Physics 114 –Summer 2008

Participation Points #2

This is an in-class work, to be turned in at the end each discussion session.

Use provided space to show your work only, use back of page for notes. Please staple this set on top of set #1.

 


Student’s Name:____________________ Team #________     Previous points

 

 

 

 

 

 

 

 

 

 

 

1.-           (a) Can we cool the kitchen by leaving the refrigerator door open? Circle one: Yes    No. Explain:

 

 

(b) To maximize the COP o a refrigerator, should you strive for large or small temperature difference? Large     Small. Explain:

 

 

(c) If new materials were developed that could withstand higher temperatures than those encountered in power plants, how would that help us save energy?

 

 

 

2.-            2nd Law TD. 4L of water at 11 oC is placed in a refrigerator. The refrigerator 125W motor runs for 4 min to cool the water to the refrigerator low temperature of 1oC. What is the COP of the refrigerator as compared to the maximum possible COP if it exhausts heat at 25oC. Hints: COP=heat extracted at cold temperature/work received

 

 

 

 

 

 

 

 

3.-            Entropy.5-mol of an ideal diatomic gas is initially at 1 atm pressure and 300 K. What are the entropy changes if the gas is heated to 600K at constant volume and at constant pressure? Hints: they are connected in the same way as the corresponding specific heats.

 

 

 

 

 

 

 

 

 

4.-            (a)The electrical force between an electron and a proton is 1040 stronger than the gravitational force, matter consists largely of electrons and protons, then how can the gravitational force be important at the macroscopic level? One phrase answer:

 

 

(b) The field of a dipole decreases with the inverse cube of the distance, does this violate our assertion that the field of a charge distribution approaches that of a point charge at large distances, which follows an inverse-square law? Explain in one phrase.

 

 

(c) Why should a test  charge used to measure the electric field be small?

 

 

 

5.-            Electric field. You are 1.5m from a charge distribution whose size is much less than 1m, measuring an electric field of 282N/C. You moved to 2.0m and the new field strength is 119N/C. What is the net charge of the distribution? Hint: does this field satisfy the inverse-square law?

 

 

 

 

 

 

 

 

 

 

 

6.-            Matter in Electric Fields. Ink jet printers work by deflecting moving ink droplets with an electric field so they hit the right place on the paper. E.g. droplets with m=1.1x10-10 kg, charge 2.1 pC speed 12m/s, passing through a uniform 100kN to be deflected through a 10o angle, what is the length of the field region? (Hint: vy=at=aΔx/vx )

 

 

 

 

 

 

 

 

 

7.-            (a) The electric flux through a closed surface is zero, must the electric field be zero on that surface? Yes   No. Draw an example.

 

 

 

(b) The field of an infinite line of charge falls as 1/r, state the inverse-square law such that this is not a contradiction.

 

 

(c) Why must the electric field at the surface of a conductor in electrostatic equilibrium be perpendicular to the surface?

 

 

8.-            Gauss’s Law. A thick, spherical shell of inner and outer radii a and b carries a uniform charge density ρ. What is an expression for the electric field strength for a<r<b? (Hint:draw appropriate Gaussian surface and determine the total charge enclosed by this surface)

 

 

 

 

 

 

 

 

 

 

 

 

9.-            Conductors. A point charge placed at the center of an uncharged spherical conducting shell o inner radius 2.5cm and outer radius 4cm. As a result, the outer surface of the shell acquires a surface charge density σ = 70 nC/cm2. What is the point charge and the surface charge density on the inner wall of the shell? (Hint: charges are induced on the surfaces of the conductor such that Gauss’s Law give zero electric field inside the conductor and nonzero outside)