Electric Charges and Fields quiz

The SI unit of electric charge is:

Quantisation of charge means:

When two charges are placed in a medium of dielectric constant K, the force between them becomes:

Electric field lines:

The flux through a closed surface enclosing no charge is:

An electric dipole placed in a uniform electric field experiences:

The electric field inside a charged conducting spherical shell is:

The SI unit of electric field is:

The electric field of a dipole at a large distance r on its axis varies as:

The charge on an electron is:

What is charging by induction? Explain with an example. (2 marks)

Why do two field lines never cross each other? (2 marks)

What is the principle of superposition of electric forces? (2 marks)

Define electric flux. Is it a scalar or vector? (2 marks)

Write the expression for the electric field of a dipole on its equatorial line. (2 marks)

Why is the electric field due to an infinite plane sheet independent of distance? (2 marks)

A positive charge is released from rest in a uniform electric field. Describe its motion. (2 marks)

What is a Gaussian surface? Why is it chosen to match the symmetry of the charge? (2 marks)

When is the torque on a dipole in a uniform field maximum and when is it zero? (2 marks)

Why can an insulator not be charged by induction easily while a conductor can? (2 marks)

Two equal point charges placed 1 m apart in air repel each other with a force of 0.9 N. Find the magnitude of each charge. (3 marks)

Find the ratio of the electrostatic force to the gravitational force between an electron and a proton (k = 9 x 10^9 SI, G = 6.67 x 10^-11 SI, me = 9.11 x 10^-31 kg, mp = 1.67 x 10^-27 kg, e = 1.6 x 10^-19 C). (3 marks)

An electric dipole has a dipole moment of 1 x 10^-8 C m. Find the electric field at a distance of 10 cm from its centre on (i) the axial line and (ii) the equatorial line, assuming the dipole is short. (3 marks)

A uniform electric field of 3 x 10^3 N/C acts along the x-axis. Find the flux through a square of side 10 cm whose plane is (i) parallel to the yz-plane and (ii) whose normal makes 60 degrees with the x-axis. (3 marks)

An electron is placed in a uniform electric field of 2 x 10^4 N/C. Find the force on it and its acceleration (e = 1.6 x 10^-19 C, m = 9.1 x 10^-31 kg). (3 marks)

Read the passage and answer the questions. During a thunderstorm, charges separate in clouds; the lower part of a cloud often becomes negatively charged and induces a positive charge on the ground. When the field becomes very strong, a lightning discharge occurs. (i) Name the process by which the ground gets charged. (ii) Why are lightning conductors pointed? (iii) Why is it safe to stay inside a car during lightning? (5 marks)

Read the passage and answer the questions. In a photocopier, a drum is given a uniform positive charge. Light from the bright parts of the document makes those parts conducting so that they lose charge. Negatively charged toner sticks to the remaining charged parts. (i) Which property of charges is used for the toner to stick? (ii) Why does toner not stick to discharged areas? (iii) State the principle of conservation of charge. (5 marks)

Read the passage and answer the questions. Sensitive electronic instruments are kept inside metal boxes so that external electric fields do not affect them. (i) What is this arrangement called? (ii) Why is the field inside a closed conductor zero? (iii) Give one everyday application. (5 marks)

Read the passage and answer the questions. A water molecule has a permanent dipole moment of about 6.2 x 10^-30 C m. In a microwave oven, an alternating electric field makes these dipoles rotate rapidly, heating the food. (i) What is the torque on a dipole in a field E at angle theta? (ii) Find the maximum torque in a field of 1 x 10^3 N/C. (iii) Why does the rotation produce heat? (5 marks)

Read the passage and answer the questions. A student rubs a plastic comb with dry hair and finds that it attracts small bits of paper, but the effect disappears on a humid day. (i) How does the comb get charged? (ii) Why does it attract neutral bits of paper? (iii) Why does it fail on a humid day? (5 marks)

Derive expressions for the electric field due to a dipole at a point on (i) its axial line and (ii) its equatorial line, with diagrams. (6 marks)

State Gauss's law and use it to derive the electric field due to an infinitely long, uniformly charged straight wire. (6 marks)

Using Gauss's law, derive the electric field due to an infinite plane sheet of charge. Extend it to two parallel sheets with equal and opposite charge densities. (6 marks)

Using Gauss's law, find the electric field due to a uniformly charged thin spherical shell at points outside and inside it. Draw the graph of E against r. (6 marks)

Derive the expression for the torque on an electric dipole in a uniform electric field. Explain stable and unstable equilibrium positions with diagrams. (6 marks)