Electric & Magnetic Fields – Comprehensive Study Notes

Definition:
- Region surrounding an electric charge where its influence (force) can be felt.
- Operational definition: if a test charge is placed at any point, an electric force acts on it.
Field Strength (Intensity):
- Varies point-to-point; higher near the source charge, lower farther away.
- Units: Newton per Coulomb (N C⁻¹) or Volt per metre (V m⁻¹).
Action-at-a-distance: produces forces without physical contact.
Source Charge: any charged object that generates the field.

Originate on positive charges, terminate on negative charges.
Never intersect one another.
Like-charge configurations:
- Lines from identical charges diverge; they never merge into a single straight path.
Unlike-charge configurations:
- Lines emanate from + and terminate on -, forming continuous paths.
Lines meet the surface of any object at right angles (normal incidence).
Conductors:
- Lines do not penetrate; they end or begin on the surface.
- Hollow spherical conductor with no internal charge → no internal electric field (field exists only on the outer surface).

Magnet: material whose atomic/molecular moments are orderly and can exert magnetic force.
Suspension test: a free magnet aligns N-pole toward geographic North, S-pole toward geographic South ⇒ Earth behaves as a giant magnet.
Interaction rules:
- Unlike poles attract.
- Like poles repel.

Always possess two poles: North (N) & South (S).
Naturally align along the N–S geographic direction.
Magnetic field lines:
- Outside a magnet: emerge from N, enter S.
- For Earth: emerge near geographic South, enter near geographic North (reversed labeling – Earth’s magnetic south ≈ geographic north).
Magnetic Flux: total number of magnetic field lines (symbol \Phi, units Weber, Wb).
Neutral Point: location where the vector sum of magnetic fields = 0; field intensity cancels.

Instrument containing a freely-pivoted magnetic needle.
Needle aligns with Earth’s field; arrowhead points to geographic North (marked ‘N’).
Nearby magnets disturb the reading by superposing their own field.

Definition: number of magnetic field lines crossing a unit area normally.
Formula: B = \dfrac{\Phi}{A}
- B: magnetic flux density (Tesla, T or Wb m⁻²).
- \Phi: magnetic flux (Weber, Wb).
- A: area perpendicular to the flux (m²).

Region between two bar magnets where their fields cancel, producing B = 0.
Often appears as one or more points along the axial or equatorial line, visualised by absence of iron-filing pattern.

Core composition: mostly iron.
Inner core: solid (high pressure, higher temperature).
Outer core: viscous liquid iron.
Heat from inner to outer core → convection currents.
Moving conductive fluid generates electric currents → self-sustaining magnetic field (geodynamo model).

Cutting a Magnet:
- Each fragment becomes an independent magnet with its own N & S poles (cannot isolate a single pole).
Electromagnetism:
- Rubbing iron with a permanent magnet aligns domains → temporary magnetisation.
- Passing electric current through a coil around an iron core generates a magnetic field (electromagnet).
- Field strength ∝ current magnitude & number of coil turns; disappears when current stops (temporary magnet).

Electric & Magnetic fields enable non-contact forces; foundation for electromagnetic induction, motors, generators.
Field line conventions help predict force directions, interactions, shielding effects (Faraday cage, magnetic shielding).
Hollow conductor property underlies electrostatic shielding (no field inside Faraday cage).
Knowledge of neutral points aids in mapping composite fields and designing magnetic instruments.
Geodynamo explains compass navigation, auroras, and space weather interactions.