Electricity and Magnetism Answers

High-voltage power supplies are sometimes designed intentionally to have rather large internal resistance as a safety precaution. Why is such a power supply with a large internal resistance safer than a supply with the same voltage but lower internal resistance?

A rule of thumb used to determine the internal resistance of a source is that it is the open-circuit voltage divided by the short-circuit current. Is this correct? Why or why not? 

Two copper wires with different diameters are joined end to end. If a current flows in the wire combination, what happens to electrons when they move from the larger-diameter wire into the smaller-diameter wire? Does their drift speed increase, decrease, or stay the same? If the drift speed changes, what is the force that causes the change? Explain your reasoning.

A conductor is an extreme case of a dielectric, since if an electric field is applied to a conductor, charges are free to move within the conductor to set up “induced charges.” What is the dielectric constant of a perfect conductor? Is it K = 0, K S q or something in between? Explain your reasoning.

A capacitor made of aluminum foil strips separated by Mylar film was subjected to excessive voltage, and the resulting dielectric breakdown melted holes in the Mylar. After this, the capacitance was found to be about the same as before, but the breakdown voltage was much less. Why?

Two parallel-plate capacitors, identical except that one has twice the plate separation of the other, are charged by the same voltage source. Which capacitor has a stronger electric field between the plates? Which capacitor has a greater charge? Which has greater energy density? Explain your reasoning.

Suppose the two plates of a capacitor have different areas. When the capacitor is charged by connecting it to a battery, do the charges on the two plates have equal magnitude, or may they be different? Explain your reasoning.

two parallels wires, wire P and Q which carries a current of 30A and 50A, respectively in the same direction.The wires are placed 1.25 mm apart in a vacuum.

a) Calculate the magnetic force per unit length on wire P.

b) Determine the position x from wire P in between the wires where the magnetic field is zero.

. A charge of 20 x 10−6 C is 20 cm from another charge of 20 x 60−6 C

a. Find the force between them.

b. Determine the 𝐸𝑝 midway between them.

c. How much force will be experienced by an electron placed midway between them?

The relative permittivity of a dielectric is 2.25

A) what is its permittivity?

B) what will be the capacitance if this inserted into a 7.8 uF air capacitor