Unit 28
Machines at work.
Energy, pressure and forces at work in an engine, a fridge, a hydraulic lift, a wing, a turbine and a bridge.
Before this unit
The unit, lesson by lesson
Lessons
6 lessons · 44 practice questions · about 3 hours
Engines
A petrol engine is a heat engine: four strokes every two turns, an efficiency set by the compression ratio, heat that must leave in the exhaust, and power that grows with speed.
Refrigerators and heat pumps
A fridge does not make cold, it moves heat from the box to the room. How much heat it moves for each joule of electricity, why a freezer costs more, and how the same loop heats a house.
Hydraulics
A hydraulic lift raises a car with a hand's push: the pressure is the same throughout the oil, the force grows with the area, and the distance shrinks by the same ratio, so the work is conserved.
Flight
A wing lifts a plane because it turns air downward. How much lift each square metre gives, why it goes with the square of the speed, and what happens at too steep an angle.
Wind turbines
A wind turbine takes part of the kinetic energy of the air passing through it. The power grows with the cube of the wind speed and the square of the diameter, and it has a limit.
Structures and bridges
A bridge hands the lorry's weight on to the banks. Where it is squeezed and where stretched, how hard the supports push, and why an arch pushes the banks out while cables pull them in.