Electricity, Magnetism and Measurement

Electric current is the flow of charge; its SI unit is the Ampere (A). Ohm's Law states V = IR, where V is voltage (Volt), I is current (Ampere), and R is resistance (Ohm). Memory trick: cover the quantity you need in the V-I-R triangle. Resistances in series add up: R = R1 + R2 + ... For parallel: 1/R = 1/R1 + 1/R2 + ... Electric power P = VI = I squared R = V squared/R. SI unit of power is Watt. A good conductor (copper, silver) has low resistance; an insulator (rubber, glass) has high resistance. Fuse wire has low melting point and protects circuits from overload.

A magnet has two poles: North and South. Like poles repel, unlike poles attract. A freely suspended magnet always points North-South (used in compass). The Earth itself behaves like a giant magnet. An electric current produces a magnetic field around it (Oersted's discovery); this is the principle of the electromagnet and electric motor. A coil carrying current behaves like a bar magnet (solenoid). Magnetic substances: iron, cobalt, nickel. Memory aid for magnetic metals: 'I Co Ni' (Iron, Cobalt, Nickel). The unit of magnetic flux density is Tesla. An electric generator (dynamo) converts mechanical energy to electrical energy using electromagnetic induction.

Example 1: A 6 V battery drives a current of 2 A through a resistor. Resistance R = V/I = 6/2 = 3 Ohm. Example 2: Two resistors of 3 Ohm and 6 Ohm in series: total R = 3 + 6 = 9 Ohm. In parallel: 1/R = 1/3 + 1/6 = 3/6, so R = 2 Ohm (parallel resistance is always less than the smallest resistor). Example 3: Power of a device drawing 5 A at 220 V: P = VI = 220 x 5 = 1100 W. Tip: In series, current is the same; in parallel, voltage is the same. Always check whether the question asks for series or parallel.

Cover the answer, recall, then check. 13 cards on electricity, magnetism and measurement for RPF Constable.

Q1. State Ohm's law and its formula.
A1. At constant temperature, current through a conductor is directly proportional to the potential difference across it. V = IR.

Q2. Give the SI units of current, charge, potential difference and resistance.
A2. Current — ampere (A); charge — coulomb (C); potential difference — volt (V); resistance — ohm (Ω).

Q3. How is electric current related to charge?
A3. Current = charge / time (I = Q/t). One ampere = one coulomb per second.

Q4. Write the formulas for electric power.
A4. P = VI = I²R = V²/R. SI unit of power is the watt (W).

Q5. What is a fuse and how does it protect a circuit?
A5. A safety device with a low-melting-point wire. When current exceeds a safe value, the wire melts and breaks the circuit, preventing fire.

Q6. Who discovered that a current-carrying wire produces a magnetic field?
A6. Hans Christian Oersted. A magnetic field forms around any conductor carrying current.

Q7. What does an electric motor do, and what does a generator do?
A7. A motor converts electrical energy into mechanical energy. A generator (dynamo) converts mechanical energy into electrical energy.

Q8. What is electromagnetic induction, and who discovered it?
A8. The production of current in a coil by a changing magnetic field. Michael Faraday discovered it; it is the basis of generators.

Q9. How does resistance combine in series and in parallel?
A9. Series: R = R₁ + R₂ + R₃ (adds up). Parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃ (total resistance decreases).

Q10. Give one example each of a good conductor and a good insulator.
A10. Conductor: copper, silver, aluminium. Insulator: rubber, glass, dry wood, plastic.

Q11. Name the seven SI base units.
A11. Metre (length), kilogram (mass), second (time), ampere (current), kelvin (temperature), mole (amount of substance), candela (luminous intensity).

Q12. Why is the earth wire used in domestic wiring?
A12. It is a safety connection to the ground that carries leakage current away, protecting the user from electric shock if the appliance's metal body becomes live.

Electricity and magnetism is among the highest-yield physics topics for the RPF Constable CBT. Within the 50-question General Awareness section, questions on Ohm's law, electrical units, the fuse, motors and generators, and the SI base units are near-certain. They are recall-based, so memorisation converts directly into marks.

Must-know facts and laws

• Ohm's law: V = IR (at constant temperature).
• Electrical units: current — ampere (A); charge — coulomb (C); potential difference — volt (V); resistance — ohm (Ω); power — watt (W).
• Current I = Q/t (charge per second); power P = VI = I²R = V²/R.
• Fuse: a low-melting-point safety wire that melts and breaks the circuit on excess current.
• Oersted: a current produces a magnetic field. Faraday: electromagnetic induction (basis of the generator).
• Motor: electrical → mechanical. Generator/dynamo: mechanical → electrical.
• Household appliances are wired in parallel (full voltage, independent switching).

Devices and their energy conversion

Device	Converts	Into
Electric motor	electrical	mechanical
Generator / dynamo	mechanical	electrical
Electric bulb	electrical	light + heat

Exam Tricks & Tips

• 🎯 "VIR" = V is I times R — Ohm's law in three letters; rearrange to I = V/R or R = V/I as needed.
• 🎯 Motor: electric→motion; Generator: motion→electric — they are exact opposites (a common swap trap).
• 🎯 Seven SI base units: metre, kilogram, second, ampere, kelvin, mole, candela (length, mass, time, current, temperature, amount, luminous intensity).
• 🎯 Fuse melts, saves the circuit — it works on the heating effect of current (P = I²R).
• 🎯 Home wiring is parallel, not series, so one appliance failing doesn't switch off the rest.
• ❌ Common mistake: confusing the unit of current (ampere) with charge (coulomb). Current = coulomb per second; I = Q/t.

Expected exam pattern

Expect: "Ohm's law / its formula," "SI unit of current/resistance/charge," "device that converts mechanical to electrical energy," "safety device that melts on overload," or "name an SI base unit." Simple V = IR numericals appear.

Quick recap

V = IR; units — ampere, volt, ohm, coulomb, watt. Power = VI = I²R = V²/R. Fuse protects via the heating effect. Motor: electric→mechanical; generator: mechanical→electric (Faraday). SI base units: metre, kg, second, ampere, kelvin, mole, candela.