Magnetic Effects of Electric Current

Magnetic Field and Field Lines

Introduction

• Electricity’s Other Effects: Besides heating, electric current can also create magnetic effects. An electric current-carrying wire behaves like a magnet.

Key Takeaway

• Connection: Electricity and magnetism are linked. Electric current produces a magnetic effect.

Hans Christian Oersted

• Discovery: In 1820, Oersted discovered that a compass needle deflects near a current-carrying wire, showing the relationship between electricity and magnetism.
• Legacy: His work led to technologies like radio, television, and fiber optics. The unit of magnetic field strength is named “oersted” in his honor.

Magnetic Field and Field Lines

Basics

• Compass Needle: A small bar magnet. Points north (north pole) and south (south pole). Like poles repel; unlike poles attract.

Explanation

• Magnetic Field: The region around a magnet where its force is felt.
• Field Lines: Represent the magnetic field; filings align along these lines.

Drawing Field Lines

Properties of Magnetic Field Lines

• Direction: Field lines emerge from the north pole and merge at the south pole.
• Inside Magnet: Lines go from south pole to north pole, forming closed curves.
• Strength: Closer lines indicate a stronger field.
• No Intersection: Field lines never cross; a compass needle can’t point in two directions at once.

Magnetic Field Due to a Current-Carrying Conductor

Magnetic Field Around a Straight Conductor

• Experiment:
  • Use a thick copper wire, battery, rheostat, ammeter, and a cardboard.
  • Pass the wire through the cardboard.
  • Sprinkle iron filings on the cardboard.
  • Pass current through the wire.
• Observation:
  • Iron filings form concentric circles around the wire.
  • These circles represent magnetic field lines.
  • The direction of the magnetic field can be found using a compass.
• Effects of Changing Current and Distance:
  • Increasing current increases needle deflection.
  • Moving the compass away decreases needle deflection.
  • Magnetic field strength decreases with distance from the wire.

Right-Hand Thumb Rule

• How to Find Magnetic Field Direction:
  • Hold the wire in your right hand with thumb pointing in the current direction.
  • Fingers curl in the direction of magnetic field lines.
  • This rule helps to visualize the magnetic field direction.

Example 12.1

• Problem: Current flows from east to west in a horizontal power line.
• Solution:
  • Using the right-hand thumb rule, the magnetic field below the wire turns clockwise, and above it, anti-clockwise.

Magnetic Field Due to a Circular Loop

Circular Loop Magnetic Field

• Observation:
  • Concentric circles of the magnetic field lines become straight at the loop’s center.
  • Every part of the loop contributes to the magnetic field in the same direction at the center.
• Effect of Turns:
  • Magnetic field strength increases with more turns in the loop.
  • For a coil with n turns, the field is n times stronger than a single turn.

Magnetic Field Due to a Solenoid

Solenoid Magnetic Field

• Description:
  • A solenoid is a coil of many circular turns.
  • The magnetic field pattern is similar to a bar magnet.
  • One end acts as the north pole, the other as the south pole.
  • The magnetic field inside the solenoid is uniform and strong.
• Usage:
  • A strong solenoid field can magnetize materials like soft iron, forming an electromagnet.

Force on a Current-Carrying Conductor in a Magnetic Field

• Magnetic Fields and Forces: A current-carrying conductor produces a magnetic field, which exerts a force on nearby magnets. According to Ampere, magnets also exert an equal and opposite force on the conductor.

Key Observations

• Force Direction: The direction of force on the rod depends on the current direction and magnetic field direction.
• Force Magnitude: The force is strongest when the current is perpendicular to the magnetic field.

Fleming’s Left-Hand Rule

• Finding Force Direction:
  • Stretch thumb, forefinger, and middle finger of the left hand perpendicular to each other.
  • First finger points in the direction of the magnetic field.
  • Second finger points in the direction of the current.
  • Thumb points in the direction of the force or motion.

Applications

• Devices: Electric motors, generators, loudspeakers, microphones, and measuring instruments use this principle.

Example 12.2

• Problem: An electron enters a magnetic field at right angles. Determine the force direction.
• Solution: Using Fleming’s left-hand rule, the force direction is into the page.

More to Know: Magnetism in Medicine

• Magnetic Fields in the Body: Nerve cells produce weak magnetic fields when they carry electric impulses. Significant magnetic fields are produced in the heart and brain.
• MRI: Magnetic fields are used in Magnetic Resonance Imaging (MRI) to create images of body parts for medical diagnosis.

Domestic Electric Circuits

Main Supply

• Source: Electricity comes to our homes through overhead poles or underground cables.
• Wires:
  • Live Wire: Red insulation, also called positive.
  • Neutral Wire: Black insulation, also called negative.
• Voltage: The potential difference between live and neutral wires is 220 V in India.

Meter-Board and Circuits

• Meter-Board:
  • Wires pass through an electricity meter and a main fuse.
  • Connected to line wires in the house via a main switch.
• Circuits:
  • Two types:
    • 15 A for high-power appliances (geysers, air coolers).
    • 5 A for low-power appliances (bulbs, fans).

Earth Wire

• Insulation: Green color.
• Connection: Linked to a metal plate in the earth.
• Purpose: Safety for metallic appliances (toasters, refrigerators) to prevent electric shock by providing a low-resistance path for current leakage.

Domestic Circuit Diagram

• Appliances: Connected across live and neutral wires.
• Switches: Each appliance has its own switch.
• Parallel Connection: Ensures equal potential difference for all appliances.

Electric Fuse

• Function: Prevents damage due to overloading by breaking the circuit when current is too high.
• Overloading Causes:
  • Short-circuiting: Live wire touches neutral wire.
  • High supply voltage.
  • Too many appliances connected to one socket.
• Action: Fuse melts due to Joule heating, stopping the current flow and protecting the circuit.

Chapter Summary:

• A compass needle is a small magnet.
  • One end points north (north pole).
  • The other end points south (south pole).
• A magnetic field exists around a magnet.
  • It is the region where the magnet’s force can be detected.
• Field lines represent a magnetic field.
  • A field line shows the path a free north pole would move along.
  • The direction of the magnetic field at a point is the direction a north pole would take.
  • Field lines are closer where the magnetic field is stronger.
• A metallic wire carrying an electric current has a magnetic field.
  • The field lines form concentric circles.
  • The direction of the field is given by the right-hand rule.
• The magnetic field around a conductor depends on its shape.
  • A solenoid’s magnetic field is like that of a bar magnet.
• An electromagnet has a soft iron core wrapped with insulated copper wire.
• A current-carrying conductor in a magnetic field experiences a force.
  • If the current and field are perpendicular, the force is also perpendicular.
  • This is explained by Fleming’s left-hand rule.
• In houses, we receive AC power of 220 V at 50 Hz.
  • Red insulation wire: live wire.
  • Black insulation wire: neutral wire.
  • Potential difference between live and neutral is 220 V.
  • Green insulation wire: earth wire, connected to a metal body in the earth for safety.
• A fuse is a crucial safety device.
  • Protects circuits from short-circuiting or overloading.