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## Electromagnetism (Essentials) - Class 12th

### Course: Electromagnetism (Essentials) - Class 12th>Unit 2

Lesson 2: Electric charges- what are they? And how do they behave?

# Conservation of charge review

Review the conservation of charge and how charge is transferred in various scenarios.

## Key terms

TermMeaning
Law of conservation of chargeCharge is neither created nor destroyed, it can only be transferred from one system to another.
ConductorMaterials that permit electrons to move freely through them, such as most metals.

## How does an object get negatively charged?

A neutral object can become negatively charged when electrons get transferred from another object (see Figure 1).
In Figure 1, a negatively charged conducting rod with net charge $-4e$ touches a neutral conducting sphere. During the transfer, $2$ electrons move from the rod to the sphere because the electrons are repulsed from each other and want to spread out. Afterward, the rod has a net charge of $-2e$ and the sphere has a net charge of $-2e$.
The total charge between the two objects is $-4e$ before and after the transfer, so charge is conserved. The electrons redistribute so the rod becomes less negatively charged and the sphere becomes negatively charged.

## How does an object get positively charged?

A neutral object can become positively charged when electrons get transferred to another object (see Figure 2).
In Figure 2, a positively charged conducting rod with net charge $+4e$ touches a neutral conducting sphere. During the transfer, $2$ electrons move from the neutral sphere to the rod because the electrons are attracted to the positive charge and want to spread out from other electrons. Afterward the rod has a net charge of $+2e$ and the sphere has a net charge of $+2e$.
The total charge between the two objects is $+4e$ before and after transfer, so charge is conserved. The electrons redistribute so the rod becomes less positively charged and the sphere becomes positively charged, and the protons remain fixed.

## Common mistakes and misconceptions

People mistakenly think that an object gets positive charge by receiving extra positive charges. Protons have positive charge and are bound to the object, while electrons can be transferred between objects. When an object’s net charge becomes more positive, it has lost electrons.

For deeper explanations of conservation of charge, see our video about the law of conservation of charge.
To check your understanding and work toward mastering these concepts, check out our exercises:

## Want to join the conversation?

• a neutral object has positive and neagtive charges both. so when a postively charged object comes close to neutral one why electrons from neutral gets transferred because if there is attraction between opposite charges it must be within neutral object also so electrons of neutral object must be attracted to protons of same object. what is so special about the positive charge on other object
• Your question is really an interesting one and actually you came up with a valid point. The first thing which you have to keep in mind that 'Electron Transfer' only happens with metals. In insulators and semiconductors, the scenario which takes place is called 'Electron Localisation'. The electrons which are transferred or localised are called 'Valence Electrons' which means the electrons residing on the last orbit in an atom. The valence electrons are loosely bound in all the atoms may it be of a conductor or of an insulator. 'Loosely Bound' means that if we can apply enough force which supercedes the attraction force between the nucleus and the electrons, the electrons will be transferred to a positively charged atom(for conductors) or be localised at a side of an atom(for insulators).

If we take a positively charged atom to a neutral one, the electrostatic force occurring between the positive charge and the valence electrons of the atom(because of Coulomb's Law) is more than the force between the valence electrons and the protons of its own atom. So, the electrons will be transferred to the positively charged atom as those will have enough energy to get 'free'. That is why they are called 'Free Electrons'.

For insulators and semiconductors, almost the same incident happens; but the electrons can't be transferred, instead, they get localised at a side of the atom. The reason is that to get free, the electrons have to be in 'Conduction Band'(kind of an energy state), but here they are in 'Valence Band' which is a lower energy state. Though they don't have enough energy to get free, they localise at a side of the atom.

If you want to know more about 'Electron Bands', you can google a bit and I hope you'll find some YouTube videos which might clear some of your clouds in mind.

And remember, all the 'electrons' I used to explain are 'Valence Electrons'.
• What happens when the net charge of the two objects is an odd multiple of e?
• The material with more protons get the extra electron
• Does the shape of the conductor matter? Because with the cube and the cylinder examples, we just looked at charge transfer but when does Volume start to matter (if ever?) Or the shape?

Because at some point I was thinking that the cube would have more matter distributed and therefore hold more distribution of negative charge.
• Yes the shape of the conductor does matter. If a conductor has a pointy end, then the charges on the conductor would tend to accumulate at the pointy end.