PH5. Work, Energy, and Power

What is energy, and what are its forms?

Energy is the ability to do work or cause change, measured in joules

What does The Principle of Conservation of Energy state?

- Energy cannot be created or destroyed, only transferred between forms

- So total amount of energy in a closed system remains constant

What is work done, and how is it calculated?

- Work done is force multiplied by the distance travelled in the direction of the force

- Work Done=Fdcosθ

if θ=90∘, cos90∘=0, so no work is done

How is work represented on a force-displacement graph?

Area under force-displacement graph = work done

What happens to the work done on an object when it is lifted?

- Work is done against gravity = Fd = mgΔh

- This work increases the object’s gravitational potential energy ΔEp​ = mgΔh

What energy change takes place when an object is allowed to fall?

- Ep decreases, Ek increases

- If air resistance is negligible: Loss of Ep=Gain in Ek → mg(h0-h)=½​mv²

What is the effect on the kinetic energy of a car if its speed is doubled?

- Kinetic energy depends on the square of the speed

- So doubling speed (v→2v) increases Ek by four times (4 × ½mv²)

How is kinetic energy derived from work done?

How can you calculate the energy lost due to friction in a fairground ride?

Work done to overcome friction/air resistance (energy lost)=

Initial Ep-Final Ek = mgh−1/2​mv²

What is power?

How could you develop more power?

Increase energy transferred or reduce time taken

Why is a 100 W light bulb brighter than a 40 W light bulb when both operate at the same voltage?

- 100W bulb transfers 100 J/s, while 40W bulb transfers 40 J/s

- So at the same voltage, higher-powered bulbs use more current for a greater energy transfer rate, which increases brightness

How is the output power of an engine calculated when a vehicle moves at constant velocity?

Output power = Fv = Motive/Resistive Forces×Speed

What happens to the output power of an engine when a vehicle gains speed?

- When a vehicle accelerates, engine power overcomes resistive forces (as heat) and increases kinetic energy

- Output power = Energy wasted per sec + Gain of Ek per sec

What is efficiency and how is it calculated for a machine?

What are the causes of inefficiency in power stations?

- Not all energy is used

- Some energy is always wasted as heat or sound due to factors like friction

Why don't we stop energy from being wasted as heat?

Heat dissipation is essential to prevent overheating, ensuring safe operation