Flashcards · 7408 · Topic 11

Engineering physics flashcards

15 cards on Topic 11, subtopics 3.11.1 to 3.11.2. Pick a subtopic or study them all.

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Moment of inertia: equation and what it depends on: I = Σmr² (kg m²). It depends on the mass, how far the mass is from the axis, and where the axis is. Mass at the rim counts most.
Rotational kinetic energy, torque and angular momentum: Ek = ½Iω². T = Iα (T is the resultant torque). L = Iω.
A wheel turns at 300 rpm. What is ω?: 300 rpm = 5.0 rev s⁻¹. ω = 2π × 5.0 = 31.4 rad s⁻¹.
Flywheels: what increases the energy stored, and three uses: Energy = ½Iω², so a large I (heavy rim) and high ω (limited by the material's strength). Uses: smoothing torque and speed in engines, storing braking energy in vehicles, releasing energy quickly in presses.
Conservation of angular momentum: a spinning body pulls its mass inwards: With no external resultant torque, Iω is constant. I falls, so ω rises. Kinetic energy rises because work is done pulling the mass in.
Work and power for a torque: W = Tθ. P = Tω. At constant speed, driving torque = frictional torque.
Angular velocity–time graph: gradient and area: Gradient = angular acceleration α. Area = angular displacement θ.

First law of thermodynamics (AQA form) and the meaning of each term: Q = ΔU + W. Q: energy supplied to the gas by heating. ΔU: increase in internal energy. W: work done by the gas.
Isothermal vs adiabatic change: Isothermal: T constant, pV = constant, ΔU = 0, so Q = W. Adiabatic: no heat flow, pV^γ = constant, Q = 0, so W = −ΔU. Adiabatic compression heats the gas.
Work done in a p–V cycle: The area enclosed by the loop. ΔU = 0 over a complete cycle.
Petrol vs diesel engine: Petrol: fuel–air mixture compressed and ignited by a spark; theoretical heat input at constant volume. Diesel: air only compressed; fuel ignites in the hot air; theoretical heat input at constant pressure.
Indicated power, brake power and friction power: Indicated power = loop area × cycles per second × number of cylinders. Brake power = Tω at the output shaft. Friction power = indicated − brake.
Overall, thermal and mechanical efficiency: Overall = brake ÷ input power. Thermal = indicated ÷ input. Mechanical = brake ÷ indicated. Overall = thermal × mechanical.
Maximum efficiency of a heat engine with T_H = 800 K, T_C = 300 K: (T_H − T_C)/T_H = 500 ÷ 800 = 0.625 (62.5%). Temperatures must be in kelvin.
Coefficient of performance of a refrigerator and a heat pump: Refrigerator: COP = Q_C/W, maximum T_C/(T_H − T_C). Heat pump: COP = Q_H/W, maximum T_H/(T_H − T_C). COP can be greater than 1.