How we got there
Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.
Explain in another language · 11
1. What is the question asking?
Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: Two identical metal spheres carry +6 µC and −2 µC. They touch and are then placed 0.10 m apart. Find the charge on each sphere and the force between them. k = 9 × 10⁹ N·m²·C⁻².
In other words — as if you have never seen this before
If you have never seen this before: Identical spheres share the total charge equally when they touch. Then use Coulomb’s law with the new charges. We will name a method in words, write the formula with no numbers, then substitute one line at a time.
2. What do we already know?
Q₁=+6 µC, Q₂=−2 µC, r=0.10 m after separation.
3. Which method — and why this one?
Identical spheres share the total charge equally when they touch. Then use Coulomb’s law with the new charges.
4. Step-by-step working
01 / 06
Share the charge
Q=(Q₁ + Q₂)2=(6 + (−2))2=2 µC each.
Q=2 µCWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
02 / 06
Apply Coulomb’s law
F=kQ²r²=(9 × 10⁹)(2.0 × 10⁻⁶)²(0.100)²=3.60 N.
F=3.60 NWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
03 / 06
Interpret
Both spheres now carry the same sign, so the force is repulsive.
repulsiveWhy this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.
04 / 06
Step G — Sense-check the size
Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.
Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.
05 / 06
Step F — Box the answer with its unit
2 µC on each sphere; F = 3.60 N, repulsive.
Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.
06 / 06
Step H — What the marker is looking for
On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.
Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.
5. The answer
2 µC on each sphere; F = 3.60 N, repulsive.
Copy onto your answer sheet — in this order
Formula: Q=2 µC
Answer: 2 µC on each sphere; F = 3.60 N, repulsive.
6. Check it like a marker
Do not use the charges from before the spheres touched. Add the charges WITH their signs before halving.
7. A trap first-timers fall into
First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”