NCERT Science: Electricity and Magnetism

The one-line takeaway

Electric current is the flow of charge, driven by potential difference (voltage) and opposed by resistance, related by Ohm's law. A current produces heat, light and a magnetic field; a changing magnetic field in turn produces a current (electromagnetic induction). These two facts power the motor, the generator, the transformer and almost all modern technology.

The basic quantities

• Electric current is the rate of flow of electric charge; SI unit the ampere. Conventional current flows from the positive to the negative terminal (opposite to the actual electron flow).
• Electric charge is measured in coulomb; one ampere is one coulomb per second.
• Potential difference (voltage) is the work done to move a unit charge between two points; SI unit the volt. A cell or battery maintains the potential difference that drives current.
• Resistance is the opposition to current flow; SI unit the ohm. It increases with the length of a conductor and decreases with thicker cross-section, and depends on the material and temperature. Good conductors (copper, silver) have low resistance; insulators (rubber, glass) have very high resistance.

Ohm's law

• Ohm's law: at constant temperature, the current through a conductor is directly proportional to the potential difference across it. In symbols, voltage equals current times resistance (V = IR).

Circuits

• A series circuit has components in a single path; the same current flows through all, and the total resistance is the sum. If one component fails, the whole circuit breaks (old fairy lights).
• A parallel circuit has components on separate branches; the voltage across each is the same, and the total resistance is less than the smallest. Household wiring is in parallel, so each appliance gets full voltage and can be switched independently.

The heating effect of current

• A current passing through a resistor produces heat (Joule heating); the heat is proportional to the square of the current, the resistance and the time.
• Uses: the filament of an incandescent bulb, the electric heater, iron, toaster and geyser. The electric fuse and MCB are safety devices that melt or trip and break the circuit when the current is dangerously high (a short circuit or overload).
• Electric power equals voltage times current (P = VI), measured in watt; energy used equals power times time (the kilowatt-hour on the bill).
• Safety: the three-pin plug has live, neutral and earth wires; earthing safely carries leakage current to the ground and prevents shocks.

Magnetism and the magnetic effect of current

• A magnet has two poles (north and south); like poles repel, unlike poles attract. The Earth itself behaves like a giant magnet, which is why a compass needle points north-south.
• Oersted's discovery: a current-carrying conductor produces a magnetic field around it. A coil carrying current (a solenoid) behaves like a bar magnet; an electromagnet (a solenoid with an iron core) can be switched on and off and is used in cranes, bells, relays and loudspeakers.
• A current-carrying conductor in a magnetic field experiences a force (the principle of the electric motor). Fleming's left-hand rule gives the direction of this force.

Electromagnetic induction

• Faraday's electromagnetic induction: a changing magnetic field through a coil induces a current in it. Fleming's right-hand rule gives the direction of the induced current.
• This is the principle of the electric generator (dynamo), which converts mechanical energy into electrical energy, the basis of all large-scale power generation, and of the transformer, which steps voltage up or down for transmission and distribution.

Motor and generator

• The electric motor converts electrical energy into mechanical energy (current in a magnetic field experiences a force). Used in fans, pumps, mixers and electric vehicles.
• The electric generator does the reverse, converting mechanical energy into electrical energy by induction. A motor and a generator are essentially the same machine run in opposite directions.

AC versus DC

• Direct current (DC) flows in one direction (a cell or battery). Alternating current (AC) reverses direction periodically; in India the domestic supply is AC at 220 to 240 volt and 50 hertz. AC is used for transmission because transformers can change its voltage efficiently, reducing transmission losses.

Key terms to fix

• Ampere, volt, ohm: current, potential difference, resistance.
• Ohm's law: V = IR.
• Series versus parallel: single path versus branches; household wiring is parallel.
• Heating effect: the basis of the bulb, heater and fuse; power P = VI.
• Induction (Faraday): a changing magnetic field induces current, the basis of the generator and transformer.

CAPF angle

Reliable power and communications are operational necessities in the field, and protecting the power grid and substations is part of critical-infrastructure security (a CISF mandate). The magnetic effect underlies electromagnetic devices in detection and communication. Electrical-safety knowledge (earthing, fuses, short circuits) matters in barracks and field installations. The physics here also feeds the wider technology base for radar, communications and electronic warfare.

Authored practice

1. Household electrical appliances are connected in parallel rather than in series mainly because: (a) it saves wire (b) each appliance gets the full supply voltage and can be operated independently (c) it increases total resistance (d) it prevents heating. (Answer: b.) Authored practice, not a verbatim PYQ.
2. The electric generator is based on which principle? (Answer: electromagnetic induction, a changing magnetic field induces a current.) Authored practice, not a verbatim PYQ.