Question 1
A student rubs a rubber balloon on their hair. Which of the following correctly describes what happens to the electrons during this process?No clue? Show me the answer
Correct answerCorrect!
IncorrectStep-by-step walkthrough
Choose a solution method
Method #1Direct ApproachStep 1: Identify the particle that moves
In all electrostatic situations caused by friction, only electrons move. Protons are fixed in the nucleus and never transfer between objects under normal conditions.
Step 2: Apply the triboelectric principle
Rubber is lower in the triboelectric series than human hair. This means rubber gains electrons from hair when they are rubbed together. Electrons travel from hair → to balloon.
Step 3: Determine resulting charges
The balloon gains electrons → it now has more electrons than protons → it becomes negatively charged. The hair loses electrons → it now has fewer electrons than protons → it becomes positively charged.
Step 4: Select the correct answer
The correct description is: electrons transfer from hair to balloon, balloon becomes negative, hair becomes positive.
Method #2Process of EliminationStep 1: Identify the key concept being tested
This question tests whether you know which particle moves during friction charging and in which direction it moves between hair and rubber balloon.
Step 2: Eliminate the proton transfer option
"Protons transfer from the balloon to the hair" is incorrect. Protons never leave the nucleus in electrostatic situations — only electrons are transferred. Eliminate this option.
Step 3: Eliminate the reversed direction option
"Electrons transfer from the balloon to the hair" would make the balloon positive and hair negative. Rubber gains electrons from hair (not the other way around), so this direction is wrong. Eliminate this option.
Step 4: Eliminate the no-charge option
"Both protons and electrons transfer equally" contradicts the fact that only electrons move, and rubbing two different materials always results in charge transfer, not neutrality. Eliminate this option.
Step 5: Select the correct answer
The remaining option — electrons transfer from hair to balloon, balloon becomes negative, hair becomes positive — is correct and consistent with the physics of charging by friction.
Question 2
Two plastic rods are each charged by rubbing with wool. When one rod is suspended and the other is brought near it, what will happen and why?No clue? Show me the answer
Correct answerCorrect!IncorrectStep-by-step walkthrough
Choose a solution method
Method #1Direct ApproachStep 1: Identify what charging with wool does to plastic rods
When a plastic rod is rubbed with wool, electrons transfer from the wool to the plastic (plastic is lower in the triboelectric series and gains electrons). Both rods become negatively charged.
Step 2: Apply the Law of Electrostatics
The Law of Electrostatics states: like charges repel and unlike charges attract. Both rods have the same charge (negative), so they will repel each other.
Step 3: Note that contact is not required
The electrostatic force is a non-contact force — the rods do not need to touch to exert a force on each other. The force acts across the space between them.
Step 4: Select the correct answer
The rods repel each other because both carry the same type of charge (negative), and like charges repel.
Method #2Process of EliminationStep 1: Identify what the question is asking
The question asks what force two identically charged plastic rods exert on each other, and why. Both rods undergo the same charging process, so they must have the same type of charge.
Step 2: Eliminate the attraction option
"They attract because opposite charges are created" is wrong. Both rods are made of the same material and both rubbed with wool — they both gain electrons and become negative. They do not have opposite charges. Eliminate this option.
Step 3: Eliminate the electron flow option
"They repel because electrons flow between them" is incorrect. Plastic is an insulator — charges cannot flow freely through it or jump between the rods (they don't even touch). Eliminate this option.
Step 4: Eliminate the no-force option
"Nothing happens because plastic is an insulator" is wrong. Insulators can carry static charge on their surface and still exert electrostatic forces. Electrostatic force is a non-contact force. Eliminate this option.
Step 5: Select the correct answer
The correct answer is that the rods repel because they carry the same type of charge (both negative), consistent with the Law of Electrostatics.
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