Chapter 11: Electricity
Electric Current and Ohm’s Law
Electric current is the flow of electric charge, measured in amperes (A). One ampere equals one coulomb of charge passing through a point per second. A complete, closed path is needed for current to flow—this is called an electric circuit. The potential difference (voltage) across a conductor causes charges to move and is measured in volts (V). Ohm’s Law states that current (I) is directly proportional to the potential difference (V) across a resistor, provided the temperature remains constant: V=IRV=IR. Resistance (R) is the opposition to current flow and is measured in ohms (Ω).
Factors Affecting Resistance and Combination of Resistors
The resistance of a conductor depends on its length (directly), cross-sectional area (inversely), and the material. The resistivity (Ω·m) is a property specific to the material. In series combination, total resistance is the sum of individual resistances Rs=R1+R2+…Rs=R1+R2+…. In parallel combination, the reciprocal of total resistance equals the sum of reciprocals 1/Rp=1/R1+1/R2+…1/Rp=1/R1+1/R2+…. Series circuits carry the same current through each resistor, while parallel circuits allow different currents but maintain equal potential across branches.
Heating Effect and Power of Electric Current
As current flows through a resistor, energy is dissipated as heat. The heat produced follows Joule’s Law: H=I²Rt . This effect is utilized in electric heaters, irons, and fuses. Electric power is the rate at which energy is used: P=VI = I²R = V²/R (Ohm’s Law V=IR) ; measured in watts (W). The energy consumed is measured in kilowatt-hours (kWh), the commercial unit of electricity. Devices like bulbs and motors are rated by their power consumption, which helps calculate energy costs and safety limits.