=VsIs, so Is=(200×5 mA)/400=2.5 mA.

Question 36: What happens if earthing is not proper in a telegraph?

Status: Partially correct
Show model answer and explanation
The earth return path becomes high-resistance or incomplete, so insufficient current flows. The receiver electromagnet may respond weakly or not at all, and messages cannot be received reliably.

Question 37: What makes the rectangular coil in a DC motor rotate?

Status: Partially correct
Show model answer and explanation
Opposite magnetic forces act on the two current-carrying sides in the magnetic field. They form a couple/torque that rotates the coil; split rings reverse current each half-turn to maintain torque direction.

Question 38: Parallel conductors: what happens for currents in the same and opposite directions?

Status: Unanswered
Show model answer and explanation
Currents in the same direction attract; currents in opposite directions repel. Each wire lies in the other's magnetic field and experiences a force.

Question 39: How can a galvanoscope be modified to measure current?

Status: Incorrect
Show model answer and explanation
Add a calibrated scale and suitable control/damping so deflection is proportional to current; this makes a galvanometer-type measuring instrument. For larger currents, a low-resistance shunt is connected in parallel.

Question 40: Does a straight conductor have poles? What shape gives magnetic poles?

Status: Correct
Show model answer and explanation
No distinct poles exist for a straight wire. Form a circular coil or solenoid so one face/end is north and the other south.

Question 41: A 50 Hz sinusoidal voltage becomes zero how many times in 5 s?

Status: Correct
Show model answer and explanation
A sinusoid crosses zero twice per cycle. In 5 s there are 50×5=250 cycles, so zero occurs 500 times.

Question 42: In induction, what is the source of electrical energy?

Status: Correct
Show model answer and explanation
The source is the mechanical work/kinetic energy used to move the magnet or conductor.

Question 43: Two parallel W-E wires carry 5 A and 2 A. How does a midpoint compass deflect, and after reversal?

Status: Partially correct
Show model answer and explanation
At the midpoint the fields oppose, but the 5 A wire's field is stronger, so the compass follows that net field. Reversing both currents reverses the net field and therefore reverses the deflection. A complete answer must state the actual compass direction from the diagram's vertical placement.

Question 44: A copper spring moves toward a magnet: what if ends are connected vs open?

Status: Partially correct
Show model answer and explanation
With ends connected, induced current flows; by Lenz's law its field opposes approach, producing repulsion and tending to stretch/separate turns. With ends open, EMF exists but no sustained current flows, so there is essentially no magnetic force or shape change.

Question 45: How can a concealed wall conductor carrying DC be located?

Status: Correct
Show model answer and explanation
Move a galvanoscope/compass along the wall. Its needle deflects near the current-carrying wire; strongest systematic deflection traces the conductor.

Question 46: Can a transformer step up or down DC voltage?

Status: Correct
Show model answer and explanation
No. Steady DC creates no continuously changing magnetic flux, so after the switching instant no EMF is induced in the secondary. Transformers require AC.

Question 47: A magnet falls through an aluminium loop at room temperature and when hot. Compare fall time.

Status: Unanswered
Show model answer and explanation
Heating aluminium increases resistance, reducing induced eddy current and opposing magnetic force. Therefore the magnet falls faster through the hot loop and more slowly through the cooler loop.

Question 48: What if a galvanoscope coil plane is east-west instead of along the magnetic meridian?

Status: Partially correct
Show model answer and explanation
The coil's field and Earth's horizontal field geometry becomes unsuitable for useful deflection; the needle may align with the resultant with little or no sensitive response. The magnetic-meridian orientation is used for maximum clear deflection.

Question 49: Why does more turns per unit length strengthen a solenoid's field?

Status: Partially correct
Show model answer and explanation
Each turn contributes field in the same axial direction. More turns per unit length means more reinforcing fields over the same distance; for a long solenoid, B is proportional to nI.

Question 50: Why use concave poles in a DC motor? What if poles are flat?

Status: Correct
Show model answer and explanation
Concave poles produce a strong nearly radial field, keeping force/torque effective through rotation. Flat poles produce a less radial, more non-uniform field, causing weaker and uneven torque.
Main revision priorities: electromagnetic induction and energy conversion; clock-face/right-hand rules; transformer ratios; solenoid field properties; construction and working of the motor, generator, bell and telegraph; and the distinction between galvanoscope and galvanometer.

Grading is based on the legible content of the scanned handwriting. A response was marked partial when the core idea was present but required a missing definition, direction, calculation step, label or explanation.

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