THERMAL AND MACHINAL ENGERY TEST

What is energy?

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

State the law of conservation of energy.

Energy cannot be created or destroyed. It can only be transferred from one form to another.

What is the energy conservation relationship?

Total energy before = total energy after.

What should you say instead of saying energy is lost?

Energy is transferred to the surroundings, usually as thermal energy and sound.

What is work done?

Work is done when a force causes an object to move in the direction of the force.

What is the work done formula?

W = Fd, where W is work in J, F is force in N, and d is distance in m.

When is no work done?

When the object does not move or does not move in the direction of the force.

A 40 N force moves an object 6 m. Find the work done.

W = Fd = 40 × 6 = 240 J.

What is the difference between mass and weight?

Mass is the amount of matter in kg. Weight is the gravitational force acting on an object in N.

What is the weight formula?

Fg = mg. Usually use g = 10 N/kg unless another value is given.

What is gravitational potential energy?

Stored energy an object has because of its height.

What is the GPE formula?

GPE = mgh, where m is mass in kg, g is gravitational field strength, and h is vertical height in m.

What increases gravitational potential energy?

A greater mass, greater vertical height, or greater gravitational field strength.

Which height is used in a GPE calculation?

The vertical height, not the length of a ramp or slope.

A 5 kg bag is lifted 4 m. Find its GPE gain.

GPE = mgh = 5 × 10 × 4 = 200 J.

What is kinetic energy?

The energy an object has because it is moving.

What is the kinetic energy formula?

KE = ½mv², where m is mass in kg and v is speed in m/s.

What happens to KE if speed doubles or triples?

If speed doubles, KE becomes four times greater. If speed triples, KE becomes nine times greater.

A 4 kg object moves at 5 m/s. Find its KE.

KE = ½mv² = ½ × 4 × 5² = 50 J.

What happens to energy as an object falls?

GPE decreases and is transferred into KE, so the object speeds up.

What happens at the highest point?

GPE is maximum and KE is minimum.

What happens at the lowest point?

GPE is minimum and KE is maximum.

What is the energy relationship when there is no friction?

Loss in GPE = gain in KE.

What happens when friction or air resistance is present?

Some mechanical energy is transferred into thermal energy and sound.

What is power?

Power is the rate at which work is done or energy is transferred.

What is the power formula?

P = W ÷ t or P = E ÷ t.

What is the unit of power?

The watt, W. One watt means one joule transferred every second.

A machine does 900 J of work in 3 seconds. Find its power.

P = W ÷ t = 900 ÷ 3 = 300 W.

Who produces more power when two people do the same work?

The person who finishes in less time because they transfer the same energy faster.

What is efficiency?

Efficiency measures how much of the input energy becomes useful output energy.

What is the efficiency formula?

Efficiency = useful energy output ÷ total energy input × 100%.

Why are machines not 100% efficient?

Some input energy is transferred to the surroundings as unwanted thermal energy and sound.

A machine receives 800 J and gives 600 J useful output. Find its efficiency.

Efficiency = 600 ÷ 800 × 100 = 75%.

How do you calculate wasted energy?

Wasted energy = total input energy − useful output energy.

What steps should you show in calculations?

Write the formula, substitute the values, calculate, and include the correct unit.

What are common mechanical energy mistakes?

Forgetting the ½ or v² in KE, using slope length instead of vertical height, and forgetting ×100% for efficiency.

What are all materials made from?

Particles that are always moving.

Describe particles in a solid.

They are closely packed, vibrate in fixed positions, and have strong attractive forces.

Describe particles in a liquid.

They are close together but can move and slide past each other.

Describe particles in a gas.

They are far apart and move quickly in random directions.

What happens to particles when heated or cooled?

Heating gives particles more kinetic energy, so they move faster. Cooling makes them move more slowly.

What does temperature measure?

The average kinetic energy of the particles.

What is thermal energy?

The total energy of all the particles in an object.

What does thermal energy depend on?

The mass, temperature, and type of material.

Which direction does thermal energy transfer?

From a hotter object to a colder object.

What is thermal equilibrium?

When objects reach the same temperature and there is no overall heat transfer.

What is conduction?

The transfer of thermal energy through particle collisions and vibrations, mainly in solids.

How does conduction work?

Heated particles vibrate faster and collide with neighbouring particles, transferring energy through the material.

Why are metals good conductors?

They contain free electrons that move through the metal and transfer energy quickly.

What are thermal insulators?

Materials that transfer energy slowly, such as air, wool, plastic, wood, and foam.

What is convection?

The transfer of thermal energy by the movement of liquids or gases.

Why can convection not happen in solids?

Solid particles cannot move from place to place.

How does a convection current form?

A fluid is heated, expands, becomes less dense and rises. Cooler, denser fluid sinks to replace it.

Why does warm fluid rise and cool fluid sink?

Warm fluid expands and becomes less dense. Cool fluid is more dense, so it sinks.

What is radiation?

The transfer of thermal energy by infrared electromagnetic waves.

Does radiation need particles?

No. Radiation can travel through a vacuum.

How does energy from the Sun reach Earth?

By radiation through space.

What are dark, dull surfaces like?

They are good absorbers and good emitters of infrared radiation.

What are light, shiny surfaces like?

They are poor absorbers, poor emitters, and good reflectors of infrared radiation.

Compare conduction, convection, and radiation.

Conduction uses particle collisions, convection uses moving fluids, and radiation uses infrared waves.

How does insulation reduce heat loss?

It traps air, which is a poor conductor, and prevents convection currents.

How does double glazing reduce heat loss?

The gas between the panes is a poor conductor, and the narrow gap prevents convection currents.

How does shiny foil reduce heat loss?

It reflects infrared radiation back into the room.

How do draught excluders reduce heat loss?

They prevent warm air escaping and cold air entering, reducing convection.

What is melting?

The change from solid to liquid.

What is freezing?

The change from liquid to solid.

What is boiling?

The change from liquid to gas throughout the liquid at the boiling point.

What is evaporation?

The change from liquid to gas at the surface and at any temperature.

What is condensation?

The change from gas to liquid.

What is sublimation?

The change directly from solid to gas.

Which state changes absorb energy?

Melting, boiling, evaporation, and sublimation.

Which state changes release energy?

Freezing and condensation.

Why does temperature stay constant during melting or boiling?

The energy is used to overcome attractive forces between particles instead of increasing their kinetic energy.

What is the difference between evaporation and boiling?

Evaporation happens at the surface at any temperature. Boiling happens throughout the liquid at its boiling point.

Why does evaporation cause cooling?

The fastest particles escape, leaving particles with a lower average kinetic energy.

What is specific heat capacity?

The energy needed to raise the temperature of 1 kg of a substance by 1°C.

What is the specific heat capacity formula?

Q = mcΔT.

What do the symbols in Q = mcΔT mean?

Q is energy in J, m is mass in kg, c is specific heat capacity, and ΔT is temperature change.

How is temperature change calculated?

ΔT = final temperature − initial temperature.

What does a high specific heat capacity mean?

The substance needs lots of energy to change temperature, so it heats and cools slowly.

Calculate Q for 2 kg of water heated by 5°C, where c = 4200 J/kg°C.

Q = mcΔT = 2 × 4200 × 5 = 42,000 J.

What is specific latent heat?

The energy needed to change the state of 1 kg of a substance without changing its temperature.

What is the latent heat formula?

Q = mL.

What are latent heat of fusion and vaporisation?

Fusion is for solid and liquid changes. Vaporisation is for liquid and gas changes.

When do you use Q = mcΔT and Q = mL?

Use Q = mcΔT when temperature changes. Use Q = mL when state changes at constant temperature.

What happens on sloping and flat parts of a heating graph?

Sloping means temperature changes and use Q = mcΔT. Flat means state changes and use Q = mL.

What is the energy, power, and time formula?

E = Pt.

A 500 W heater runs for 20 seconds. Find the energy transferred.

E = Pt = 500 × 20 = 10,000 J.

What unit conversions must you know?

g to kg divide by 1000; cm to m divide by 100; minutes to seconds multiply by 60; kJ to J multiply by 1000; kW to W multiply by 1000.

What are common thermal energy mistakes?

Saying heat rises instead of warm fluid rises, using the wrong formula, forgetting unit conversions, and leaving off units.