Magnetic Effects of Electric Current Class 10: Notes and Questions

Magnetic Effects of Electric Current Class 10 shows that an electric current behaves like a magnet, and uses that idea to explain field patterns, electromagnets, the force on a wire, and how our homes are wired safely. This guide follows the chapter in order with the right-hand thumb rule and Fleming's left-hand rule worked through on examples, a current calculation for domestic circuits, and important questions with model answers.

Oersted's discovery

In 1820, Hans Christian Oersted, a Danish scientist, passed a current through a copper wire during a demonstration and noticed that a nearby compass needle moved. Until then, only magnets were known to affect a compass. He concluded that moving charges in the wire create a magnetic field around it.

This showed that electricity and magnetism are linked: a current behaves like a magnet, and moving magnets can produce electricity. The unit of magnetic field strength, the oersted, is named after him.

Magnetic field and field lines

A magnetic field is the region around a magnet where its force can be felt. It is a vector quantity: it has both strength (magnitude) and direction. A compass needle is a tiny bar magnet that lines up with the local field. Like poles repel and unlike poles attract. We can map a field with iron filings or by moving a compass around a magnet.

Field lines never cross because a compass at the crossing point would have to point in two directions at once, which is impossible. Around a bar magnet, the field is strongest at the poles, where the lines are most crowded.

Field around a straight wire and the right-hand thumb rule

Around a long straight current-carrying wire, the field lines are concentric circles centred on the wire, in a plane perpendicular to it. Iron filings on a card pierced by the wire show these circles.

• Field strength is directly proportional to the current. Double the current and the field doubles.
• Field strength is inversely proportional to the distance from the wire. Move away and the compass deflects less.
• Reverse the current and the field direction reverses.

Right-hand thumb rule (Maxwell's corkscrew rule): hold the wire in your right hand with the thumb along the current. Your curled fingers show the direction of the field lines. Equivalently, turn a corkscrew so it moves forward along the current; the direction of turning is the field direction.

• Current vertically upward: field is anticlockwise seen from above.
• Current downward: clockwise seen from above.

Worked example: A horizontal power line carries current from east to west. Point your right thumb west. Seen from the east end, the field curls clockwise. Directly below the wire the field points south; directly above it, north.

Circular loop and coil

Bend the wire into a loop and every small part acts like a straight wire. Inside the loop the fields from all parts add up. At the centre, the field lines are nearly straight and perpendicular to the plane of the loop, so one face of the loop behaves like a north pole and the other like a south pole.

The field at the centre grows if you increase the current or the number of turns. A coil of n turns gives a field n times that of one turn, because the current flows the same way in every turn. So a 100-turn coil gives 100 times the field of a single loop.

Solenoid and electromagnet

A solenoid is a long cylindrical coil of many closely wound turns of insulated copper wire. Its field looks like a bar magnet's, with a north end and a south end. Inside, the field lines are parallel straight lines, so the field is uniform: the same strength and direction everywhere inside.

Polarity tip: look at one end of the coil. If the current there flows clockwise, that end is a south pole; if anticlockwise, a north pole.

Put a soft iron rod inside and the strong field magnetises it, making an electromagnet. Its magnetism lasts only while current flows. Electromagnets are useful because:

• their strength changes with the current or the number of turns, and
• their polarity reverses when the current reverses.

They are used in electric bells, cranes and MRI machines.

Force on a conductor in a magnetic field

André-Marie Ampère reasoned that if a current exerts a force on a magnet, the magnet must exert an equal and opposite force on the current (Newton's third law). An aluminium rod hung between the poles of a horseshoe magnet jerks to one side when current flows.

• Reverse the current or the field, and the force reverses.
• Reverse both, and the force stays the same.
• The force is largest when the current is at 90° to the field and zero when the wire is parallel to the field.

This is the principle of the electric motor, which turns electrical energy into mechanical work.

Fleming's left-hand rule

Stretch the thumb, forefinger and middle finger of your left hand so that each is at right angles to the other two.

• Forefinger: magnetic field (B)
• Middle finger: current (I)
• Thumb: force or motion (F)

Remember the order F-B-I for thumb, forefinger, middle finger. "Current" means conventional current, from positive to negative. For electrons, current is opposite to their motion.

Worked example 1: Electrons move to the right in a field pointing downward (in the plane of the page). Conventional current is to the left. The rule gives a force out of the page, towards the reader.

Worked example 2: A proton moves north in a field pointing east. For a positive charge, current is along its motion (north). The rule gives a downward force.

Magnetism in medicine

Nerve impulses are tiny electric currents, so they make very weak magnetic fields, about one billionth of the Earth's field. The heart and brain produce the most significant ones. MRI (Magnetic Resonance Imaging) uses a strong applied magnetic field and radio signals to produce images of structures inside the body for diagnosis.

Domestic electric circuits

NCERT's examples use an AC supply of 220 V at 50 Hz. Live and neutral provide the potential difference; the earth wire is a safety connection.

These colours are the textbook's convention, not a guide to identifying real wiring.

Why parallel? Every appliance gets the full 220 V and has its own switch, so one appliance being off or faulty does not stop the others.

There are two kinds of circuit: 15 A for high-power appliances (air conditioners, geysers, refrigerators) and 5 A for fans, bulbs and chargers.

Path of supply: main supply → main fuse → electricity meter → distribution board (separate 5 A and 15 A circuits) → appliances in parallel.

Safety: fuse, earthing and current check

• Short circuit: live and neutral touch directly (often through damaged insulation). Resistance drops almost to zero and current shoots up.
• Overloading: too many appliances on one socket, or an accidental rise in supply voltage.
• Fuse: a wire with a low melting point. Too much current heats it (Joule heating) and it melts, breaking the circuit. MCBs do the same job and can be reset.
• Earthing: gives fault current a low-resistance path if live wiring touches a metal case, which helps protective devices cut the supply and reduces shock risk.

Worked example: can a 2000 W appliance run on a 5 A circuit at 220 V? I = P/V = 2000/220 ≈ 9.09 A. This exceeds 5 A, so the appliance is unsuitable for that circuit.

Compare: a 440 W appliance draws I = 440/220 = 2 A, which is within 5 A.

Remember this

• Field lines: N to S outside, S to N inside, closed loops, never cross.
• Straight wire: concentric circles; right-hand thumb rule.
• Solenoid: uniform field inside; clockwise end is south.
• Force: Fleming's left-hand rule, F-B-I; maximum at 90°, zero when parallel.
• Appliances in parallel; fuse melts on excess current; I = P/V.

Important questions with answers

1. What did Oersted observe? A compass needle near a current-carrying wire was deflected, showing the current produced a magnetic field.

2. Why can two field lines never cross? A compass at the crossing point would point two ways at once, which is impossible.

3. What happens to the field near a straight wire if the current is doubled? The field doubles, since it is directly proportional to the current.

4. Describe the field inside a long solenoid. Parallel straight lines, so the field is uniform in strength and direction.

5. When is the force on a current-carrying conductor largest? When the current is perpendicular (90°) to the magnetic field.

6. What happens to the force if both current and field are reversed? It keeps the same direction, because the two reversals cancel.

7. Why are household appliances connected in parallel? Each gets the full 220 V and can be switched on or off independently.

8. What is a short circuit? Live and neutral touch directly, so resistance falls almost to zero and current rises sharply.

9. How does a fuse protect a circuit? Excess current heats its low-melting-point wire until it melts, breaking the circuit.

Common mistakes to avoid

• Using the left hand for the field around a wire, or the right hand for force.
• Forgetting that electron motion is opposite to conventional current.
• Drawing field lines from N to S inside a magnet.
• Thinking a fuse protects against every kind of voltage surge; it opens the circuit on excessive current.

When you want to practise these rules with guided questions, study this chapter with Joy on Learnijoy.