An inquiry-based lab where students use the PhET Coulomb's Law simulation to discover the relationships between charge, distance, and electrostatic force.
Analysis: When you increased the distance by 4x (2cm to 8cm), what happened to the force?
4 The Prediction Challenge
"If the force between two charges is 100 N at a distance of 2 cm, what will the force be if the distance is increased to 6 cm (3 times the original distance)?"
Your Prediction
Calculations / Thinking
Sim Verification
Simulated Force: _________ N
Final Synthesis
Coulomb's Law states that force is directly proportional to ________ and inversely proportional to the square of ________.
In your own words, why does the force decrease so rapidly as the charges move apart?
PAGE 2
CER Scoring (Page 4)
Claim:
Force is directly proportional to the product of charges and inversely proportional to the square of the distance.
Evidence:
Must cite Graph 1 (linear trend) and Graph 2 (exponential decay). Student should mention specific N values for r=2cm vs r=4cm.
Instructional Pitfalls
Sum vs. Product
Students often think if you double both charges, the force doubles (2+2) rather than quadruples (2x2). Challenge them to check the sim!
The "1/2" Trap
In Experiment 2, students frequently assume doubling distance cuts force in half. Use Graph 2 to show the non-linear drop to 1/4th.
Teacher Resource: Electric Tug Investigation Page 2 of 2
Evidence
Provides specific numeric evidence from both Experiment A and B to support the claim.
Provides numbers from only one experiment or general observations without numbers.
Little to no evidence provided; numbers are incorrect.
Reasoning
Explicitly links evidence to Coulomb's Law formula \(F = k \frac{q_1q_2}{r^2}\).
Refers to the law or math but doesn't explain how the numbers prove the relationship.
Reasoning is absent or circular ("it is because the data says so").
Implementation Note:
Removing the synthesis questions allows for a deeper dive into the CER. Encourage students to draft their Evidence section in bullet points first, ensuring they have at least 3 trials mentioned (one from charge, two from distance for the square relationship).
Part 2: Charge vs Force (at 4cm, q2=10μC)
q1
Force (N)
Relationship
2 μC
~112.4 N
Linear: If charge doubles, force doubles.
4 μC
~224.8 N
8 μC
~449.6 N
Part 3: Distance vs Force (q1=q2=10μC)
Distance
Force (N)
Analysis
2 cm
~2247 N
Double the distance (2 to 4) → 1/4 force.
Quadruple distance (2 to 8) → 1/16 force.
4 cm
~562 N
8 cm
~141 N
10 cm
~90 N
Part 4: The Prediction
Question: 100N at 2cm. What is it at 6cm? Answer: Since distance increased by 3x, the force decreases by \(3^2\) (9x). Calculation: \(100 \div 9 = 11.11 N\).
Synthesis
1. Directly proportional to charge magnitude; inversely proportional to square of distance.
2. Rapid decrease: Because it is an inverse square law, even small increases in distance result in massive drops in force.
2. TREND CHECK (DISTANCE)
Compare Trial 1 (2cm) to Trial 3 (4cm). As distance doubled, by what factor did the force decrease?
Page 2
Prediction Challenges
Now that you understand the relationships, use them to predict outcomes for complex scenarios. Write your prediction BEFORE checking the simulation.
Challenge A: The Force Multiplier
Starting Setup: q1 = 4 μC, q2 = 4 μC, r = 4 cm.
If you double q1 AND double q2 but keep the distance at 4 cm, what will the new force be?
Mathematical Reasoning:
Prediction:
________ N
Simulated Value:
________ N
Challenge B: The Balancing Act
Starting Setup: q1 = 10 μC, q2 = 10 μC, r = 4 cm.
If you double both charges, what must you do to the distance to bring the force back to its original value?
Algebraic Reasoning:
Prediction (Distance):
________ cm
Simulated Value:
________ cm
Page 3
Final Scientific Argument (CER)
The Investigation Question:
"How do charge magnitude and separation distance interact to determine the total strength of the electrostatic force between two objects?"
Claim Answer the question in one complete sentence
Evidence Cite specific numeric values from your data tables
Reasoning Connect evidence to Coulomb's Law (\(F = k \frac{q_1q_2}{r^2}\))
Page 4
8.0 cm
64.0
10.0 cm
100.0
Q3.1: When you doubled the distance (2cm to 4cm), what happened to the force?
Q3.2: Why must distance be measured from the center of the spheres rather than the edges?
4 The Prediction Challenge
Scenario A
"If the force is 100 N at 2 cm, what will the force be at 6 cm?"
Calculations
Prediction: ________ N
Scenario B
"Force is 200 N. You double both charges AND double the distance. What is the new force?"
Calculations
Prediction: ________ N
Final Synthesis
Q5.1: Look at the values for 'Force on q1' and 'Force on q2' in any setup. How do they compare? Which law of motion does this demonstrate?
Q5.2: Coulomb's Law states that force is directly proportional to ________ and inversely proportional to the square of ________.
Q5.3: If you move two negative charges very far apart, does the force ever truly reach zero? Explain.
PAGE 2
2. TREND CHECK (DISTANCE)
Compare Trial 1 (2cm) to Trial 3 (4cm). As distance doubled, by what factor did the force decrease?
Page 2
Prediction Challenges
Now that you understand the relationships, use them to predict outcomes for complex scenarios. Write your prediction BEFORE checking the simulation.
Challenge A: The Force Multiplier
Starting Setup: q1 = 4 μC, q2 = 4 μC, r = 4 cm.
If you double q1 AND double q2 but keep the distance at 4 cm, what will the new force be?
Mathematical Reasoning:
Prediction:
________ N
Simulated Value:
________ N
Challenge B: The Balancing Act
Starting Setup: q1 = 10 μC, q2 = 10 μC, r = 4 cm.
If you double both charges, what must you do to the distance to bring the force back to its original value?
Algebraic Reasoning:
Prediction (Distance):
________ cm
Simulated Value:
________ cm
Page 3
Final Scientific Argument (CER)
The Investigation Question:
"How do charge magnitude and separation distance interact to determine the total strength of the electrostatic force between two objects?"
Claim Answer the question in one complete sentence
Evidence Cite specific numeric values from your data tables
Reasoning Connect evidence to Coulomb's Law (\(F = k \frac{q_1q_2}{r^2}\))
Page 4
Synthesis
Q5.1: They are equal in magnitude and opposite in direction. This demonstrates Newton's Third Law (Action-Reaction pairs).
Q5.2: Charge magnitude / square of distance.
Q5.3: Theoretically no, as \(1/r^2\) never equals zero. Practically yes, as the force eventually becomes smaller than ambient noise or other forces (like gravity).
Part 4: Predictions
Scenario A (Distance change)
Force at 2cm is 100N. New distance 6cm.
Answer: 11.11 N
Logic: Distance increases by 3x. Force decreases by \(3^2\) (9x). \(100 / 9 = 11.11\).
Scenario B (Multi-variable)
Force is 200N. Double both charges and double distance.
Describe the relationship between charge magnitude and electrostatic force. Is it linear, exponential, or inverse? Cite two specific data points from Experiment A as evidence.
2. MATHEMATICAL TREND (EXP B)
Describe the relationship between separation distance and force. How does the shape of Graph B differ from Graph A? Why can't we draw a straight line for this data?
3. THE SYMMETRY RULE
Look at the force arrows in the simulation when the charges are unequal (e.g., 10μC and 2μC). Is one arrow larger? Explain why this confirms Newton's 3rd Law of Motion.
Page 2
Section 3: Analysis & Logic (Part 2)
4. ZERO FORCE LOGIC
Explain why Trial 1 of Experiment A (0μC) resulted in exactly 0 Newtons. Why is it impossible for a "tug" to occur if only one object is charged?
5. SCALING CHALLENGE
If you repeated Experiment A but moved the charges closer (distance = 2cm instead of 4cm), would the slope of your Graph A be steeper or flatter? Explain your reasoning.
6. INVERSE SQUARE PROOF
Compare Trial 1 (2cm) and Trial 3 (4cm) in Experiment B. As distance doubled, by what mathematical factor did the force decrease? (Show the math: F2cm / F4cm).
Prediction Challenges
Challenge A: The Force Multiplier
Starting Setup: q1 = 4 μC, q2 = 4 μC, r = 4 cm.
If you double q1 AND double q2 but keep the distance at 4 cm, what will the new force be?
Mathematical Reasoning:
Prediction:
________ N
Simulated Value:
________ N
Challenge B: The Balancing Act
Starting Setup: q1 = 10 μC, q2 = 10 μC, r = 4 cm.
If you double both charges, what must you do to the distance to bring the force back to its original value?
Algebraic Reasoning:
Prediction (Distance):
________ cm
Simulated Value:
________ cm
Page 3
Final Scientific Argument (CER)
The Investigation Question:
"How do charge magnitude and separation distance interact to determine the total strength of the electrostatic force between two objects?"
Claim Answer the question in one complete sentence
Evidence Cite specific numeric values from your data tables
Reasoning Connect evidence to Coulomb's Law (\(F = k \frac{q_1q_2}{r^2}\))
Page 4
This data supports the inverse square law \(F \propto 1/r^2\). Because distance is squared in the denominator, any change in distance \(x\) results in a force change of \(1/x^2\). Doubling distance (\(2r\)) results in \((2)^2 = 4\) times less force, which matches our collected trial data exactly.
8
10.0
Force (Newtons)
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Distance r (cm)
Section 3: Analysis & Logic (Part 1)
1. MATHEMATICAL TREND (EXP A)
Describe the relationship between charge magnitude and electrostatic force. Is it linear, exponential, or inverse? Cite two specific data points from Experiment A as evidence.
2. MATHEMATICAL TREND (EXP B)
Describe the relationship between separation distance and force. How does the shape of Graph B differ from Graph A? Why can't we draw a straight line for this data?
3. THE SYMMETRY RULE
Look at the force arrows in the simulation when the charges are unequal (e.g., 10μC and 2μC). Is one arrow larger? Explain why this confirms Newton's 3rd Law of Motion.
Page 2
Section 3: Analysis & Logic (Part 2)
4. ZERO FORCE LOGIC
Explain why Trial 1 of Experiment A (0μC) resulted in exactly 0 Newtons. Why is it impossible for a "tug" to occur if only one object is charged?
5. SCALING CHALLENGE
If you repeated Experiment A but moved the charges closer (distance = 2cm instead of 4cm), would the slope of your Graph A be steeper or flatter? Explain your reasoning.
6. INVERSE SQUARE PROOF
Compare Trial 1 (2cm) and Trial 3 (4cm) in Experiment B. As distance doubled, by what mathematical factor did the force decrease? (Show the math: F2cm / F4cm).
Prediction Challenges
Challenge A: The Force Multiplier
Starting Setup: q1 = 4 μC, q2 = 4 μC, r = 4 cm.
If you double q1 AND double q2 but keep the distance at 4 cm, what will the new force be?
Mathematical Reasoning:
Prediction:
________ N
Simulated Value:
________ N
Challenge B: The Balancing Act
Starting Setup: q1 = 10 μC, q2 = 10 μC, r = 4 cm.
If you double both charges, what must you do to the distance to bring the force back to its original value?
Algebraic Reasoning:
Prediction (Distance):
________ cm
Simulated Value:
________ cm
Page 3
Final Scientific Argument (CER)
The Investigation Question:
"How does the separation distance affect the electrostatic force between the two charges?"
Claim Answer the question in one complete sentence
Evidence Cite specific numeric values from your data tables
Reasoning Connect evidence to Coulomb's Law (\(F = k \frac{q_1q_2}{r^2}\))
Page 4
Page 2
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Force (Newtons)
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Distance r (cm)
Section 3: Analysis & Logic (Part 1)
1. MATHEMATICAL TREND (EXP A)
Describe the relationship between charge magnitude and electrostatic force. Is it linear, exponential, or inverse? Cite two specific data points from Experiment A as evidence.
2. MATHEMATICAL TREND (EXP B)
Describe the relationship between separation distance and force. How does the shape of Graph B differ from Graph A? Why can't we draw a straight line for this data?
3. THE SYMMETRY RULE
Look at the force arrows in the simulation when the charges are unequal (e.g., 10μC and 2μC). Is one arrow larger? Explain why this confirms Newton's 3rd Law of Motion.
Page 2
Section 3: Analysis & Logic (Part 2)
4. ZERO FORCE LOGIC
Explain why Trial 1 of Experiment A (0μC) resulted in exactly 0 Newtons. Why is it impossible for a "tug" to occur if only one object is charged?
5. SCALING CHALLENGE
If you repeated Experiment A but moved the charges closer (distance = 2cm instead of 4cm), would the slope of your Graph A be steeper or flatter? Explain your reasoning.
6. INVERSE SQUARE PROOF
Compare Trial 1 (2cm) and Trial 3 (4cm) in Experiment B. As distance doubled, by what mathematical factor did the force decrease? (Show the math: F2cm / F4cm).
Prediction Challenges
Challenge A: The Force Multiplier
Starting Setup: q1 = 4 μC, q2 = 4 μC, r = 4 cm.
If you double q1 AND double q2 but keep the distance at 4 cm, what will the new force be?
Mathematical Reasoning:
Prediction:
________ N
Simulated Value:
________ N
Challenge B: The Balancing Act
Starting Setup: q1 = 10 μC, q2 = 10 μC, r = 4 cm.
If you double both charges, what must you do to the distance to bring the force back to its original value?
Algebraic Reasoning:
Prediction (Distance):
________ cm
Simulated Value:
________ cm
Page 3
Final Scientific Argument (CER)
The Investigation Question:
"How does the separation distance affect the electrostatic force between the two charges?"
Claim Answer the question in one complete sentence
Evidence Cite specific numeric values from your data tables
Reasoning Connect evidence to Coulomb's Law (\(F = k \frac{q_1q_2}{r^2}\))
Page 4
10.0
Force (Newtons)
02505007501000125015001750200022502500
012345678910
Distance r (cm)
Section 3: Analysis & Logic (Part 1)
1. MATHEMATICAL TREND (EXP A)
Describe the relationship between charge magnitude and electrostatic force. Is it linear, exponential, or inverse? Cite two specific data points from Experiment A as evidence.
2. MATHEMATICAL TREND (EXP B)
Describe the relationship between separation distance and force. How does the shape of Graph B differ from Graph A? Why can't we draw a straight line for this data?
3. THE SYMMETRY RULE
Look at the force arrows in the simulation when the charges are unequal (e.g., 10μC and 2μC). Is one arrow larger? Explain why this confirms Newton's 3rd Law of Motion.
Page 2
Section 3: Analysis & Logic (Part 2)
4. ZERO FORCE LOGIC
Explain why Trial 1 of Experiment A (0μC) resulted in exactly 0 Newtons. Why is it impossible for a "tug" to occur if only one object is charged?
5. SCALING CHALLENGE
If you repeated Experiment A but moved the charges closer (distance = 2cm instead of 4cm), would the slope of your Graph A be steeper or flatter? Explain your reasoning.
6. INVERSE SQUARE PROOF
Compare Trial 1 (2cm) and Trial 3 (4cm) in Experiment B. As distance doubled, by what mathematical factor did the force decrease? (Show the math: F2cm / F4cm).
Prediction Challenges
Challenge A: The Force Multiplier
Starting Setup: q1 = 4 μC, q2 = 4 μC, r = 4 cm.
If you double q1 AND double q2 but keep the distance at 4 cm, what will the new force be?
Mathematical Reasoning:
Prediction:
________ N
Simulated Value:
________ N
Challenge B: The Balancing Act
Starting Setup: q1 = 10 μC, q2 = 10 μC, r = 4 cm.
If you double both charges, what must you do to the distance to bring the force back to its original value?
Algebraic Reasoning:
Prediction (Distance):
________ cm
Simulated Value:
________ cm
Page 3
Final Scientific Argument (CER)
The Investigation Question:
"How does the separation distance affect the electrostatic force between the two charges?"
Claim Answer the question in one complete sentence
Evidence Cite specific numeric values from your data tables
Reasoning Connect evidence to Coulomb's Law (\(F = k \frac{q_1q_2}{r^2}\))
Page 4
0
2
4
6
8
10
Distance (cm)
Electrostatic Force (N)
Plot points and sketch trend line.
Scientific Argument (CER)
Question: Based on your experimental data, how does the electrostatic force between two objects change as the distance between them increases?
Claim
Evidence (Cite specific data points from Part 3)
Reasoning (Apply Coulomb's Law and physics principles)
Page 2
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Force (Newtons)
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Distance r (cm)
C PART C: Prediction Challenges
C1: Double Double
If you double q1 AND double q2 (distance 4cm), what is the new force?
Prediction
Measured
C2: The Balance
Double both charges. What distance returns the force to original?
Prediction
Measured
Page 2
PART D: Data Analysis & Logic
1. Mathematical Trend (Charge)
Describe the relationship between charge magnitude and force. Is it linear, exponential, or inverse? Cite two specific data points from Part A.
2. Mathematical Trend (Distance)
Describe the relationship between distance and force. How does the shape of Graph B differ from Graph A? Why can't we use a straight line?
3. The Symmetry Rule
When charges are unequal (e.g., 10μC and 2μC), is one force arrow larger? Explain why this confirms Newton's 3rd Law.
4. Zero Force Logic
Why did Trial 1 (0μC) result in exactly 0N? Can a "tug" occur with only one charged object?
5. Scaling Challenge
If you repeat Part A at 2cm instead of 4cm, would the slope be steeper or flatter? Why?
6. Inverse Square Proof
Compare 2cm and 4cm in Part B. As distance doubled, by what mathematical factor did the force decrease? (Show math: F2cm / F4cm).
Page 3
PART E: Scientific Argument (CER)
The Investigation Question:
"How does the separation distance affect the electrostatic force between the two charges?"
Claim Answer the question in one complete sentence
Evidence Cite specific numeric values from your data tables
Reasoning Connect evidence to Coulomb's Law (\(F = k \frac{q_1q_2}{r^2}\))
Page 4
2500
2000
1500
1000
500
0
0
2
4
6
8
10
Distance r (cm)
Electrostatic Force F (N)
Plot points and sketch trend line.
Scientific Argument (CER)
Question: Based on your experimental data, how does the electrostatic force between two objects change as the distance between them increases?
Claim (Answer the question in one complete sentence)
Evidence (Cite specific data points from your Distance table or Graph 2)
Reasoning (Explain the physics behind your evidence using Coulomb's Law)
Page 2
2000
1500
1000
500
0
0
2
4
6
8
10
Distance r (cm)
Force F (Newtons)
Plot points and sketch an inverse square curve.
Scientific Argument (CER)
Question: Based on your experimental data, how does the electrostatic force between two objects change as the distance between them increases?
Claim (Answer the question in one complete sentence)
Evidence (Cite specific data points from your Distance table or Graph 2)
Reasoning (Explain the physics behind your evidence using Coulomb's Law)
Page 2
10.0
Force (Newtons)
02505007501000125015001750200022502500
012345678910
Distance r (cm)
PART C: Prediction Challenges
C1: Double Double
If you double q1 AND double q2 (at fixed 4cm distance), what is the new force?
Prediction
______ N
Measured
______ N
C2: The Balance
If you double both charges, what new distance returns the force to its original value?
Prediction
______ cm
Measured
______ cm
Page 2
PART D: Data Analysis & Logic
1. Mathematical Trend (Charge)
Describe the relationship between charge magnitude and force. Is it linear, exponential, or inverse? Cite two specific data points from Part A.
2. Mathematical Trend (Distance)
Describe the relationship between distance and force. How does the shape of Graph B differ from Graph A? Why can't we use a straight line?
3. The Symmetry Rule
When charges are unequal (e.g., 10μC and 2μC), is one force arrow larger? Explain why this confirms Newton's 3rd Law.
4. Zero Force Logic
Why did Trial 1 (0μC) result in exactly 0N? Can a "tug" occur with only one charged object?
5. Scaling Challenge
If you repeat Part A at 2cm instead of 4cm, would the slope be steeper or flatter? Why?
6. Inverse Square Proof
Compare 2cm and 4cm in Part B. As distance doubled, by what mathematical factor did the force decrease? (Show math: F2cm / F4cm).
Page 3
PART E: Scientific Argument (CER)
The Investigation Question:
"How does the separation distance affect the electrostatic force between the two charges?"
Claim Answer the question in one complete sentence
Evidence Cite specific numeric values from your data tables
Reasoning Connect evidence to Coulomb's Law (\(F = k \frac{q_1q_2}{r^2}\))
Page 4
Claim:
Evidence: Students should cite that at 2cm, F = ~2247N, but when distance doubled to 4cm, the force dropped to ~562N (exactly 1/4 the original value).
Reasoning: Coulomb's Law states \(F = k \frac{q_1q_2}{r^2}\). Because distance is in the denominator and squared, any increase in 'r' results in a significant reduction in 'F'. This explains why the graph is a curve rather than a straight line.
Grading Rubric (CER)
Claim (2 pts)
Complete, accurate sentence answering the prompt.
Evidence (4 pts)
Uses at least 2 numeric data points from the lab; identifies the trend.
Reasoning (4 pts)
Connects data to the mathematical formula; uses physics terminology (inverse square).
Simulation Tip
"If students get 'scientific notation' values (e.g. 5.62 x 10^2), help them convert to standard Newtons if they are struggling with the graph scales."
Teacher Guide: Coulomb Lab Page 2 of 2
Part E: CER Exemplar (Distance Focus)
Claim:
"The electrostatic force is inversely proportional to the square of the separation distance between the two charges."
Evidence:
- In Part B, Trial 1 (2cm) resulted in a force of 2247 N.
- In Part B, Trial 3 (4cm) resulted in a force of 561 N.
- When distance was doubled (from 2cm to 4cm), the force decreased by exactly a factor of 4.005.
Reasoning:
This observation perfectly matches the Inverse Square Law (\(F \propto 1/r^2\)). According to the formula, if distance r is doubled (\(2r\)), it must be squared in the denominator, becoming \(4r^2\). This causes the total force to become 1/4 as strong as the original, which is exactly what our data trials showed.
Part D: Analysis Key (4-6)
4. Zero Force Logic
"Force requires an interaction. Since \(F = k \frac{q_1 \cdot 0}{r^2}\), the result is 0. You can't pull on nothing."
5. Scaling Challenge
"The slope would be steeper. Because the objects are closer, every increase in charge has a much larger impact on total force."
6. Inverse Square Proof
"At 2cm, Force is ~2247N. At 4cm, Force is ~561N. \(2247 / 561 \approx 4\). Doubling distance reduces force by 4."
Part E: CER Exemplar (Distance Focus)
Claim:
"The electrostatic force is inversely proportional to the square of the separation distance between the two charges."
Evidence:
- In Part B, Trial 1 (2cm) resulted in a force of 2246.8 N.
- In Part B, Trial 3 (4cm) resulted in a force of 561.7 N.
- When distance was doubled (from 2cm to 4cm), the force decreased by exactly a factor of 4.
Reasoning:
This observation perfectly matches the Inverse Square Law (\(F \propto 1/r^2\)). According to the formula, if distance r is doubled (\(2r\)), it must be squared in the denominator, becoming \(4r^2\). This causes the total force to become 1/4 as strong as the original, which is exactly what our data trials showed.
q1, q2 = Magnitude of Charges
r = Separation Distance
k = Coulomb's Constant (\(9 \times 10^9\))
The Product of Charges
"More Charge = More Pull"
The force is directly proportional to the product of the two charges. If you double the charge on one sphere, the force doubles. If you double the charge on both spheres, the force quadruples (\(2 \times 2 = 4\)).
The Inverse Square Law
"Farther Apart = Much Weaker"
Distance has a geometric impact. Because r is squared in the denominator, the force drops off rapidly as objects move apart. If you triple the distance, the force becomes \(1/3^2\), or 1/9th as strong.
Electricity vs. Gravity
Mathematically, Coulomb’s Law looks nearly identical to Newton’s Law of Universal Gravitation (\(F = G \frac{m_1 m_2}{r^2}\)). Both are inverse-square laws. However, there are two massive differences:
Scale: Gravity is incredibly weak; electricity is incredibly strong. The constant k is 20 orders of magnitude larger than G.
Repulsion: Gravity only pulls. Charge can push back.
"If you could remove all the electrons from your body and stand 1 meter away from them, the repulsive force would be enough to lift the entire Earth."
Magnitude
Force increases with the amount of charge.
Distance
Force decreases by the square of the distance.
Direction
Opposites attract; identical charges repel.
Page 2 / 2
A The same as the original force
B 2 times the original force
C 4 times the original force
D 1/4 of the original force
3. Symmetry: Forces are identical. Confirms Newton's 3rd Law.
4. Zero Force: \(F \propto q_1q_2\). If \(q=0\), \(F=0\). No interaction possible.
5. Scaling: Steeper slope at 2cm. Force is 4x stronger for the same charge.
6. Proof: \(F_{2cm} / F_{4cm} \approx 4\). Doubling distance reduces force by factor of 4.
Part E: CER Exemplar (Distance Focus)
Claim:
"The electrostatic force is inversely proportional to the square of the separation distance between the two charges."
Evidence:
- In Part B, Trial 1 (2cm) resulted in a force of 2246.8 N.
- In Part B, Trial 3 (4cm) resulted in a force of 561.7 N.
- When distance was doubled (from 2cm to 4cm), the force decreased by exactly a factor of 4.
Reasoning:
This matches the Inverse Square Law (\(F \propto 1/r^2\)). According to the formula, if distance r is doubled (\(2r\)), it becomes \(4r^2\) in the denominator, causing the force to become 1/4 as strong.
Quiz Answer Key
Electric Tug Concept Check
1. C
Electrons
Electrons are the mobile charge carriers in atomic transfer.
2. B
Repel each other
Like charges (- and -) repel.
3. C
1/4 as strong
Inverse Square: \(2^2 = 4\), so force is \(1/4\).
4. A
Triple
Force is directly proportional to each individual charge.
5. C
Both experience same force
Newton's 3rd Law: Force pairs are always equal in magnitude.
6. B
Exert force without touching
Fields explain "action at a distance".
7. C
Quadruple (4x)
\(2 \times 2 = 4\). Force scales with the product of charges.
8. C
Force drops to zero
If either q is 0, the product \(q_1q_2\) is 0.
9. A
Can be repulsive
Gravity only attracts; electricity can push or pull.
