Electricity and Magnetism

Overview

Electricity and Magnetism forms a core physics unit in the Assam TET Paper II Mathematics and Science section. This topic connects directly to everyday life—from household wiring to electric motors—making it both practically relevant and a frequent source of exam questions. Understanding the relationship between electric current and magnetic effects is essential, as questions often test conceptual clarity rather than mere formula recall.

For Assam TET, you must master the basics of electric circuits (current, voltage, resistance, Ohm's law), series and parallel connections, heating effects, and how current produces magnetic fields. The magnetic effects of current—electromagnets, electric motors, and electromagnetic induction—are high-yield areas. Expect questions that require applying Ohm's law, calculating equivalent resistance, or explaining the working of devices like electric bells and motors.

Key Concepts

• **Electric current** is the rate of flow of electric charge through a conductor. Measured in amperes (A). Current flows from positive to negative terminal in conventional direction, though electrons actually flow opposite.

• **Potential difference (voltage)** is the work done per unit charge to move charge between two points. Measured in volts (V). It acts as the "push" that drives current through a circuit.

• **Resistance** opposes the flow of current. Measured in ohms (Ω). Depends on material, length (directly proportional), cross-sectional area (inversely proportional), and temperature.

• **Ohm's Law** states that current through a conductor is directly proportional to potential difference across it, provided temperature remains constant: V = IR.

• **Series circuit**: Components connected end-to-end; same current flows through all. Total resistance = R₁ + R₂ + R₃...

• **Parallel circuit**: Components connected across same two points; same voltage across all. Total resistance: 1/R = 1/R₁ + 1/R₂ + 1/R₃...

• **Heating effect of current**: When current flows through a resistor, electrical energy converts to heat. This principle operates in electric heaters, bulbs, and fuses.

• **Magnetic effect of current**: A current-carrying conductor produces a magnetic field around it. This is the basis of electromagnets, electric motors, and generators.

Formulas / Key Facts

| Formula/Fact | Context | |--------------|---------| | V = IR | Ohm's Law: Voltage = Current × Resistance | | P = VI = I²R = V²/R | Electric power in watts | | H = I²Rt | Heat produced (Joule's law of heating) | | E = Pt = VIt | Electrical energy consumed | | R(series) = R₁ + R₂ + R₃ | Resistances in series add up | | 1/R(parallel) = 1/R₁ + 1/R₂ | Reciprocals add for parallel resistances | | 1 kWh = 3.6 × 10⁶ J | One unit of electricity | | Right-hand thumb rule | Thumb points to current direction; curled fingers show magnetic field direction around wire | | Fleming's left-hand rule | For motor: First finger (Field), Second finger (Current), Thumb (Motion/Force) | | Fleming's right-hand rule | For generator: First finger (Field), Thumb (Motion), Second finger (Induced current) |