Physics 204 Exam 1

Lecturer: Dr. Min Xu

Oct 1, 2003
LAST NAME: ___________________________________________________________________________________________________ FIRST NAME: __________________________________________________________________________________________________ SS# (last 4 digits): _____________________________________________________________________________________________

General Instruction

You have 1 hour 30 minutes to answer 7 questions on this exam. Work each problem only on the sheet provided for this problem, using the back if necessary. Please write the name and recitation section on each sheet at the top of the page.

You must indicate the reasoning and show all the steps.

If you are unable to obtain the answer to part of a problem and you need the answer to solve the subsequent parts, use an appropriate symbol for the answer to the previous part and solve in terms of that symbol.

1. (5 points) A loudspeaker produces a sound wave. Does the wavelength of the sound increase, decrease, or remain the same, when the wave travels from air to water? Justify your answer. The frequency of sound does not change.

2. (10 points) Different standing wave patterns can be obtained by varying the tension of a string driven by an oscillator with a fixed frequency at one end of the string. The other end of the string is fixed. Should the tension be increased or decreased to form a standing wave of twice denser patterns. What is the ratio of the two tensions?

3. (10 points) On a thin, nonconducting rod, positive charges are spread evenly over the left half, and the same amount of negative charges are spread evenly over the right half. Deduce the direction of the electric field and the electric potential at a point h directly above the midpoint of the rod.

4. (10 points) Two loudspeakers at A and B are vibrating in phase. They are set up as in the figure. The speed of sound is 343m/s. The speakers play the same tone. What is the smallest frequency that will produce destructive interference at point C?

PIC

5. (15 points) When a car is at rest, its horn emits a frequency of 600Hz. A person standing in the middle of the street hears the horn with a frequency of 580Hz.


(a) (5 points) In which direction is the car moving (leaving or approaching the person)?
(b) (10 points) Find the speed of the car.

6. (35 points) Two point charges are lying on the y axis. The charges are q1 = 9mC at (0, 1)m and q2 = 9mC at (0,-1)m. A testing charge q0 = 1mC is put on the x axis at the position (1, 0)m.


(a) (10 points) Find the electric force acting upon q0.
(b) (5 points) What is the electric potential of q0?
(c) (5 points) What is the electric potential energy of q0?
(d) (10 points) What is the work done by an external force which moves q0 to the position (2, 0)m.
(e) (5 points) What is the total electric potential energy of the three charges?

PIC

7. (15 points) A 12V battery is used to operate a radio. The radio has a resistance of 10_O_. The internal resistance of the battery is 0.1_O_.


(a) (5 points) Find the current in the circuit.
(b) (5 points) What is the voltage applied on the radio?
(c) (5 points) What is the power consumed by the radio?