Physics 202, Section I, Test 2 Mar 6, 2002

Problem 1 : Your electronics lab instructor gives you a 2 $k\Omega$ resistor and a bunch of 2, 4 and 8$\mu F$ capacitors, and asks you to make an RC circuit with a 24 ms time constant. Which capacitor(s) do you choose, and how do you wire your circuit (make a sketch of the wiring diagram)?

Problem 2 : A velocity separator makes use of electric and magnetic fields to force only the particle of the desired velocity to travel in a straight path. State what electric and magnetic fields (specify both magnitude and directions) could be used to select electrons traveling at $2\times 10^{3}~m/s$. (Stay within the limits $B\le~2T,~E\le 10^{6} V/m$). What would happen if you were to send through the same separator a) protons traveling with the same velocity b) electrons traveling twice as fast ?(it is sufficient to specify direction of deflection, if any)

Problem 3 : A compass is positioned to the South of a wire running vertically. When no current runs through the wire, the compass points due North, under the influx of the $2.5\times 10^{-5}T$ earth magnetic field. When a 4.3 A current runs through the wire, the compass needle is deflected and it points in a North-Easterly direction, 60 degrees away from North. How far is the compass from the wire? and what is the direction of the current in the wire?

Problem 4 : A square wire loop moving with constant velocity = 0.125 m/s, at t=0 enters a region of constant magnetic field = 0.2 T, perpendicular to the plane of the loop. If each side of the square has a 2 $\Omega$ resistance and measures 50 cm, what is the average current in the loop a) during the first 3 seconds and b) during the next 3 seconds? If the B field is into the page, will the current (if any) be clockwise or counterclockwise ?

1. A physics graduate student and his professor look at the bubble chamber picture sketched below. The student says: "we know that there is a constant B field coming out of the page, therefore this pictures represents the path of a negative particle losing energy", while the professor says "sorry, I think this is a positive particle gaining energy". Who is correct?
   1. both
   2. neither
   3. only the professor
   4. only the student
   5. not enough information to reach any conclusion

2. A circular loop of wire and a long straight wire are positioned as in problem 22.33, with the wire directly above the loop's center. If the current in the wire, always going from left to right, first decreases and then increases, there will be a current in the loop
   1. first going clockwise, then counterclockwise
   2. first going counterclockwise, then clockwise
   3. always clockwise
   4. always counterclockwise
   5. none of the above

3. Which of the following statements is not true
   1. an ideal voltmeter has a very high resistance
   2. an ideal ammeter has a very high resistance
   3. the purpose of a shunt resistor is to limit the current going through an ammeter's coil
   4. a voltmeter is to be connected in parallel to the circuit whose voltage one wants to measure
   5. mark this choice if you believe that all the above statements are true

4. Three parallel wires carrying equal current in the same direction are positioned at the three corners of a square. If the current is going into the page, what is the angle the magnetic field makes with the x-axis at the empty corner ?
   1. no angle, the field is zero
   2. not enough information to evaluate the angle
   3. 135$^0$
   4. -45$^0$
   5. none of the above

5. If the current sketched below is sent to the primary coils of a transformer, which graph describes the current in the secondary ?

Physics 202, Section II, Test 2 Mar 7, 2002

Problem 1 : Your lab instructor gives you a galvanometer and a drawerful of resistors, and asks you to assemble a voltmeter capable of measuring voltages as high as 20 V. As a first step, you determine that the galvanometer reaches full scale at a current of 10 ma, and its internal resistance is 100 $\Omega$. What additional resistor(s) do you pick, and how do you connect it (them) to the galvanometer? ( a sketch might be more clear than many words)


Problem 2 : A long wire carries a 5 A current from left to right, while another runs vertically with a 20 A current flowing upwards. If the shortest distance between the two wires is 40 cm, what is the magnitude of the magnetic field at the midpoint between the two wires?


Problem 3 : An electron is traveling in a region where the electric and magnetic fields are respectively oriented along the x and z axes, with magnitudes $6\times 10^{4} V/m$ and 0.15 T. At one instant, the electron's velocity is parallel to the E field, and its acceleration vector is at +45 degrees in the x-y plane. What is the electron's velocity in that instant?

Problem 4 : An electric mixer with a 20 $\Omega$ internal resistance connected to the 120 V AC supply draws a 1.5 A current when turning at full speed. What current will it draw if the thickness of the mixture slows it down to half speed ?

1. A wire loop centered above a current carrying wire is sliding back and forth along the wire. If the current in the wire is going from left to right, there will be a current in the loop
   1. clockwise when the loop is going left, counterclockwise when it's going right
   2. clockwise when the loop is going right, counterclockwise when it's going left
   3. always clockwise
   4. always counterclockwise
   5. none of the above

2. What is the flux of a 5 T magnetic field through a 27 m$^3$ cube positioned with one face perpendicular to the field?
   1. 0 Tm$^2$
   2. 135 Tm$^2$
   3. 45 Tm$^2$
   4. 90 Tm$^2$
   5. 270 Tm$^2$

3. One could cause a charged particle to move with constant velocity (both direction and magnitude) by means of
   1. a suitable choice of non-zero magnetic field, with zero electric field
   2. a suitable choice of non-zero electric field, with zero magnetic field
   3. both choices would work
   4. neither would work, the only choice is zero electric and magnetic fields
   5. not enough information to draw any inference

4. When the current through two parallel wires flows in the same direction
   1. the force among them is always attractive, regardless of the relative values of the current
   2. the force among them is always repulsive, regardless of the relative values of the current
   3. the force among them is for sure attractive only if the currents are the same, otherwise it depends on the relative current values
   4. the force among them is for sure repulsive only if the currents are the same, otherwise it depends on the relative current values
   5. none of the above statements is correct

5. The graph below shows the current going through an inductor. Make a sketch of the self-induced emf in the inductor over the same time interval.