8. Energy: Forces Doing Work Flashcards
State the law of energy conservation.
Energy cannot be created or destroyed (it can only be transferred into different forms).
Describe the energy changes involved when a ball is thrown upwards and then returns to its starting position. Ignore air resistance.
Upwards: KE is converted to GPE
Peak: maximum GPE, zero KE
Downwards: GPE is converted to KE
State any changes in the total energy of a ball that is kicked, assuming that no external forces act.
The total energy of the system remains constant due to the conservation of energy.
Describe the energy changes that occur in a filament light bulb.
- Electrical energy is transferred into light and heat energy.
- Light is a useful energy form, heat is waste energy.
Describe the energy transfers for a bungee jumper.
- When falling, the GPE is converted to KE of jumper.
- As the cord tightens, KE is converted and stored as elastic
potential energy (EPE). - At lowest point, the jumper’s initial GPE equals the EPE stored in the cord.
Explain why a bungee jumper slows down once the cord begins to stretch.
- Kinetic energy decreases since it is converted to elastic potential energy.
- Since KE is proportional to (velocity)2, as KE decreases, so does velocity.
Describe the energy changes in a power station.
- Heat energy released in the reaction heats water to make steam.
- Steam moves the turbine, converting kinetic energy.
- Kinetic energy turns the generator.
- Electrical energy as the generator generates electricity.
In what ways can the energy of a system be changed?
- Through work done by forces.
- Through electrical input.
- Through heating.
How can energy be changed through work done by forces?
A force which moves an object (kinetic energy) causes work to be done over a distance.
Why do wasteful mechanical processes cause a rise in the temperature of the surroundings?
They cause energy to be transferred by heating the surroundings.
What is the definition of ‘power’?
The rate at which energy is transferred (or rate at which work is done).
What is the unit of power?
Watt, W
What is one Watt equivalent to?
1 joule per second (J/s)
What does a power rating of 10W mean for an electrical device?
It converts 10 J of energy every second.
Two motors lift the same mass through the same height. Motor A does this in half the time of Motor B. Which dissipates the most power?
Motor A
The energy transferred is the same but the time taken is less
