Electromagnetism Revision

Overview:

• Electromagnetism is an important topic in A-level physics and consists of magnetic fields and their relation to electric fields via Faraday's Law.

Magnetic Fields

Magnetic Field around the Earth:

• Acts like a bar magnet with North and South poles.
• Rules:
  • Magnetic field lines go from North to South.
  • Direction of line represents the movement of a free North pole.
  • Uniform fields have equally spaced, parallel lines.

Magnetic Field around a Current-Carrying Wire:

• Notations:
  • Circle with a cross: Current into the screen.
  • Circle with a dot: Current out of the screen.
• Right Hand Grip Rule:
  • Thumb: Direction of conventional current.
  • Finger curve: Direction of magnetic field lines.

Force on a Current-Carrying Wire:

• Formula: F = BIL sin θ
  • B: Magnetic flux density (Tesla)
  • I: Current (Amps)
  • L: Length of wire (meters)
  • θ: Angle between wire and field
  • Max force when θ = 90° (Field perpendicular to wire)
  • Zero force when θ = 0° (Field parallel to wire)
• Direction: Determined by Fleming's Left Hand Rule.

Charged Particles in a Magnetic Field:

• Positive charge in a magnetic field experiences a centripetal force.
• Formula: mv²/r = qvB
  • Rearrange to find radius: r = mv / qB

Magnetic Flux and Faraday's Law

Magnetic Flux (Φ)

• Formula: Φ = BA cos θ
  • B: Magnetic flux density
  • A: Area
  • θ: Angle between field lines and the normal
• Max flux when field is perpendicular to area.
• Zero flux when field is parallel to area.

Faraday's Law of Electromagnetic Induction:

• Expression: E = -Δ(NΦ)/Δt
  • Induced EMF directly proportional to rate of change of magnetic flux linkage.
  • Minus sign represents Lenz's Law: Induced current opposes change causing it.

AC Generators and Transformers

AC Generator:

• Flux linkage changes as coil rotates in magnetic field.
• Maximum and zero flux linkage correspond to coil’s orientation.

Transformers:

• Working:
  • AC current produces changing magnetic flux.
  • Induces EMF in secondary coil via Faraday's Law.
• Equation: Ns/Np = Vs/Vp
  • Ns, Np: Number of turns in secondary and primary coils
  • Vs, Vp: Voltages in secondary and primary coils

Power Conservation in Transformers:

• Equation: Vs/Vp = Ip/Is
  • Based on power conservation: Pp = Ps
  • Power (P) = Voltage (V) x Current (I)

Important Notes:

• Always use right hand for Right Hand Grip Rule and left hand for Fleming's Left Hand Rule.
• Conventional current flows from positive to negative.

Conclusion:

• Covered all major aspects of electromagnetism, beneficial for revision and exams.