What is the primary difference between a closed system and an isolated system?

A closed system can exchange energy (but not matter) with its surroundings, whereas an isolated system cannot exchange either energy or matter.

How does an open system differ from a closed system regarding matter?

An open system allows matter to cross its boundary to or from the surroundings, while a closed system prevents any matter exchange.

What is the difference between useful energy and dissipated energy?

Useful energy is transferred to the intended store for a specific purpose, while dissipated energy is spread out into non-useful stores, typically as heat in the surroundings.

What common error occurs when calculating energy using time in minutes?

Standard energy formulas (like $E = Pt$) require time in seconds. Using minutes will result in an answer that is 60 times too small.

Why is it incorrect to say energy is 'lost' in a system?

According to the conservation of energy, energy cannot be destroyed; it is only transferred to less useful stores, such as the thermal store of the air.

What mistake is often made when using the specific heat capacity formula with mass in grams?

The formula $\Delta E = mc\Delta\theta$ requires mass in kilograms (kg). Using grams will result in an energy value 1000 times larger than the true value.

Define 'Specific Heat Capacity'.

It is the amount of energy required to raise the temperature of one kilogram of a substance by one degree Celsius.

What is the formula for electrical work done in terms of charge and potential difference?

The formula is $E = Q \times V$, where $E$ is energy in Joules, $Q$ is charge in Coulombs, and $V$ is potential difference in Volts.

How many Joules are in one kilowatt-hour (kWh)?

There are $3.6 \times 10^6$ Joules in $1 \text{ kWh}$ ($1000 \text{ W} \times 3600 \text{ s}$).

What does 'Work Done' represent in physics?

Work done is a measure of the energy transferred when a force acts on an object to cause displacement.

Changes in Energy | OCR GCSE Physics

1. Definition & Core Concepts

Thermodynamic Systems: A system is a defined object or group of objects. Its behavior is categorized by how it interacts with its surroundings regarding matter and energy.
Open Systems: These allow both energy and matter to be exchanged with the surroundings. For example, a boiling pot without a lid loses energy through heat and matter through steam.
Closed Systems: These can exchange energy with the surroundings but not matter. The total amount of energy in a closed system remains constant, though it may change form.
Isolated Systems: These do not allow the transfer of either matter or energy to or from the surroundings, representing a perfectly contained environment.

Diagram showing Open, Closed, and Isolated systems with arrows representing energy and matter flow.

2. Underlying Principles

Conservation of Energy: Energy cannot be created or destroyed; it can only be transferred from one store to another. In a closed system, the sum of all energy transfers must equal zero.
Energy Dissipation: During any energy transfer, some energy is typically spread out (dissipated) to the surroundings, usually as thermal energy. This is often referred to as 'wasted' energy because it is no longer in a useful store.
Energy Stores: Common stores include kinetic (movement), thermal (temperature), gravitational potential (height), and chemical (bonds). Changes in energy involve moving energy between these specific stores.

3. Mechanical and Electrical Work

Mechanical Work Done: Work is done when a force moves an object through a distance. It is a measure of energy transfer via mechanical means.

Formula: $W = F \times s$

$W$ : Work done in Joules (J)
$F$ : Force applied in Newtons (N)
$s$ : Distance moved in the direction of the force in meters (m)
Electrical Work Done: This occurs when a power supply provides a potential difference to move charge through a circuit. The energy transferred depends on the current, voltage, and time.

Formula: $E = I \times V \times t$

$E$ : Energy transferred in Joules (J)
$I$ : Current in Amperes (A)
$V$ : Potential Difference in Volts (V)
$t$ : Time in seconds (s)

4. Thermal Energy Changes

Specific Heat Capacity: This property defines how much energy is required to raise the temperature of 1 kg of a substance by 1 degree Celsius. It links energy changes to temperature changes.

Formula: $\Delta E = m c \Delta \theta$

$\Delta E$ : Change in thermal energy (J)
$m$ : Mass of the substance (kg)
$c$ : Specific heat capacity ( $J/kg^{\circ}C$ )
$\Delta \theta$ : Change in temperature ( $^{\circ}C$ )
Heating Mechanism: Energy transfer by heating increases the kinetic energy of the particles within a system, which is reflected as an increase in the system's thermal store.

5. Key Distinctions

Feature	Mechanical Work	Electrical Work	Thermal Transfer
Driver	Physical Force	Potential Difference	Temperature Gradient
Medium	Displacement	Charge Flow	Particle Vibration/Radiation
Primary Store	Kinetic/Potential	Thermal/Chemical	Thermal

Useful vs. Wasted Energy: Useful energy is the energy transferred to the intended store (e.g., kinetic energy in a motor), while wasted energy is dissipated to the surroundings (e.g., heat from friction).

6. Exam Strategy & Tips

Unit Consistency: Always ensure mass is in kilograms (kg) and time is in seconds (s) before using energy formulas. A common mistake is using grams or minutes.
The Kilowatt-Hour (kWh): In domestic contexts, energy is often measured in kWh. To convert to Joules, multiply by $3.6 \times 10^6$ . Remember that $1 \text{ kWh}$ is the energy used by a $1 \text{ kW}$ appliance in $1 \text{ hour}$ .
Identifying Stores: When describing a process, identify the 'start' store and the 'end' store. For example, a falling object transfers energy from the gravitational potential store to the kinetic store.

7. Common Pitfalls & Misconceptions

Energy 'Loss': Students often say energy is 'lost'. In physics, energy is never lost; it is dissipated or transferred to a non-useful store (usually the thermal store of the surroundings).
Temperature vs. Heat: Heat is the transfer of energy, while temperature is a measure of the average kinetic energy of particles. A change in energy ( $\Delta E$ ) causes a change in temperature ( $\Delta \theta$ ).
Power vs. Energy: Power is the rate of energy transfer ( $P = E/t$ ). Ensure you do not confuse the two when selecting formulas.

Changes in Energy

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Mechanical and Electrical Work

4. Thermal Energy Changes

5. Key Distinctions

6. Exam Strategy & Tips

7. Common Pitfalls & Misconceptions

Changes in Energy

Summary

1. Definition & Core Concepts

2. Underlying Principles

3. Mechanical and Electrical Work

4. Thermal Energy Changes

5. Key Distinctions

6. Exam Strategy & Tips

7. Common Pitfalls & Misconceptions