SAM Teachers Guide Electrostatics Overview Students learn that oppositely charged particles attract each other and particles with the same charge repel each other. In addition, they discover that Coulombʹs force between a pair of particles is determined by the distance between the particles and the charge of each particle. Students explore the screening effect in systems, such as a water environment. Learning Objectives Students will be able to: • Explain how a neutral atom can become a charged particle, an ion. • Define Coulomb forces as a result of like charges repelling each other and unlike charges attracting. • Determine that the force generated between atoms is dependent on the amount of charge they carry and the distance between them. • Explore the mathematical relationship described in Coulomb equations. • Explain polarization in terms of charge redistribution. • Give an example of how screening is at work in a biological system, such as a cell. Possible Student Pre/Misconceptions • Gravity is the strongest force in the universe. • An atom has a positively charged nucleus with negatively charged electrons that orbit the nucleus on set paths, similarly to how the planets orbit the sun. • The electrostatic force between two atoms is independent of the distance between them. • A charged atom can only attract another charged atom. Models to Highlight and Possible Discussion Questions After completion of Part 1 of the activity: Models to Highlight: • Page 1 – Atomic Structure and Electric Charge o Review that changing the number of protons would change not just the charge, but also the identity of the element. Uneven numbers of protons and electrons result in a charged particle, an ion. o Link to other SAM activities: Atomic Structure. Students will need to understand the current model of an atom to understand •
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upcoming topics (such as Newton’s Laws, Electric Current, etc.) in the atomic world. Page 2 – Attraction and Repulsion o Highlight / review the fact that opposites attract and like charges repel as shown in the model. Page 3 – Hands On Coulomb Force o You may need to walk students through this model and highlight the different tools (how to increase and decrease steering force, and how to pull the green atom) prior to students running the model. Discuss what students learned from the model and highlight the conclusion that the greater the charge difference, the greater the force. Page 4 – The Distance Between Charged Particles Matters o Have students share their strategies for hitting the target and discuss the connection between distance and force. o Link to other SAM activities: Newton’s Laws. Forces are at work at the atomic level when atoms are in close proximity to each other. Page 5 – Maze Game o Review with students the strategies they used to beat the maze game. Possible Discussion Questions: • What is an ion? Why do particles attract or repel? • What would be the result in our world if there were no Coulomb force? • Does increasing the difference in charge affect attraction or repulsion? Does increasing the distance between particles affect attraction or repulsion? Why? • Demonstration/Laboratory Ideas: o Electrostatic Force Lab with electroscopes. o Use magnets to review charge attraction and polarity. After completion of Part 2 of the activity: Models to Highlight: • Page 6 – Polarization Model o Highlight the fact that the wall is neutral. What happens to atoms in the wall when the balloon is either positively or negatively charged? Why? Does that amount of charge have an effect? o Link to other SAM activities: Chemical Bonds. In a future chemistry lesson, students will explore how polarity affects chemical bonding. •
Page 7 – Screening Model o Review what happens in the model to the positive and negative charges. What happened when there were no electrons around? Possible Discussion Questions: • What does charge induction mean? • How can a material become polarized? What effects might this have? • What is a screening effect? What is a real‑world example of the screening effect? Why do salt ions not come together when placed in water? • Demonstration Idea: Static electricity, such as balloon and hair. Connections to Other SAM Activities The focus of this activity is to teach students that atoms can be positively or negatively charged. Opposite charges attract and like charges repel. This activity is supported by Newton’s Laws at the Atomic Scale, which shows that forces can impact the straight‑line motion of atoms. The impact of like or unlike charges on atomic motion is the focus of Electrostatics. This activity is also supported by Atomic Structure. To understand electrostatics, students must recognize that atoms can be positively or negatively charged because of a difference in their number of protons and electrons. The Electrostatics unit supports many other SAM activities because they deal with attraction of unlike charges and interaction of charges in general. First, Electricity, a natural extension, refers to the motion of charged particles, or an electric current. The source of Intermolecular Attractions is the attraction of positive and negative charges and Phase Change explores the role of these intermolecular attractions when substances change state. Chemical Bonds illustrates the different types of bonds that are a result of the shifting of charge density due to differences in electronegativity. The notion of “like dissolving like” as discussed in Solubility is the outcome of attraction between unlike charges. Molecular Geometry is based on the theory of VSEPR (Valence Shell Electron Pair Repulsion) where molecules have specific shapes because of the repulsion of their electrons. There is also a direct connection to Diffusion, Osmosis and Active Transport. With active transport, there is a buildup of electric potential. Four Levels of Protein Structure illustrates salt bridges as well as other types of attractions. Molecular recognition demonstrates the electrostatic attraction of proteins to ligands. In Lipids and Carbohydrates, solubility and intermolecular attraction are covered. In addition, carbohydrates are defined as polar molecules while lipids are non‑polar. Finally, in both Nucleic Acids and Proteins and DNA to Proteins, bonding patterns, such as those in nucleotides (A to T and C to G) are shown. Protein folding and reactions to their environments are also a direct result of charge interaction. Activity Answer Guide Page 1:
1. Create a negative ion and record below the
number of protons and the number of
electrons for that ion. (There are 21 protons
and 21 electrons initially.)
*Answers may vary. Make sure the number of
protons is less than the number of electrons. *
Sample Answer:
Number of Protons = 21
Number of Electrons = 22
2. An atom that has a charge of -2 has: (a)
Page 2:
1. Follow the instruction below to drag in a
snapshot image that shows atoms that have
repulsive forces:
positive charge results. Unlike charges attract
and the atoms move toward one another.
3. Describe the rule for whether or not atoms
will attract or repel each other.
Review: Particles with opposite charges attract
each other. Particles with like charges repel
each other. This is known as the Coulomb force.
Page 3:
1. Let's say the purple atom has a charge of
+3 and the green atom a charge of -1. It
would be easier to separate these atoms
than which of the following? (d)
2. From this experiment it can be concluded
that the greater the charge ... (b)
Page 4:
1. Take a snapshot to show that you
succeeded in shooting an electron into the
blue rectangle.
Sample snapshot: When electrons were added
to both atoms, the like charges repel. The result
is that the two atoms move away from one
another.
2. Follow the instruction below to drag in a
snapshot image that shows atoms that have
attractive forces:
Once the position of the positively charged
particle was moved, the target could be hit.
2. Which of the following settings would
cause the shot electron to travel in a straight
line? Check all that apply.
(a) (b) (c)
Sample snapshot: When an electron is added to
the left atom, a negative charge results. When
an electron is removed on the right atom, a
3. Describe where you placed the particle
and why this made the electron move
towards the target.
2. Describe what happens to the electrons in
the wall when a balloon with positive
charges sticks to the wall.
Answers will vary.
Sample Answer: Moving the proton to the –2
location with the slider will allow you to hit the
blue rectangle. The attraction of the electron to
the positive charge will cause the electron to
follow a path that hits the lower rectangle.
The electrons in the wall are attracted to the
positive charge of the balloon. They position
themselves to be as close as possible to the
positive charge.
Page 5:
*Answers are approximated from the graph
and may vary slightly.*
1. Force at distance = 0.5 nm when charge =
-2
1
2. Force at distance = 1 nm when charge = -2
0.2
3. Force at distance = 1.5 nm when charge =
-2
0
4. Force at distance = 0.5 nm when charge =
-4
2
5. Force at distance = 1 nm when charge = -4
0.4
6. Force at distance = 1.5 nm when charge =
-4
almost 0
Page 7:
1. Compare what happens to the yellow and
blue particles when there are charges
between them and when there are no
charges between them. With charges between them, the negative
particles attract to the positive charge (yellow)
and repel from the other negative charge (blue).
With no charges between them, positive and
negative are attracted and move toward one
another.
2. Why does it make sense that a positive
and a negative particle cannot find each
other if there are other charged particles
around?
The force of attraction of the charged particles is
interrupted by the other charged particles that
are in closer proximity to them. Once that
happens, their charge is screened or "hidden"
from other charged particles in the area.
7. When the charge is double what happens
to the force at 0.5 nm?
Page 8:
When the charge doubles, the force doubles at a
set distance.
1. Which amount is greatest? Explain how
you can tell.
Page 6:
Charge amount C is the greatest. The force of
repulsion between the positive charges is the
greatest, causing them to move the farthest
away from each other.
1. The balloon sticks to the wall as long as it
is charged, regardless of the sign (positive
or negative). Can you explain why?
The wall is neutral and contains both positive
and negative charges. Regardless of the charge
on the balloon, it will be attracted to the opposite
charge present in the wall.
2. Suppose you shot an electron into the
container pictured. The container has a
particle that has a negative charge. Describe
the path the electron will follow and why it
follows this path.
Answers will vary. Student responses should
include their logic that the electron shot into the
box repels from the negative charge that is
already present.
3. Particle A has a charge of +3 and particle
B a charge of -3. They are 4 nm apart from
each other. How would the forces change if
the distance was halved? Explain your
answer.
As the distance is decreased, the force between
the two charged particles will increase.
4. Two positively charged particles are
placed on a line. A third particle is placed on
the line and it starts to move towards the
particle on the right. What is the charge of
the third particle? Why does it travel towards
the right? Choose all that are possible.
(a) (b)
5. Suppose the Coulomb force is the main
force responsible for most interactions
between atoms and molecules. The Coulomb
force gets weaker at further distance. What
can you guess about the forces between
molecules in a gas versus the forces
between molecules in a liquid?
Gas particles are further apart from one another
so you would expect the Coulomb forces at work
in a gas to be weaker than those at work in a
liquid. Particles in a liquid are closer to each
other, so they would have more opportunities for
attraction and repulsion.
SAM HOMEWORK QUESTIONS
Electrostatics
Directions: After completing the unit, answer the following questions to review.
1. When does a neutral atom become charged?
2. Sodium has an atomic number of 11, which means it has 11 protons in its nucleus. If a
sodium ion has a charge of +1, what does that mean about its number of electrons?
3. Below are two sets of atoms illustrating Coulomb’s Law. Explain what is happening in
each picture and why it is happening in the space provided.
a)
b)
a)
b)
4. How is the Coulomb force between atoms dependent upon distance between atoms?
5. Mathematically, Coulomb’s Law is stated as: F = kQ1Q2 / r2. Explain what this formula
means in words.
6. Have you ever rubbed a balloon against your hair? When you pull the balloon away, your
hair sticks to the balloon and stands up! Explain what is happening in this example.
*Hint: Think back to the balloon sticking to the wall in the unit you just completed.
7. Career connection: The force between charged objects is sometimes called the Coulomb
or electrostatic force. This is particularly important for nanoscale-sized devices or objects
(sometimes made from single molecules). Research nanotechnology and find one way the
electrostatic force is related to this.
SAM HOMEWORK QUESTIONS
Electrostatics – With Suggested Answers for Teachers
Directions: After completing the unit, answer the following questions to review.
1. When does a neutral atom become charged?
When electrons are lost or gained, a neutral atom becomes charged. A charged atom is called an ion.
2. Sodium has an atomic number of 11, which means it has 11 protons in its nucleus. If a
sodium ion has a charge of +1, what does that mean about its number of electrons?
It means that sodium has lost one electron, giving it a net charge of +1.
3. Below are two sets of atoms illustrating Coulomb’s Law. Explain what is happening in
each picture and why it is happening in the space provided.
a)
b)
a) Like charges repel, so the atoms move apart.
b) Opposite charges are attracted, so the two atoms move closer to each other.
4. How is the Coulomb force between atoms dependent upon distance between atoms?
The closer the atoms are to one another, the greater the Coulomb force.
5. Mathematically, Coulomb’s Law is stated as: F = kQ1Q2 / r2. Explain what this formula
means in words.
The interaction force between two charged particles is proportional to the product of the charges divided
by the square of the distance between them.
6. Have you ever rubbed a balloon against your hair? When you pull the balloon away, your
hair sticks to the balloon and stands up! Explain what is happening in this example.
*Hint: Think back to the balloon sticking to the wall.
When you rub a balloon against your hair, electrons are transferred from your hair to the balloon. Your
hair, with net positive charges, stands on end because like charges are repelling.
7. Career connection: Nanotechnology is a rapidly developing field, so you will have to
search for the latest online. One problem for developing molecular-sized devices is that
most molecules carry at least some partial charge and everything tends to be “sticky” at
the atomic level. Some people think this may prevent the promises of nanotech futurists
from ever coming true. However, some devices may use this very property, such as
molecule-sized sensors. http://pubs.acs.org/doi/abs/10.1021/jp910946v