The crackle when you pull off a sweater in winter happens because electrons cross from one rubbed object to the other, leaving the two with opposite charges.
Before rubbing, both objects are neutral, with equal amounts of (+) and (−) charge, and only the light electrons can move. Protons stay locked in the atomic nucleus.
The object that loses electrons becomes (+), the one that gains them becomes (−), and like charges push apart while unlike charges pull together.
In the diagram, raising the number of transferred electrons opens the same number of empty spots in the glass rod and adds the same number of extra electrons to the cloth.
The Crackle of a Sweater
To understand static electricity you first have to decide what actually moves. Before rubbing, the glass rod and the cloth each hold equal numbers of (+) and (−) charges and are neutral; rubbing pulls a few electrons from the surface of the rod onto the cloth. The rod is then short of electrons and positive, the cloth has extra electrons and is negative, and since the cloth gained exactly what the rod lost, the total charge of the pair is the same as before. No charge was created; electrons that were already there simply changed places.
🧤 Static Electricity in Daily Life
①Rub a balloon on your hair and the hair rises toward the balloon
②On a dry day, touching a doorknob gives a small shock
③A plastic ruler rubbed on a sleeve picks up scraps of paper
④Each time, electrons crossed from one object to another while they were rubbed
Rubbing Moves Electrons
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Electrons crossing from a glass rod to a cloth as they are rubbed together
Each box in the diagram above starts with six (+) charges paired with six (−) charges. When the slider sets how many electrons move, that many (−) leave the glass rod and remain as dotted empty spots, while the same number of green electrons line up along the bottom of the cloth. Notice that no (+) ever moves, and that the bottom line always shows the rod's +n and the cloth's −n with the same size: the diagram itself shows that charge is conserved. Set the slider to 0 and both boxes are neutral again.
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💡 How Rubbing Creates Charge
①Before rubbing, both objects have equal amounts of (+) and (−) charge and are neutral
②Rubbing moves only electrons; protons stay inside the nucleus
③The object that loses electrons becomes (+) and the one that gains them becomes (−)
④Which object loses electrons depends on the materials: glass rubbed with silk becomes (+), and plastic rubbed with fur becomes (−)
Forces Between Charges
Because the glass rod ends up positive, it is tempting to think positive charge moved onto it, but the rod is positive because electrons left it. Protons are held tightly in the nucleus and rubbing cannot pull them off. In the lower diagram, choosing like signs pushes the two spheres apart and unlike signs draws them together; this force depends only on the kinds of charge, not on how the charge got there. The leaves of an electroscope spread for the same reason: both leaves carry the same kind of charge and repel each other.
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Direction of the force between like and unlike charges
Force between charges (electric force)
Like charges → repulsion, unlike charges → attraction
(+) and (+), or (−) and (−), push apart; (+) and (−) pull together. The closer they are, the stronger the force
Charging and the Electroscope
🔬 Detecting Charge with an Electroscope
①An electroscope is a metal plate, a metal rod and two thin metal leaves joined into one piece
②Bringing a charged object near the plate pulls or pushes the electrons inside, so both leaves end up with the same kind of charge
③Like charges repel, so the leaves spread apart
④If they spread, there is charge, and the wider they spread, the more charge there is
Ways an Object Becomes Charged
Friction / contact / electrostatic induction
Friction: electrons move when rubbed | Contact: electrons are shared when touching a charged object | Induction: just bringing a charge near pushes the object's electrons to one side
Summary
Static Electricity in One Line
Rubbing → electrons move → charging → electric force
Losing electrons gives (+), gaining them gives (−), and the amount lost equals the amount gained
🎯 Common Test Points
①What moves during rubbing is electrons (−); protons (+) do not move
②The object that loses electrons is (+); the one that gains them is (−)
③Like charges repel and unlike charges attract
④An uncharged object is neutral, with equal amounts of (+) and (−)
⑤Spread electroscope leaves mean a charged object is nearby or the electroscope itself is charged
Examples and Unit Review
Example 1
After a glass rod is rubbed with a cloth, the rod is positively charged. Which way did electrons move, and what charge does the cloth have?
1
Only electrons move when objects are rubbed; losing electrons gives (+) and gaining them gives (−).
2
The rod became (+), so it lost electrons, and those electrons went to the cloth, which is therefore (−).
▸ Electrons moved from the glass rod to the cloth, and the cloth is negatively charged
Whatever number of electrons you choose in the diagram above, the rod's + and the cloth's − always come out the same size.
Example 2
What force acts between two negatively charged objects brought close together, and what force acts between a positive and a negative object?
1
Charges of the same kind push each other away (repulsion).
2
(−) and (−) are the same kind and repel; (+) and (−) are different kinds and pull together (attraction).
▸ Repulsion between (−) and (−); attraction between (+) and (−)
The direction of the electric force depends only on whether the two charges are the same kind or different.
Unit-test style
Which statement about the generation of electricity is correct?
①When rubbed, protons move to give a charge
②When rubbed, electrons move to give a charge
③Charges of the same kind attract each other
④An object that gains electrons becomes positively charged
⑤Charges of different kinds repel each other
▸ ② When rubbed, electrons move to give a charge
1
What moves when rubbed is electrons; protons are fixed in the nucleus and do not move.
2
What moves is electrons (①), like charges repel (③), gaining electrons gives a negative charge (④), and unlike charges attract (⑤).