Orientation
Lesson goal: explain charge interactions, charging methods, conservation of charge, and electrostatic force.
The key category separation is charge, force, field, and potential. Do not use these as interchangeable words.
Core Content
Electric charge is conserved and quantised. Like charges repel and unlike charges attract. Charged objects may interact by contact, friction, induction, or polarisation.
Key model:
$$F = k\frac{|q_1q_2|}{r^2}$$
This gives the magnitude of the force between point charges. Direction comes from the signs of the charges: attraction for opposite charges and repulsion for like charges.
| Idea | Meaning |
|---|---|
| conservation | total charge in an isolated system remains constant |
| quantisation | charge occurs in integer multiples of elementary charge |
| conductor | charges move freely through the material |
| insulator | charges are not free to move through the material |
| induction | charge separation or charging without direct contact |
Concept Check
-
Like charges:
- A. attract
- B. repel
- C. cancel mass
- D. become neutral automatically
Answer: B.
-
Doubling separation in Coulomb's law changes force by:
- A. half
- B. double
- C. one quarter
- D. four times
Answer: C.
-
Charging by induction involves:
- A. direct rubbing only
- B. charge separation without direct contact
- C. creating charge from nothing
- D. destroying electrons
Answer: B.
Applied Practice
Two point charges, +3.0 uC and -2.0 uC, are separated by 0.15 m.
Calculate the force magnitude and state the direction type.
-
Formula:
$$F = k\frac{|q_1q_2|}{r^2}$$
-
Substitute:
$$F = 8.99\times10^9\frac{(3.0\times10^{-6})(2.0\times10^{-6})}{(0.15)^2}$$
-
Result:
$$F = 2.40\ \text{N}$$
Final answer: $2.40\ \text{N}$, attractive.
Deep Practice And Writing
Prompt: explain why charge conservation matters when a neutral object is charged by friction or induction.
Maintenance Loop
Recall Coulomb's law, the inverse-square distance relation, and the difference between conductor and insulator.