Energy stores and systems

Systems and Energy Stores

Think of energy like money in a bank; it can be moved between different accounts, but the total amount remains exactly the same. In physics, we track these energy movements by defining a system, which is simply an object or a group of objects.

• A closed system is a system where no matter or energy can enter or leave.
• Because nothing can escape a closed system, the total energy remains constant and never changes.

Energy is kept in one of eight energy stores: Kinetic, Gravitational Potential (GPE), Elastic Potential, Thermal, Chemical, Electrostatic, Magnetic, and Nuclear. When a system changes, energy moves between these stores via four energy transfer pathways:

• Mechanical working: A force moving an object through a distance.
• Electrical working: Charges moving due to a potential difference.
• Heating: Energy transferred due to temperature differences.
• Radiation: Energy transferred by waves (e.g., light or sound).

Energy Changes in Common Situations

Whenever an everyday action occurs, energy is transferred between specific stores. AQA requires you to describe the exact step-by-step changes for five common scenarios:

• Object projected upwards: An object thrown upwards initially has energy in its kinetic store. As it rises, energy is transferred to the gravitational potential energy store. At its peak, the kinetic store is zero and GPE is maximum, before transferring back as it falls.
• Moving object hitting an obstacle: A moving object has energy in its kinetic store. Upon impact, mechanical work is done by the collision force. Energy is transferred to the thermal energy store of the object and the obstacle, and is dissipated as sound to the surroundings.
• Object accelerated by a constant force: When a motor or muscle applies a force, work is done. Energy transfers from a chemical store (fuel/muscles) or electrical store (battery) into the kinetic store of the object via the mechanical pathway.
• Vehicle slowing down: A moving vehicle transfers energy from its kinetic store when braking. Friction between the brakes and wheels does work, transferring energy to the thermal energy store of the brakes and dissipating it as heat and sound.
• Bringing water to a boil: Energy from the mains supply is transferred from the electrical store to the thermal energy store of the heating element. The element then transfers energy by heating into the thermal energy store of the water.

Calculating Energy Changes

To understand exactly how much energy moves during a transfer, physicists calculate the work done. Work done is defined as the amount of energy transferred, where 1 Joule is equivalent to 1 Newton-metre.

The rate at which this energy is transferred is called power. The energy required to change temperature relies on the material's specific heat capacity, which is the amount of energy required to raise the temperature of one kilogram of a substance by one degree Celsius.

Energy changes can be calculated using these key formulas:

$$W = Fs$$ $$E = Pt \quad \text{or} \quad E = IVt$$ $$\Delta E = mc\Delta\theta$$

Worked Example: Mechanical Work Done

A man pushes a box with a force of $500\text{ N}$ over a distance of $25\text{ m}$. Calculate the work done.

• Step 1: Write out the formula and identify knowns ($F = 500\text{ N}$, $s = 25\text{ m}$).

• Step 2: Substitute the values into the equation: $W = 500 \times 25$

• Step 3: Calculate the final answer with units: $W = 12,500\text{ J}$

Worked Example: Electrical Work Done

A $1.2\text{ kW}$ heater is switched on for $5\text{ minutes}$. Calculate the energy transferred.

• Step 1: Convert units to standard forms ($1.2\text{ kW} = 1200\text{ W}$, $5\text{ minutes} = 300\text{ s}$).

• Step 2: Substitute into the equation: $E = P \times t = 1200 \times 300$

• Step 3: Calculate the final answer: $E = 360,000\text{ J}$ (or $360\text{ kJ}$).

Conservation of Energy and Common Scale Redistribution

No matter how energy moves, the universe's total energy balance sheet always adds up perfectly. The Law of Conservation of Energy states that energy can be transferred usefully, stored, or dissipated, but it cannot be created or destroyed.

During transfers, some energy inevitably becomes dissipated energy, meaning it is transferred to the surroundings in less useful ways (usually heating the surroundings). To prove conservation in an exam, you must show energy redistribution on a common scale, which simply means calculating all stores using the exact same unit (Joules).

Worked Example: Proving Conservation on a Common Scale

A $2\text{ kg}$ ball is dropped from a height of $5\text{ m}$. At the bottom, its speed is $9.9\text{ m/s}$. Show that energy has been redistributed on a common scale.

• Step 1: Calculate the initial energy in the GPE store: $E_p = mgh = 2 \times 9.8 \times 5 = 98\text{ J}$.

• Step 2: Calculate the final energy in the kinetic store: $E_k = 0.5mv^2 = 0.5 \times 2 \times (9.9)^2 \approx 98\text{ J}$.

• Step 3: Conclude that the $98\text{ J}$ originally in the GPE store is redistributed entirely into the kinetic store on a 1:1 scale (assuming no air resistance).