Try these "busters" to exercise your brain ... they should help you grasp the concepts underlying electric charges, forces, fields and Gauss's Law. To gain the maximum effect you should attempt to answer them before looking at the answers!
[1] A rod made from insulating material carries a net charge, while a copper sphere is neutral. The rod and the sphere do not touch. Is it possible for the rod and sphere to (a) attract one another and (b) repel one another?
[2] If you blow up a balloon and rub it against your shirt a number of times you give it a net electric charge. If you touch the balloon to the ceiling, or the wall, on being released it will remain "stuck". Why?
[3] Three point charges are fixed to the corners of a square, one to a corner, in such a way that the electric field at the empty corner is zero. Do these charges all have the same sign or the same magnitude but different signs, etc, or what? Explain your reasoning.
[4]
The figure shows three charges, Q1, Q2 and Q3, with a Gaussian surface drawn around Q1 and Q2. Which charges determine the electric flux through the Gaussian surface? Which charges produce the electric field at the point P?
[5] A charge +Q is placed inside a spherical Gaussian surface; it is not located at the center. Can Gauss's Law tell us where the charge is located?
[6] Two charges, +Q and -Q, are inside a Gaussian surface. Since the net charge inside the Gaussian surface is zero, Gauss's Law tells us that the electric flux through the surface is also zero. Does this fact imply that the electric field at any point on the Gaussian surface is also zero? Explain your reasoning.
[7] A positively charged particle is moving horizontally when it enters a region between two charged, parallel plates as shown below.
What does the trajectory of the particle look like as it travels through the plates and out the other side? When the particle is between the plates which of the following four vectors, if any, are parallel to the electric field:
[8] Two metal balls have charges +Q and -Q. How will the force between them change if the balls are placed in water with the same separation?
[9] Two point charges, +Q and -Q, are placed a distance, d, apart. How will the force between them change if an initially uncharged hollow conducting sphere, of radius R < d, is placed around, and centered on, the +Q charge?
[10] At the Earth's surface there is an electric field of about 200N/C directed towards the center of the Earth. Does this mean the Earth is charged? Explain.
[11] If the electric field is zero everywhere on a closed surface, is the net flux through the surface necessarily zero? What, then, is the net charge inside the surface?
[12] A cylindrical piece of (neutral) insulating material is placed in an external electric field as shown below.
What is the net electric flux passing through the surface? Is it
[13] An electric dipole is placed in different electric fields. In which case(s) is there no net force on the dipole?
Why?
[14] If a positively charged comb is brought near a stream of water falling from a faucet, the stream is deflected towards the comb. Why?
What would happen if a negatively charged rod were brought near the stream of water instead ... would the stream deflect away from the rod?
[15] Can you think of an example where electric field lines cross each other?
[16] Two charged conductors are brought close to each other. The electric field lines are shown below.
What can you say about the signs and relative magnitudes of their charges? Can you explain how such a set of electric field lines could be produced?
[17] You hold in your hands two objects that have identical charge Q.
Estimate the magnitude of Q if, when you exert maximum force, you are only able to bring them as close together as 10cm because of the repulsive force between them.
[18] Can charge objects exist but with no electric field(s)?
[19] An electron and a proton are held some distance apart. After they are released they will move towards each other and collide.
(ii) Which, if either, is traveling faster when the collision occurs?
[20] In a typical TV or monitor, electrons leave an electron gun with speeds of about 5 × 107 m/s. The electric field due to the Earth is about 200 N/C directed towards the Earth's surface. Can you estimate the amount of deviation an electron will experience in a typical TV or monitor between the exit of the gun and the screen?