Magnetism and Magnetic Effects of Current PYQs – Physics, SSC & Railway

Correct Answer: Option 1 — Magnetic field
The right-hand thumb rule (also called Maxwell's corkscrew rule) is used to find the direction of the magnetic field around a straight current-carrying conductor. Hold the wire in your right hand so that the stretched thumb points along the direction of the current; then the curled fingers give the direction in which the magnetic field lines encircle the wire.
So in this rule the thumb = current and the curled fingers = magnetic field. The field lines around a straight wire are concentric circles lying in a plane perpendicular to the wire, and they get farther apart — that is, the field gets weaker — as you move away, since B ∝ I/r.
❌ Why the other options are incorrect
✗ Current — the current is shown by the stretched thumb, not by the curled fingers; the two are commonly interchanged in a hurry.
✗ Force — the force on a current-carrying conductor placed in a magnetic field is found by Fleming's left-hand rule, a different rule altogether.
✗ Deflection of needle — the deflection of a compass needle is only the experimental evidence (Oersted's experiment) that a field exists; it is not what the fingers represent.
📌 SSC & Railway Exam Facts
• Right-hand thumb rule: thumb → current, curled fingers → magnetic field.
• Fleming's left-hand rule → motor (force); Fleming's right-hand rule → generator (induced current).
• Hans Christian Oersted (1820) discovered that a current produces a magnetic field.
• For a straight wire, B ∝ I and B ∝ 1/r; SI unit of B is the tesla (T).
• Inside a long solenoid the field is uniform and its pattern is like that of a bar magnet.

Correct Answer: Option 2 — bar magnet
A solenoid is a long cylindrical coil made of many circular turns of insulated copper wire wound closely together. When a current flows through it, the fields of all the turns add up, and inside the solenoid the field lines become straight, parallel and equally spaced — a uniform magnetic field. Outside, the lines emerge from one end, curve round and re-enter at the other end.
This whole pattern is exactly like the pattern of a bar magnet. One end of the solenoid behaves as a north pole and the other as a south pole, which is why a solenoid carrying current can be used to magnetise a piece of steel. If a soft iron core is placed inside, we get a strong electromagnet.
❌ Why the other options are incorrect
✗ disc magnet — a flat, thin magnet magnetised across its thickness; its external field pattern is short and spread out, not like the long uniform interior of a solenoid.
✗ horseshoe magnet — a bar magnet bent into a U so that both poles face each other; it gives a strong field only in the narrow gap between the poles.
✗ ring magnet — magnetised along a circle, so its field lines close on themselves and it shows no free poles like a solenoid does.
📌 SSC & Railway Exam Facts
• Field inside a long solenoid is uniform; outside it is like a bar magnet.
• Poles of a solenoid are found by the clock rule or the right-hand thumb rule applied to the turns.
• A soft iron core in a solenoid makes an electromagnet — soft iron is used because it loses magnetism quickly.
• Permanent magnets are made of steel / alnico, which retain magnetism.
• William Sturgeon made the first electromagnet (1824).

Correct Answer: Option 1 — We can use Fleming's left-hand rule to determine the direction of magnetic field.
A straight current-carrying conductor produces a magnetic field whose lines are concentric circles around the wire. The direction of this field is found by the right-hand thumb rule (also called Maxwell's corkscrew rule): if the thumb of the right hand points along the current, the curled fingers give the direction of the magnetic field lines.
Fleming's left-hand rule is not used for this purpose at all. It is used to find the direction of the force on a current-carrying conductor placed in a magnetic field — the principle of the electric motor. Its three mutually perpendicular fingers stand for: forefinger → magnetic field, centre finger → current, thumb → force/motion. Hence Option 1 is the statement that is NOT true.
❌ Why the other options are incorrect (i.e. they are true statements)
✗ The magnetic field will reverse the direction if we reverse the current — true; reversing the current reverses the sense of the circular field lines, as the right-hand thumb rule shows.
✗ The magnetic field decreases as we move away from the wire — true; for a long straight wire B ∝ 1/r, so the circles get farther apart as r grows.
✗ The strength of the magnetic field increases if we increase the current — true; B ∝ I, so a larger current gives a stronger field.
📌 SSC & Railway Exam Facts
• Field of a long straight wire: B = μ₀I/2πr — directly proportional to I and inversely proportional to r.
• Right-hand thumb rule → direction of the magnetic field of a current; Fleming's left-hand rule → force on a conductor (motor); Fleming's right-hand rule → induced current (generator).
• Oersted (1820) first showed that a current-carrying wire deflects a magnetic compass needle.
• μ₀ = 4π × 10⁻⁷ T m A⁻¹; SI unit of magnetic field is the tesla (T), and 1 T = 10⁴ gauss.
• Inside a long solenoid the field is B = μ₀nI and is uniform; magnetic field lines never intersect.

Correct Answer: Option 1 — The magnitude of the magnetic field outside but close to the solenoid is extremely large.
A solenoid is a long cylindrical coil of many closely wound circular turns of insulated copper wire. When current flows through it, the field of every turn adds up inside the coil, so the field inside a long solenoid is strong and uniform, and the field lines there are parallel straight lines.
Outside the solenoid the field lines spread far apart, so the field there is extremely weak — practically zero for an ideal long solenoid. Hence the statement in Option 1 is incorrect and is the required answer.
The field of a solenoid is B = μ₀nI, where n is the number of turns per unit length. It can be increased by increasing the current, increasing the number of turns, or inserting a soft-iron core (which makes an electromagnet).
❌ Why the other options are incorrect (i.e. they are true statements)
✗ The solenoid in a way acts similar to the bar magnet — true; one end behaves as a north pole and the other as a south pole, and its field pattern matches that of a bar magnet.
✗ The strength of the magnetic field increases if we increase the number of turns — true; B ∝ n, so more turns per unit length give a stronger field.
✗ The magnetic field near the centre of long solenoid is nearly constant — true; the field in the central region of a long solenoid is uniform.
📌 SSC & Railway Exam Facts
• Field inside a long solenoid: B = μ₀nI; μ₀ = 4π × 10⁻⁷ T m A⁻¹.
• SI unit of magnetic field (magnetic flux density) is the tesla (T); smaller unit gauss, 1 T = 10⁴ gauss.
• The polarity of a solenoid is found by the right-hand thumb rule or the clock rule.
• A soft iron core is used in electromagnets because it is easily magnetised and demagnetised; steel is used for permanent magnets.
• Magnetic field lines never intersect, and they run from N to S outside and S to N inside a magnet.

Correct Answer: Option 1 — parallel and equidistant straight lines
A magnetic field is said to be uniform when its magnitude and direction are the same at every point in the region. Magnetic field lines show both these properties at a glance: the direction of the tangent gives the direction of the field, and the closeness of the lines gives its strength.
Therefore, for a uniform field the lines must be straight and parallel (same direction everywhere) and equally spaced (same strength everywhere). This is exactly the pattern found inside a long current-carrying solenoid and in the central region between the poles of a large horseshoe magnet. The Earth's magnetic field over a small laboratory region is also treated as uniform.
❌ Why the other options are incorrect
✗ elliptical lines — curved lines mean the direction changes from point to point, so the field is non-uniform.
✗ parallel but irregular straight lines — the direction is the same but unequal spacing means the strength changes, hence non-uniform.
✗ concentric circular lines — this is the pattern around a straight current-carrying conductor, where the field decreases as distance from the wire increases (B ∝ 1/r), so it is non-uniform.
📌 SSC & Railway Exam Facts
• Magnetic field lines are closed continuous curves and never intersect each other.
• Outside a magnet they run from north to south; inside the magnet, from south to north.
• Field around a straight current-carrying wire: concentric circles, direction given by the right-hand thumb rule.
• B = μ₀nI inside a long solenoid gives a uniform field; SI unit tesla (T).
• Oersted (1820) discovered that a current-carrying conductor produces a magnetic field.

Answer: Option 4 — electric generators
Fleming's right-hand rule gives the direction of the induced current when a conductor moves in a magnetic field — the principle of the electric generator.
Revision note — Fleming's rules and electromagnetic induction
• Right hand: thumb = motion of conductor, forefinger = magnetic field, middle finger = induced current — all three mutually perpendicular.
• Generator converts mechanical energy → electrical energy; works on electromagnetic induction (Faraday).
• Fleming's left-hand rule gives the direction of force on a current-carrying conductor — used for the electric motor (electrical → mechanical).
• AC generator uses slip rings; DC generator and DC motor use a split-ring commutator.
• Induced current is larger when the coil turns faster, has more turns, or the field is stronger.
• In India, AC is supplied at 220 V, 50 Hz.
Do not mix these up
• Right hand = generator (current is produced) · left hand = motor (force is produced).
Also asked from this topic: the part that makes an AC generator's output alternate (slip rings); the effect of doubling the speed of rotation of a generator coil (induced potential difference doubles).

Answer: Option 1 — Perpendicular to both the current and magnetic field
By Fleming's left-hand rule, the force on a current-carrying conductor in a magnetic field acts at right angles to both the current and the field.
This force is what turns the coil of an electric motor.
Revision note — Force on a conductor and the electric motor
• Fleming's left-hand rule: forefinger → field, middle finger → current, thumb → force (motion), all mutually perpendicular.
• Force F = BIL sin θ: maximum when current ⊥ field, zero when current ∥ field.
• Reversing either the current or the field reverses the force.
• An electric motor converts electrical energy → mechanical energy.
• The split-ring commutator reverses the current in the coil every half rotation, so the coil keeps turning one way.
• Parts of a motor: armature coil, field magnet, split rings, carbon brushes.
Do not mix these up
• Left-hand vs right-hand rule — left hand gives the force in a motor; right hand gives the induced current in a generator.
Also asked from this topic: the finger that shows the field in Fleming's left-hand rule (index finger); the force when the current is reversed (it reverses); doubling the current (force doubles).

Answer: Option 3 — When the current is perpendicular to the magnetic field
Force on the conductor: F = BIL sin θ, θ = angle between current and field.
sin θ is largest (1) at θ = 90°, so the force is maximum when current ⊥ field.
Revision note — Force on a current-carrying conductor
• Discovered with the "rod-in-magnet" experiment; basis of the electric motor.
• θ = 0° (parallel) or 180° (opposite) ⇒ sin θ = 0 ⇒ no force.
• Force doubles when the current or the field doubles.
• Direction by Fleming's left-hand rule: forefinger → field, middle finger → current, thumb → force.
• Reversing the current or the field reverses the force.
Do not mix these up
• Opposite to the field is still along the field line (θ = 180°) — the force is zero, not maximum.
Also asked from this topic: the effect of doubling the current in a conductor perpendicular to the field (the force doubles); the finger that shows the field in Fleming's left-hand rule (index finger).

Answer: Option 2 — Because of the magnetic field
The region around a magnet has a magnetic field.
Each iron filing becomes a tiny magnet in this field and lines up along the field lines, making the pattern.
Revision note — Magnetic field and field lines
• Field lines go from the north pole to the south pole outside the magnet and S to N inside — they are closed curves.
• Two field lines never cross — else the field would have two directions at one point.
• Closer lines = stronger field; lines are crowded near the poles.
• A compass needle placed on a line points along it; its north end shows the field direction.
• Magnetic field is a vector: it has both magnitude and direction.
• SI unit of magnetic field: tesla (T).
Also asked from this topic: what crowded field lines indicate (a stronger magnetic field); what the deflection of a compass near a current-carrying wire shows (a magnetic field around the wire).
The filings are pulled by magnetism, not by gravity or electric charge — iron is magnetic, not charged.

Answer: Option 2 — Straight and parallel
Inside a current-carrying solenoid the magnetic field is uniform, so its field lines are parallel straight lines.
Revision note — Magnetic field of a solenoid
• Solenoid: a coil of many circular turns of insulated wire wound close together in the shape of a cylinder.
• Its field pattern outside is like that of a bar magnet — one end acts as north, the other as south.
• Inside: field lines are straight and parallel — equal strength and direction at every point (uniform field).
• The field is stronger with more turns, larger current, and a soft-iron core.
• A soft-iron core inside a solenoid forms an electromagnet — a temporary magnet (used in electric bells, cranes, relays).
• Direction of the field: right-hand thumb rule (curl fingers along current; thumb gives the north end).
Do not mix these up
• Straight wire vs solenoid — a straight wire gives concentric circles; inside a solenoid the lines are parallel straight lines.
Also asked from this topic: the field pattern of a solenoid (parallel lines inside, closed loops outside); why a solenoid can magnetise soft iron (strong uniform field inside).

Answer: Option 3 — Direction of motion
In Fleming's right-hand rule the thumb points along the motion of the conductor.
The rule gives the direction of the induced current in a conductor moving in a magnetic field.
Revision note — Fleming's right-hand rule (Class 10)
• Stretch the thumb, forefinger and middle finger of the right hand mutually perpendicular.
• Thumb → direction of motion of the conductor.
• Forefinger → direction of the magnetic field.
• Middle finger → direction of the induced current.
• Used for the electric generator (electromagnetic induction — mechanical energy → electrical energy).
Do not mix these up
• Right-hand vs left-hand rule — right-hand rule = generator (induced current).
Left-hand rule = motor (force on a current-carrying conductor).
Also asked from this topic: induced p.d. when a generator coil spins twice as fast (doubles); induced current for a conductor moving up in a north-to-south field (towards east).

Answer: Option 3 — is the same at all points
Inside a long current-carrying solenoid the field lines are parallel straight lines.
Parallel, equally spaced lines mean a uniform field — the same at every point inside.
Revision note — Magnetic field of a solenoid (Class 10)
• Solenoid = a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.
• Its field pattern is like that of a bar magnet: one end acts as the north pole, the other as the south pole.
• Inside: parallel straight lines ⇒ uniform field; outside: lines loop from N to S.
• A strong uniform field inside can magnetise a soft iron piece placed in it — this makes an electromagnet.
• Field strength rises with more turns, larger current and a soft iron core.
Do not mix these up
• Solenoid vs circular loop — a single circular loop's field is strongest at its centre and not uniform; a solenoid's field is uniform inside.
Also asked from this topic: the shape of field lines inside a current-carrying solenoid (parallel straight lines); why a solenoid can magnetise soft iron (strong, uniform field inside).

Answer: Option 3 — anti-clockwise
Look at the face of the loop from the front.
If the current flows anti-clockwise, that face behaves as a North pole; if clockwise, it is a South pole.
Revision note — Magnetic field of a current-carrying loop
• Clock-face rule: anti-clockwise current ⇒ North pole; clockwise current ⇒ South pole (as seen by the viewer).
• Right-hand thumb rule: curl the fingers along the current, the thumb points along the magnetic field inside the loop.
• At the centre of the loop the field lines are almost straight and perpendicular to the plane of the loop.
• Field at the centre increases with the current and the number of turns, and decreases as the radius increases.
• A solenoid (many turns) behaves like a bar magnet; with a soft iron core it becomes an electromagnet.
Do not mix these up
• Direction of current vs direction of field — the pole depends on how the current circulates, not on "north" or "south" as a direction of flow.
Also asked from this topic: the rule used for the field at the centre of a circular loop (right-hand thumb rule).

Answer: Option 4 — the direction of force on a current-carrying conductor
Fleming's left-hand rule tells the direction of the force (motion) on a conductor carrying current in a magnetic field.
It gives direction only — the magnitude comes from F = BIL.
Revision note — Fleming's left-hand rule
• Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular.
• Forefinger → magnetic field; middle finger → current; thumb → force / motion.
• Force is maximum when the current is perpendicular to the field and zero when parallel.
• Reversing either the current or the field reverses the force.
• Devices based on it: electric motor, loudspeaker, galvanometer.
Do not mix these up
• Left-hand vs right-hand rule — left for motor (force on a current); right for generator (direction of induced current).
Also asked from this topic: which finger shows the magnetic field (forefinger); what happens to the force if the current is reversed (it reverses).

Answer: Option 1 — Fuse
A fuse is a wire of low melting point placed in series with the circuit.
When the current becomes too large, the wire heats up and melts, breaking the circuit and saving the appliances.
Revision note — Electric fuse and household safety
• A fuse works on the heating effect of current (Joule's law, H = I²Rt).
• Fuse wire is an alloy (e.g. tin–lead) with low melting point and suitable resistance; it sits in a porcelain cartridge.
• It is always put in the live wire, in series with the appliance.
• Indian homes use a 5 A circuit for lights and fans and a 15 A circuit for geysers, ACs and heaters.
• Overloading (too many appliances on one socket) and short-circuit (live touching neutral) both cause excess current.
• Wire colours: live — red/brown, neutral — black/blue, earth — green; modern homes use an MCB in place of a fuse.
Do not mix these up
• Fuse vs earthing — a fuse guards against excess current; earthing guards the user against shock from a leaking metal body.
Also asked from this topic: the main use of a fuse in a household circuit (to break the circuit when excess current flows); the current rating for heavy appliances (15 A).

Answer: Option 4 — bar magnet
A current-carrying solenoid has field lines like a bar magnet.
One end behaves as a north pole, the other as a south pole.
Revision note — Magnetic field of a solenoid
• Solenoid — a coil of many closely wound turns of insulated copper wire in the shape of a cylinder.
• Inside the solenoid the field lines are parallel straight lines — the field is uniform there.
• Outside, the lines run from the N end to the S end, exactly as around a bar magnet.
• Field strength rises with more turns, more current and a soft iron core.
• A solenoid with a soft iron core is an electromagnet — magnetism stays only while the current flows.
• The polarity of each end follows the right-hand thumb rule (clockwise current seen from an end = S pole).
Do not mix these up
• Electromagnet vs permanent magnet — electromagnet uses soft iron, temporary and adjustable; permanent magnet uses steel / alnico.
Also asked from this topic: the shape of field lines of a solenoid (parallel straight lines inside); why a solenoid can magnetise soft iron placed inside it (strong, uniform field inside).

Answer: Option 3 — A magnet is a dipole with north and south poles.
Every magnet has two poles — north and south — so it is a magnetic dipole.
Breaking a magnet only gives smaller magnets, each with both poles.
Revision note — Magnets and magnetic poles
• Isolated magnetic poles (monopoles) do not exist; poles always come in N–S pairs.
• A freely suspended magnet rests in the north–south direction.
• Like poles repel, unlike poles attract; repulsion is the sure test of a magnet.
• Magnetic field lines go from N to S outside and S to N inside, forming closed loops.
• Field lines never cross each other; they are crowded where the field is strong (near the poles).
• A stationary magnet produces only a magnetic field, no electric field.
Do not mix these up
• Magnetic vs electric force — a magnet attracts iron, nickel, cobalt; it does not attract a stationary electron (charge).
Also asked from this topic: the property of field lines showing there are no monopoles (they form closed loops); the pattern of field lines of a solenoid (like a bar magnet).

Answer: Option 2 — wood particles
Wood is non-magnetic, so its particles do not line up along the field and cannot trace the field lines.
Iron filings, a magnetic compass and another magnet all respond to the field, so each can show the pattern.
Revision note — Magnetic field lines (Class 10)
• Outside a magnet the field lines run from the north pole to the south pole; inside, from south to north — they are closed curves.
• Two field lines never cross — at a crossing the compass would point in two directions at once.
• Where lines are closer together, the field is stronger (crowded near the poles).
• The tangent to a field line at a point gives the direction of the field there.
• A compass needle is a small bar magnet; its north end points along the field line.
• Iron filings sprinkled round a magnet and gently tapped arrange themselves along the field lines.
Do not mix these up
• Magnetic vs non-magnetic — iron, nickel, cobalt respond to a magnet; wood, plastic, paper, copper do not.
Also asked from this topic: what closely spaced field lines show (a stronger magnetic field); the pattern of field lines inside a current-carrying solenoid (parallel straight lines).
A compass needle is itself a tiny bar magnet — that is why it can plot the field line by line.

Answer: Option 2 — wood particles
Wood is a non-magnetic material, so its particles do not line up along a magnetic field.
Iron filings and a compass needle are magnetic, so they show the field pattern.
Revision note — Magnetic field lines
• Field lines of a bar magnet are traced with iron filings or by marking the positions of a compass needle step by step.
• Outside the magnet the lines run from the north pole to the south pole; inside, from south to north — they are closed curves.
• No two field lines ever cross — at one point the field has only one direction.
• Where lines are closer together, the field is stronger — crowded near the poles.
• The tangent to a field line gives the field's direction; a compass needle points along it.
• SI unit of magnetic field: tesla (T).
Also asked from this topic: what closely spaced field lines show (a stronger magnetic field); the shape of field lines inside a current-carrying solenoid (parallel straight lines).

Answer: Option 3 — Faraday's law
Faraday's law of electromagnetic induction states that an emf is induced in a circuit whenever the magnetic flux linked with it changes.
Revision note — Electromagnetic induction
• Michael Faraday discovered electromagnetic induction in 1831.
• Induced emf = rate of change of magnetic flux: e = −dΦ/dt (for N turns, e = −N dΦ/dt).
• Lenz's law gives the direction — the induced current opposes the change that causes it (conservation of energy).
• The induced current is larger with more turns, a stronger magnet and faster motion.
• Fleming's right-hand rule gives the direction of induced current in a moving conductor.
• Applications: AC generator (dynamo), transformer, induction cooker.
Do not mix these up
• Faraday's law vs Lenz's law — Faraday tells how much emf is induced; Lenz tells which direction the current flows.
Also asked from this topic: the law that gives the direction of induced current (Lenz's law — it opposes the change in flux); how to increase the induced current (use more turns in the coil).

Answer: Option 1 — reverse the direction of current flowing through the coil
The split-ring commutator reverses the current in the coil every half rotation.
So the force on each side of the coil keeps pushing the same way, and the coil keeps turning in one direction.
Revision note — Electric motor
• An electric motor converts electrical energy → mechanical energy.
• Principle: a current-carrying conductor in a magnetic field feels a force; its direction is given by Fleming's left-hand rule.
• Parts: armature coil, field magnet, split rings (commutator), carbon brushes, axle.
• The brushes touch the split rings and carry current into the coil.
• A commercial motor uses an electromagnet, a soft-iron core and many turns of wire to increase power.
• Uses: fans, mixers, washing machines, water pumps, computers.
Do not mix these up
• Motor vs generator — a motor turns current into motion (left-hand rule); a generator turns motion into current (right-hand rule).
• Split ring vs slip rings — split ring (commutator) in a DC motor / DC generator; slip rings in an AC generator.
Also asked from this topic: the device that turns electrical energy into mechanical energy (electric motor); what an AC generator uses to take current from the coil (slip rings).

Answer: Option 4 — Fleming’s Left-Hand Rule
In a motor, a current-carrying conductor in a magnetic field feels a force.
Its direction is given by Fleming's left-hand rule — the motor rule.
Revision note — Fleming's left-hand rule and the electric motor
• Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular.
• Forefinger → magnetic field; middle finger → current; thumb → force (motion).
• Force is maximum when current is perpendicular to the field and zero when parallel; F = BIL.
• Reversing either the current or the field reverses the force.
• Electric motor converts electrical energy → mechanical energy; the split-ring commutator reverses current every half turn.
• Fleming's right-hand rule gives the direction of induced current — used for the generator.
• Right-hand thumb rule gives the direction of the magnetic field around a straight wire.
Do not mix these up
• Left hand vs right hand — Left for motor (force); Right for generator (induced current).
Also asked from this topic: what the middle finger shows in Fleming's left-hand rule (direction of current); the finger for the field (index finger); the effect of reversing the current (the force reverses).

Answer: Option 3 — To prevent overloading and short circuits
A fuse is a safety device. When too much current flows (overloading or a short circuit), the fuse wire melts and breaks the circuit, protecting wiring and appliances.
Revision note — Electric fuse and domestic circuits
• Works on the heating effect of current (H = I²Rt).
• Fuse wire has a low melting point — usually an alloy of tin and lead.
• A fuse is connected in series with the live wire.
• Ratings: 5 A for lights and fans; 15 A for heavy appliances like geysers, ACs, heaters.
• Short circuit — live and neutral wires touch directly; resistance falls and current rises sharply.
• Overloading — too many appliances on one socket, or a voltage surge.
• Earth wire (green insulation) protects the user from shocks from metal-body appliances; MCBs are the modern replacement for fuses.
Do not mix these up
• Fuse vs earthing — a fuse protects the circuit and appliances from excess current; earthing protects the user from electric shock.
Also asked from this topic: the main purpose of a household fuse (to break the circuit if excess current flows); the rating for geysers and coolers (15 A); the colour of the earth wire in India (green).

Answer: Option 3 — uniform and parallel to the axis of the solenoid
Inside a long current-carrying solenoid the field lines are parallel straight lines along the axis.
Equally spaced parallel lines mean the field is the same at all points — uniform.
Revision note — Magnetic field of a solenoid
• Solenoid — a coil of many circular turns of insulated copper wire wrapped close together in the shape of a cylinder.
• Its field pattern outside is like a bar magnet — one end acts as N, the other as S.
• Field strength increases with more turns, more current, and a soft iron core.
• A soft iron rod inside a solenoid becomes an electromagnet — a temporary magnet.
• Electromagnets are used in electric bells, cranes, relays, MRI and loudspeakers.
• The direction of the field is found by the right-hand thumb rule (curl fingers along the current; thumb gives N).
Do not mix these up
• Inside vs outside — inside: uniform, parallel lines; outside: lines spread out from N and curve back to S, like a bar magnet.
Also asked from this topic: why a solenoid can magnetise soft iron placed inside it (it produces a strong and uniform magnetic field inside); the shape of field lines of a solenoid (parallel straight lines inside).

Answer: Option 4 — mechanical energy
A current-carrying coil in a magnetic field feels a force and rotates.
So the motor turns electrical energy into mechanical energy (rotation).
Revision note — Electric motor
• Principle: a current-carrying conductor in a magnetic field experiences a force.
• Direction of the force: Fleming's left-hand rule (First finger — field, seCond finger — current, thuMb — motion).
• Parts: armature coil, permanent magnet (field magnet), split-ring commutator, brushes, axle.
• The split ring reverses the current every half rotation, so the coil keeps turning in one direction.
• Commercial motors use an electromagnet, many turns of wire and a soft-iron core to raise power.
• Uses: fans, mixers, washing machines, water pumps, electric trains.
Do not mix these up
• Motor vs generator — a motor turns electrical → mechanical (left-hand rule); a generator turns mechanical → electrical (right-hand rule).
Also asked from this topic: the device that uses the force on a current-carrying conductor in a magnetic field to give motion (electric motor).

Answer: Option 4 — Perpendicular to the plane of the paper and coming out.
Right-hand thumb rule: hold the wire with the thumb along the current (upward); the curled fingers give the field.
The fingers come out of the paper on the left of the wire and go into the paper on the right.
Revision note — Magnetic field of a straight current-carrying conductor
• Discovered by Hans Christian Oersted (1820) — a current deflects a compass needle.
• Field lines are concentric circles centred on the wire, in a plane perpendicular to it.
• Right-hand thumb rule is also called Maxwell's corkscrew rule.
• Field strength ∝ current I and ∝ 1/distance from the wire.
• Reversing the current reverses the direction of the field lines.
• Symbols: ⊙ (dot) = out of the paper; ⊗ (cross) = into the paper.
Do not mix these up
• Right-hand thumb rule vs Fleming's rules — thumb rule gives the field around a current; Fleming's left-hand rule gives the force on a conductor; Fleming's right-hand rule gives the induced current.
Also asked from this topic: the other name of the right-hand thumb rule (Maxwell's corkscrew rule); the effect of reversing the current in a straight conductor (the field direction reverses).

Answer: Option 2 — direction of the current of a current carrying conductor
Fleming's right-hand rule gives the direction of the induced current when a conductor moves in a magnetic field.
It is the generator rule.
Revision note — Fleming's rules
• Stretch the thumb, forefinger and middle finger mutually perpendicular.
• Right hand: thumb = motion of conductor, forefinger = magnetic field, middle finger = induced current.
• Left hand: forefinger = field, middle finger = current, thumb = force / motion of the conductor.
• Right-hand rule → electric generator (mechanical → electrical energy).
• Left-hand rule → electric motor (electrical → mechanical energy).
• Induced current arises from electromagnetic induction, discovered by Michael Faraday (1831).
Do not mix these up
• Right-hand rule vs left-hand rule — right finds the current (generator); left finds the force / motion (motor).
• Fleming's right-hand rule vs right-hand thumb rule — the thumb rule gives the magnetic field around a current.
Also asked from this topic: the effect of rotating a generator coil at double speed (the induced potential difference doubles); the induced current in a rod moving north in an eastward field (vertically downward).

Answer: Option 4 — The direction and strength of the magnetic field
The tangent to a field line at any point gives the direction of the field there, and the closeness of the lines shows its strength.
Revision note — Magnetic field lines
• Outside a magnet they run from north pole to south pole; inside, from south to north — so they are closed loops.
• Lines are crowded near the poles, where the field is strongest.
• No two field lines ever cross — at a crossing a compass would point two ways at once.
• The field direction at a point is the way the north pole of a compass points.
• Field lines are traced with iron filings or a compass needle; SI unit of magnetic field: tesla (T).
Also asked from this topic: what closely spaced field lines indicate (a stronger magnetic field); the pattern of field lines of a current-carrying solenoid (parallel straight lines inside, like a bar magnet outside).

Answer: Option 3 — The compass needle is a small bar magnet.
A compass needle is itself a tiny magnet with a north and a south pole.
Near a bar magnet, like poles repel and unlike poles attract, so the needle turns.
Revision note — Magnets and the magnetic compass
• A freely suspended magnet rests in the north–south direction; the north-seeking end is the north pole.
• Like poles repel, unlike poles attract; repulsion is the sure test of a magnet.
• Magnetic field lines go from north to south outside the magnet and south to north inside — they form closed loops.
• Field lines never cross; they are crowded where the field is strong (near the poles).
• A compass is used to trace field lines and to detect a field — Oersted (1820) saw a compass deflect near a current-carrying wire.
• The Earth behaves like a huge magnet whose geographic north has a magnetic south pole — that is why the needle's north end points north.
Do not mix these up
• Iron vs magnet — a plain iron piece is only attracted by a magnet; a compass needle, being a magnet, can be attracted or repelled and so deflects to line up with the field.
Also asked from this topic: what a compass needle deflecting near a straight current-carrying wire shows (a magnetic field is present around the conductor).