Physics 212
Section
Group
Force on a Point Charge Due to an Electric Field I
Name
1
Signature
Grade
After this activity you should know:  know the basic relation for the force on a point charge due to a electric
field and the direction and magnitude of the force. ● know the difference between electric field and electric force.
1. The electric field at a point P is ⃗𝑬. What is the electric force if a new charge q is placed at point P? Answer in
⃗ . This is the basic relation you will use in this activity. Be careful with notation – make sure
terms of 𝑞 and 𝑬
to put arrows on quantities that are vectors.
⃗ of 120,000 𝑁/𝐶
2. A region has a uniform electric field 𝑬
𝑜
directed at an angle 120 ccw from +𝑥. Here uniform
means constant in space. The electric field is caused by
charges not shown.
a. In the left diagram, draw an arrow showing the
direction of the electric force on a new charge of
+4 𝜇𝐶 placed at the point marked x. Hint: what is
the direction when you multiply a vector by a
positive scalar?
+y
+x
⃗𝑬
⃗
×
⃗𝑬
⃗
×
b. In the right diagram, draw an arrow showing the
direction of the electric force on a new charge of
−4 𝜇𝐶 placed at x. Hint: what is the direction when
a vector is multiplied by a negative scalar?
c. What is the magnitude of the electric force that the electric field exerts on the −4 𝜇𝐶 charge?
d. What is the direction of the electric force this electric field exerts on the −4𝜇𝐶 charge? Give angle
counterclockwise from +x between 0𝑜 and 360𝑜 .
Physics 212
Force on a Point Charge Due to an Electric Field I
2
3. A pith ball (a small object that picks up charge easily) of mass 2.5 grams sits in
an electric field of magnitude 300,000 N/C pointing straight downwards. The
pith ball is negatively charged with a charge of −0.15 𝜇𝐶 (micro-coulomb).
𝑔
e. Draw the free body diagram showing the force on the pith ball. Include
gravity in your analysis. Label the electric force 𝐹𝐸 (not 𝐸) to avoid
confusion between the electric force and electric field.
f.
𝐸
Apply Newton’s 2nd law to determine the magnitude and direction
(up/down) of the acceleration of the pith ball.
4. A pith ball (a small object that picks up charge easily) of
mass 𝑀 is tied to a string. The pith ball sits in a uniform
electric field of magnitude 𝐸𝑜 (created by other charges)
directed as shown. The string is at an angle 𝜃 when
equilibrium is reached. Use the free body diagram method
to determine the charge 𝑞 of the pith ball.
g. Draw a label free body diagram showing the forces acting
on the pith ball. (Remember that the labels in the FBD are
the magnitudes of the forces. Label the electric force as 𝐹𝐸
– labeling the electric force as 𝐸 will confuse the force with
the electric field.)
𝜃
𝐸𝑜
𝑔
+𝑦
+𝑥
h. Use Newton’s 2nd law in the 𝑥 and 𝑦 directions to determine the charge 𝑞 in terms of 𝑀, 𝐸𝑜 , 𝜃, and/or 𝑔.