Workshop Calculation: Heat, Temperature and Simple Electrical Quantities
Heat, temperature and simple electrical quantities
This domain tests temperature conversion, sensible heat, and basic electrical relationships of voltage, current, resistance, power and energy. Workshop contexts include heating a coolant or estimating the energy used by a heater or motor.
Core rules
- Temperature conversion: °F = °C × 9/5 + 32; K = °C + 273.15. A temperature difference of 1°C equals 1 K, but 1°F difference equals 5/9°C.
- Heat energy Q = mcΔT, where m is mass, c is specific heat capacity and ΔT is temperature change. Water c ≈ 4186 J/(kg·°C).
- Ohm’s law: V = IR. Power: P = VI = I²R = V²/R.
- Electrical energy in kWh = power in kW × time in hours.
Worked example
2 kg of water is heated from 20°C to 70°C. ΔT = 50°C, so Q = 2 × 4186 × 50 = 418,600 J = 418.6 kJ. If a 2 kW heater supplies this energy without losses, time = energy/power = 418.6 kJ/2 kW = 209.3 s ≈ 3.49 min. The reasoning is to use the temperature change, not the final temperature, and to convert seconds to minutes only at the end.
Traps
- Using 70°C instead of ΔT = 50°C in Q = mcΔT.
- Applying the Fahrenheit offset to a temperature difference.
- Confusing kW (power) with kWh (energy).
- Mixing series and parallel resistance rules when a circuit is described.
- Ignoring heat losses in a practical heating estimate.