Flip a switch, and a bulb glows instantly. Behind that simple act is a whole system of rules governing charge, current, and resistance.
Electricity is one of the most heavily tested topics in competitive exam physics, especially Ohm’s law and circuit calculations.
MCQ Questions🏛️ Must Know
Electric Current and Potential Difference
- Current Electric current is the rate of flow of charge through a conductor. It is measured in amperes (A).
- Ampere One ampere means one coulomb of charge flows past a point every second. The SI unit of charge is the coulomb.
- Conventional Direction Conventional current flows from the positive to the negative terminal, opposite to the actual flow of electrons.
- Potential Difference Potential difference is the work done to move one unit of charge between two points. It is measured in volts (V).
- Measuring Instruments An ammeter measures current and is connected in series. A voltmeter measures potential difference and is connected in parallel.
🏛️ Must Know
Ohm's Law and Resistance
- Ohm's Law Potential difference across a conductor is directly proportional to the current through it, at constant temperature.
- Formula This gives V = IR, where R is the conductor's resistance, measured in ohms (Ω).
- Resistance Resistance is a conductor's opposition to the flow of charge. Higher resistance means less current for the same voltage.
- Rheostat A rheostat is a variable resistance used to regulate current in a circuit, without changing the voltage source.
- Conductors vs Insulators Good conductors have very low resistance. Insulators have very high resistance. Resistors fall in between.
📘 Good to Know
What Determines a Wire's Resistance
- Length and Area Resistance is directly proportional to a conductor's length, and inversely proportional to its cross-sectional area.
- Resistivity Resistivity is a material's own characteristic resistance, given by R = ρl/A. It is measured in ohm-metres.
- Metals vs Insulators Metals and alloys have very low resistivity, around 10⁻⁸ ohm-metres. Insulators can exceed 10¹² ohm-metres.
- Alloys for Heating Alloys like nichrome resist oxidising at high temperatures and have higher resistivity than pure metals, so heaters use them.
- Transmission Wires Copper and aluminium have low resistivity, so they are used for electrical transmission lines, not heating elements.
📘 Good to Know
Resistors in Series and Parallel
- Series In series, the same current flows through every resistor. The equivalent resistance equals the sum of the individual resistances.
- Parallel In parallel, every resistor gets the same potential difference. The reciprocal of equivalent resistance equals the sum of reciprocals.
- Total Resistance Adding a resistor in parallel always lowers the total resistance, below the smallest individual resistance in the group.
- Household Wiring Household circuits use parallel wiring, so each appliance gets full voltage and keeps working even if another one fails.
- Series Drawback Series components must share one current, so a single failed bulb, as in old fairy lights, breaks the whole circuit.
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🌟 Great to Know
Heating Effect: Joule's Law, Fuses and Filaments
- Joule's Law Heat produced in a resistor equals I²Rt: proportional to the square of current, to resistance, and to time.
- Practical Uses Electric irons, heaters, toasters, and kettles all convert electrical energy into heat using this same principle.
- Bulb Filaments Tungsten is used for bulb filaments. It has a very high melting point, around 3380°C, so it glows without melting.
- Fuse Wires A fuse wire must conduct electricity but have a low melting point, so it melts and breaks the circuit during an overload.
- Fuse Ratings Household fuses are rated in amperes. The rating chosen must exceed the appliance's normal operating current.
🌟 Great to Know
Electric Power and the Cost of Electricity
- Power Formula Electric power equals P = VI. It can also be written as P = I²R or P = V²/R.
- SI Unit The SI unit of power is the watt. One watt is one joule of energy consumed per second.
- Commercial Unit Electricity bills use kilowatt-hours, called a "unit." One kilowatt-hour equals 3.6 × 10⁶ joules.
- Voltage Sensitivity An appliance's power depends on the square of its operating voltage, so power drops sharply if the supply voltage falls.
📝 Exam Point of View
NDA GAT — Four Real Electricity Questions
- Question NDA (I) 2025 GAT Q148: three resistors of 1 ohm each are connected in parallel. The correct answer for the resultant resistance is 1/3 ohm.
Why For resistors in parallel, reciprocals add: 1/R = 1/1 + 1/1 + 1/1 = 3, so R = 1/3 ohm. - Question NDA (I) 2025 GAT Q138: if the length of a copper wire is increased to twice its length, its resistivity will be the same.
Why Resistivity is a fixed property of the material, copper, and does not change with a wire's length or thickness. Only its resistance changes. - Question NDA (I) 2023 GAT Q147: a bulb rated 220 V and 80 W is operated on 110 V. The correct answer for its new power is 20 W.
Why Power varies with the square of voltage at fixed resistance. Halving the voltage to 110 V cuts power to one-quarter: 80 W ÷ 4 = 20 W. - Question NDA-I 2019 Q70: a fuse wire must be conducting and of low melting point.
Why It must conduct electricity to carry normal current, but melt quickly on an overload, so it protects the circuit by breaking it.
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