Coulomb’s & Newton’s law


a) Why does a shirt or blouse taken from clothes dryer sometimes cling to your body? (1 point)
b) If a metal object receives a positive charge, does it mass increase, decrease or stay the same? What happens to its
mass if the object receives a negative charge? explain your reasoning. (1 point)
c) In the operating rooms, doctors and technicians must take care not to create an electric spark, since the presence
of oxygen gas used during operations increases the risk of a deadly fire. Why should operating-room personnel wear
conducting shoes instead of non-conducting? (1 point)
d) The form Coulomb’s law is very similar to that for Newton’s law of universal gravitation. What are the differences
between these two laws? (1 point)
e) When probing the electric field direction, why it is necessary to specify that the magnitude of the charge of the test
charge be very small? (1 point)
Conceptual Question 2 (2 points): Three point charges have equal magnitudes and are located on the same line. The
separation d between A and B is the same as that between B and C. One of the charges is positive and two are
negative, as shown in the figure below. Which charge experiences the greatest net electrostatic force due to the
other two charges? Please justify your answer.
Conceptual Question 3 (3 points): Three point charges have equal magnitudes and are fixed to the corners of an
equilateral triangle. Two of the charges are positive and one is negative, as the drawing shows. Which charge
experiences a net electrostatic force directed along the x-axis? Please explain your reasoning.
Problem 1 (3 points): Near the surface of the Earth, an electric field points radially downward and has a magnitude of
approximately 100 N/C.
a) What charge (magnitude and sign) would have to be placed on a penny that has a mass of 3.11 grams to make it
float in equilibrium on air i.e. make it levitate? (2 points)
b) What charge (magnitude and sign) would have to be placed on the penny to cause it to rise into the air with an
initial upward acceleration of 0.190 m/s2? (1 point)
Problem 2 (7 points): A small sphere with mass of 0.002 g and carrying a charge of +5.00×10-8 C hangs from a thread
near a very large, charged conducting sheet, as shown in the figure below. The charge density ? on the sheet is
-2.50×10-9 C/m2.
a) Draw clearly the electric field lines between the conductive plane and the sphere. (2 points)
b) Calculate the magnitude of the electrostatic force exerted on the sphere by the electric field produced by the
sheet. The electric field of a conducting sheet is given by E=?/2eo where eo=8.85×10-12 C2/Nm2 is the permittivity of
free space. (1 point)
c) Determine the angle ? of the thread with respect to the vertical. (3 points)
d) If the thread is cut, calculate the net acceleration which the small sphere will attain. (1 point)
Problem 3 (5 points): Two point-charges +q1 and -q2 are held in place 4.50 cm apart. Another point negative charge
Q= -1.75 µC of mass 5.00 grams is initially located 3.00 cm from each of these charges as shown in the figure below.
You observe that the initial acceleration of Q is +324 m/s2 upward (as shown in the figure), parallel to the line
connecting the two point-charges.
a) Determine the charges q1 and q2. (3 points)
b) Calculate the magnitude and direction of the electric field produced at the location of charge Q. (2 points)