Electromagnetism
Electrostatics, Coulomb's Law, circuits, Ohm's Law, magnetism and electromagnetic induction.
A. Electrostatics
Electric Charge & Coulomb's Law
- Electric charge: positive (+) and negative (−)
- Like charges repel; unlike charges attract
- Charge measured in Coulombs (C)
- Charge of electron = −1.6 × 10⁻¹⁹ C
Coulomb's Law
F = k × q₁ × q₂ / r²
k = 9 × 10⁹ N·m²/C² (Coulomb's constant) · q = charges · r = distance
Electric Field
E = F / q = kQ / r²
Unit: N/C or V/m · Field lines point from + to −
Electric Potential
V = W / q
Unit: Volt (V) = Joule per Coulomb
B. Capacitors
Capacitance & Combinations
Capacitance
C = Q / V
Unit: Farad (F). Charge stored per unit voltage.
| Configuration | Formula | Result |
|---|---|---|
| Series | 1/C_total = 1/C₁ + 1/C₂ + … | Total capacitance DECREASES |
| Parallel | C_total = C₁ + C₂ + … | Total capacitance INCREASES |
C. Current, Voltage & Resistance
Electrical Quantities
| Quantity | Symbol | Unit | Formula |
|---|---|---|---|
| Electric Current | I | Ampere (A) | I = Q/t |
| Voltage (PD) | V | Volt (V) | V = W/Q |
| Resistance | R | Ohm (Ω) | R = V/I (Ohm's Law) |
| Power | P | Watt (W) | P = IV = I²R = V²/R |
| Energy | E | Joule (J) | E = Pt = VIt |
Ohm's Law
V = I × R
Voltage = Current × Resistance
Electric Power
P = IV = I²R = V²/R
Series vs Parallel Circuits
| Property | Series Circuit | Parallel Circuit |
|---|---|---|
| Current | Same through all | Different in each branch |
| Voltage | Divided across components | Same across all |
| Total Resistance | R = R₁ + R₂ + … | 1/R = 1/R₁ + 1/R₂ + … |
| One fails? | All stop (breaks circuit) | Others continue working |
Series Resistance
R_total = R₁ + R₂ + …
Parallel Resistance
1/R_total = 1/R₁ + 1/R₂ + …
D. Magnetism
Magnetic Fields & Induction
- Magnets have two poles: North (N) and South (S)
- Like poles repel; unlike poles attract
- Magnetic field lines go from N to S outside the magnet
- Earth has a magnetic field — compass needle points to magnetic north
- Electromagnet: coil of wire carrying current produces a magnetic field
- Increasing current or turns increases electromagnet strength
Faraday's Law
EMF ∝ −ΔΦ/Δt
Changing magnetic flux induces an EMF (electromagnetic induction)
⚡ MCQ Tip
Ohm's Law: V = IR. Series: R adds, current same. Parallel: V same, 1/R adds.
Coulomb's Law: F = kq₁q₂/r². Like charges repel, unlike attract. Faraday = induction. Lenz's Law = opposes change.
Live Animation: Magnetic Field Around a Wire
Current-Carrying Wire — Magnetic Field Lines
Right-hand rule: curl fingers show field direction around a wire
Current (I): 5 A
Field rings: 5
Direction: Upward ↑
Quick MCQ Revision
| Formula / Fact | Meaning |
|---|---|
| V = IR | Ohm's Law — Voltage = Current × Resistance |
| P = IV = I²R = V²/R | Electric Power formulas |
| F = kq₁q₂/r² | Coulomb's Law (k = 9×10⁹ N·m²/C²) |
| C = Q/V | Capacitance = Charge ÷ Voltage |
| Series R | R_total = R₁ + R₂ (current same) |
| Parallel R | 1/R_total = 1/R₁ + 1/R₂ (voltage same) |
| I = Q/t | Current = charge per second |
| Like charges | Repel each other |
| Unlike charges | Attract each other |
| Faraday's Law | Changing magnetic flux → induces EMF |
| Lenz's Law | Induced current opposes the change that caused it |