Rub a balloon on your hair and it sticks to the wall. Walk across carpet and you zap the doorknob. Every one of these everyday surprises comes down to one tiny particle that can move: the electron. Protons stay locked in the nucleus, but electrons can be transferred from one object to another, and that single fact explains how anything becomes charged.
You rub a plastic rod with a cloth and the rod becomes negatively charged.
Before reading: which particle moved to make the rod negative, protons or electrons? Did the rod gain or lose them? Write your prediction and one sentence explaining why.
Why electrons, not protons, are the particles that move
Every atom has a heavy nucleus of positive protons surrounded by light negative electrons, and only the outer electrons are loosely held enough to move.
A neutral atom has equal numbers of protons and electrons, so their charges cancel. The protons are bound tightly inside the nucleus by the strong nuclear force, but the outermost electrons are held only weakly. When two surfaces are brought into close contact, these loosely held electrons can hop from one material to the other.
Protons are bound in the nucleus and do not move during everyday charging. Only electrons, negative and loosely held, transfer between objects. Charging is always the movement of electrons.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? True or false: an object can be charged by adding protons to it.
How electron movement decides the sign of the charge
We just saw that only electrons move. That raises a question: if an object ends up positive or negative, what exactly happened to its electrons? This card answers it → gaining electrons makes an object negative; losing electrons leaves it positive.
An object's charge is just a count: more electrons than protons makes it negative, fewer electrons than protons makes it positive.
Because the proton number is fixed, the sign of an object's charge depends entirely on its electron count. The size of the charge is the number of extra or missing electrons multiplied by the charge of one electron, $e = 1.6 \times 10^{-19}\ \text{C}$.
A plastic rod gains a charge of $-3.2 \times 10^{-9}\ \text{C}$ after being rubbed. How many electrons were transferred onto it?
Gaining electrons makes an object negative; losing electrons makes it positive. The charge magnitude is $q = n\,e$, where $n$ is the number of electrons moved and $e = 1.6 \times 10^{-19}\ \text{C}$.
Add the highlighted rule and equation to your notes before the check below.
Quick check: A glass rod loses electrons when rubbed with silk. What is the charge on the glass rod?
Pattern A, Reason then calculate
A balloon is rubbed on hair and gains a charge of $-8.0 \times 10^{-9}\ \text{C}$. In your book: (1) state whether the balloon gained or lost electrons and why; (2) state the sign of the charge left on the hair, explaining how conservation of charge requires this; (3) calculate how many electrons were transferred using $n = q/e$.
Two truths, one lie: three of these statements about charging are correct. Which one is the lie?
Dot point: Explain the role of the movement of electrons in charging particles and objects
Why charging one object always charges another oppositely
We just learned that gaining or losing electrons sets the sign of charge. That raises a question: where do those electrons go? This card answers it → they move to another object, which becomes equally and oppositely charged.
Electrons are never destroyed when an object is charged, they simply leave one object and arrive on another, so the two end up with equal and opposite charges.
In an isolated system the total charge cannot change; this is the law of conservation of charge. When a balloon rubbed on hair gains electrons and becomes negative, those very electrons came from the hair, which is left with an equal positive charge. Add the two charges together and you get the same total you started with, zero.
This is exactly the answer to the opening hook: rubbing did not create charge. It moved electrons from your hair to the balloon, separating charge that was already there.
Charge is conserved: the electrons gained by one object are exactly those lost by another, so the two objects end up with equal and opposite charges and the total charge is unchanged.
Pause, copy the highlighted conservation statement into your book before the check below.
Last check: Decide if the statement is true or false.
If a cloth gives electrons to a rod, the cloth becomes positive and the rod becomes negative.
Fill the blanks: Complete the charge bookkeeping.
A neutral sphere loses $5.0 \times 10^{9}$ electrons. Its charge becomes , with magnitude $q = ne$ where $n$ is the of electrons transferred.
Dot point: Explain the role of the movement of electrons in charging particles and objects
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 4(4 marks) 1. A balloon is rubbed against a woollen jumper. Afterward the balloon is negatively charged. Explain, in terms of the movement of electrons and the conservation of charge, how both the balloon and the jumper became charged.
1 mark: only electrons move (protons fixed) · 1 mark: balloon gained electrons → negative · 1 mark: jumper lost those electrons → positive · 1 mark: charge conserved, electrons transferred not created
ApplyBand 4(3 marks) 2. A plastic rod is rubbed and gains a charge of $-4.8 \times 10^{-9}\ \text{C}$. Calculate the number of electrons transferred to the rod, and state what happened to the charge of the cloth used to rub it.
1 mark: uses $n = q/e$ · 1 mark: $n = 3.0 \times 10^{10}$ electrons · 1 mark: the cloth is left with an equal positive charge of $+4.8 \times 10^{-9}\ \text{C}$
Dot point: Explain the role of the movement of electrons in charging particles and objects
AnalyseBand 5(3 marks) 3. A student claims that rubbing a balloon on their hair "creates" negative charge on the balloon. Explain what is wrong with the word "creates", and describe what has actually happened at the particle level.
1 mark: charge is not created, it is transferred · 1 mark: electrons move from hair to balloon · 1 mark: total charge of the system is unchanged, hair is left equally positive
Dot point: Explain the role of the movement of electrons in charging particles and objects
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): Only electrons move during charging because protons are bound tightly in the nucleus, while the outer electrons are loosely held. Rubbing brings the surfaces into close contact, transferring electrons from the wool to the balloon. The balloon therefore has more electrons than protons and becomes negatively charged. The jumper, having lost exactly those electrons, now has more protons than electrons and is left equally positively charged. No charge was created or destroyed, by the law of conservation of charge the electrons were simply transferred, so the total charge of the balloon-and-jumper system stays the same.
Q2 (3 marks): The magnitude of the charge is $4.8 \times 10^{-9}\ \text{C}$ and each electron carries $1.6 \times 10^{-19}\ \text{C}$, so the number transferred is $n = q/e = (4.8 \times 10^{-9})/(1.6 \times 10^{-19}) = 3.0 \times 10^{10}$ electrons. Because the rod became negative it gained those electrons, and they had to come from the cloth. Charge is conserved, so the cloth is left short of exactly the same number of electrons and carries a charge of $+4.8 \times 10^{-9}\ \text{C}$.
Q3 (3 marks): Charge cannot be created or destroyed, so "creates" is the wrong word. Rubbing transfers electrons that already existed: the balloon holds electrons more strongly than hair does, so electrons move from the hair onto the balloon. The balloon now has more electrons than protons and is negative, while the hair has lost the same number and is left equally positive. The total charge of the balloon-and-hair system is exactly what it was before rubbing, which is the principle of conservation of charge.
You can now explain any charging event as electron movement and use conservation of charge to find the sign on each object. Next lesson uses this to compare the three ways objects get charged: friction, induction and conduction.
↩ Back to focus-area hubAt the start you predicted which particle moved to charge a rubbed rod. The answer is electrons: the rod became negative because it gained electrons from the cloth, which was left positive. If you guessed protons moved, revisit Card 1, protons stay locked in the nucleus.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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