Energy transfers and system calculations

Principles of energy • Energy stores and changes

Flashcards

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How to compare different energy transfers on the same diagram?

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Convert every transfer to joules and draw energy bars or list numeric values so that each store's magnitude is directly comparable.

Key concepts

What you'll likely be quizzed about

Definition of a system and energy stores

A system is an object or group of interacting objects. Energy is stored in named stores such as kinetic, gravitational potential, elastic potential, chemical and internal (thermal). In any process, energy moves between stores but the total energy in an isolated system remains constant (conservation); some energy may dissipate to the surroundings and become less useful. Energy transfer mechanisms include heating, mechanical work, electrical work and radiation.

Mechanical work by forces

When a force moves an object through a distance in the direction of the force, work is done and energy transfers between stores. Work done is W = F × d (force times distance). Lifting a mass increases its gravitational potential energy by ΔE = m × g × Δh. Friction or inelastic deformation converts some work into thermal energy (internal store) and possibly sound. Example calculations using work show how kinetic or potential energy stores change when forces act.

Electrical work when a current flows

A power supply (cell or battery) does electrical work when it drives charge around a circuit. Electrical energy transferred equals E = V × I × t (potential difference × current × time) or E = Q × V where Q is charge (Q = I × t). Electrical energy can transfer to kinetic, thermal or other stores via components (motors, heaters, lamps). Transformers and transmission affect currents and therefore heating losses in cables. fileciteturn0file6turn0file15

Heating and specific heat capacity

Heating transfers energy into the internal (thermal) store, raising temperature unless a phase change occurs. The energy required to change temperature is ΔE = m × c × Δθ, where m is mass (kg), c is specific heat capacity (J/kg°C), and Δθ is temperature change (°C). The equation applies only when the material remains in the same state (no melting or boiling) and c is approximately constant over the temperature range. Typical specific heat capacity values must be used correctly. fileciteturn0file13turn0file16

Representing redistribution on a common scale

All energy transfers and stores use the joule (J), allowing direct comparison on a common scale. Energy bar charts or numeric lists use the same unit to show initial and final store magnitudes. Conservation of energy requires that the sum of changes in all stores equals zero for an isolated system (energy lost by some stores equals energy gained by others), with dissipation accounted for as transfers to the surroundings. Numerical calculations from work, electrical energy and heating combine to produce a single consistent energy balance. fileciteturn0file6turn0file17

Key notes

Important points to keep in mind

Always convert every energy transfer into joules before comparing stores.

Use ΔE = m × c × Δθ only when no phase change occurs and c is approximately constant. fileciteturn0file13turn0file16

Work done by a force requires displacement in the force direction; W = F × d.

Electrical energy supplied equals V × I × t; charge-based form is E = Q × V with Q = I × t.

Account for dissipation: some transferred energy appears as thermal energy in the surroundings and cannot be reused.

Show redistribution using an energy bar chart or proportional numbers on the same scale to demonstrate conservation and losses.