Grade 10 physics – Electrostatics Quiz
1. What does the study of electrostatics focus on?
Electrostatics studies electric charges when they are stationary and the forces and fields they produce. Charges in motion and magnetic effects are studied in electromagnetism or circuits.
2. What is the SI unit of electric charge?
The SI unit of electric charge is the coulomb (C). Volt is unit of potential difference, ampere is unit of current, and newton is unit of force.
3. What is the magnitude of the elementary charge (the magnitude of the charge of an electron)?
The elementary charge (magnitude of electron or proton charge) is approximately 1.6 × 10^-19 coulombs.
4. Which statement best describes conservation of electric charge?
Charge conservation means the net charge of an isolated system does not change. Charges can be separated or moved, but the total remains constant.
5. According to Coulomb's law, how does the electrostatic force between two point charges vary with distance r between them?
Coulomb's law states the magnitude of force is proportional to the product of charges and inversely proportional to the square of the separation distance (1/r^2).
6. What is the direction of the electrostatic force between two like charges?
Like charges (both positive or both negative) exert forces that push them apart, i.e., they repel each other.
7. How is the electric field at a point defined?
Electric field E is defined as the force experienced by a small positive test charge per unit charge: E = F/q.
8. What is the magnitude of the electric field produced by a point charge Q at distance r (in vacuum)?
The electric field magnitude from a point charge is E = kQ/r^2 where k is the Coulomb constant (≈9.0×10^9 N·m^2/C^2).
9. What does the principle of superposition state for electrostatic forces?
Superposition means each pairwise force is independent and the net force is the vector sum of all individual forces.
10. Which of the following is a correct example of charging by friction?
Frictional charging transfers electrons between materials; rubbing a comb on wool can transfer electrons to the comb, making it negatively charged.
11. What happens when a negatively charged rod touches a neutral metal sphere (charging by conduction)?
When a negatively charged object touches a neutral conductor, excess electrons move onto the conductor, leaving it negatively charged.
12. How does charging by induction produce a charge on an isolated conductor?
Induction involves rearranging charges by a nearby charged object and using a ground connection to allow charges to flow away or in, producing net charge without direct contact.
13. Which material is a good electrical conductor suitable for wiring in Kenyan schools?
Copper is an excellent electrical conductor used for wiring because its free electrons move easily. Plastic, rubber, and glass are insulators.
14. What is electric potential (voltage) at a point?
Electric potential is defined as the work done per unit positive charge in bringing it from a reference point (often infinity) to the point in question.
15. If two identical positive point charges are brought closer together, how does their electric potential energy change?
Bringing like charges closer requires work against their repulsion, which increases the system's electric potential energy.
16. Which of the following is true about electric field lines around a positive point charge?
Field lines represent the direction a positive test charge would move: away from a positive source. Lines never cross and do not form closed loops for electrostatics.
17. What is meant by polarization of an insulating material when a charged object is brought near?
In insulators, charges cannot move freely, but the electron clouds shift slightly, creating induced positive and negative regions (polarization) without net charge transfer.
18. If an isolated conducting sphere carries a net positive charge, where does this charge reside?
In electrostatic equilibrium, excess charges on a conductor reside on its surface because like charges repel and move as far apart as possible.
19. Which value is closest to the Coulomb constant k used in Coulomb's law?
The Coulomb constant k, which appears in Coulomb's law, is approximately 9.0 × 10^9 newton·metre^2 per coulomb^2.
20. What is the electrostatic force between two 1 C charges placed 1 m apart in vacuum (use k = 9.0 × 10^9 N·m^2/C^2)?
Using Coulomb's law F = kQ1Q2/r^2 = (9.0×10^9)(1)(1)/(1^2) = 9.0×10^9 N.
21. If the distance between two fixed point charges is doubled, how does the magnitude of the electrostatic force change?
Force varies as 1/r^2, so doubling r reduces the force by 1/(2^2) = 1/4.
22. Between two equal positive charges separated by a distance, where is the electric field zero?
For two equal positive charges, fields from each at the midpoint are equal in magnitude but opposite in direction, so they cancel giving net zero field.
23. What does grounding (earthing) a charged object do?
Grounding provides a path for electrons to flow between an object and the Earth, which can remove or supply charge until the object is neutral.
24. A neutral metal sphere is touched by a negatively charged rod and then the rod is removed. What is the final charge of the sphere?
Direct contact with a negatively charged rod transfers electrons onto the neutral sphere, leaving it with a net negative charge.
25. Which of these observations shows that electric charge is quantised?
Quantisation of charge means charges occur in discrete amounts equal to whole-number multiples of the elementary charge (≈1.6×10^-19 C).
26. Why do electric field lines never cross each other?
If lines crossed, that point would have two different directions for the electric field, which is impossible; the field at a point is unique.
27. A plastic comb becomes charged after being rubbed on dry hair. Which statement correctly explains the charging process?
Rubbing transfers electrons between materials; in this case electrons are transferred to the comb, making it negatively charged and the hair positive.