Electricity
This chapter covers electric current, potential difference, Ohm's law, factors affecting resistance, series and parallel combinations of resistors, and electric power and energy.
Ohm's law states that the current flowing through a conductor is directly proportional to the potential difference across its ends, provided the temperature remains constant. V = IR, where V is potential difference, I is current, and R is resistance.
In series, the same current flows through all resistors and total resistance is the sum: R = R1 + R2 + R3. In parallel, the voltage across each resistor is the same and the reciprocal of total resistance is the sum of reciprocals: 1/R = 1/R1 + 1/R2 + 1/R3.
Q. Why are household circuits wired in parallel rather than in series?
In a parallel circuit, each appliance gets the same full voltage and can be switched on or off independently without affecting other appliances, and if one appliance fails or is switched off, the rest keep working, unlike in series where the whole circuit would break.
Resistance of a conductor depends on its length (directly proportional), area of cross-section (inversely proportional), and the nature of the material (resistivity), as given by R = (resistivity x length) / area.
Q. Why does the connecting wire of an electric heater not glow, while its heating element does?
The heating element is made of a material (like nichrome) with high resistivity, so it heats up strongly and glows when current passes through it. Connecting wires are made of low-resistivity materials like copper, so they get heated very little and do not glow.
The SI unit of electrical resistance is:
Correct answer: (C) Ohm
The rate at which electrical energy is consumed or produced in a circuit is called electric _______.
Power
Connecting more resistors in parallel decreases the total resistance of the circuit.
True — adding more resistors in parallel provides additional paths for current to flow, which decreases the overall resistance of the combination below that of any individual resistor.
An electric bulb is rated 220V and 100W. Calculate the current drawn and the resistance of the bulb's filament.
Power P = VI, so I = P/V = 100/220 = 0.4545 A (approximately 0.45 A). Resistance R = V/I = 220/0.4545 = 484 ohms (approximately).