Electromagnetism

Page 1: Magnetism and Electromagnetism

Poles of a Magnet

• Definition: A magnet has two ends known as poles: north pole and south pole.

• Magnetic Forces: The magnetic forces are strongest at the poles.

• Inter-pole Interaction:

  • Like poles repel each other (e.g., N-N or S-S).

  • Opposite poles attract (e.g., N-S).

• Nature of Forces: The forces between the poles are a type of non-contact force; they do not require physical contact to exert their effect.

Magnetic Materials

• Only specific metals such as iron, cobalt, and nickel (and their alloys) exhibit magnetic properties.

• Permanent Magnet: These magnets have their own constant magnetic field (e.g., bar magnet, horseshoe magnet).

• Induced Magnet: Becomes magnetic only when placed in a magnetic field; loses magnetism once removed (e.g., iron filings).

Magnetic Fields

• Definition: The area around a magnet where magnetic forces can act on other magnets or magnetic materials.

• Observation: The magnetic field can be observed using a compass.

• Field Lines:

  • Magnetic field lines indicate the direction of the magnetic field.

  • Show strong force where lines are close together, especially at the poles.

• Earth’s Magnetism: Earth itself generates a magnetic field which aids navigation using a compass.

Creating Electric Magnetic Fields

• When a current flows through a wire, it produces a circular magnetic field.

• Strength of the magnetic field is greater near the wire and can be manipulated by:

  • Increasing the current.

  • Coiling the wire into a solenoid.

• Solenoid: A coil of wire that generates a strong and uniform magnetic field when current passes through it.

Enhancing Electromagnetic Strength

• To strengthen the magnetic field generated by a solenoid, one can:

  • Add an iron core.

  • Increase the number of coils of wire.

  • Increase the current passing through the wire.

• Electromagnet: A solenoid with an iron core; it can be turned on and off thus is an induced magnet.

• Applications: Used in devices like electric motors, loudspeakers, electric bells, and remote-controlled locks.

Plotting Magnetic Field Lines

• Method using a Compass:

  1. Place the bar magnet in the center of plain paper.

  2. Position a magnetic compass around the magnet.

  3. Mark the position of the compass needle and draw dots aligning with the needle's direction.

  4. Repeat for various positions and join the dots with arrows that indicate the magnetic field direction (from North to South).

Page 2: The Motor Effect and Fleming’s Left-Hand Rule

Electric Motors

• Coiled Wire Rotation: When current passes through a coiled wire, the motor effect causes it to rotate due to forces acting in opposite directions on the coil.

• Stopping Mechanism: Upon reaching a vertical position, the forces become parallel to the magnetic field lines, leading to a loss of motion.

• Split Ring Commutator: Used to reverse the current direction in order to maintain continuous rotation of the coil.

• Connection Maintenance: Achieved through brushes made from graphite or metal that allow for free rotation.

Understanding the Motor Effect

• The interaction of a current-carrying wire with a magnetic field produces a force perpendicular to both the magnetic field and the current.

• Motor Effect Equation:

  • Formula: **Force (N) = Magnetic Flux Density (T) × Current (A) × Length (m)

  • Example Calculation**: If 8A current flows through 75cm of wire in a magnetic field of 0.5T,

  • Convert length to meters: 75cm = 0.75m.

  • F = 0.5 × 8 × 0.75 = 3N.

Fleming’s Left-Hand Rule

• Purpose: Helps determine the direction of the force produced by the motor effect.

• Configuration:

  • Use the left hand with:

    • Thumb: Direction of the force.

    • Index Finger: Direction of the magnetic field.

    • Middle Finger: Direction of the current flowing through the wire.

• All fingers should be at right angles to each other to follow the rule correctly.