<< Chapter < Page Chapter >> Page >

Force between charges

The force exerted by non-moving (static) charges on each other is called the electrostatic force. The electrostatic force between:

  • like charges is repulsive
  • opposite (unlike) charges is attractive .

In other words, like charges repel each other while opposite charges attract each other. This is different to the gravitational force which is only attractive.

The closer together the charges are, the stronger the electrostatic force between them.

Experiment : electrostatic force

You can easily test that like charges repel and unlike charges attract each other by doing a verysimple experiment.

Take a glass rod and rub it with a piece of silk, then hang it from its middle with a piece string so that it is free to move. If you then bring another glass rod which you have also charged in the same way next to it, you will see the rodon the string turn away from the rod in your hand i.e. it is repelled . If, however, you take a plastic rod, rub it with a piece of fur and then bring it close to the rod on thestring, you will see the rod on the string turn towards the rod in your hand i.e. it is attracted .

This happens because when you rub the glass with silk, tiny amounts of negative charge are transferred from the glassonto the silk, which causes the glass to have less negative charge than positive charge, making it positively charged . When you rub the plastic rod with the fur, you transfer tiny amounts ofnegative charge onto the rod and so it has more negative charge than positive charge on it, making it negatively charged .

Two charged metal spheres hang from strings and are free to move as shown in the picture below. The right hand sphere is positively charged. The charge on the left hand sphere is unknown.

The left sphere is now brought close to the right sphere.

  1. If the left hand sphere swings towards the right hand sphere, what can you say about the charge on the left sphere and why?
  2. If the left hand sphere swings away from the right hand sphere, what can you say about the charge on the left sphere and why?
  1. In the first case, we have a sphere with positive charge which is attracting the left charged sphere. We need to find the charge on the left sphere.

  2. We are dealing with electrostatic forces between charged objects. Therefore, we know that like charges repel each other and opposite charges attract each other.

    1. In the first case, the positively charged sphere is attracting the left sphere. Since an electrostatic force between unlike charges is attractive, the left sphere must be negatively charged.
    2. In the second case, the positively charged sphere repels the left sphere. Like charges repel each other. Therefore, the left sphere must now also be positively charged.

Interesting fact

The word 'electron' comes from the Greek word for amber. The ancient Greeks observed that if you rubbed a piece of amber, youcould use it to pick up bits of straw.


Unlike conductors, the electrons in insulators (non-conductors) are bound to the atoms of the insulator and cannot move around freely through the material. However, a charged object can stillexert a force on a neutral insulator due to a phenomenon called polarisation .

If a positively charged rod is brought close to a neutral insulator such as polystyrene, it can attract the bound electronsto move round to the side of the atoms which is closest to the rod and cause the positive nuclei to move slightlyto the opposite side of the atoms. This process is called polarisation . Although it is a very small (microscopic) effect, if there are many atoms and the polarised object islight (e.g. a small polystyrene ball), it can add up to enough force to cause the object to be attracted onto thecharged rod. Remember, that the polystyrene is only polarised, not charged. The polystyrene ball is still neutral since no charge was added or removed from it. The picture showsa not-to-scale view of the polarised atoms in the polystyrene ball:

Some materials are made up of molecules which are already polarised. These are molecules which havea more positive and a more negative side but are still neutral overall. Just as a polarised polystyrene ball can be attracted to a charged rod, these materialsare also affected if brought close to a charged object.

Water is an example of a substance which is made of polarised molecules. If a positively charged rod is brought close to a stream of water, the molecules can rotateso that the negative sides all line up towards the rod. The stream of water will then be attracted to the rod since opposite charges attract.

Conservation of charge

The principle of conservation of charge states that the net charge of an isolated system remains constant during any physical process, e.g. when two charges make contact and are separated again.

Questions & Answers

Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
The fundamental frequency of a sonometer wire streached by a load of relative density 's'are n¹ and n² when the load is in air and completly immersed in water respectively then the lation n²/na is
Mukesh Reply
Properties of longitudinal waves
Sharoon Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Siyavula textbooks: grade 10 physical science [caps]. OpenStax CNX. Sep 30, 2011 Download for free at http://cnx.org/content/col11305/1.7
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Siyavula textbooks: grade 10 physical science [caps]' conversation and receive update notifications?