Class 10 Science Chapter 12: Magnetic Effects of Electric Current

Magnetic Effects of Electric Current is Chapter 12 of NCERT Class 10 Science. It explains magnetic fields and field lines, the field produced by a current in a straight wire, a circular loop and a solenoid, the force on a current-carrying conductor placed in a magnetic field, and how electricity is safely wired in our homes. Watch the three video lessons below, then revise the rules, diagrams and solved examples.

What you will learn

• What a magnetic field is and the properties of magnetic field lines
• The field around a straight conductor, a circular loop and a solenoid
• The right-hand thumb rule for finding the direction of a magnetic field
• The force on a current-carrying conductor and Fleming's left-hand rule
• Domestic electric circuits, earthing, fuses, short-circuiting and overloading

Video Lessons

Magnetic Effects of Electric Current - Part 1

Magnetic Effects of Electric Current - Part 2

Magnetic Effects of Electric Current - Part 3

Key Concepts and Formulas

Magnetic Field and Field Lines

In 1820, Hans Christian Oersted saw a compass needle deflect near a current-carrying wire, showing that electricity and magnetism are linked.

• Magnetic field: the region around a magnet in which its force can be detected; it has both magnitude and direction
• The direction of the field at a point is the direction in which the north pole of a compass points there
• Field lines emerge from the north pole and enter the south pole outside a magnet, and run from south to north inside it, so they are closed curves
• Field lines never intersect, because the field cannot have two directions at the same point
• Where field lines are closer together, the field is stronger

Field Due to a Straight Conductor

A current through a straight wire produces a field in the form of concentric circles centred on the wire.

Field strength ∝ current I
Field strength ∝ 1 / distance from the wire

• Right-hand thumb rule: hold the wire in your right hand with the thumb pointing along the current; the curled fingers give the direction of the field lines
• Reversing the current reverses the direction of the field

Field Due to a Circular Loop

Near the centre of a circular loop, the field lines look almost like straight lines.

Field at centre ∝ current I
Field at centre ∝ number of turns n
Smaller radius → stronger field at centre

• Inside the loop, the field from every part of the wire points in the same direction, so it adds up

Field Due to a Solenoid

A solenoid is 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 a north pole and the other as a south pole
• Inside the solenoid, field lines are parallel straight lines, so the field is uniform
• A soft iron core placed inside a solenoid makes an electromagnet, which is magnetic only while current flows

Force on a Current-Carrying Conductor

A rod carrying current between the poles of a magnet experiences a force and moves. Ampère suggested that the magnet feels an equal and opposite force.

Fleming's left-hand rule:
Forefinger    → magnetic field
Middle finger → current
Thumb         → force (motion)

• The force is maximum when the current is perpendicular to the magnetic field, and zero when it is parallel
• Reversing either the current or the field reverses the force
• In the rule, the thumb, forefinger and middle finger of the left hand are held mutually perpendicular
• A moving charge acts like a current; for electrons, the current is taken opposite to their motion

Domestic Electric Circuits

Electricity reaches homes through a main supply with a potential difference of 220 V between the live and neutral wires.

Supply voltage V = 220 V
Current drawn: I = P ÷ V
Maximum safe power = V × fuse rating

• Live wire: red insulation; neutral wire: black insulation; earth wire: green insulation
• 15 A circuit for high-power appliances (geysers, air coolers); 5 A circuit for bulbs and fans
• All appliances are connected in parallel so each gets 220 V and can be switched on or off independently
• Earthing: the earth wire connects metal bodies of appliances to a metal plate buried deep in the ground, giving leaked current a low-resistance path and protecting the user from electric shock
• Short-circuit: live and neutral wires touch directly, so the current rises sharply; overloading: too many appliances on one socket or a sudden rise in supply voltage
• A fuse melts due to heating and breaks the circuit when the current exceeds a safe value; MCBs do the same job

Solved Examples

Example 1: A long horizontal wire on a table carries current from south to north. What is the direction of the magnetic field at a point just above the wire?

Answer: With the right thumb pointing north, the fingers curl over the top of the wire towards the east, so the field above the wire is towards the east.

Example 2: A horizontal conductor carrying current from west to east is placed in a magnetic field acting vertically upwards. Find the direction of the force.

Answer: Using Fleming's left-hand rule with the forefinger pointing up (field) and the middle finger towards the east (current), the thumb points towards the south.

Example 3: A beam of electrons moves towards the north inside a downward magnetic field. Which way is it deflected?

Answer: The current due to electrons is towards the south. With the forefinger down and the middle finger south, the thumb points towards the east.

Example 4: A 2 kW geyser runs on a 220 V supply. What current does it draw, and which circuit should it use?

I = P ÷ V
I = 2000 ÷ 220
I ≈ 9.09 A, which is more than 5 A
∴ About 9.1 A; use the 15 A circuit

Example 5: A room circuit on 220 V has a 5 A fuse. How many 60 W bulbs can glow together safely?

Maximum power = V × I = 220 × 5 = 1100 W
Number of bulbs = 1100 ÷ 60 ≈ 18.3
∴ At most 18 bulbs

Example 6: The current in a straight wire is doubled. What happens to the magnetic field at a fixed point near it?

Field ∝ I
New current = 2I
∴ The field doubles

Common Mistakes to Avoid

• Using the right hand for force; the left hand is for force (Fleming's rule), while the right-hand thumb rule is for field direction.
• Treating electron motion as current; always reverse the direction for negatively charged particles.
• Drawing field lines that cross or that start and stop in empty space; they must form smooth closed loops.
• Saying a fuse is connected in parallel; the fuse is always connected in series with the live wire.

Quick Revision

Fleming's left hand: Field, Current, Thrust
Straight wire: field ∝ I, weaker farther away
I = P ÷ V; domestic supply 220 V

• Field lines go from N to S outside a magnet and never intersect
• Straight wire: concentric circles; use the right-hand thumb rule
• Solenoid: uniform field inside, behaves like a bar magnet
• Live wire red, neutral black, earth green
• Earthing prevents shocks; fuses protect against short-circuit and overloading

Contact us to join our Class 10 Science batch in Janakpuri, or to book a free demo class at our centre.