PHYS 202 Test #1 2/4/2009 Dr. Holmes NAME
DO ALL EIGHT PROBLEMS. THE WORTH OF EACH PROBLEM IS MARKED NEXT TO THE PROBLEM. SHOW YOUR WORK FOR PARTIAL CREDIT.
1) Consider two charged particles (q1 = -3 microCoul, m1 = 31 grams; q2 = -7 microCoul, m2 =17 grams). The first particle is 34 cm to the North of the second one.
a) What is the magnitude and direction of the electric FORCE on the 1st
particle due to the presence of the 2nd particle? [When answering this direction question, answer with North,
East, South, West, Up or Down.]
1.63 Nt, North
b) Is the magnitude of the force on the second particle due to the first particle bigger, the same as, or smaller than the force on the first particle due to the 2nd particle?
Same.
c) Is the direction of the force on the first particle due to the 2nd particle the same or opposite that of the force on the second particle due to the 1st particle?
Opposite.
d) Is the magnitude of the acceleration of the second particle due to the presence of the first particle bigger, the same as, or smaller than the acceleration of the first particle due to the second particle? (Assume there are no other forces acting on the two particles.)
Bigger.
2) Consider the same two point charges as in problem #1: (q1 = -3 microCoul, m1 = 31 gm; q2 = -7 microCoul, m2 = 17 gm). The first particle is 34 cm to the North of the second one.
a) What is the electric field due to charge #1 at the location of charge #2 ?
2.34 x 105 Nt/Coul,
North.
b) What is the voltage due to charge #1 at the location of charge #2 ?
- 79,412 Volts .
3) a) What is the magnitude of the electrical force between the proton and the electron when they are separated by 0.04 nm?
1.44 x 10-7 Nt.
b) What speed would the elecgron have to orbit with if it were to orbit the proton at this distance (radius = 0.04 nm)?
2.52 x 106 m/s.
4) A dust particle of charge -3 nC and mass m = 38 micrograms is initially at rest and at a location where the potential is –10 volts.
a) Will it tend to go to a position where the voltage is higher or lower?
Higher.
b) What will be the speed of the particle at a new position that is different by 30 volts (i.e., either +20 volts or -40 volts, based on your answer to the first part) if there is no friction or air resistance on the particle?
2.18 m/s.
5) A 22 pF capacitor has a voltage of 50 volts applied across it.
a) How much charge is stored on the capacitor?
1.10 x 10-9 Coul. = 1.10 nC.
b) How much energy is stored on the capacitor?
2.75 x 10-8 Joules = 27.5 nJ.
c) If the capacitor is a parallel plate type of capacitor, and the charge on the plates is doubled, which of the following quantities will change:
(1) capacitance, (2) energy stored, (3) charge stored?
[if none change, then answer none; if one changes, simply indicate which changes, if two change, then indicate both of those that change, if all three change then answer all]
2 (energy stored) and 3 (charged stored)
6) A particular light bulb is rated at 60 Watts when a voltage of 110 volts is placed across it.
a) What is the electric current through the light bulb in this circumstance?
0.55 amps.
b) What is the resistance of the light bulb?
202 W .
c) In designing a new wattage light bulb, should the resistance of the light bulb be lowered or raised from that in part b above if the power of the light bulb is to be increased when used with the same 110 volts power supply?
Lowered.
7) Consider three capacitors: C1 = 3 nF, C2 = 9 nF, C3 = 15 nF .
a) Connect the three capacitors in a circuit (make a circuit drawing) such that the effective capacitance is the smallest it can be:
b) Are the three capacitors above connected in series, parallel, or some other combination?
Series.
c) What is the effective capacitance of this circuit?
1.96 nF.
8) Consider three resistors: R1 = 3 W , R2 = 9 W , R3 = 15 W .
a) Connect the three resistors in a circuit (make a circuit drawing) such that the effective resistance is the smallest it can be:
b) Are the three resistors above connected in series, parallel, or some other combination?
Parallel.
c) What is the effective resistance of this circuit?
1.96 W .
d) Connect the three resistors in a circuit (make a circuit drawing) such that the effective resistance is between 3 and 9 ohms.