Electric Charge & Fields — Practice · Grade 12 Science
Chapter: Electricity, Magnetism & CircuitsWhat this practice covers
By the end of Electric Charge & Fields you should be able to:
- I can explain that charge comes in two types, is quantized, and is conserved.
- I can use Coulomb's law (F = kq₁q₂/r²) to calculate the electrostatic force between two charges.
- I can describe an electric field, calculate it with E = F/q, and sketch its field lines.
10 questions, multiple choice, numeric and written answer, each with a worked explanation.
Vocabulary used: electric charge, Coulomb's law, electric field, conductor, insulator.
Problem 1
- attractive
- repulsive
- zero
- always gravitational
Answer and explanation
repulsive
Like charges repel, so two positive charges push apart.
Problem 2
Answer and explanation
50000 N/C
E = F ÷ q = 0.1 ÷ (2 × 10⁻⁶) = 50,000 N/C.
Problem 3
Answer and explanation
0.6 N
F = kq₁q₂/r² = (9 × 10⁹)(2 × 10⁻⁶)(3 × 10⁻⁶) ÷ (0.3)² = 0.054 ÷ 0.09 = 0.6 N.
Problem 4
- glass
- copper
- dry wood
- rubber
Answer and explanation
copper
Copper is a metal with free-moving electrons, making it an excellent conductor — which is why wires are made of it.
Problem 5
Answer and explanation
0.9 N
F = kq₁q₂/r² = (9 × 10⁹)(3 × 10⁻⁶)(3 × 10⁻⁶) ÷ (0.3)² = 0.081 ÷ 0.09 = 0.9 N.
Problem 6
Answer and explanation
In a conductor, such as copper, some electrons are free to move through the material, so charge flows easily. In an insulator, such as rubber or glass, the electrons are held tightly to their atoms and cannot move freely, so charge does not flow. That is why wires use a copper conductor coated in a rubber insulator.
Problem 7
Hints
Distance is center-to-center under the point-charge model.
Answer and explanation
Away from the other charge: like charges repel, with equal and opposite interaction forces.
Problem 8
Hints
State what the supplied observations or conditions establish, and separate that from any assumption.
Answer and explanation
F=qE=6.0×10⁻⁴ N along the field for this positive charge.
Problem 9
Answer and explanation
Left, because the electron’s charge is negative and F=qE.
An electric field assigns force per positive test charge: E=F/q for a sufficiently small nonperturbing test. A positive source has outward field direction; a negative source has inward direction. A positive test charge feels force along E while a negative one feels force opposite E. Field units N/C differ from force units N. Field arrows are local direction indicators, not paths every released charge must follow.
Left, because the electron’s charge is negative and F=qE.
Response criteria
- Interpret labels, structures, axes, units and directions correctly.
- Explain the physical or environmental relationship or mechanism.
- Respect stated assumptions and distinguish observation from inference.
Problem 10
Answer and explanation
F=qE gives+0.0010 N east for+2 μC and−0.0015 N, i.e.west, for−3 μC. The field is unchanged in the ideal nonperturbing-test model.
F=qE gives+0.0010 N east for+2 μC and−0.0015 N, i.e.west, for−3 μC.
The field is unchanged in the ideal nonperturbing-test model.
Response criteria
- Use the stated measurements or conditions, with correct units.
- Connect the result to the physical or environmental mechanism or evidence relationship.
- Identify the assumption, model limit or further evidence requested.