Electric Charge and Electrostatics

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.

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

1. Like charges:

   • A. attract
   • B. repel
   • C. cancel mass
   • D. become neutral automatically

   Answer: B.

2. Doubling separation in Coulomb's law changes force by:

   • A. half
   • B. double
   • C. one quarter
   • D. four times

   Answer: C.

3. Charging by induction involves:

   • A. direct rubbing only
   • B. charge separation without direct contact
   • C. creating charge from nothing
   • D. destroying electrons

   Answer: B.

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.

1. Formula:

   $$F = k\frac{|q_1q_2|}{r^2}$$

2. Substitute:

   $$F = 8.99\times10^9\frac{(3.0\times10^{-6})(2.0\times10^{-6})}{(0.15)^2}$$

3. Result:

   $$F = 2.40\ \text{N}$$

Final answer: $2.40\ \text{N}$, attractive.

Prompt: explain why charge conservation matters when a neutral object is charged by friction or induction.