⚡ Physics — Electricity and Magnetism
1. Electric Charge and Field
Electric charge is an intrinsic property of protons and electrons. Only two types exist: positive and negative.
KEY PROPERTIES
- Like charges repel, unlike charges attract
- Charge on electron: −1.6 × 10⁻¹⁹ C
- Charge on proton: +1.6 × 10⁻¹⁹ C
- SI unit: Coulomb (C)
COULOMB'S LAW
- F = (1/4πε₀) · q₁q₂/r²
- ε₀ = permittivity of free space
- k = 9 × 10⁹ Nm²/C²
ELECTRIC FIELD
- E = F/q₀ (Force per unit test charge)
- Field lines: Begin at positive charge, end at negative charge
- Two lines never intersect
- E ∝ 1/r²
ELECTRIC POTENTIAL & CAPACITOR
- Potential (V): Work done per unit charge
- V = W/q₀
- Capacitance: C = q/V (Unit: Farad, F)
- 1 μF = 10⁻⁶ F
- Charge resides on outer surface of conductor
- Electric field inside conductor is zero
TYPES OF MATERIALS
- Conductors: Have free electrons (metals)
- Insulators: No free electrons (wood, plastic, rubber)
- Semiconductors: Conduct at higher temperature (silicon, germanium)
2. Electric Current and Resistance
ELECTRIC CURRENT
- I = q/t (Unit: Ampere, A)
- Direct Current (DC): Magnitude and direction constant
- Alternating Current (AC): Magnitude and direction change periodically
- Inverter: Converts DC to AC
RESISTANCE
- R = ρl/A (Resistivity × length / Area)
- SI unit: Ohm (Ω)
- Conductance: G = 1/R (Unit: Siemens or mho)
- Resistivity (ρ): Depends on temperature and material
- For metals: ρt = ρ₀(1 + αt)
OHM'S LAW
- V = IR (Voltage = Current × Resistance)
- Valid when physical conditions (temperature) remain constant
- Not applicable to semiconductors, discharge tubes
COMBINATION OF RESISTANCES
- Series: R = R₁ + R₂ + R₃ (Same current)
- Parallel: 1/R = 1/R₁ + 1/R₂ + 1/R₃ (Same voltage)
3. Electric Cell and Effects
ELECTRIC CELL
- Converts chemical energy to electrical energy
- Primary cell: Cannot be recharged (Voltaic, Daniell, Leclanche)
- Secondary cell: Can be recharged (Acid/alkali accumulators)
- EMF: Work done to move unit charge from one terminal to other
JOULE'S LAW OF HEATING
- H = I²Rt = VIt = V²t/R
- Applications: Electric bulb, kettle, heater
- Fuse: Tin-lead alloy (63% + 37%); high resistance, low melting point
ELECTRICAL POWER
- P = VI = I²R = V²/R
- 1 kWh = 3.6 × 10⁶ J (1 unit of electricity)
ELECTROLYSIS
- Decomposition of electrolyte by electric current
- Faraday's First Law: m = ZIt (mass ∝ charge)
- Faraday's Second Law: m₁/m₂ = E₁/E₂ (mass ∝ chemical equivalent)
- Electroplating: Coating base metal with expensive metal
4. Magnetism
MAGNET BASICS
- Natural magnet: Magnetite (Fe₃O₄) or lodestone
- Artificial magnet: Bar magnet, horseshoe magnet
- Freely suspended magnet aligns in North-South direction
- Like poles repel, unlike poles attract
- Electromagnet: Current-carrying coil with soft iron core
- Permanent magnets: Made of steel
- Temporary magnets: Made of soft iron
MAGNETIC FIELD
- Magnetic flux: φ = B·A·cos θ (Unit: Weber)
- Force on moving charge: F = Bqv·sin θ
- Force on current-carrying conductor: F = BIl·sin θ
- Fleming's Left Hand Rule: For direction of force
EARTH'S MAGNETISM
- Earth behaves like a magnetic dipole
- Magnetic North pole near geographic South
- Magnetic South pole near geographic North
- Magnetic storm: Disturbances caused by charged particles from Sun
MAGNETIC SUBSTANCES
| Type | Examples | Properties |
|---|---|---|
| Paramagnetic | Aluminium, Platinum | Weakly attracted; decreases with temperature |
| Diamagnetic | Gold, Silver, Copper | Weakly repelled; independent of temperature |
| Ferromagnetic | Iron, Nickel, Cobalt | Strongly attracted; loses magnetism at Curie temperature |
CURIE TEMPERATURE
- Temperature at which ferromagnetic becomes paramagnetic
- Iron: 770°C | Nickel: 358°C
- Curie Law: χm ∝ 1/T
5. Electromagnetic Induction
FARADAY'S LAWS
- EMF is induced when magnetic flux changes
- E = −N·dφ/dt
- Lenz's Law: Induced current opposes the cause producing it
MOTIONAL EMF
- E = B·v·l·sin θ
- Fleming's Right Hand Rule: For direction of induced current
EDDY CURRENTS
- Whirlpool-like currents in metal placed in varying magnetic field
- Also called Foucault's currents
- Used in: Dead beat galvanometer, induction furnaces, speedometers
- Used in diathermy for deep heat treatment
SELF & MUTUAL INDUCTION
- Self induction: EMF induced in coil due to change in its own current
- φ = LI (L = coefficient of self induction)
- Mutual induction: EMF induced in coil due to change in current of neighbouring coil
- Unit: Henry (H)
6. Alternating Current and Devices
ALTERNATING CURRENT
- I = I₀·sin ωt | V = V₀·sin ωt
- Frequency in India: 50 Hz
- RMS value: Irms = I₀/√2 = 0.707I₀
- Vrms = V₀/√2 = 0.707V₀
- AC ammeter/voltmeter read RMS values
- Wattless current: Current through pure capacitor/inductor (zero power)
AC GENERATOR (DYNAMO)
- Converts mechanical energy to electrical energy
- Based on electromagnetic induction
- e = NBAω·sin ωt
- Parts: Armature, Field magnets, Slip rings, Brushes
DC MOTOR
- Converts electrical energy to mechanical energy
- Uses commutator (not slip rings)
- τ = NBI A·sin θ
- Efficiency: η = Back EMF / Applied EMF
TRANSFORMER
- Based on mutual induction
- Step-up: Ns > Np, Vs > Vp, Ip > Is
- Step-down: Ns < Np, Vs < Vp, Ip < Is
- Transformation ratio: K = Ns/Np = Vs/Vp = Ip/Is
- Energy losses: Iron loss, Flux loss, Hysteresis loss, Humming loss
Summary: Electricity and Magnetism at a Glance
| Topic | Key Formula | Key Point |
|---|---|---|
| Coulomb's Law | F = kq₁q₂/r² | k = 9 × 10⁹ |
| Ohm's Law | V = IR | Unit: Ohm (Ω) |
| Power | P = VI | 1 kWh = 3.6 × 10⁶ J |
| Faraday's Law | E = −N·dφ/dt | Lenz's Law: opposes change |
| Transformer | K = Ns/Np | Step-up: K > 1 |
| Curie Temperature | χm ∝ 1/T | Iron: 770°C |
| AC RMS | Irms = I₀/√2 | India: 50 Hz |
KEY TAKEAWAYS FOR UPSC
- Charge: Electron = −1.6 × 10⁻¹⁹ C
- Ohm's Law: V = IR
- Series: Current same; Parallel: Voltage same
- Fuse: Tin-lead alloy, low melting point, high resistance
- Transformer: Based on mutual induction
- AC frequency in India: 50 Hz
- Ferromagnetic: Iron, Nickel, Cobalt
- Curie temperature: Iron = 770°C, Nickel = 358°C
High-Yield Interactive UPSC Quiz (Physics — Electricity and Magnetism)
Q1. What is the SI unit of electric charge?
Correct Answer: B. The SI unit of electric charge is Coulomb (C).
Q2. According to Ohm's law, V = IR. What is the SI unit of resistance?
Correct Answer: B. The SI unit of resistance is Ohm (Ω).
Q3. In a series combination of resistors, which quantity remains the same?
Correct Answer: B. In series combination, current is the same through all resistors.
Q4. A transformer works on the principle of:
Correct Answer: B. A transformer works on the principle of mutual induction.
Q5. Which of the following is a ferromagnetic material?
Correct Answer: C. Iron, Nickel, and Cobalt are ferromagnetic materials.
Q6. The frequency of alternating current in India is:
Correct Answer: B. The frequency of alternating current in India is 50 Hz.
Q7. What is the RMS value of AC current?
Correct Answer: B. The RMS value of AC current is I₀/√2 = 0.707I₀.
Q8. Lenz's law is based on which principle?
Correct Answer: B. Lenz's law is based on the principle of conservation of energy.
Q9. What is the Curie temperature of iron?
Correct Answer: B. The Curie temperature of iron is 770°C.
Q10. A step-up transformer has:
Correct Answer: B. In a step-up transformer, Ns > Np (more turns in secondary coil).