Physics Units Formulas - Quick Revision
Key Points (One-Liners)
- SI base units: kg (mass), m (length), s (time), A (current), K (temp), mol (amount), cd (luminosity).
- Force → Newton (N) = kg·m·s⁻².
- Energy → Joule (J) = N·m = kg·m²·s⁻².
- Power → Watt (W) = J·s⁻¹ = kg·m²·s⁻³.
- Pressure → Pascal (Pa) = N·m⁻² = kg·m⁻¹·s⁻².
- Frequency → Hertz (Hz) = s⁻¹ (per second).
- Charge → Coulomb (C) = A·s.
- Potential → Volt (V) = J·C⁻¹ = kg·m²·s⁻³·A⁻¹.
- Capacitance → Farad (F) = C·V⁻¹ = kg⁻¹·m⁻²·s⁴·A².
- Resistance → Ohm (Ω) = V·A⁻¹ = kg·m²·s⁻³·A⁻².
- Magnetic field → Tesla (T) = N·A⁻¹·m⁻¹ = kg·s⁻²·A⁻¹.
- 1 kWh = 3.6 × 10⁶ J.
- 1 atm = 101325 Pa = 760 mm-Hg.
- 1 hp = 746 W.
- 1 eV = 1.6 × 10⁻¹⁹ J.
| Formula/Rule |
Application |
| F = ma |
Force from mass & acceleration |
| W = F·d·cosθ |
Work done by constant force |
| P = W/t |
Power = work ÷ time |
| KE = ½mv² |
Kinetic energy of moving body |
| PE = mgh |
Gravitational potential energy |
| P = F/A |
Pressure = force ÷ area |
| Q = mcΔT |
Heat gained/lost (no phase change) |
| V = IR |
Ohm’s law for ohmic conductors |
| P = VI |
Electrical power in any device |
| c = fλ |
Wave speed, frequency, wavelength |
Memory Tricks
- “My King Had A Dog Called Max” → metre, kilogram, second, ampere, kelvin, mole, candela (SI base units).
- “VIR” → V = I × R (say “Vee-eye-are” like a word).
- “Work is Force × Distance, think of lifting a WALlet” (W = F·d).
- “Twinkle, Twinkle little star, Power is I × V” (P = I V).
- “Great Cats Catch Mice” → Giga-10⁹, Mega-10⁶, kilo-10³, centi-10⁻², milli-10⁻³.
Common Mistakes
| Mistake |
Correct Approach |
| Confusing weight (N) with mass (kg) |
Weight = mg; always convert kg → N via g = 9.8 m/s² |
| Writing kW h as “kW/h” |
It’s kilowatt-hour (energy), not per hour |
| Using cm or km in formulas without conversion |
Always switch to metres, kg, s before substituting |
| Forgetting cosθ in work/pressure |
Include angle when force & displacement aren’t parallel |
| Mixing kWh with Joules |
1 kWh = 3.6 × 10⁶ J—convert first, then solve |
Last Minute Tips
- Recite the 7 SI base units while entering the hall—free marks.
- Write units beside every numerical answer; no unit = zero.
- Double-check powers of 10: 1 MHz = 10⁶ Hz, not 10³.
- If options differ by 10³ or 10⁶, suspect unit-conversion trap.
- Use dimensional analysis when formula is forgotten—units must balance.
Quick Practice (5 MCQs)
1. The unit of magnetic flux is:
- (A) Tesla (B) Weber (C) Henry (D) Gauss
**Answer:** B (Weber = T·m²)
2. 1 horsepower equals approximately:
- (A) 476 W (B) 646 W (C) 746 W (D) 846 W
**Answer:** C
3. Which pair has same units?
- (A) Work & torque (B) Pressure & energy (C) Power & force (D) Momentum & impulse
**Answer:** A (both N·m or J)
4. The correct unit of spring constant k is:
- (A) N·m (B) N·m⁻¹ (C) kg·m·s⁻¹ (D) J·m
**Answer:** B
5. 1 eV converted to joules is:
- (A) 1.6 × 10⁻¹⁹ J (B) 1.6 × 10⁻²³ J (C) 9.1 × 10⁻³¹ J (D) 6.6 × 10⁻³⁴ J
**Answer:** A
BETA
