Magnetic Effects of Electric Current

1. The big idea — electricity and magnetism are linked

In the previous chapter you studied the heating effect of current. This chapter studies its magnetic effect. The story begins with a famous accident in 1820: Hans Christian Oersted noticed that a compass needle placed near a current-carrying wire deflects the moment current flows (NCERT Activity 12.1).

When the current is switched off, the deflection vanishes; when the direction of current is reversed, the needle deflects the opposite way. So the magnetic field depends on the current and on its direction. The reverse effect — a moving magnet producing an electric current (electromagnetic induction) — is studied later in this chapter.

2. Magnetic field and magnetic field lines

A compass needle is itself a tiny bar magnet. Its north-seeking end is the north pole (N), the south-seeking end is the south pole (S). Like poles repel; unlike poles attract. A compass deflects near a bar magnet because the magnet exerts a force on its poles.

The region around a magnet where its force can be detected is its magnetic field. If you sprinkle iron filings around a bar magnet (Activity 12.2), they line up in a definite pattern — along the magnetic field lines. You can also trace these lines using a small compass moved step by step around the magnet (Activity 12.3).

Properties of magnetic field lines (very important):

• Field lines emerge from the north pole and merge into the south pole outside the magnet; inside the magnet they run from S to N. So they are closed, continuous curves.
• The direction of the field at a point is the tangent to the field line there (direction a north pole moves).
• Field lines are crowded where the field is strong (near the poles) and spread out where it is weak. Relative closeness shows relative strength.
• No two field lines ever cross. If they did, a compass at that point would have to point in two directions at once — impossible.

Diagram in words (field of a bar magnet): draw the bar with N on the right, S on the left. Curved lines come out of N, loop around the outside, and enter S. The loops are tight and dense right at each pole and become wide, gently curving arcs further out. Arrows on every line point away from N and into S.

3. Magnetic field due to a current — the straight conductor

Pass a current through a straight vertical wire stuck through a horizontal cardboard sprinkled with iron filings (Activity 12.5). The filings settle into concentric circles centred on the wire.

Two key dependences observed:

• Strength ↑ with current. Increase the current and the compass deflection at a point increases — the field is stronger.
• Strength ↓ with distance. Move the compass away from the wire and deflection decreases — the circles grow larger and the field weakens with distance.

4. Right-hand thumb rule (Maxwell's corkscrew rule)

This rule tells us the direction of the circular field around a straight wire.

It is also called Maxwell's corkscrew rule: if a corkscrew (right-handed screw) is driven so it advances in the direction of the current, the direction in which its head rotates gives the direction of the field.

5. Magnetic field due to a current through a circular loop

Bend the straight wire into a circular loop. At every point of the loop the field is a set of concentric circles; as we approach the centre of the loop these arcs become so large they look like straight lines. By the right-hand rule, every part of the loop pushes the field through the centre in the same direction, so the contributions add up.

If the loop has n turns instead of one, the field at the centre becomes n times as strong, because the current in each turn flows the same way and their fields add.

6. Magnetic field due to a current in a solenoid — electromagnets

A solenoid is a coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder.

Diagram in words (solenoid field): draw a cylinder of close loops. Outside, curved lines leave one flat end (N), loop around, and enter the other flat end (S) — exactly like a bar magnet. Inside, the lines are straight, parallel and evenly spaced, all pointing N→S inside, i.e. from the S end towards the N end through the core.

Electromagnet: place a piece of soft magnetic material (soft iron) inside the solenoid. The strong field magnetises it, turning the rod into a powerful magnet that works only while current flows. This is an electromagnet.

• Core: always soft iron — it magnetises and demagnetises easily, so the magnet can be switched on/off.
• Strength ↑ with more turns, more current, and a soft-iron core.

7. Force on a current-carrying conductor in a magnetic field

A current produces a field that pushes a magnet near it. By Newton's third law (and as Ampere reasoned), the magnet must push back on the wire with an equal and opposite force.

In Activity 12.7, a horizontal aluminium rod AB carrying current is placed between the poles of a horseshoe magnet (field vertical). When current flows, the rod is displaced sideways.

• Reverse the current → force (displacement) reverses.
• Reverse the field (swap magnet poles) → force reverses.
• The force is largest when current is perpendicular to the field (at right angles), and the three directions — field, current, force — are mutually perpendicular.

8. Fleming's left-hand rule

Memory aid: Fore=Field, Centre=Current, Thumb=Thrust. Devices using this force include the electric motor, loudspeakers, microphones and measuring instruments (galvanometer, ammeter).

9. Electric motor

An electric motor converts electrical energy into mechanical (rotational) energy. It uses the force on a current-carrying coil in a magnetic field.

Construction: a rectangular coil ABCD of insulated wire placed between the poles of a magnet (field N→S). The two ends of the coil connect to the two halves of a split ring (commutator), P and Q, which press against carbon brushes X and Y joined to a battery.

Working: current enters along ABCD. Side AB carries current one way, side CD the other way, both in the same field, so by Fleming's left-hand rule AB is pushed (say) down and CD up — the coil rotates. After a half turn the split ring reverses the current direction in the coil, so the force on each side keeps turning the coil the same way continuously.

Commercial motors use an electromagnet (strong field), many turns of wire on a soft-iron core (armature, boosting power) and a large number of turns.

10. Electromagnetic induction

Oersted showed current → magnetism. Michael Faraday asked the reverse: can magnetism produce a current? Yes — this is electromagnetic induction (EMI).

• Move a bar magnet in and out of a coil joined to a galvanometer → the galvanometer needle deflects, showing an induced current, but only while the magnet is moving (i.e. while the field through the coil is changing).
• Faster motion → bigger deflection. A stationary magnet gives no current.
• The same happens with two coils: changing the current in one coil (primary) induces a momentary current in a nearby coil (secondary).

This is used to find the direction of induced current; Fleming's left-hand rule is for the force on a current-carrying conductor — do not mix them up.

11. Electric generator

An electric generator (dynamo) converts mechanical energy into electrical energy using electromagnetic induction — it is essentially a motor run in reverse.

Construction: a rectangular coil ABCD rotated mechanically between the poles of a magnet. The coil ends connect to slip rings (for AC) or a split ring (for DC) pressing on brushes joined to the external circuit.

Working: as the coil rotates, the magnetic field through it keeps changing, so a current is induced in it (direction by Fleming's right-hand rule). In an AC generator, two slip rings keep the same wire connected to the same brush, so the induced current reverses direction every half rotation — an alternating current. In a DC generator, a split-ring commutator switches the connections each half turn so the output current stays in one direction.

12. Domestic electric circuits

Power reaches homes through a main supply at 220 V AC, frequency 50 Hz. Three wires:

• Live wire — usually red insulation (positive).
• Neutral wire — usually black insulation (negative). Potential difference between live and neutral = 220 V.
• Earth wire — green insulation, connected to a metal plate buried in the earth. A safety wire: it connects to the metal body of appliances and provides a low-resistance path, so any leaked current goes to earth and the user is not shocked.

Homes usually have two circuits: a 15 A circuit for heavy appliances (geyser, air cooler) and a 5 A circuit for lights and fans. All appliances are connected in parallel across live and neutral, so each gets the full 220 V and has its own switch.

Overloading and short circuit: Short-circuiting happens when live and neutral touch directly (damaged insulation), making the current shoot up. Overloading happens when too many appliances draw too much current, or the supply voltage spikes. An electric fuse — a short wire of low melting point in series with the live wire — melts and breaks the circuit by Joule heating when current exceeds a safe value, protecting wiring and appliances.

13. NCERT in-text Questions — answered

Page 196, Q1. Why does a compass needle get deflected when brought near a bar magnet? A compass needle is itself a small magnet. A bar magnet exerts a magnetic force on its poles (like poles repel, unlike attract), so the needle turns and aligns along the magnet's field — i.e. it gets deflected.

Page 200, Q1. Draw magnetic field lines around a bar magnet. Lines come out of the N pole, curve around, and enter the S pole (inside, S→N). They are closed curves, dense near the poles, with arrows pointing N→S outside. (See §2 description.)

Page 200, Q2. List the properties of magnetic field lines. (i) They run from N to S outside the magnet, forming closed loops; (ii) direction at a point = tangent (direction a free N pole moves); (iii) closer lines mean a stronger field; (iv) no two field lines cross each other.

Page 200, Q3. Why don't two magnetic field lines intersect each other? If they crossed, at the crossing point the field would have two directions, so a compass placed there would have to point two ways at once — which is impossible. Hence they never intersect.

Page 201, Q1. A circular loop lies flat on a table, current clockwise (seen from above). Use the right-hand rule for the field inside and outside the loop. Curl the right-hand fingers in the current direction (clockwise from above) so the thumb points downward: the field is directed downwards (into the table) inside the loop and upwards just outside it. The top face acts as a south pole, the bottom face as a north pole.

Page 201, Q2. The magnetic field in a region is uniform. Draw a diagram. A uniform field is shown by equally spaced parallel straight arrows all pointing the same way (e.g. parallel horizontal arrows pointing right).

Page 202, Q3. The magnetic field inside a long straight current-carrying solenoid is — (d) is the same at all points (the field inside a long solenoid is uniform).

Page 203, Q1. Which property of a proton can change while it moves freely in a magnetic field? (c) velocity and (d) momentum. The magnetic force changes the proton's direction, so velocity (a vector) and momentum change; its mass and speed stay the same (the force does no work).

Page 204, Q2. In Activity 12.7, how does the displacement of rod AB change if (i) current is increased, (ii) a stronger magnet is used, (iii) the rod's length is increased? In each case the force increases, so the displacement increases. Force on a conductor grows with current, with field strength, and with the length of conductor in the field.

Page 204, Q3. A positively-charged alpha particle moving west is deflected towards the north by a magnetic field. The field direction is — (d) upward. Current direction = direction of positive charge motion = west; force = north. By Fleming's left-hand rule (middle finger west = current, thumb north = force), the forefinger (field) points upward.

Page 205, Q1. Name two safety measures commonly used in electric circuits and appliances. (i) Use of a correctly rated fuse (or MCB) in the live wire; (ii) earthing the metal body of appliances with the earth wire. (Also: proper insulation.)

Page 205, Q2. An electric oven of 2 kW power is run in a 220 V circuit rated 5 A. What result do you expect? Current drawn = P/V = 2000/220 ≈ 9.1 A, which is more than the 5 A rating. The circuit is overloaded: the fuse will melt (or the wires overheat) and the circuit breaks — the oven cannot be safely run on this circuit.

Page 205, Q3. What precaution avoids overloading of domestic circuits? Do not connect too many high-power appliances to one socket/circuit; use separate properly rated circuits (15 A for heavy loads, 5 A for light loads); use a correct fuse/MCB; and avoid voltage surges. Essentially, keep total current within the circuit's rated value.

14. NCERT Exercises — fully answered

Q1. Which correctly describes the field near a long straight wire? (d) concentric circles centred on the wire.

Q2. At the time of short circuit, the current in the circuit — (c) increases heavily (resistance drops sharply, so current shoots up).

Q3. State true/false. (a) "The field at the centre of a long circular coil carrying current will be parallel straight lines." — True. (b) "A wire with green insulation is usually the live wire." — False (green is the earth wire; live is red).

Q4. List two methods of producing magnetic fields. (i) By a permanent magnet (or bar magnet); (ii) by passing an electric current through a conductor (straight wire, coil, or solenoid/electromagnet).

Q5. When is the force on a current-carrying conductor in a magnetic field largest? When the conductor is placed perpendicular (at right angles) to the direction of the magnetic field.

Q6. You sit with your back to one wall; an electron beam moving horizontally from the back wall to the front wall is deflected to your right by a strong field. Find the field direction. The current is opposite to the electron motion, i.e. current points from front wall to back wall. Force is to the right. Apply Fleming's left-hand rule (middle finger = current = backward, thumb = force = right): the forefinger (field) points vertically downward. So the magnetic field is directed downwards.

Q7. State the rule for the direction of — (i) the magnetic field around a straight current-carrying conductor: right-hand thumb rule (thumb = current, curled fingers = field). (ii) the force on a current-carrying conductor in a field perpendicular to it: Fleming's left-hand rule. (iii) the current induced in a coil rotating in a field: Fleming's right-hand rule.

Q8. When does an electric short circuit occur? When the live and neutral wires come into direct contact (e.g. due to damaged insulation or a fault), so the resistance becomes very small and the current rises abruptly.

Q9. What is the function of an earth wire? Why earth metallic appliances? The earth wire is a safety wire that connects the metal body of an appliance to the earth and provides a low-resistance path to the ground. If current leaks to the metal body, it flows away to earth instead of through the user, so we avoid a severe electric shock. That is why metallic appliances must be earthed.

15. Common mistakes to avoid

• Using the left-hand rule for induced current — that's the right-hand rule. Left hand = force on a current; right hand = induced current.
• Saying field lines start and stop at the poles — they are closed loops continuing inside the magnet (S→N).
• Confusing the commutator (split ring) of a motor/DC generator with the slip rings of an AC generator.
• Writing the green wire as live — green is earth; red is live, black is neutral.
• Forgetting that the magnetic force is largest only when current is perpendicular to the field, and zero when current is parallel to the field.
• Saying an electromagnet's core is steel — an electromagnet uses soft iron (so it can be switched off).

16. Quick revision checklist

• Oersted: current → magnetic field; reverse current → reverse field.
• Right-hand thumb rule: thumb = current, fingers = field.
• Straight wire → concentric circles; loop → field through centre; solenoid → bar-magnet field, uniform inside; soft-iron core → electromagnet.
• Fleming's left hand → force (motor). Fore=Field, Centre=Current, Thumb=Thrust.
• EMI (Faraday): changing field → induced current; Fleming's right hand for its direction; basis of generator.
• Motor & DC generator use a split ring; AC generator uses slip rings.
• Domestic: 220 V, 50 Hz; live=red, neutral=black, earth=green; fuse + earthing are the safety devices.