What does a power source do when a battery charges?
You will be able to: Explain how external energy drives a thermodynamically unfavorable process.
What does a power source do when a battery charges?
A rechargeable battery delivers energy while discharging and requires energy to restore its earlier chemical state. Charging does not reverse the definition of energy conservation.
A useful starting point: How can a favorable reaction help an unfavorable one? →
Words and symbols before equations
- External energy source
- A source such as electricity or light supplying energy to a selected system.
- Charging
- Driving a battery’s chemical changes opposite their discharge direction under suitable conditions.
- Minimum reversible work
- Ideal lower bound on required useful work at constant T and pressure.
- System boundary
- The chosen division between the device and its energy source/surroundings.
What this picture assumes
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Specified transformation at constant T and pressure. Positive ΔG is the ideal reversible minimum non-expansion work. Input sufficiency alone does not prove kinetic or device feasibility; real devices have losses.
Read the picture in three steps
- Read the species and labels first. Identify what each symbol and line represents. Read the units and fixed conditions before comparing quantities.
- Chemical transformation requires +10 kJ; work input 14 kJ. Input exceeds the ideal minimum by 4 kJ. Kinetics, losses and device feasibility require further information.
- Check what the picture assumes below. Use the Explore task to predict one change before moving a control.
Connect the picture to the chemistry
An unfavorable process for the isolated reacting chemicals can occur when coupled to a sufficiently favorable external energy transfer. Electrolysis uses electricity; photosynthesis uses absorbed light.
At constant temperature and pressure, positive ΔG gives an ideal minimum non-expansion work input for a reversible transformation of the stated amount. Real devices need additional input because of losses.
Supplying energy is different from adding a catalyst. A catalyst changes the accessible pathway, while an external source can drive a net process that is uphill for the chemical subsystem.
The explorer compares a supplied chemical free-energy requirement with external work input. It is an energy-budget illustration, not a charger design, operating-voltage prediction or claim of 100% real efficiency.
A worked example, step by step
A stated charging transformation requires a chemical free-energy increase of 10 kJ. A device supplies 14 kJ for that transformation. Interpret the energy budget.
- The chemical subsystem gains 10 kJ of Gibbs free energy under the given conditions.
- The supplied 14 kJ exceeds the ideal reversible minimum.
- The remaining 4 kJ is energy not stored as that chemical free-energy increase, such as dissipated energy.
- This simplified budget does not imply an exact operating mechanism or universal battery efficiency.
External energy can drive an uphill chemical change; a catalyst alone does not supply that missing free energy.
Does a power source make the chemical subsystem’s original ΔG disappear?
Compare with an explanation
No. It supplies energy through the larger coupled system.
Predict. Change one thing. Explain.
Compare input below, equal to and above the supplied chemical free-energy requirement. Explain why equality is an ideal limit, not a guarantee of useful charging current.
On narrow screens, swipe or scroll diagrams sideways to read all labels.
Chemical transformation requires +10 kJ; work input 14 kJ. Input exceeds the ideal minimum by 4 kJ. Kinetics, losses and device feasibility require further information.
Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Specified transformation at constant T and pressure. Positive ΔG is the ideal reversible minimum non-expansion work. Input sufficiency alone does not prove kinetic or device feasibility; real devices have losses.
Explain what you noticed: Answer the investigation prompt above. State one observation and explain it using energy and entropy contributions, electron and ion bookkeeping, or the stated cell reaction. Identify what the representation cannot tell you.
Apply the idea to a fresh problem Practice →Show what you understand.
Two original questions are a starting check, not proof of mastery. Explain your choice before revealing the answer.
Original written challenge
4 points · self-check · not an official AP questionA specified uphill transformation requires +6 kJ of free energy and receives 9 kJ of electrical work. Explain what is possible, the ideal minimum and a limitation of this calculation.
This response is not submitted or saved. Copy it before leaving.
Compare with the answer and four-point rubric
- 1 point: An external source can drive the uphill chemical transformation.
- 1 point: The reversible minimum input is 6 kJ under the stated constant-T,p conditions.
- 1 point: The 9 kJ budget leaves 3 kJ not stored as the specified chemical free-energy increase.
- 1 point: Energy sufficiency alone does not establish kinetics, current, mechanism or device feasibility.
Accept equivalent correct methods and explanations. This is a Refresh Kid teaching rubric, not an official AP scoring guideline.
Retrieve it before you reveal it.
RECALL 1What drives electrolysis?
External electrical energy.
RECALL 2Does a catalyst supply missing ΔG?
No.
RECALL 3Why state a system boundary?
To account for energy transferred from the source.
Revisit these tomorrow and a week later. Try a fresh problem and explain why the method applies.
What does a power source do when a battery charges?
- Ideal reversible work input at constant T,p is at least the positive ΔG for the specified transformation.
- Real processes generally require more input because of losses.
Remember: External energy can drive an uphill chemical change; a catalyst alone does not supply that missing free energy.
Conditions: Original teaching model with supplied rounded data. Numerical states, units and assumptions are specified below; no measured reaction rate is implied. Specified transformation at constant T and pressure. Positive ΔG is the ideal reversible minimum non-expansion work. Input sufficiency alone does not prove kinetic or device feasibility; real devices have losses.
Refresh Kid · AP Chemistry Unit 9 · Objectives 9.7.A · Review edition
Framework, scope and review status
Mapped to College Board CED, Topic 9.7, objective 9.7.A. CED effective Fall 2024 and June 2026 clarifications checked September 17, 2026. Unit 9: Thermodynamics and Electrochemistry, Topics 9.1–9.11. Focused lesson names, examples, models and assessments are original Refresh Kid teaching materials, not additional official topics or official AP questions. Official corrections.
The model states its assumptions beside the diagram. Numerical thermodynamic examples state standard conditions, temperature, reaction scaling and unit conventions. Supplied data and schematic geometry are teaching models. Standard ΔG° describes standard-state favorability and relates to K; actual direction depends on composition. Thermodynamic favorability does not predict rate. Nonstandard cell potential is taught through Q, distance from equilibrium and qualitative Nernst reasoning; algorithmic substitution alone does not demonstrate the assessed understanding. Electrode positive/negative labeling is excluded from assessed scope. Oxidation at the anode and reduction at the cathode remain essential. Faraday calculations assume the stated current efficiency and electron stoichiometry. Rotatable particle models are schematic inventories, not measured molecular trajectories. Virtual models do not replace required supervised laboratory work.
Teaching resources: The Organic Chemistry Tutor video titles/descriptions and topic coverage were checked for optional links; no claim is made to have watched every video. No creator scripts, examples, worksheets or artwork were copied. GitHub’s 3D website collection and its Three.js camera-control example informed the idea of controllable spatial inspection. Scientific diagrams, geometry and interactions here are original. The self-hosted Three.js runtime retains its MIT license. Camera rotation changes the view, not the chemistry.
Independent teacher review and observation of students remain pending. Implementation checks do not certify scientific accuracy, accessibility or learning effectiveness. This is a review edition.
Optional official resource: Released AP Chemistry questions and scoring guides. This archive contains questions across units; it is not an assignment of every question to this lesson.
The teaching sequence is informed by the IES learning guide; this exact implementation has not been evaluated with learners.
Want to work through this with a tutor?
Bring your question about What does a power source do when a battery charges? Your explanation and answers remain free to access.