Electromagnetism

ELECTROMAGNETISM

Chapter 21,22

4.5 Electromagnetic Effects

4.5.1 Electromagnetic Induction

• A conductor moving across a magnetic field or having a changing magnetic field linked to it can induce an electromotive force (e.m.f.) in the conductor.

• Experiment to Demonstrate Electromagnetic Induction:

  • Slide a magnet in and out of a coil to show e.m.f. induction.

  • Current is induced when magnet moves relative to the coil.

• Factors Affecting Induced e.m.f.:

  • Speed of movement of the conductor or magnetic field

  • Number of turns in the coil

  • Size of coils

  • Strength of the magnetic field

4.5.2 The A.C. Generator

• Description: A simple a.c. generator consists of a coil rotating within a magnetic field.

• Using Slip Rings and Brushes:

  • Slip rings conduct electricity as the coil rotates.

  • Brushes maintain electrical contact with the rotating coil.

• Graph Interpretation: e.m.f. varies over time in a sinusoidal pattern (peaks, troughs, zero).

4.5.3 Magnetic Effect of a Current

• Magnetic Field Direction & Pattern:

  • Currents in straight wires produce circular magnetic fields.

  • Solenoids create stronger, uniform magnetic fields resembling bar magnets.

• Experiment to Observe Magnetic Fields:

  • Use iron filings or plotting compasses around current-carrying wires and solenoids to observe field patterns.

• Applications:

  • Utilized in relays and loudspeakers based on the magnetic effect of currents.

4.5 Electromagnetic Effects Continued

4.5.4 Force on a Current-Carrying Conductor

• Experiment Demonstration:

  • Show force exerted on a current-carrying conductor in a magnetic field by reversing current or the field direction.

4.5.5 The D.C. Motor

• Turning Effect: Current-carrying coils in a magnetic field experience a turning effect.

• Increasing the Effect:

  • Increase number of turns, current, or strength of the magnetic field.

4.5.6 The Transformer

• Operation of a Transformer:

  • Induces e.m.f. due to alternating current in primary coil creating a changing magnetic field in the secondary coil.

  • Transformer design includes primary coil, secondary coil, and soft iron core.

• Types:

  • Step-up Transformer: More turns on secondary coil (Ns > Np).

  • Step-down Transformer: Fewer turns on secondary coil (Ns < Np).

• Equations for Transformers:

  • Vp/Np = Vs/Ns

  • Power conservation: VpIp = VsIs

INDUCED EMF

Experiment of Inducing EMF

• Movement Inducing EMF:

  • When a conductor is moved in a magnetic field, an EMF is produced.

• When Stationary:

  • No EMF induced when the magnet is stationary in the coil.

Measuring Induced EMF

• Reading Induced Current:

  • Use an ammeter to show current when EMF is induced in a circuit.

Demonstrating Induced EMF

1. Coil and Voltmeter Setup with Magnet

• EMF induced by entering/exiting a magnet.

2. Examine Conditions for Induced EMF

• No induced EMF with still magnet; movement is critical.

Factors Affecting Induced EMF

• Magnitude Determinants:

  • Speed of movement, number of turns, coil size, magnetic field strength.

• Direction Determinants:

  • Orientation of magnet poles.

Right Hand Dynamo Rule

• Used to determine the direction of induced EMF related to current and magnetic field.

A.C. Generator

Structure and Function

• Components:

  • Permanent magnet (N & S), rectangular coil, turning axis, slip rings, carbon brushes.

• Output:

  • Alternating potential difference with a changing direction from A.C. generator operation.

Increasing A.C. Output

• Increase output by:

  • More turns of the coil, stronger magnets, larger coils or faster rotation.

Magnetic Field Around Conductors

Straight Wire and Solenoid

• Direction of Current:

  • Use right-hand rule to determine magnetic field direction around wires and solenoids.

Applications of Magnetic Effects

Electromagnets

• Uses:

  • Relay circuits, electric bells, loudspeakers, and headphones.

Relay Circuit Mechanics

• Functionality: Relay switches are activated by electromagnets, allowing circuit control.

• Operation:

  • Closing and opening circuitry through electromagnet interaction.

Electric Bell Functionality

1. Button activation creates current and magnetic field.

2. Electromagnet pulls armature to strike bell, then breaks circuit to reset.

Loudspeakers and Headphones

• Working Principle: Convert electrical signals to sound using the motor effect in a coil wrapped in a permanent magnet's field.

Investigation of Magnetic Fields

Around Wire and Solenoid

• Use compasses and iron filings to visualize magnetic field patterns around wire and solenoid setups.

Force on Current-Carrying Conductor in Magnetic Fields

• Current Interaction:

  • Electron movement through a conductor experiences force in the magnetic field.

Fleming's Left-Hand Rule

• Tool for predicting force direction in motors based on current direction and field orientation.

D.C. Motor Operation

• Coil rotates due to alternating direction of current and magnetic effects.

• Enhancement of Functionality: Improve speed, force, and direction by adjusting current and magnetic configurations.

Lenz's Law

• Induced potential difference opposes the change that creates it.

Transformer Overview

Basic Operation

• A device for voltage transformation utilizing alternating current and induction principles.

Efficiency Considerations

• Evaluating power loss with resistance effects in conducting cables based on heating during transmission.

High-Voltage Transmission

1. Increases potential difference to reduce current for efficient long-distance transmission.

2. Lower heat losses due to lower current in wires.

Calculating Power Loss

• Use equation P = I²R to determine electrical losses due to resistance in wires.