Charging by conduction
Rub a comb through dry hair and it picks up charge by friction — but touch that charged comb to an uncharged metal pin and something different happens: charge quietly flows across the point of contact, and the pin becomes charged too, without any rubbing at all. This is charging by conduction, one of the three standard ways a body acquires static charge, and the one defined by a single non-negotiable condition — the two objects must actually touch. This article builds a tight, exam-ready picture of what conduction is, why it needs a conductor on both ends to work well, why the newly charged object always ends up with the same sign of charge as the one that charged it, and how the charge splits between two identical touching spheres. A worked example and a quick reasoning derivation turn the qualitative rule into something you can apply confidently to any conduction question.
Charging by conduction is the direct transfer of charge through physical contact between a charged conductor and another conductor, leaving the second body with the same type of charge as the first, and — for two identical conductors — sharing the original charge roughly equally between them.
The core idea
Definition. Charging by conduction is the transfer of electric charge from a charged object to another object through direct physical contact. When a charged conductor touches an uncharged (or differently charged) conductor, some of the charge on the first object flows across the point of contact onto the second, and the charge keeps redistributing until it stops moving — which, for two identical conductors, means it ends up shared roughly equally between them.
Contact is the defining feature. This is what separates conduction from the other two charging methods you meet in this chapter. Charging by friction needs rubbing between two different materials, and charging by induction needs the charged object to be brought near another object without ever touching it. Conduction needs neither rubbing nor mere proximity — it needs the two bodies pressed together, however briefly, so that charge has a path to cross from one to the other.
Why the receiving body keeps the same sign. Whatever type of charge — positive or negative — sits on the charging object is the type that physically moves across the contact point. Since it is actual charge migrating (not a temporary rearrangement of charge already present, as happens in induction), the object that receives it ends up with the same sign as the object that charged it, and the charging object is left with correspondingly less of that charge than it started with.
Why conductors matter so much. Conduction spreads charge efficiently only when both bodies are conductors — materials such as metals, in which charge carriers can move freely through the whole body. Touch a charged metal sphere to a plastic ruler and almost nothing visible happens, because the ruler is an insulator: charge transferred at the point of contact has nowhere to go and stays stuck exactly there, never spreading out to charge the object as a whole.
Sharing between identical conductors. If the two touching conductors are identical in shape and size, symmetry gives a simple rule: once contact is broken, the total charge that was on the charging object is shared essentially equally between the two, because neither identical sphere has any physical reason to end up with more than the other.
where: is the total charge originally on the single charged sphere before contact, and , are the final charges on the two identical spheres after they touch and separate. This is a qualitative sharing rule at this level — you are not expected to calculate charge in coulombs, only to reason about how it splits.
The goal is to reason out, using only the conservation of charge and the symmetry of two identical conductors, why touching a charged sphere to an identical uncharged sphere leaves both spheres with the same sign of charge and roughly half the original amount each, without appealing to any formula beyond simple division. Step 1. Charge is conserved throughout the process: no charge is created or destroyed when the two spheres touch, so whatever total charge existed on sphere A before contact must still be the combined total on sphere A and sphere B after they separate, meaning . Step 2. Charge only moves because the two spheres are conductors in contact, giving free charge carriers a path from one sphere to the other across the point of contact; this rules out any charge simply vanishing or a different type of charge appearing, so whatever charge does move retains the same sign it had on sphere A. Step 3. Because the two spheres are identical in size and shape, there is no physical feature that favours one sphere over the other for holding extra charge once they are in contact and free charge has had time to redistribute — any asymmetry in the split would have no cause, so the stable outcome is the symmetric one, . Step 4. Combining the conservation equation from Step 1 with the equality from Step 3 gives , so each sphere ends up with — half the original charge, and, since no charge type was created or destroyed, both halves carry the same sign that sphere A carried before contact.
Worked example
Question. A positively charged metal sphere A is brought into contact with an identical, initially uncharged metal sphere B, and then separated. State what type of charge sphere B ends up with, and describe how the charge on sphere A compares with what it started with.
Step 1. Identify the starting condition. Sphere A carries a positive charge; sphere B is uncharged, and both are conductors, so charge is free to move once they touch.
Step 2. Apply the conduction rule. When the spheres touch, positive charge flows from sphere A across the contact point onto sphere B, because a charged conductor in contact with an uncharged conductor always shares its charge this way.
Step 3. Apply the equal-split rule for identical conductors. Since the two spheres are identical, the total original charge on sphere A redistributes to leave each sphere with roughly half of it once they are separated.
Step 4. State the answer. Sphere B ends up positively charged — the same sign as sphere A — and sphere A is left with about half of its original positive charge, since the two identical spheres share the total charge roughly equally between them.
Competitive stretch
If sphere A had instead started with charge and sphere B had already carried a small charge of the same sign before contact, the reasoning changes only at Step 1: the conserved total becomes the sum of both original charges, and that combined total is what splits equally between two identical spheres after contact — the final charge on each is still found by conservation and symmetry, just applied to a different starting total.
Common traps
- Mixing up conduction with induction — conduction needs the two objects to physically touch and leaves the receiving object with the same sign of charge, while induction needs no contact and works through temporary charge separation instead.
- Assuming any two touching objects will charge up equally well, regardless of material — conduction works efficiently between conductors but barely at all when one object is an insulator, since charge cannot move freely through it.
- Thinking the charging object loses all of its charge to the other object — in conduction between two identical conductors, the charge is shared, not entirely transferred away.
- Forgetting that the charge type never flips during conduction — the previously uncharged object always ends up with the same sign of charge as the object that charged it, never the opposite sign.
Ask the tutor: want to compare conduction, friction, and induction side by side with a few more worked scenarios, or see what changes when the two conductors are different sizes — ask the AI tutor to walk through it with you.
Why does the receiving object end up with the same type of charge, not the opposite type?
Because actual charge physically crosses over from the charged object to the uncharged one during contact — it is not a case of charges merely rearranging themselves as in induction. Since the charge that migrates is whatever type the first object already carried, the second object necessarily ends up carrying that same type. If the first object is positively charged, only positive charge is available to flow across the contact point, so the second object can only pick up a positive charge, never a negative one.
Why does touching a charged metal sphere to a plastic ruler barely charge the ruler?
Charging by conduction relies on charge being free to move from the point of contact and spread across the whole receiving object, and that only happens well in a conductor. Plastic is an insulator, so any tiny amount of charge that does transfer at the contact point has no free path to travel through the rest of the ruler — it stays stuck right where the contact happened instead of spreading out. Because the charge never spreads, the ruler as a whole does not show any noticeable sign of being charged, even though a metal object touched the same way would charge up clearly.
Does the charging sphere always end up with exactly half its original charge?
Only when the two conductors are identical in size and shape, which is the case usually tested at this level. Symmetry between identical conductors means neither one has any reason to end up with more charge than the other, so the original charge splits roughly equally. If the two conductors were different sizes, the split would not be equal — a larger conductor generally ends up holding a larger share — but working out the exact unequal split needs ideas beyond this syllabus, so you only need the equal-split rule for identical conductors here.