Grade 10 physics – Electrostatics Quiz

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.

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.

The elementary charge (magnitude of electron or proton charge) is approximately 1.6 × 10^-19 coulombs.

Charge conservation means the net charge of an isolated system does not change. Charges can be separated or moved, but the total remains constant.

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).

Like charges (both positive or both negative) exert forces that push them apart, i.e., they repel each other.

Electric field E is defined as the force experienced by a small positive test charge per unit charge: E = F/q.

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).

Superposition means each pairwise force is independent and the net force is the vector sum of all individual forces.

Frictional charging transfers electrons between materials; rubbing a comb on wool can transfer electrons to the comb, making it negatively charged.

When a negatively charged object touches a neutral conductor, excess electrons move onto the conductor, leaving it negatively charged.

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.

Copper is an excellent electrical conductor used for wiring because its free electrons move easily. Plastic, rubber, and glass are insulators.

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.

Bringing like charges closer requires work against their repulsion, which increases the system's electric potential energy.

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.

In insulators, charges cannot move freely, but the electron clouds shift slightly, creating induced positive and negative regions (polarization) without net charge transfer.

In electrostatic equilibrium, excess charges on a conductor reside on its surface because like charges repel and move as far apart as possible.

The Coulomb constant k, which appears in Coulomb's law, is approximately 9.0 × 10^9 newton·metre^2 per coulomb^2.

Using Coulomb's law F = kQ1Q2/r^2 = (9.0×10^9)(1)(1)/(1^2) = 9.0×10^9 N.

Force varies as 1/r^2, so doubling r reduces the force by 1/(2^2) = 1/4.

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.

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.

Direct contact with a negatively charged rod transfers electrons onto the neutral sphere, leaving it with a net negative charge.

Quantisation of charge means charges occur in discrete amounts equal to whole-number multiples of the elementary charge (≈1.6×10^-19 C).

If lines crossed, that point would have two different directions for the electric field, which is impossible; the field at a point is unique.

Rubbing transfers electrons between materials; in this case electrons are transferred to the comb, making it negatively charged and the hair positive.

Electrostatics focuses on electric charges when they are stationary and the forces and fields they produce. Current and magnetic effects involve moving charges and belong to other topics.

Electric charge is classified as positive or negative. North/south refers to magnets, hot/cold to temperature, and massive/massless to mass, so they are not charge types.

The coulomb (C) is the SI unit of electric charge. Volt measures potential, ampere measures current, and ohm measures resistance.

Coulomb's law states the electrostatic force magnitude is proportional to 1/r^2, so doubling the distance reduces the force by four.

Coulomb's law is F = k |Q1 Q2| / r^2, where k is Coulomb's constant (≈ 9.0 × 10^9 N·m^2/C^2) and the absolute value ensures magnitude is positive.

Metals like copper have free electrons that allow charges to move easily, making them good conductors. Rubber, glass and many plastics are insulators.

Charging by friction (triboelectric charging) involves transferring electrons when two different materials are rubbed together. Induction and conduction are different processes; heating does not directly charge.

When a charged rod touches a neutral conducting sphere, charge transfers by conduction. A positively charged rod removes electrons, leaving the sphere net positive. Induction is without contact.

Induction uses a charged object to separate charges in the conductor; grounding removes or supplies charges. Removing ground then the external object leaves the conductor with net charge.

Earthing provides a path for charges to move between an object and the vast reservoir of the Earth, removing excess charge or supplying electrons to neutralise or alter the object's net charge.

Electric field lines by convention point away from positive charges and toward negative charges. They do not form closed loops around single charges.

In electrostatic equilibrium, free charges in a conductor rearrange so that the electric field inside the conducting material and in an empty cavity is zero if there are no charges inside the cavity.

Field lines start on positive and end on negative charges and do not cross because that would imply two different field directions at the same point. They do not necessarily show exact particle paths (which depend on velocity and force).

Superposition means each pairwise force is calculated independently and then vectorially added to find the total force; directions and magnitudes are both important.

Charge is quantised: the net charge on an object is an integer multiple of the elementary charge e (except for complex cases with quarks), so charges change in discrete amounts.

Coulomb's constant k is approximately 8.99 × 10^9 N·m^2/C^2, often rounded to 9.0 × 10^9. The other values are Planck's constant, speed of light, and elementary charge respectively.

Like charges repel due to electrostatic forces; the force acts along the line joining them and pushes them apart.

Insulators do not have free charges, but their molecules polarise: positive and negative charges shift slightly in opposite directions creating induced dipoles, reducing the effective field inside.

Excess charges in a conductor move to the outer surface because they repel each other and reach equilibrium when there is no field inside the conductor's material.

Charges accumulate more at sharp points (high curvature), creating stronger local electric fields. This is why lightning rods and corona discharge occur at pointed ends.

Electric potential is scalar and represents potential energy per unit charge. Electric field is a vector showing force per unit charge and has direction and magnitude.

On an equipotential surface the potential is the same at every point, so moving a charge along it requires no work. Field lines are perpendicular to equipotential surfaces.

A dielectric polarises in an electric field, reducing the effective field and thus lowering the force between charges compared with vacuum separation.

An electric dipole consists of two equal and opposite charges separated by a distance; it has a dipole moment that characterises its strength and orientation.

When two identical conductors touch, charge redistributes equally. Total charge = +5e, so each sphere gets +2.5e (assuming identical spheres).

Rubbing transfers electrons; plastics often gain electrons and become negatively charged. Protons do not move in these processes and magnetism is unrelated.

If two equal and opposite charges are placed symmetrically, their forces at the midpoint cancel. Two like charges on same side or unequal charges will not cancel at the midpoint.

Electrostatic sparks can ignite flammable vapours, so avoid such hazards and discharge (earth) charged objects safely. Wearing metal jewellery can be dangerous, and touching charged conductors is unsafe.