Electromagnetic EffectsCopy

Cheat Sheet: Electromagnetic Effects, Motors, Transformers, and Oscilloscopes (O Level / IGCSE Physics)

4.5.1 Electromagnetic Induction

1. Demonstration of Electromagnetic Induction

• Move a magnet into/out of a coil connected to a sensitive galvanometer
• Needle deflects, showing induced current
• Reversing direction of motion or magnet polarity reverses deflection

2. Factors Affecting Induced e.m.f.
   (a) Faster movement or more rapid magnetic field change = greater e.m.f.
   (b) More coil turns = greater total e.m.f.
3. Lenz’s Law

• Induced current opposes the change causing it
• Demonstration: Drop a strong magnet through a metal pipe → falls slowly due to opposing currents

4.5.2 The a.c. Generator

1. Structure

• Rotating coil in magnetic field (or rotating magnet)
• Uses slip rings and carbon brushes to maintain contact in a.c. output

2. e.m.f. vs Time Graph

• Sine wave pattern
• Maximum e.m.f. when coil is horizontal (cuts field lines most rapidly)
• Zero e.m.f. when coil is vertical (parallel to field lines)

4.5.3 Magnetic Effect of a Current

1. Field Patterns

• Straight wire: circular field around wire (right-hand grip rule)
• Solenoid: field like a bar magnet, stronger with:
  • More turns
  • Higher current
  • Iron core

2. Uses in Devices

• Relay: small current energizes coil → attracts armature → switches on larger circuit
• Loudspeaker: current through coil in magnetic field causes vibration → sound

4.5.4 Force on a Current-Carrying Conductor

1. Demonstration

• Place wire in magnetic field (e.g. between poles of horseshoe magnet)
• Pass current → wire moves
• Reversing:
  • Current → reverses force direction
  • Field → reverses force direction

2. Relative Directions – Fleming’s Left-Hand Rule:

• Thumb = Force (motion)
• First finger = Field (B)
• Second finger = Current (I)

3. Currents in Parallel Wires

• Same direction → attract
• Opposite direction → repel
• Field lines between wires interact accordingly

4.5.5 The d.c. Motor

1. Turning Effect

• Coil in magnetic field experiences torque
• Turning effect increases with:
  (a) More turns
  (b) Stronger current
  (c) Stronger magnetic field

2. Operation

• Current flows in coil → forces on sides → coil turns
• Split-ring commutator: reverses current every half turn → continuous rotation
• Brushes: maintain electrical contact

4.5.6 The Transformer

1. Structure & Operation

• Two coils (primary and secondary) wound on iron core
• a.c. in primary → changing magnetic field
• Induces a.c. in secondary via electromagnetic induction

2. Types

• Step-up: more turns in secondary (increases voltage)
• Step-down: more turns in primary (decreases voltage)

3. Equation
   Vp / Vs = Np / Ns

where:
V = voltage
N = number of turns
p = primary, s = secondary

4. High-Voltage Transmission

• High voltage → low current for same power
• Less energy loss in wires: P = I^2 R
• Transformers step up for transmission, step down for use

4.6 Oscilloscope Use

1. Displaying Waveforms

• Plots voltage vs time
• Shows wave shape of a.c. or d.c. supply
• Horizontal axis = time
• Vertical axis = voltage

2. Measuring with Oscilloscope

• Voltage: measure vertical deflection → volts/div × number of divisions
• Time: measure wavelength (period) → frequency = 1 / time period
• (Use time base in sec/div)