Magnetic Effect of Electric Current

Introduction

A compass can turn when a nearby wire carries current, even though the wire never touches it. Change the current’s direction and the magnetic effect changes direction too. This chapter asks you to become a field detective: use a compass, a drawing and a few carefully chosen changes to work out what is happening in the space around a conductor.

Scope: NCERT Class 10 Science, Chapter 12, reprint 2026–27. CBSE’s 2026–27 Unit IV lists magnetic fields, current-carrying conductors, force, AC/DC and domestic circuits. It identifies motors, electromagnetic induction and generators as formative topics; those sections below are labelled accordingly. A broader teacher note in the same syllabus is inconsistent with the detailed unit listing, so this page makes no chapter-wide annual-exam guarantee. Follow your school’s clarification for assessment planning.

The ideas belong together: current produces a magnetic field; a magnetic field can exert force on a current; a changing magnetic situation can induce a voltage. Keep the three statements separate while looking for the connections.

Magnetic field and field lines

A bar magnet has north and south poles: like poles repel and unlike poles attract. A freely turning compass needle is a small magnet. Its north-seeking end aligns approximately with the local magnetic field; near another magnet, that field can differ from Earth’s field alone.

A magnetic field describes magnetic influence at each point, with both magnitude and direction. Field lines are a drawing tool, not threads or tracks floating in space. At a point, the tangent to a field line shows the direction in which a small compass’s north-seeking end points.

• Outside a bar magnet, arrows run from north towards south; inside, they continue from south towards north. The usual bar-magnet diagram therefore forms closed loops.
• Closer spacing represents a stronger field when the same drawing convention is used. Adding extra pencil lines cannot strengthen a real magnet.
• Two distinct field lines do not cross at a point where the field has a definite direction. A crossing would assign two directions to the same field at that point.

These conventions connect a picture with a possible observation. A compass samples direction locally; a pattern of iron filings alone does not label which way the field points. OpenStax: magnetic field lines.

A sketch has eight field lines and another has sixteen. Is the second magnet twice as strong?

You cannot conclude that without knowing the drawing scale and convention. Field-line density represents strength within a consistently drawn map; the total number an illustrator chooses is not a measurement by itself.

Magnetic field due to a current through a straight conductor

Around a long straight wire, the field lines are circles centred on the wire, in planes perpendicular to it. The field points along a tangent to each circle, not directly towards or away from the wire.

Right-hand thumb rule: include the observer

Point your right thumb in the direction of conventional current; your curled fingers show the field’s circulation. In a metal, electron drift is opposite to conventional current. Using electron motion for the thumb without reversing it gives the wrong answer.

Look straight at the page. A dot ⊙ represents current coming out towards you, like an arrow tip; a cross ⊗ represents current going in, like an arrow’s tail. Outward current produces an anticlockwise field as you see the page; inward current produces a clockwise field. “Clockwise” without a stated viewpoint is incomplete.

Keeping the distance fixed, a larger current produces a stronger field. Keeping current fixed, the field is weaker farther from a long straight wire. In the ideal long-wire model, field magnitude is proportional to current divided by perpendicular distance, B ∝ I/r. OpenStax: fields produced by currents.

A proportional-reasoning check

Suppose one observation point is 2 cm from the wire. Compare it with a point 4 cm away while doubling the current. The factor from current is 2; the factor from distance is 1/2. Together, B₂/B₁ = 2 × 1/2 = 1: the wire’s field magnitude is unchanged in this ideal model. Moving away did matter; another change compensated for it.

Current comes out of the page. Which way does the wire’s field point immediately to its right?

Up the page. Trace the anticlockwise circle: at its rightmost point, the tangent points up. At the top it points left; on the left it points down; at the bottom it points right. These directions describe the wire’s contribution; an actual compass responds to the total local field.

Reverse the current, but keep the same magnitude and observation point. What changes?

The field direction reverses. Its magnitude stays the same for the same wire geometry and medium. A change of direction is not automatically a change of strength.

Magnetic field due to a current through a circular loop

Bending the wire into a loop lets contributions from different parts reinforce one another near its centre. The central field points along the loop’s axis, perpendicular to its plane. Near a small part of the wire the local pattern resembles the straight-wire pattern, but the field of the whole loop is the combined result, not a collection of independent circles that never interact.

Look at one face of the loop: anticlockwise conventional current gives a field towards you at the centre, and that face behaves as a north pole. Clockwise current gives a field away from you, making that face south. Reverse your viewing side and the apparent clockwise/anticlockwise description reverses too.

More current strengthens the central field. For closely packed turns of the same radius carrying the same current in the same sense, their contributions add: N turns give approximately N times the single-turn field at the centre. Keep those conditions in the sentence. Adding wire to a fixed-voltage circuit can change resistance and current, so extra turns do not automatically guarantee that factor in a real experiment.

Two equal coils have 20 and 40 closely packed turns. Both carry the same current and have the same radius. Compare their central fields.

The 40-turn coil gives about twice the central field. If only the supply voltage were stated to be equal, you would also need the currents: the longer wire may have a different resistance.

Magnetic field due to a current in a solenoid

A solenoid is a cylindrical coil with many closely wound turns of insulated wire. With current flowing, one end behaves as north and the other as south. Its external field pattern resembles a bar magnet’s, while contributions inside reinforce one another.

Inside a long solenoid, away from the ends, the field is approximately uniform: similar magnitude and direction across the central region. Parallel, equally spaced arrows represent that approximation. A real finite coil has a changing field near its ends; “exactly the same everywhere inside any solenoid” overstates the model.

A soft-iron core can strengthen the field, producing a useful electromagnet. Adjusting or reversing current changes its field. Soft iron is useful when magnetisation should largely disappear after switching off, although real materials can retain some magnetism. This is different from choosing a permanent magnet specifically to keep its magnetisation.

For a long coil in the simple model, both current and turns per unit length matter. Adding turns while lengthening the solenoid proportionally leaves the turn density unchanged. Geometry, current and core material all deserve attention when comparing designs.

Force on a current-carrying conductor in a magnetic field

A wire carrying current can experience a force in an external magnetic field. For a straight segment, the force depends on current, field strength, the length within the field and the angle between current and field. With the other factors fixed, it is greatest at right angles and zero when the current is parallel or antiparallel to the field.

Fleming’s left-hand rule

Arrange your left thumb, forefinger and middle finger mutually perpendicular. The forefinger gives the magnetic field, the middle finger conventional current, and the thumb force. The thumb shows the force direction, which need not be the object’s existing direction of motion.

For example, let conventional current point right across the page and the external field point into the page. The force is upwards. Reverse only current: the force is downwards. Reverse only the field: also downwards. Reverse both: the force is upwards again. Each reversal changes one factor; two reversals restore the original direction. OpenStax: force on a conductor.

A current-carrying wire is parallel to a strong uniform field. Must it move?

No. The magnetic force on the straight segment is zero in this arrangement. A stronger field does not remove the importance of direction. Other forces or other parts of the circuit would have to be considered separately.

An electron moves right through a field into the page. Is its magnetic force also upwards?

No: it is downwards. An electron has negative charge, so the equivalent conventional-current direction is opposite to its motion. The upwards result above was for conventional current towards the right.

Electric motor

Formative extension in CBSE 2026–27. A motor transfers electrical energy into mechanical work. A current-carrying loop placed in a magnetic field can experience opposite forces on its opposite sides. Because these forces act at different positions, they can produce a turning effect even when their net force is zero.

In the simple school DC motor, a coil rotates on an axle between magnetic poles. Stationary brushes connect the supply to two insulated halves of a split-ring commutator. The rotating split ring reverses the current in the coil each half-turn. After the sides swap places, their force directions swap too, keeping the turning effect in the same rotational sense.

The coil’s momentum carries it through positions where the turning effect vanishes. A one-loop classroom model is therefore not a complete description of every practical motor or every starting position. Real motors use different winding, magnet and control arrangements. OpenStax: torque and motors.

Why not just leave the current direction unchanged throughout a rotation?

In this simple motor, after the coil passes its alignment position, the turning effect would reverse and tend to bring it back. Correctly timed current reversal lets the turning effect continue in the intended rotational sense.

Electromagnetic induction

Formative extension. A changing magnetic flux through a circuit can induce an electromotive force, or emf, measured in volts. Think of flux here as accounting for the magnetic field passing through the loop’s area and orientation. If the circuit is closed, the induced emf can drive current. An open circuit can have an induced voltage without a continuous current around it.

Move a magnet towards a stationary coil connected to a sensitive galvanometer: its needle can deflect. Hold the magnet still: after transient effects settle, the reading returns to zero. Withdraw the same pole: the deflection reverses. Moving the coil instead can also change the flux. In this arrangement, a faster change gives a larger induced emf. OpenStax: induced emf.

The essential condition is change in flux, not movement by itself. A steady current in one stationary coil does not continuously induce a current in a second stationary coil merely because both share a core. Switching or changing the first current changes its field and can produce induction in the second.

Predict using the change

Lenz’s law says the induced current’s magnetic effect opposes the change that produced it. Push a north pole towards a coil: the coil’s near face tends to act as north, opposing approach. Pull that pole away: the near face tends to act as south, opposing withdrawal. It opposes the change, not necessarily the original field in every situation. The work you supply is part of the energy account. OpenStax: Lenz’s law.

For the moving-conductor arrangement, Fleming’s right-hand rule uses the forefinger for field, thumb for conductor motion and middle finger for induced conventional current in a closed circuit. Keep it separate from the right-hand thumb rule, which finds the field created by an existing current.

Would a stronger magnet held completely still keep an ordinary closed coil’s lamp lit?

Not by this induction arrangement alone. Once the field through the stationary coil is unchanging, the induced emf is zero. Strength and rate of change are different ideas. A sustained output needs a continuing source of change and energy.

Electric generator

Formative extension. A generator transfers mechanical energy into electrical energy. In the simple rotating-coil model, an external drive turns the coil through a magnetic field, changing the flux through it. Two separate slip rings keep its ends connected to stationary brushes without exchanging the connections every half-turn.

The induced emf reverses after each half-turn, producing an alternating output. A split-ring commutator instead reverses the external connections each half-turn and gives a unidirectional, pulsating output. “DC” need not mean a perfectly constant magnitude. OpenStax: generators. A generator does not create energy from nothing: someone or something must drive it. EIA: magnets and electricity.

AC, DC and frequency

Direct current flows in one direction; its magnitude can still vary. Alternating current reverses direction periodically. For a 50 Hz sinusoidal supply, there are 50 complete cycles each second: T = 1/50 s = 0.020 s = 20 ms. Direction reverses every half-cycle, after 10 ms, so there are 100 reversals per second.

AC’s practical advantage in a conventional grid is that transformers can readily raise and lower its voltage. Higher-voltage transmission allows a smaller current for a given transferred power, reducing resistive losses. Substations then reduce the voltage for distribution. EIA: electricity delivery.

Myth to retire: “DC cannot travel efficiently over long distances.” Modern high-voltage DC links do exactly that. Power electronics and the design of a complete link matter. AC is not inherently lossless or automatically more efficient in every situation. US Department of Energy: AC and HVDC.

A 50 Hz supply reverses direction every 20 ms. Find the mistake.

20 ms is a full cycle. A sinusoidal current has one positive half-cycle and one negative half-cycle, so it reverses direction every 10 ms. Frequency counts cycles, not individual reversals.

Domestic electric circuits

Appliances are connected in parallel so each branch receives the supply potential difference and can be switched independently. The supply current is the sum of branch currents. This is why adding appliances can overload a shared circuit even when each appliance works normally on its own.

The live conductor supplies the alternating voltage relative to neutral. Neutral is part of the normal return circuit; it is not a permanent negative terminal. Protective earth serves a different purpose: a correctly designed protective connection provides a path for fault current and helps protective equipment disconnect a dangerous fault. It is not the intended return conductor for ordinary appliance operation.

NCERT uses 220 V and 50 Hz in its school examples. Use the voltage specified in a numerical problem. Do not turn 220 V into a guarantee about every real supply: for example, Delhi’s published supply-code amendment specifies 230/240 V for single-phase supply. Nor should the older red/black/green textbook drawing be used to identify real conductors. Delhi supply-code amendment, 2018.

Overload, short circuit and protection

An overload is excessive current through an otherwise intended circuit, such as too many high-power loads on one branch. A short circuit is an unintended low-resistance connection that bypasses the normal load, allowing a very large current. Both can produce dangerous heating; their causes differ.

A correctly selected fuse melts when excessive current heats its element enough, interrupting the circuit. A circuit breaker can also disconnect an overcurrent. A fuse is not a guarantee against every electric shock: leakage protection, earthing and insulation have different jobs. Residual-current devices can reduce shock risk but cannot protect against every situation. HSE: electrical protection limitations.

Use diagrams for this investigation. Do not open plugs, sockets, appliances or distribution boards, connect bare wire directly across a battery, or test mains circuits. Practical wiring and fault repair belong with a qualified electrician.

Worked load example

For an ideal resistive appliance rated 880 W at the problem’s 220 V supply, I = P/V = 880/220 = 4 A. Two such appliances in parallel draw 8 A in total. That exceeds a circuit rating of 5 A. The calculation identifies an overload; it is not a reason to replace a protective device with a larger one without redesigning the circuit.

One of two parallel appliances is switched off. What happens to the other in the ideal model?

It still receives the same supply voltage and draws its own operating current. The total supply current falls. In a series connection, opening one appliance would break the only shared path; that is why independent household use needs parallel branches.

Why it still matters: from your notes to the rare-earth race

Conceptual extension. Some motor designs use high-performance permanent magnets containing rare-earth elements; others produce their rotor field in different ways. Motor engineers weigh efficiency, size, temperature performance, cost and material supply. The US Department of Energy’s motor programme specifically researches reducing reliance on rare-earth materials. That is a real design question, not proof that every motor needs a rare-earth permanent magnet. DOE: motor research.

Try this design conversation: one team wants compact permanent magnets, another wants to reduce material dependence. What performance evidence would you request before choosing? The school model helps you ask how a magnetic field and a current create a turning effect; it cannot, by itself, decide a manufacturing or supply-chain tradeoff.

Another connection: does MRI simply photograph the weak magnetic fields made by nerves?

No. MRI uses a strong applied magnetic field, radiofrequency excitation and detected signals associated with nuclei, especially hydrogen in body tissues. It does not simply photograph the weak magnetic fields produced by ordinary nerve currents. This is an enrichment connection, not a complete account of MRI physics. NIBIB: how MRI works.

Make a field map and try to break your explanation

Draw a large dot for a wire carrying current out of the page. Mark four points around it, like the top, right, bottom and left of a clock. Before looking back, draw an arrow for the field at each point. Now replace the dot with a cross and change only the arrows that need changing.

Next, move one point twice as far from the wire and double the current. Predict both direction and relative strength. Ask a partner to turn the paper around: has the physical field changed, or just your way of describing its direction? A useful answer names both the current and the observer’s viewpoint.

If you have a classroom bar magnet and a small compass, map several compass directions on paper around the magnet without moving it. Predict the needle’s next direction before placing it. Move the magnet away and repeat one point to reveal the background field. Keep ordinary magnets away from sensitive devices; use paper drawings if equipment is unavailable. No current-carrying home-made circuit or loose iron filings are required.

What should the field map reveal?

Around the outward-current wire, arrows run anticlockwise: top left, right up, bottom right, left down. Reversing current reverses all four. Doubling current while doubling distance leaves the ideal wire-field magnitude unchanged. A real compass responds to the combined field, so the magnet-away observation is a useful control rather than an inconvenient result to discard.

• State the viewpoint before using clockwise or anticlockwise.
• Distinguish the field created by current from the force exerted by an external field.
• Keep geometry and current fixed when comparing turn counts.
• Induction needs a changing flux; a closed path is needed for sustained circulating current.
• Motors and generators transfer energy; safety devices address specific kinds of fault.

Sources

Curriculum: NCERT Science Chapter 12, reprint 2026–27 and CBSE Science 2026–27. Further primary explanations are linked where used. Worked examples, prediction questions and map-making prompts are original, not attributed past-paper questions. No other-board assessment alignment or official endorsement is claimed.

Connect the load calculations with Electricity and return to the NCERT and CBSE study library.