Energy

"What is the principle of conservation of energy?

Energy cannot be created or destroyed — only transferred, stored, or dissipated."

"What is a closed system in terms of energy?

A system where there is no net change to the total energy — energy may move between stores, but the total stays the same."

"What does it mean that energy is 'dissipated'?

In every system change, some energy is transferred to thermal stores in the surroundings, ending up stored in less useful ways. This is often called 'wasted' energy."

"Give two ways to reduce unwanted energy transfers.

Lubrication (reduces friction between moving parts) and thermal insulation (reduces conduction through materials)."

"What is thermal conductivity?

A measure of how quickly energy is transferred by conduction through a material. The higher the thermal conductivity, the faster the rate of energy transfer."

"What two factors of a building's walls affect its rate of cooling?

The thickness of the walls and the thermal conductivity of the material. Thicker walls and lower thermal conductivity both slow cooling."

"What is efficiency?

The proportion of total input energy that is usefully transferred. (A measure of how 'wasteful' a device or process is.)"

"What is the equation for efficiency in terms of energy?

Efficiency = useful output energy transfer ÷ total input energy transfer."

"What is the equation for efficiency in terms of power?

Efficiency = useful power output ÷ total power input."

"How can efficiency be expressed?

As a decimal (e.g. 0.6) or as a percentage (e.g. 60%). It can never be greater than 1 (or 100%)."

"What is specific heat capacity?

The amount of energy required to raise the temperature of 1 kg of a substance by 1 °C."

"What is the equation linking energy, mass, specific heat capacity and temperature change?

ΔE = m c Δθ (change in thermal energy = mass × specific heat capacity × temperature change)."

"What three factors does the temperature rise of a heated substance depend on?

The mass of the substance, the type of material (its specific heat capacity), and the energy input to the system."

"Why does water take a long time to heat up compared with most metals?

Water has a very high specific heat capacity, so a large amount of energy is needed to raise its temperature even by a small amount."

"What is latent heat?

The energy needed for a substance to change state, supplied without changing the temperature. (It changes the internal energy by breaking or forming bonds between particles.)"

"What is specific latent heat?

The amount of energy required to change the state of 1 kg of a substance with no change in temperature."

"What is the equation linking energy, mass and specific latent heat?

E = m L (energy for change of state = mass × specific latent heat)."

"What is specific latent heat of fusion?

The energy needed to change 1 kg of a substance from solid to liquid (melting), with no temperature change."

"What is specific latent heat of vaporisation?

The energy needed to change 1 kg of a substance from liquid to vapour (boiling), with no temperature change."

"On a heating curve, what does a flat (plateau) section show?

A change of state is happening. Energy is being supplied but the temperature is not changing — energy is going into latent heat, not increasing particle speed."

"What is the difference between specific heat capacity and specific latent heat?

Specific heat capacity is the energy needed to change temperature (no state change). Specific latent heat is the energy needed to change state (no temperature change)."

"What is a 'system' in physics?

An object or group of objects being considered together."

"What happens to energy when a system changes?

Energy is redistributed between different stores — some stores increase, others decrease — but the total energy is conserved."

"Describe the energy changes when an object is projected upwards.

Kinetic store decreases as the object slows down. Gravitational potential store increases as it rises. (Some energy is dissipated to thermal stores by air resistance.)"

"Describe the energy changes when a vehicle slows down using brakes.

Kinetic store decreases. Thermal store of the brakes (and surroundings) increases due to work done against friction."

"Describe the energy changes when an object is accelerated by a constant force.

Chemical/elastic/other input store decreases. Kinetic store of the object increases (and some energy is dissipated to thermal stores)."

"Describe the energy changes when a kettle boils water.

Chemical/electrical input store decreases. Thermal store of the water increases. (Some energy is dissipated to the surroundings.)"

"What is the equation for kinetic energy?

Eₖ = ½ m v² (kinetic energy = ½ × mass × speed²)."

"What happens to kinetic energy if speed doubles?

Kinetic energy quadruples (×4), because Eₖ depends on v² — speed has a squared effect."

"What is the equation for gravitational potential energy?

Eₚ = m g h (GPE = mass × gravitational field strength × height)."

"For a falling object (ignoring air resistance), how do GPE and KE relate?

GPE lost = KE gained. All the gravitational potential energy is transferred into the kinetic store as the object falls."

"What is work done?

The energy transferred when a force causes an object to move through a distance."

"What is the equation for work done?

W = F s (work done = force × distance moved along the line of action of the force)."

"How are joules and newton-metres related?

1 joule = 1 newton-metre. (1 J of work is done when a force of 1 N moves an object 1 m.)"

"What is the effect of work done against friction on an object?

It causes a rise in the temperature of the object — energy is transferred to the thermal store."

"What is power?

The rate at which energy is transferred, or the rate at which work is done. Measured in watts (W)."

"What is the equation for power in terms of energy?

P = E ÷ t (power = energy transferred ÷ time)."

"What is the equation for power in terms of work?

P = W ÷ t (power = work done ÷ time)."

"What is 1 watt equal to?

1 watt = 1 joule per second (1 W = 1 J/s)."

"Two motors lift the same load to the same height, but motor A is faster. What does this tell us?

Motor A has a higher power, because it transfers the same amount of energy in less time."

"Why is more than one force needed to change the shape of a stationary object?

A single force would just make the object move (or accelerate) rather than deform it. Two or more forces are needed to stretch, compress or bend it."

"What is elastic deformation?

When an object returns to its original shape after the deforming forces are removed."

"What is inelastic deformation?

When an object does not return to its original shape after the deforming forces are removed — it stays permanently deformed."

"What is Hooke's law?

The extension of a spring is directly proportional to the force applied, provided the limit of proportionality is not exceeded."

"What is the equation linking force, spring constant and extension?

F = k e (force = spring constant × extension). Linear case only — must be below the limit of proportionality."

"What is the spring constant?

The force per unit extension of a spring (in N/m). It is a measure of stiffness — a stiffer spring has a higher spring constant."

"What is the limit of proportionality?

The point on a force–extension graph beyond which extension is no longer directly proportional to force. Beyond this, the line stops being straight."

"What does a force–extension graph look like for a spring obeying Hooke's law?

A straight line through the origin, up to the limit of proportionality. Beyond that limit it curves."

"What is elastic potential energy?

The energy stored in a spring (or elastic object) when work is done to stretch or compress it."

"What is the equation for elastic potential energy?

Eₑ = ½ k e² (elastic potential energy = ½ × spring constant × extension²). Assumes limit of proportionality not exceeded."

"What happens to the elastic potential energy stored if the extension doubles?

It quadruples (×4), because Eₑ depends on e² — extension has a squared effect."

"RP14 — what is the aim of the specific heat capacity practical?

To determine the specific heat capacity of one or more materials by measuring the energy supplied and the resulting temperature rise of a known mass."

"In RP14, why is the block insulated?

To reduce energy loss to the surroundings, so that more of the input energy goes into raising the temperature of the block — making the calculated SHC closer to the true value."

"RP18 — what is the aim of the spring practical?

To investigate the relationship between force and extension for a spring."

"In RP18, why measure extension rather than total length?

Extension is the change from the original (unstretched) length. Using extension means the data starts at zero and directly tests the proportional relationship in F = k e."