Identify positive negative and neutral charge from particle balance
Use a fixed-positive particle model to distinguish an electron deficit, an electron excess and zero net charge.
Mapped to the named 0654 section using teaching from the corresponding single-science course. A 0654-specific review of every Core and Supplement objective is still in progress. Written practice is self-checked; it does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Add and subtract signed quantities.
- Distinguish a whole object's net value from its individual particles.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which sum represents balanced equal-magnitude positive and negative units?
What you will learn
- Name the two signs of electric charge.
- Determine net charge from a stated balance of positive and negative units.
Count both signs
Electric charge has positive and negative signs. A proton carries positive charge and an electron carries an equal-magnitude negative charge. In a simple illustrative model, six protons and six electrons have zero net charge because their contributions balance; neutral does not mean there are no charged particles.
Keep the positive count fixed when describing ordinary charging of a solid. Fewer electrons than positive units leaves a positive net charge; more electrons gives a negative net charge. An object with ten positive units and twelve electrons has net −2 elementary charge units, not twelve negative net units.
Distinguish the model from a real object
The small particle counts are a bookkeeping model, not a claim about the actual number or positions of particles in a macroscopic rod. Neutral solids contain very large numbers of positive and negative charges. The sign depends on their imbalance rather than which total is large by itself.
Charge sign is not a magnetic pole label, and neutral is not a third charge sign. State positive, negative or zero net charge for the specified balance. This lesson uses supplied equal-magnitude units; converting them to coulombs is introduced in the Supplement sequence.
Sources for this lesson
Pause and explain
An object has ten positive units and seven electron units. What is its net charge?
Worked example
A model object has eight positive units and ten electron units. Find its net charge.
Show the worked solution
- Each positive and negative unit has equal magnitude.
- Compute 8 − 10 = −2.
- The electron excess gives a negative net charge.
Answer −2 elementary charge units; the object is negatively charged.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Find the net charge of a model with six protons and four electrons.
Give me a hint
Subtract the equal-magnitude negative count from the positive count.
Compare my reasoning
- There are six positive units.
- There are four negative units.
- 6 − 4 = +2 units.
+2 elementary charge units; positive.
Look for these in your work
- Include both contributions.
- State the sign and the unit model.
Classify models with twelve positive units and twelve electrons, and with twelve positive units and fifteen electrons.
Give me a hint
Apply the same fixed-positive count to both electron balances.
Compare my reasoning
- 12 − 12 = 0.
- 12 − 15 = −3.
- The first is neutral and the second negative.
Neutral; −3 elementary charge units (negative).
Look for these in your work
- Compute each imbalance.
- Do not confuse neutral with particle-free.
A learner says a neutral copper rod contains no electrons. Correct the statement.
Give me a hint
Separate zero net charge from absence of charged particles.
Compare my reasoning
- Copper contains positive and negative constituents.
- A neutral rod has balanced total contributions.
- Electrons can be present while the net charge is zero.
A neutral rod contains electrons; their negative charge balances its positive charge.
Look for these in your work
- Explain what neutral measures.
- Retain the charged constituents.
Common mistakes
- Neutral still contains charged particles.
- Use the imbalance, not the electron count alone.
- Charge signs are not magnetic poles.
What can you explain now?
What sign results when a solid loses electrons from an initially neutral state?
Compare with the explanation
Positive.
Removing negative charge leaves an excess of positive over negative charge.
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Make it stick
Determine charge sign from imbalance and explain why a neutral object still contains charged particles.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.