Download Electric Forces and Fields

Electric Forces and Fields
rev. 5/12
I
Electric Force
1. Two charges are positioned as shown, one charge twice the other.
A. What are the directions of the forces on the two charges?
-Q
+2Q
B. How do the magnitudes of the two forces compare?
1. The force on the +2Q is twice the force on the -Q
2. The force on the +2Q is half the force on the -Q
3. The forces on the two charges are equal in magnitude
2. A negative point charge -Q is located at x = 0 and a positive point charge +Q of equal magnitude
is located at x = 4 m as shown in the figure below
a) What is the direction of the force on a THIRD point charge –Q placed at x = +2 m?
b) at x = -4 m?
c) at x = +6 m?
d) Find the magnitude of each force for Q = 8 μC.
-Q
+Q
3. Four charges are positioned in a square, as in the figure below. The positive charges have a
charge of +6 nC. The lower left negative charge has a charge of -6 nC, and the upper right
negative charge has a charge of -3 nC.
a. What is the direction of the net force on the upper right charge?
b. What is the magnitude of the net force on the upper right charge?
4 cm
4 cm
Active Learning: Electric Forces and Fields
II
ELECTRIC FIELDS
1. What is the difference between a field and a force? Explain it below in plain English, as you
would to someone who missed class today and asked you.
2. In the picture below the negative test charge is moving in a straight line towards the stationary
negative source charge
a. What is the direction of the electric field at the location of the test charge? Draw the field
vector.
b. Is the test charge speeding up, slowing down, or moving at constant speed?
c. As the test charge moves closer to the source charge, does the field acting on the test charge
get larger, smaller, or stay the same?
v
3. The picture below shows two charges, +Q and -3Q, separated by d = 0.5 mm. Let Q = 2 nC.
A. What is the electric field (magnitude and direction) at point P located 2 cm to the left of the
positive charge?
B. Where along the x axis, other than infinity, is E = 0?
---P------------------------+Q ------ -3Q ---------------------d
Active Learning: Electric Forces and Fields
4. You have three charges: A long thin rod of length 0.75 m with a linear charge density λ = 400
nC/m, a thin square with sides 20 cm with and area charge density σ = 12 µC/m2, and a solid
sphere of radius 15 cm with volume charge density ρ = 28 µC/m3. Which charge is the largest?
5. An infinitely long rod which lies along the x axis has a linear charge density λ = 400 nC/m.
How much does the charge between x = -1 mm and x = +1 mm contribute to the electric field at
the point (0,1) m? (Hint: consider the charge along this very short line to be a point charge.)