Chapter 12: Magnetic Effects of Electric Current

1. Introduction

We already know that electricity and magnetism are closely related. When an electric current passes through a wire, it produces a magnetic effect around it. This chapter explains how current creates magnetism, what magnetic field lines are, and how electric current can be used to make devices like electromagnets and electric motors.

2. Magnetic Field

Definition:
 The region around a magnet or a current-carrying conductor where the magnetic force can be felt is called a magnetic field.

Magnetic Field Lines:
 Magnetic field lines are imaginary lines used to represent the magnetic field.

Properties of Magnetic Field Lines:
  1. Magnetic field lines emerge from the north pole and enter the south pole outside the magnet.
  2. Inside the magnet, they move from south to north, forming a closed loop.
  3. The closer the lines, the stronger the magnetic field.
  4. Field lines never intersect each other.
  5. Direction at any point gives the direction of the magnetic field.

Representation:
 The direction of the magnetic field at a point can be shown by:

  • A dot (•) for the field coming out of the page.
  • A cross (×) for the field going into the page.

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

When an electric current passes through a straight wire, a magnetic field is produced around it in concentric circles.

Experiment: (Oersted’s Experiment)

  • Place a straight wire through a cardboard and sprinkle iron filings on it.
  • When current flows, iron filings arrange in circular patterns around the wire.

Direction of Magnetic Field: Determined by Right-Hand Thumb Rule.

4. Right-Hand Thumb Rule

Statement:
 If you hold a current-carrying wire in your right hand such that the thumb points in the direction of current, then the curl of fingers gives the direction of the magnetic field around the conductor.

Example:
 If current flows upward, magnetic field lines go anticlockwise around it.

5. Magnetic Field Due to a Circular Loop

When current flows through a circular coil, the magnetic field lines become concentric circles near the wire but appear straight and parallel near the center of the loop.

This shows that the magnetic field near the center of a circular coil is similar to the field of a bar magnet.

Factors affecting magnetic field strength:

  1. Current — More current → stronger field
  2. Number of turns — More turns → stronger field
  3. Medium — Soft iron core increases the field strength

6. Magnetic Field Due to a Solenoid

A solenoid is a long coil of insulated copper wire wound in the shape of a cylinder.

When electric current flows through a solenoid:

  • It produces a magnetic field similar to that of a bar magnet.
  • One end behaves like a north pole, and the other as a south pole.

Magnetic field inside a solenoid is uniform, straight, and strong.

Polarity rule:
 If the current in the coil flows anticlockwise from the front, that end behaves as north pole; if clockwise, it behaves as south pole.

7. Electromagnet

Definition:
 When a soft iron core is placed inside a current-carrying solenoid, it becomes a temporary magnet.
 This is called an electromagnet.

Uses:

  • Electric bell
  • Electric crane (for lifting heavy iron pieces)
  • Electric motors

Advantages:

  • Strength of magnet can be increased by increasing current or number of turns.
  • Magnetism can be turned on or off by switching current.

8. Force on a Current-Carrying Conductor in a Magnetic Field

When a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force.

Key features:

  • The force is maximum when the conductor is perpendicular to the magnetic field.
  • The force is zero when it is parallel to the field.

Direction of Force:
 Given by Fleming’s Left-Hand Rule.

9. Fleming’s Left-Hand Rule

  1. Statement:
     If the thumb, forefinger, and middle finger of the left hand are held at right angles to each other:

    • Forefinger → Direction of Magnetic Field (B)
    • Middle Finger → Direction of Current (I)
    • Thumb → Direction of Force (Motion)

    This rule helps find the direction in which a current-carrying wire moves in a magnetic field.

    Application:
     Used in electric motors.

10. Domestic Electric Circuit (Overview only)

Main components:

  1. Main switch – controls supply
  2. Fuse – safety device
  3. Earth wire – prevents electric shocks
  4. Live wire (red) and Neutral wire (black)

In India, domestic supply is 220 V AC, 50 Hz.

11. Important Rules Summary

Rule

Purpose

Right-Hand Thumb Rule

Direction of magnetic field around a current-carrying wire

Fleming’s Left-Hand Rule

Direction of motion in a motor

12. Key Points

  • A current-carrying wire produces a magnetic field.
  • Solenoid behaves like a bar magnet.
  • Electromagnets are temporary magnets.
  • Motors convert electrical → mechanical energy.
  • Generators convert mechanical → electrical energy.
  • AC is used in homes; DC in batteries.
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