Download Lesson 2: Coulomb`s Law

Lesson 2: Coulomb’s Law
Physics 30 Outcome 1
Lesson 2: Coulomb’s Law
Hewitt-Drew-it! PHYSICS 88 Coulomb's Law
Charge: Symbol
unit
Rule for using charge in the Physics 30 formulas:
Charles-Augustin de Coulomb
Born on June 14, 1736, in Angoulême,
France, Charles-Augustin de Coulomb
studied engineering and plied his trade with
the military before winning accolades for his
work in torsion balances. He offered
pioneering theories in the force found
between electrical charges, as well as
magnetic attraction and repulsion. The unit
of measurement known as the coulomb is
named in his honor. He died in Paris on
August 23, 1806.
Part 1: Coulomb’s Torsion Balance:
Explain how Coulomb’s Torsion Balance was used to determine the force between two charged spheres:
Lesson 2: Coulomb’s Law
Physics 30 Outcome 1
Part 2: The Coulomb Experiment
Variables
Activity: The Physics Classroom Coulomb’s Law Interactive
Google “Coulomb's Law Interactive - The Physics Classroom” to access the
Coulomb’s Law app. Draw the frame to be a larger size so your whole group
can see it on the screen.
The app starts out with a positive and negative charge.
Manipulated
QUESTION 1: What is the magnitude of each charge? Which charge is larger?
Controlled
Responding
Move them closer together until you see force arrows.
QUESTION 2: Which way do the force arrows point and how do their sizes compare to each other? Is this
result expected? Is there a law of physics that explains/predicts this?
Move the charges around, changing the distance between them so they are closer together and further
apart.
Gather data from at least 8 points and plot the force as a function of distance using the graph below. Note
that separation distance is the distance between the centres of the objects.
Distance (m)
Force (N)
Use your graphing calculator or Desmos to plot each of the following graphs. Sketch the results below
Linear
Quadratic
Inverse Square
QUESTION 3: Which of the three graph types above looks the most like your graph of force as a function of
distance?
Lesson 2: Coulomb’s Law
Physics 30 Outcome 1
Part 3: Coulomb’s Law
Coulomb’s Law is an inverse square law that describes the force between two point charges:
Example: What is the force on the +3.0 μC charge?
Demo: Electric forces and gravitational forces
Example: Suppose you hung two pith balls from long insulating strings of
length 10 cm. You then touched the pith balls with a charged Teflon rod,
causing them both to become equally charged. The pith balls are seen to
repel each other and hang a distance d=2.5 m apart. The pass of each pith
ball is 0.50 g. Determine the magnitude of the electric
force on each ball and calculate the charge.
Lesson 2: Coulomb’s Law
Physics 30 Outcome 1
Part 4: Curve Straightening
In a Coulomb-type experiment, students were investigating the relationship between the force of electrostatic
repulsion acting on two charged spheres and their separation distance The results of their investigations
yielded the following results.
1) Draw a graph of the results shown in the table.
2) From the shape of the graph, what is the relationship between the electrostatic force and the
separation distance between two charges?
3) Make a new table of values to obtain data to straighten the graph (Curve Straightening)
Fe =
kq1q2
r2
!1"
Fe = kq1q2 ⋅ $ 2 %
&r '
you can compare this to y = m*x (equation of a line) where Fe is your y,
(1/r2) is your x.
4) Draw a Graph of the data in your new table of values
5) Determine the slope of the graph
6) What does the slope of this graph represent?
7) If the charges of the two spheres are the same, what is the value of the charge on each sphere?
Lesson 2: Coulomb’s Law
Physics 30 Outcome 1
Part 5: Vectors and Vector Addition
Vectors
Scalars
Vector notation:
Magnitude [angle] notation (Alberta high school):
Component notation (U of C first year):
Unit vector notation:
Example: Draw the force vector with a magnitude of 150 N at an angle of 30o north of east and write it in the
three different notations.
Example: A box is pushed along a road with a force of <13, -14> N. Convert this force vector into magnitude
[angle] notation.
Lesson 2: Coulomb’s Law
Physics 30 Outcome 1
Vector Operations:
Multiply by a scalar
Example: A 10 kg box is pushed with an acceleration of 15 m/s2 [12o N of W]. What is the force on the box?
Example: A car travels with a velocity of <15, -10> m/s for 15 s. What is the car’s displacement?
Add two vectors
Example: Add the force vectors F1 = 15 N [30o S of E] and F2 = 25 N [12o N of E]
Note: you are not able to add a vector to a scalar or multiply to vectors using high school vector algebra.
Part 6: Superposition of Electric Forces
The Principle of Superposition for Coulomb's Law of Electric Force | Doc Physics
The principle of superposition states that the
interaction between any two charges is unaffected by
the presence of others.
In other words, if there are multiple charges in an area
and you are interested in the total force on one of these
charges, let’s call it qo, you can calculate the force
between qo and each charge individually, ignoring the
others, and then add the forces together.
Physics 30 Outcome 1
Example: Calculate the net force on charge 3
Example: What is the net force on charge 2?
Lesson 2: Coulomb’s Law
Physics 30 Outcome 1
Example: What is the force on charge 3?
Example: What is the force on charge 1?
Lesson 2: Coulomb’s Law
Lesson 2: Coulomb’s Law
Physics 30 Outcome 1
LESSON 2 SUMMARY NOTES
Name: ___________________________________
Hand in instructions: Take a picture or scan (using a scanning app) of this page when completed and
submit the image file or pdf to the dropbox folder for Outcome 1 Summary Notes on D2L.
Fill in everything you have learned about the following topics using your lesson 1 booklet
Give the symbol and unit for charge
Describe or draw Coulomb’s Torsion Balance
Write the equation for Coulomb’s law and state the meaning and unit for each of the symbols in the equation
What is the shape of an inverse square graph and how does this relate to Coulomb’s Law?
Create a vector. Draw your vector as a vector diagram (triangle) and show it written in each of the three
notations.
Explain how to convert between the each of the three forms of vector notation.
How do you add two vectors? Give an example
What is the principle of superposition for electric force?
Physics 30 Outcome 1
Lesson 2: Coulomb’s Law
Lesson 2 Practice Problems
1. In a hydrogen atom, an electron is 5.29 x 10-11 m from a proton. An electron has a charge of -1.60 x 10-19 C,
and the proton’s charge is +1.60 x 10-19 C. Calculate the electrostatic force of attraction acting on the two
charges. [8.22 x 10-8 N [attraction]]
2. The electric force between two charged objects is 5.2 x 10-4 N when the objects are 3.11 x 10-1 m apart.
What is the electric force between the same objects if the distance is changed to 4.04 x 10-1 m? (3.1 x 10-4 N)
3. Two small metallic spheres have the same mass and volume. One of the spheres has a charge of +4.00
μC and the other a charge of -1.00 μC. If the two spheres are brought into brief contact with each other and
are then separated to a distance of 0.200 m, what is the electric force between them? (0.506 N)
4. Three charges are placed in a line, as shown in the diagram.
a) What is the net electrostatic force on charge A? [1.20 x 104 N [right]]
b) What is the net electrostatic force on charge B? [7.49 x 103 N [left]]
5. Calculate the net electrostatic force on charge B shown in the figure to the right.
[2.54 x 102 N [225o]]
6. A metal sphere with a negative charge of 3.00 µC is placed 12.0 cm from another similar metal sphere with
a positive charge of 2.00 µC. The two spheres momentarily touch, and then return to their original
positions. Calculate the electrostatic force acting on the two metal spheres. [1.56 x 10-1 N [repulsion]]
7. A metal sphere with a charge of -2.50 x 10-9 C is 1.50 cm to the left of a second metal sphere with a charge
of +1.50 x 10-9 C. A third metal sphere of -1.00 x 10-9 C is situated 2.00 cm to the right of the second charged
sphere. If all three charges form a line, determine the net electrostatic force on the second sphere. [1.16 x 10-4
N [left]]
8. Two point charged objects produce an electric force of 0.0620 N on each other. What is the electric force if
the distance between them increases three times and one of the charges is doubled? (0.0138 N)
9. From the diagram to the right determine the net
electrostatic force on C. [16.8 N at 12.6o W of S]
10. In the diagram to the right, A has a charge of +0.30 μC,
B has a charge of -0.20 μC and C has a charge of
-0.20 μC. What is the net force on A? (0.093 N [S])