There is not just one way to charge an object, there are three, and each moves electrons differently. Friction rubs them off, conduction touches them across, and induction rearranges them without any contact at all. Once you can tell these three apart, you can explain almost every static-electricity demonstration you will ever see.
You hold a negatively charged rod close to (but not touching) a neutral metal sphere.
Before reading: do the electrons in the sphere move toward the rod or away from it? Write your prediction and one sentence explaining why.
Rubbing transfers electrons between two materials
When two different materials are rubbed together, one grabs electrons more strongly than the other, so electrons move across and the two surfaces end up oppositely charged.
Different materials hold their electrons with different strengths. When you rub a plastic rod with a cloth, the rod pulls electrons off the cloth: the rod becomes negative and the cloth becomes equally positive. Glass rubbed with silk does the opposite, glass gives up electrons and becomes positive. In every case the total charge is unchanged; friction simply separates charge that was already there.
Charging by friction transfers electrons between two rubbed materials; the material that gains electrons becomes negative and the one that loses them becomes equally positive. It always requires contact (rubbing).
Pause, copy the highlighted statement into your book before moving on.
Did you get this? True or false: after charging by friction, the two materials carry charges of opposite sign.
Touching a charged object shares charge by direct contact
We just saw friction transfer electrons by rubbing. That raises a question: what happens if a charged object simply touches a neutral conductor? This card answers it → charge spreads across the contact and both objects end up the same sign.
When a charged object touches a neutral conductor, electrons flow across the contact point until the excess charge is shared, leaving both objects with the same sign of charge.
Suppose a negatively charged rod touches a neutral metal sphere. Some of the rod's excess electrons spread onto the sphere because like charges repel and push apart. When the rod is removed, the sphere keeps a share of the negative charge. Both objects are now negative, conduction never produces opposite charges. Because the charge is shared, conduction works only on conductors, where electrons can move freely.
A metal sphere carrying $-6.0 \times 10^{-8}\ \text{C}$ touches an identical neutral metal sphere, then they are separated. Assuming the charge spreads evenly, what charge does each sphere carry?
Charging by conduction shares charge through direct contact, so the charged object and the conductor it touches end up with the same sign of charge. Total charge is conserved across the two objects.
Add the highlighted rule to your notes before the check below.
Quick check: A positively charged rod touches a neutral metal can. After contact, the can is:
Pattern B, Classify and justify
For each scenario, name the charging method and state the final sign of charge on the object: (1) a comb is run through hair and then attracts paper; (2) a charged balloon touches a metal doorknob and the spark transfers charge; (3) a negative rod is held near a grounded sphere, the ground connection is removed, then the rod is removed. Justify each answer in one sentence using electron movement.
Odd one out: three of these describe charging processes that require CONTACT. Which one does not?
Dot point: Explain how friction, induction and conduction can cause objects to become electrically charged
Charging without contact, the answer to the hook
We just compared friction and conduction, both needing contact. That raises a question: how can a charged rod affect a neutral object it never touches? This card answers it → induction rearranges electrons from a distance.
A nearby charge pushes or pulls the free electrons inside a conductor, separating its charge into a near side and a far side even though the object stays neutral overall.
Bring a negative rod near a neutral metal sphere. The rod's field repels the sphere's electrons to the far side, leaving the near side positive, the sphere is now polarised, but its total charge is still zero. Because opposite charges are now closer than like charges, the sphere is attracted to the rod. This is why a charged rod attracts neutral paper, answering the hook.
To leave a net charge, you ground the sphere while the rod is near: electrons flee through the ground connection. Remove the ground first, then the rod, and the sphere keeps a charge opposite to the rod. Induction can therefore either polarise temporarily or, with grounding, charge permanently, and it never needs contact with the charging rod.
Charging by induction needs no contact: a nearby charge polarises a conductor (near side opposite, far side same). Grounding while the charge is near, then removing the ground first, leaves a net charge opposite to the inducing charge.
Pause, copy the highlighted induction summary into your book before the check below.
Last check: Decide if the statement is true or false.
A charged rod can attract a neutral object without touching it because it polarises the object's charge.
Match the pairs: Match each process to what defines it.
Dot point: Explain how friction, induction and conduction can cause objects to become electrically charged
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 negatively charged rod is brought near a neutral metal sphere on an insulating stand but does not touch it. Explain, in terms of electron movement, why the sphere becomes polarised and is attracted to the rod, and state whether the sphere has gained a net charge.
1 mark: rod's field repels the sphere's free electrons · 1 mark: far side becomes negative, near side positive (polarised) · 1 mark: opposite charges are closer, so net attraction · 1 mark: no net charge gained, total charge still zero
ExplainBand 5(4 marks) 2. A negatively charged rod is brought near, but does not touch, an isolated neutral metal sphere on an insulating stand. The far side of the sphere is briefly earthed, the earth is removed, and then the rod is taken away. Explain the final charge on the sphere and why it has that sign.
1 mark: the rod repels electrons to the far side of the sphere · 1 mark: earthing lets those repelled electrons leave the sphere · 1 mark: earth removed BEFORE the rod, so the electrons cannot return · 1 mark: sphere is left positively charged
Dot point: Explain how friction, induction and conduction can cause objects to become electrically charged
AnalyseBand 5(3 marks) 3. Contrast charging by conduction with charging by induction in terms of contact and the sign of the final charge.
1 mark: conduction requires contact, induction does not · 1 mark: conduction leaves the SAME sign as the charging object · 1 mark: induction leaves the OPPOSITE sign
Dot point: Explain how friction, induction and conduction can cause objects to become electrically charged
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): The negative rod's electric field repels the free electrons inside the metal sphere. These electrons move to the far side of the sphere, leaving the near side with a deficit of electrons and therefore a positive charge, the sphere is polarised. Because the positive near side is closer to the rod than the negative far side, the attractive force outweighs the repulsion, so the sphere is attracted to the rod overall. However, no electrons have entered or left the sphere: its total charge remains zero, so it has not gained a net charge. A net charge would only result if the sphere were grounded while the rod was near.
Q2 (4 marks): The negative rod repels the sphere's free electrons, which move as far from the rod as they can, to the far side of the sphere. Earthing the far side gives those repelled electrons a path to escape to the ground, so they leave the sphere. The earth connection is removed while the rod is still in place, so the electrons that left cannot flow back. When the rod is finally taken away, the remaining charge redistributes evenly, and the sphere is left with fewer electrons than protons, so it carries a net positive charge. Note that the induced charge is opposite in sign to the rod.
Q3 (3 marks): Charging by conduction requires the charged object to touch the conductor. Charge flows across the contact until they share it, so the object being charged ends up with the same sign as the charging object. Charging by induction requires no contact at all: the charged object is only brought nearby, and it is a separate earthing connection that removes or supplies electrons. Because the nearby charge repels or attracts the conductor's electrons before earthing, the charge that is left behind is opposite in sign to the charging object. So the two processes differ both in whether contact is needed and in the sign of the result.
You can now tell friction, conduction and induction apart and predict the sign of charge each produces. Next lesson zooms into the electric field that surrounds any charged object and explains why these forces act at a distance.
↩ Back to focus-area hubAt the start you predicted which way the electrons in a sphere move near a negative rod. The answer is away from the rod: like charges repel, so the electrons are pushed to the far side, leaving the near side positive. That polarisation is exactly why a charged rod attracts neutral objects.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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