Power and Energy Transfers Flashcards (24 cards)

What does power measure?

The rate of energy transfer or work done.

What power corresponds to transferring one joule per second?

One watt.

What remains constant in a closed system?

The total energy.

What happens to energy during changes in a closed system?

It cannot be created or destroyed.

Where is energy usually dissipated during system changes?

In the thermal store of the surroundings.

How does energy become less useful during system changes?

It is dissipated and stored in less useful ways.

How is efficiency calculated from energy output and input?

$\text{Efficiency} = \frac{\text{Useful energy output}}{\text{Total energy input}}$

What unit does efficiency have?

Efficiency has no unit.

What is the equation for work done by a force over a distance?

$W = F \times s$

What is the equation for kinetic energy?

$E_k = \frac{1}{2}mv^2$

What is the equation for gravitational potential energy?

$E_p = mgh$

What is the equation relating energy, power, and time?

$E = Pt$

What is the equation for elastic energy stored in a spring?

$E_e = \frac{1}{2}ke^2$

What happens to a tennis ball’s energy stores when it is projected upwards?

Its kinetic store decreases while its gravitational potential store increases.

What happens to a vehicle’s energy stores when it slows down?

Its kinetic store decreases while its thermal store increases.

What happens to a drill’s energy stores when a battery-powered drill is turned on?

Its chemical store decreases while its kinetic and thermal stores increase.

What happens to a person’s energy stores when they slide down a zip-wire?

Their gravitational potential store decreases while their kinetic and thermal stores increase.

How does a higher-power motor lift the same mass compared with a lower-power motor?

In a shorter time.

What is a system?

An object or group of objects.

Which five energy stores are listed?

Kinetic, gravitational potential, chemical, thermal, and elastic potential.

How can lubrication reduce unwanted energy transfers?

By reducing friction.

How can insulation reduce unwanted energy transfers?

By reducing the rate of thermal energy transfer.

How is efficiency calculated from power output and input?

$\text{Efficiency} = \frac{\text{Useful power output}}{\text{Total power input}}$

In what unit are work done and energy measured?

Joules (J).