Topic 8 - Energy Forces Doing Work

Question 1

Describe a closed system

Answer 1

• a system which experiences no external forces
• e.g. a circuit, or skydiver
• In a closed system, the total energy remains the same at all times

Question 2

Describe work done

Answer 2

when an external force transfers energy to a system

• e.g. a human doing work to push a ball up a hill
• Electric– Chemical potential energy (stored in batteries) forms electric
• Heating – Electric transfers to thermal energy, as work is done against the heating coils causing it to heat up

Question 3

Describe energy waste and example (mechanical processes)

Answer 3

• Energy is rarely 100% transferred into the desired form
• Some energy is dissipated so it is stored in less useful ways
• Mechanical processes are wasteful when they get hot (usually due to friction) energy is dissipated as heat, causing a rise in temperature
• So the energy is lost to the surroundings

Question 4

Describe power and equation

Answer 4

• it’s the rate at which energy is transferred

- Power (watt, W) = energy transferred or work done (joule, J)/ time taken (s)

Question 5

Describe efficiency and equation

Answer 5

• Measure of how much energy is not lost as waste

efficiency = useful energy output/ total energy input