21. Which of the following statements about solid conductors in electrostatics are true?

 

I. The electric field inside the conductor is always zero.
II. The electric potential inside the conductor is always zero.
III. Any net charge is on the surface.

24. A sample of radioactive material has an initial activity of 10,000 counts/minute. 30 minutes later, its activity is 2,500 counts/minute. The half-life of the material is:

25. One end of a massless rope is attached to a mass m; the other end is attached to a mass of 1.00 kg. The rope is hung over a massless frictionless pulley as shown in the accompanying figure. Mass m accelerates downward at 5.0 m/s² . What is m? 

 

 

26. As shown in the accompanying figure, a force F is exerted at an angle of θ. The block of weight mg is initially moving the right with speed v. The coefficient of friction between the rough floor and the block is μ. The frictional force acting on the block is:

 

27. A mass m of helium gas is in a container of constant volume V. It is initially at pressure P and absolute (Kelvin) temperature T. Additional helium is added, bringing the total mass of helium gas to 3m. After this addition, the temperature is found to be 2T. What is the gas pressure?

28. Four identical light bulbs K, L, M, and N are connected in the electrical circuit shown in the accompanying figure. In order of decreasing brightness (starting with the brightest), the bulbs are:

 

29. Four infinitely long wires are arranged as shown in the accompanying figure’s end-on view. All four wires are perpendicular to the plane of the page and have the same magnitude of current I. The conventional current in the wire in the upper right-hand corner is directed into the plane of the page. The other conventional currents are out of the plane of the page. Point P is a distance a from all four wires. What is the total magnetic field at point P?

 

30. The conventional current I in a long straight wire flows in the upward direction as shown in the figure to the right. (Electron flow is downward.) At the instant a proton of charge +e is a distance R from the wire and heading directly toward it, the force on the proton is:

 

31. Light of a single frequency falls on a photoelectric material but no electrons are emitted. Electrons may be emitted if the:

32. In Young’s double slit experiment, second and higher order bright bands can overlap. Which third order band would occur at the same location as a second order band of wavelength 660 nm?

33. An air track car with mass m and velocity v to the right collides elastically with a second air track car with mass 2m and initial velocity zero. What is the velocity of the 2m car after the collision?

34. A hypothetical planet orbits a star with mass one-half the mass of our sun. The planet’s orbital radius is the same as the Earth’s. Approximately how many Earth years does it take for the planet to complete one orbit?

35. A gas can be taken from state a to c by two different reversible processes, a→c or a→b→c. During the direct process a→c, 20.0 J of work are done by the system and 30.0 J of heat are added to the system. During the process a→b→c, 25.0 J of heat are added to the system. How much work is done by the system during a→b→c?

 

36. What is the current through the 6.0 W resistor shown in the accompanying circuit diagram? Assume all batteries have negligible resistance.

 

 

37. You are given three 1.0 Ω resistors. Which of the following equivalent resistances CANNOT be produced using all three resistors?

38. A force F at an angle θ above the horizontal is used to pull a heavy suitcase of weight mg a distance d along a level floor at constant velocity. The coefficient of friction between the floor and the suitcase is μ. The work done by the frictional force is:

39. A small object with charge q and weight mg is attached to one end of a string of length L. The other end is attached to a stationary support. The system is placed in a uniform horizontal electric field E, as shown in the accompanying figure. In the presence of the field, the string makes a constant angle θ with the vertical. What is the sign and magnitude of q?

 

40. A block of mass M is initially at rest on a frictionless floor, as shown in the accompanying figure. The block, attached to a massless spring with spring constant k, is initially at its equilibrium position. An arrow with mass m and velocity v is shot into the block The arrow sticks in the block. What is the maximum compression of the spring?