D.2.3—Charge conservation
- Syllabus
- First assessment 2025
- Objective
- —
- Level
- HL
Apply Charge Conservation
Core idea
Electric charge is conserved: in an isolated system, the total charge before an interaction equals the total charge after it. Charge can move between objects, but it is not created or destroyed in the transfer.
Use it at a junction
In a steady circuit, charge does not accumulate at a junction. The current entering equals the current leaving, for example I1=I2+I3. This is a consequence of charge conservation, not a separate rule that overrides it.
Track the system boundary
When charge appears to change on one object, include the other object, the conductor or the ground in the system. Electrons may move across the chosen boundary, so the object’s charge changes while the total charge of the larger isolated system remains constant.
Common trap
Do not answer “Kirchhoff’s law” alone when asked for the fundamental law behind current balance. State conservation of electric charge.
D.2.3 Exam Analysis
Assessment in practice
1 marksHow it is assessed
Questions identify the fundamental law behind current balance or explain an apparent charge change during transfer.
Command terms
State
What earns marks
State conservation of electric charge and identify the complete system boundary; at a circuit junction, current entering equals current leaving.
Watch for
Naming Kirchhoff’s law without stating conservation of electric charge, or treating transferred charge as newly created.
Representative question
Question 1
[Maximum number: 1]
The diagram shows a junction in a circuit.
The currents in the three wires are related by I1=I2+I3.
State the fundamental law of Physics from which this relation is derived.
Conservation of «electric» charge
Marking guidance:
Do not accept 'Kirchoff's law' as the
sole answer.
If conservation of charge and Kirchoff's
Law are stated award [1].
If conservation of charge is listed along with other fundamental laws e.g.
conservation of energy, award [0].
[1]
Retrieve the Core D.2 Electric and Magnetic Fields Model
D.2 core fields is secure when you can move between charge, force and field representations.
- Like charges repel and unlike charges attract
- Coulomb’s law gives inverse-square force
- Charge is conserved, quantized and transferable
- Millikan’s experiment supports q=ne
- E=F/q and field lines show direction and relative density
- Parallel plates give E=V/d
- Magnetic field lines are closed and follow current direction