physics - topic 7: energy

What is the area under a force-distance graph?

Work done

What is the area under a force-extension graph?

Gain in EPE

What is the gradient of a force-extension graph?

Spring constant

How does GPE and KE change when jumping?

• Before Jump: Maximum GPE, Zero KE.

• During Fall (Freefall): GPE decreases, KE increases.

• Landing: GPE is minimal, KE is maximum

What is mechanical energy?

Energy stored in an object due to motion/position e.g. GPE, KE, EPE.

What is the relationship between KE and GPE in a closed system?

The sum of them at all points is equal.

What are the energy stores in a kettle?

Electrical energy in the mains → electrical energy in heating element → thermal energy of the water + surroundings

Define heat.

A change in thermal energy.

Define temperature.

The measure of the average KE of all the particles in a substance.

Why is temperature constant during a change in state?

Because the KE doesn’t change (energy is used breaking up the intermolecular bonds).

When energy is transferred, some of the energy is __ .

Dissipated

How can we reduce energy loss?

• Lubrication

• Air resistance (more streamlined)

• Insulation

What causes a faster/slower transfer of energy?

Faster Transfer: Conductor, high thermal conductivity, thinner layers, larger temperature difference outside & inside

Slower Transfer: Insulator, low thermal conductivity, thicker layers, smaller temperature difference outside & inside

How does a cavity wall work?

• An air pocket reduces conduction

• This causes a lower thermal conductivity and less heat loss

How does double glazing work?

• An air vacuum (argon gas) is between two panes of glass

• Too thin for convection currents

Define power.

The rate of energy transferred or work done per second.

Define electrical power.

The rate of energy transferred or work done by an electrical component per second.

What is the unit for energy when calculating electricity bills?

Kilowatt-hours (kwh)

What is a sankey diagram? What part is important?

A diagram to represent how energy is transferred in an appliance.

Length is irrelevant: only the width.

Define efficiency.

The ratio of useful output energy to total input energy.

What is the minimum percentage where something is considered efficient?

50%