Electromagnetism Revision

Overview

• Electromagnetism is a key topic in A-level Physics, divided into magnetic fields and electric fields.
• Magnetic fields relate to electric fields via Faraday's Law.

Magnetic Fields

Earth as a Magnet

• The Earth acts like a large bar magnet with a North and South pole.
• Magnetic field lines run from North to South.
• Direction represented by arrows shows where a free North pole would move.
• Uniform fields have equally spaced parallel lines.

Magnetic Field Around Current-Carrying Wire

• Concentric circles form around a wire carrying current.
• Symbols: 'X' in a circle (current into the page), 'dot' in a circle (current out of the page).
• Right Hand Grip Rule: Thumb indicates current direction, fingers show magnetic field direction.

Force on Current-Carrying Wire

• External magnetic field B, wire length L, current I results in magnetic force.
• Force formula: F = BIL sin(θ).
• Maximum force when θ = 90°; zero force when wire is parallel to field (θ = 0°, 360°).

Charged Particles in a Magnetic Field

• Positive charge in a magnetic field experiences a centripetal force causing it to move in a circle.
• Equivalence of magnetic force and centripetal force: mv²/r = qvB.
• Rearrange for radius: r = mv/qB.

Magnetic Flux and Flux Linkage

• Magnetic flux (Φ): Product of magnetic flux density (B) and area (A) perpendicular to the field: Φ = BA cos(θ).
• Maximum flux when field is perpendicular to the area.
• Magnetic flux linkage: NΦ, where N is the number of coils.

Faraday's Law

• Moving magnets induce EMF in conductors.
• Formula: E = -d(NΦ)/dt or E = -d(NBAcosθ)/dt.
• Lenz's Law: Direction of induced current opposes change causing it (energy conservation).

AC Generator

• Coil rotates in a magnetic field, changing flux linkage over time.
• Produces AC current through continuous change in direction of the coil relative to the field.
• Gradient of flux linkage vs time graph gives induced EMF magnitude.

Transformers

• Consist of primary and secondary coils connected via a soft iron core.
• AC current in primary coil creates changing magnetic flux in secondary coil, inducing EMF.
• Transformer equation: Ns/Np = Vs/Vp = Ip/Is (based on power conservation).

Important Units

• Magnetic flux density (B): Measured in Tesla (T).
• Magnetic flux (Φ): Measured in Weber (Wb).

Base Units

• Base unit for magnetic flux density: kg A⁻¹ s⁻².
• Base unit of magnetic flux: kg m² A⁻¹ s⁻² (flux density multiplied by area).

Tips

• Use the Right Hand Grip Rule for wires.
• Use Fleming's Left Hand Rule for force direction.
• Always check the angle (θ) between field lines and the normal when calculating magnetic flux.
• Understand Faraday's and Lenz's laws for induced EMF and current.

Good luck with your studies and exams!