Physics: Electrostatics and Electric Current Practice

A comprehensive set of practice questions covering electrostatics, Coulomb's Law, electric fields, Gauss's law, potential, and dipole characteristics based on lecture notes.

How can a neutral body be made positively charged without involving other bodies?

By removing electrons from it (e.g., through heating or photoemission).

What is the relationship between the removal of electrons and the charge of a body?

Removing electrons results in an excess of protons, making the body positively charged.

What is a gold leaf electroscope and what is its working principle?

It is a device used to detect and measure electric charge based on electrostatic induction and repulsion.

Write the mathematical expression for Coulomb's law.

$$F = \frac{k q_1 q_2}{r^2}$$

What type of law is Coulomb's law classified as?

An inverse-square law.

What happens to the distance between two charges if the electrostatic force between them is quadrupled?

The distance is halved ($r$ becomes $\frac{1}{2}$).

Define electric field strength and state its units.

Electric field strength is the force per unit positive charge ($E = \frac{F}{q_0}$); units are $N/C$ or $V/m$.

In physics, how is a field defined?

A region of space where a suitable object (like a charge or mass) experiences a force.

How is the net charge of a system of point charges calculated?

It is the algebraic sum of all individual charges, taking their signs into account.

What is the direction of the electric field produced by a point positive charge?

Radiating away from the charge.

What is the direction of the electric field produced by a point negative charge?

Pointing towards the charge.

Write the formula for the electric field ($E$) of a point charge ($Q$).

$E = \frac{k Q}{r^2}$ where $k = 9 \times 10^9 N m^2/C^2$.

Distinguish between a source charge and a test charge.

A source charge ($Q$) creates the electric field, while a test charge ($q_0$) is a small positive charge used to measure the field without disturbing it.

What is electric flux?

The number of electric field lines passing through a given surface ($\text{Φ} = E \times A \times \text{cos}(\theta)$).

What is the net charge of an electric dipole?

Zero, because it consists of two equal and opposite charges ($+q$ and $-q$).

Define electric dipole moment and its direction according to the text.

The product of the charge magnitude and the separation distance ($p = q \times d$); the direction is from positive to negative charge.

State Gauss's law.

The total electric flux through a closed surface equals the enclosed charge divided by the permittivity of free space ($\text{Φ} = \frac{q_{\text{enclosed}}}{\text{ε}_0}$).

What happens to the force between two point charges if their separation distance is doubled?

The force becomes one-quarter of its original value ($F$ becomes $\frac{F}{4}$).

If charges are moved from air to water with a dielectric constant of $80$, how does the force change?

The force decreases by a factor of $80$. injection

What is the expression for the quantization of charge?

$Q = ne$, where $n$ is an integer and $e = 1.6 \times 10^{-19} C$.

Why do electric field lines never cross?

If they crossed, the electric field would point in two different directions at the same point, which is impossible.

What is the effect of a non-uniform electric field on an electric dipole?

It experiences both a net force and a torque.

What does the conservative nature of an electric field imply regarding work?

The work done moving a charge is independent of the path taken and depends only on the initial and final positions.

Define electrostatic potential ($V$).

The work done per unit positive charge in bringing it from infinity to a specific point ($V = \frac{W}{q_0}$).

What is an equipotential surface?

A surface where the electric potential is the same at every point, and the electric field lines are always perpendicular to it.

Describe the equipotential surfaces of a point charge.

Concentric spheres centered on the charge.

Why is the electric field inside a conductor in electrostatic equilibrium zero?

Free electrons redistribute themselves on the surface until they cancel any internal field.

Define the electronvolt ($eV$) and provide its conversion to Joules.

A unit of energy gained by an electron accelerated through $1 V$; $1 eV = 1.6 \times 10^{-19} J$.

Define electric current and state its formula.

The rate of flow of electric charge; $I = \frac{Q}{t}$ (measured in Amperes, $A$).

What is the superposition principle for electric forces?

The net force on a charge is the vector sum of the individual forces exerted by all other charges.

Where is the strong nuclear force present and what is its role?

It exists inside the nucleus between protons and neutrons, binding them together against electrostatic repulsion.

Give two examples of the conservation of electric charge.

1. Radioactive Beta decay (neutron becomes proton + electron + antineutrino). 2. Pair production (photon becomes electron + positron).

List the three types of charge densities with their formulas and units.

1. Linear ($\text{λ} = \frac{Q}{L}$) in $C/m$. 2. Surface ($\text{σ} = \frac{Q}{A}$) in $C/m^2$. 3. Volume ($\text{ρ} = \frac{Q}{V}$) in $C/m^3$.