Electricity covers the fundamental laws governing circuits — from Ohm\'s law and resistance to Kirchhoff\'s laws and practical circuits like potential dividers and sensors.
Current I = Q/t (A). Charge: Q = It. Electron flow: conventional current opposite to electron drift. EMF (ε): energy per unit charge from a source. PD (V): energy per unit charge transferred to a component. Resistance R = V/I (Ω). Ohm\'s law: V ∝ I at constant temperature (for ohmic conductors). Resistivity: R = ρL/A where ρ = constant for a material at given temperature. Temperature effects: metals → R increases (more lattice vibrations), thermistors (NTC) → R decreases, LDR → R decreases with more light.
Kirchhoff\'s first law (KCL): sum of currents at a junction = 0 (charge conservation). Kirchhoff\'s second law (KVL): sum of EMFs = sum of PDs around a closed loop (energy conservation). Series: I same, V adds, R_total = R₁ + R₂. Parallel: V same, I adds, 1/R_total = 1/R₁ + 1/R₂. Internal resistance: ε = V + Ir → V = ε - Ir. Terminal PD decreases with current.
Two resistors in series share the source PD. V_out = V_in × R₂/(R₁+R₂). Used with sensors: thermistor in divider → output voltage changes with temperature. LDR in divider → output voltage changes with light intensity. Design: choose where to place the variable resistor to get desired relationship (e.g., V_out increases with temperature or decreases).
EMF (electromotive force) is the energy per unit charge transferred FROM chemical/other energy TO electrical energy by a source (e.g., battery, generator). PD (potential difference) is the energy per unit charge transferred FROM electrical energy TO other forms by a component (e.g., resistor → heat, motor → kinetic). Both measured in volts (J/C). The terminal PD of a battery is less than its EMF because some energy is "lost" internally: V = ε - Ir. Only when no current flows (open circuit) does terminal PD equal EMF.
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