Work through the list without notes. If you cannot do a statement, follow its link, then come back. Guide = study guide; Practice = practice questions.
Understanding
- I can define the electric potential energy of two charges as the external work needed to assemble them from infinite separation. (Guide: “Energy stored in an arrangement of charges”; Practice Q1)
- I can explain why potential energy belongs to the system and not to one charge. (Guide: “Common misconceptions”)
- I can explain what a negative U means physically. (Guide: “Signs, sizes and the U(r) graph”; Practice Q1)
- I can explain why the work done depends only on the start and end separations, not on the path. (Guide: Worked example 2)
Calculation and skills
- I can derive U = kq₁q₂/r by integrating the external force from infinity. (Guide: “Deriving U from the work done” and Worked example 2)
- I can calculate U for a pair of charges, keeping the signs, including charges given as multiples of e. (Practice Q4)
- I can find the total U of three or more charges by adding each pair once. (Guide: Worked example 1; Practice Q3, Q5)
- I can find the work done by an external agent and by the electric forces when a system is rearranged. (Practice Q5, Q7)
- I can find a force from U(r) or U(y) using F = −dU/dr. (Practice Q6, Q7)
Reasoning
- I can predict factors of change in U and in F when charges or separations change. (Practice Q2, Q6)
- I can sketch U against separation for like and unlike charges and read the force direction from the slope. (Guide: Figure 1; Practice Q6)
- I can use the shape of a U graph to decide whether a position is stable or unstable. (Practice Q7)
All ticked? Move on to Topic 9.2, Electric Potential, or return to the course roadmap.