📚 Chapter Overview

1. Magnetic Field

A magnetic field is the region around a magnet or a current-carrying conductor in which a magnetic force can be experienced.

• A magnetic field is represented by magnetic field lines.
• A compass needle placed in a magnetic field experiences a force and gets deflected.
• The direction of the magnetic field at a point is the direction in which the north pole of a compass needle points.

2. Magnetic Field Lines

Magnetic field lines are imaginary lines used to represent a magnetic field. The tangent to a magnetic field line at any point gives the direction of the magnetic field at that point.

Important Properties

• Outside a bar magnet, field lines emerge from the North pole and enter the South pole.
• Inside the magnet, field lines travel from South to North.
• Magnetic field lines form closed and continuous curves.
• Magnetic field lines never intersect each other.
• Field lines are closer together where the magnetic field is stronger.
• The magnetic field is strongest near the poles of a bar magnet.

3. Magnetic Field Due to a Straight Current-Carrying Conductor

A current-carrying straight conductor produces a magnetic field around itself. The magnetic field lines are concentric circles centred on the conductor.

• The plane of these circular field lines is perpendicular to the conductor.
• The direction of the field is found using the Right-Hand Thumb Rule.
• As the distance from the conductor increases, the magnetic field becomes weaker.
• For a long straight conductor, magnetic field is directly proportional to current and inversely proportional to distance.

4. Magnetic Field Due to a Circular Current-Carrying Loop

When electric current flows through a circular loop, every small part of the conductor produces a magnetic field. These fields combine to produce a stronger field near the centre.

• The magnetic field pattern resembles that of a bar magnet.
• At the centre of the loop, field lines are nearly straight and parallel.
• Increasing current increases the magnetic field.
• Increasing the number of turns increases the magnetic field.
• Anticlockwise current as viewed from a face makes that face a North pole; clockwise current makes it a South pole.

5. Right-Hand Thumb Rule

Hold a straight current-carrying conductor in your right hand with the thumb pointing in the direction of current. The curled fingers show the direction of the magnetic field.

6. Factors Affecting Magnetic Field Strength

7. Solenoid

A solenoid is a long cylindrical coil consisting of many closely wound turns of insulated wire. When current flows through it, it produces a magnetic field.

• The magnetic field inside a long solenoid is strong and nearly uniform.
• Field lines inside are straight, parallel and closely spaced.
• The field pattern outside resembles that of a bar magnet.
• A current-carrying solenoid has a North pole and South pole.
• Increasing current or the number of turns per unit length makes the field stronger.

8. Electromagnet

An electromagnet is a temporary magnet produced when electric current passes through a coil, usually wound around a soft iron core.

• It behaves like a magnet while current is flowing.
• Its strength can be increased by increasing current or the number of turns.
• A soft iron core makes the electromagnet much stronger.
• Its polarity can be reversed by reversing the direction of current.

Uses of Electromagnets

• Electric bells
• Relays and electromagnetic switches
• Cranes for lifting scrap iron and steel
• Motors and electrical devices

9. Force on a Current-Carrying Conductor

A current-carrying conductor placed in a magnetic field experiences a force.

• F = force on the conductor
• B = magnetic field strength
• I = current
• L = length of conductor in the magnetic field
• θ = angle between current and magnetic field

10. Fleming's Left-Hand Rule

Stretch the thumb, forefinger and middle finger of the left hand mutually perpendicular to each other.

If the forefinger points in the direction of magnetic field and the middle finger points in the direction of current, the thumb gives the direction of force or motion.

11. Overloading

When can overloading occur?

• When there is a sudden voltage rise.
• When too many appliances are connected to a single socket.
• When circuit resistance is low, current can become high.

Overloading causes excessive heating of wires and may lead to fire.

12. Short Circuiting

A short circuit occurs when the live wire and neutral wire come into direct contact, often due to damaged insulation or a fault in an appliance.

• The resistance becomes very small.
• The current increases abruptly and becomes very large.
• This may cause overheating, sparks and fire.

13. AC and DC — Difference

⚡ Magnetic Field Due to AC and DC

14. Quick Exam Revision