Electricity (Unit 2) covers charge flow, resistance, circuit theory, emf and internal resistance, Kirchhoff\'s laws, and potential dividers — foundation for Fields in Unit 4.
Current I = ΔQ/Δt (A = C/s). Conventional current: positive to negative. Electron flow: opposite. Potential difference V = W/Q (V = J/C). Work done per unit charge. Resistance R = V/I (Ω). Ohm\'s law: V = IR for ohmic conductors (linear I-V). Resistivity ρ = RA/L (Ωm) — property of the material. Factors: temperature increases → resistance increases (metals), decreases (thermistors, NTC). I-V characteristics: filament lamp (curve), diode (threshold ~0.6V), thermistor, LDR.
Series: same current, V = V₁+V₂, R = R₁+R₂. Parallel: same voltage, I = I₁+I₂, 1/R = 1/R₁+1/R₂. Kirchhoff\'s 1st law (junction): ΣI_in = ΣI_out (charge conservation). 2nd law (loop): ΣV = 0 around any closed loop (energy conservation). EMF (ε): total energy per unit charge from source. Internal resistance r: ε = V + Ir, or V = ε − Ir. Terminal pd drops as current increases. Experimental: plot V vs I, intercept = ε, gradient = −r. Potential divider: V_out = (R₂/(R₁+R₂))V_in. Sensor circuits: LDR or thermistor in divider for automatic control. Power: P = IV = I²R = V²/R. Energy E = Pt.
EMF (electromotive force, ε) is the total energy transferred per unit charge by a source (battery, generator) — it\'s the maximum possible voltage. Potential difference (V) is the energy transferred per unit charge between two points in a circuit — it\'s the voltage across an external component. The key relationship: ε = V + Ir, where Ir is the voltage "lost" across the internal resistance of the source. When no current flows (open circuit), V = ε. When current flows, V < ε because some energy is wasted heating the internal resistance. Think of emf as what the battery provides, and pd as what the external circuit gets. Measured in the same unit (volts), but they\'re conceptually different: emf is about energy conversion, pd is about energy transfer.
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