Why does an object become negatively charged when rubbed?

Everything we see is made up of tiny particles of matter called atoms. The atoms are made up of even smaller parts called protons, electrons and neutrons. An atom usually has the same number of protons and electrons, but sometimes electrons can be moved away from their atoms.

If you comb your hair,  for example, electrons leave the atoms and molecules in your hair and travel to the plastic comb. The comb, covered in negatively charged electrons, becomes negatively charged as well, and your hair is left with a positive charge. This “separation of charge” is the reason for the collection of effects we call static electricity.

If two objects have different charges, they attract (or pull towards) each other. If two objects have the same charge, they repel (or push away) from each other. After you’ve combed your hair, every single hair has the same positive charge. Since things with the same charge repel each other, the hairs try to move away from each other by standing up and away from all the other hairs, resulting in you having a very funny-looking hairdo!

Another example: if you walk across a carpet, electrons move from the rug to you. Now you have extra electrons. If you have extra electrons piled on you, they will spill off when you touch an object like a doorknob, and give you a shock. Shocks come from gaining or losing electric charge in a hurry.

When a charged object is brought close to a neutral material, the electrons on the neutral material will either move toward the charged object (if it has a positive charge) or away from the charged object (if it has a negative charge). In other words, the neutral material “picks up” charge on its near and far side, relevant to the charged object. This phenomenon is called an induced charge. The result is that a normally neutral material will have a slight charge when near the charged object, and it is enough for the two to attract.

Rubbing a balloon on your head or dragging your feet on the carpet will build up a charge, but so will ordinary walking or repeatedly touching your head with a balloon! It’s the mere contact between two different materials that causes charge to move from one object to another. Rubbing materials together can help move charge more quickly because more surface area is being contacted. Friction has nothing to do with the charge.

An important thing to consider when doing any of these activities is the weather: humidity in the air can make it difficult to build up charges, causing experiments to behave in unexpected ways!

The effect in which two objects get charged by rubbing and remain charged is called the triboelectric effect,

http://en.wikipedia.org/wiki/Triboelectric_effect

where the root "tribo" means friction in Greek (The Greek word $\tau\rho\iota\beta\omega$ means 'to rub'). Friction is actually unnecessary: contact is enough in principle.

This effect shouldn't be confused with the (Volta or Galvani) "contact potential" between metals which only exists as long as the two metals remain in contact, and especially not with "contact electrification" which was a name of a scientifically incorrect theory of electricity at the end of the 18th century that attempted to overgeneralize the interpretation of the triboelectric effect. "Electrophorus" was a gadget, first produced by Volta, that used the triboelectric effect.

The cause of the triboelectric effect is adhesion - the atoms on the surface literally form chemical bonds. Materials such as fur are ready to lose electrons and become positively charged while the materials such as ebonite or glass gain electrons and become negatively neutral. To get some idea about which atoms are likely to lose or gain electrons, it's useful to know their electronegativity:

http://en.wikipedia.org/wiki/Electronegativity#Electronegativities_of_the_elements

The redder atom, the higher electronegativity, and the more likely it is for the atom to gain electrons and become negatively charged. That's especially true for light halogens (fluorine, chlorine) and oxygen. That's partly why glass - with lots of $SiO_2$ - likes to get negatively charged in the triboelectric effect. Even sulfur (40% of ebonite) has a higher electronegativity than e.g. carbon and hydrogen that are abundant in the fur which is why fur loses electrons and becomes positively charged.

Of course, the actual arrangement of the atoms in the molecules matters, too. So this overview of the periodic table was just an analogy, not a reliable way to find out the results of the triboelectric effect.

An electrical charge is created when electrons are transferred to or removed from an object. Because electrons have a negative charge, when they are added to an object, it becomes negatively charged. When electrons are removed from an object, it becomes positively charged.

What is an electrical charge?

An electrical charge is created when two materials come into contact or are rubbed together. When the materials are in contact, electrons can be literally rubbed off of one object and onto the other. This doesn’t mean that any two materials rubbed together will cause electrons to move. Some materials are much more willing to donate electrons, and others are much more likely to accept electrons. A list of materials ordered by their ability to donate electrons is called a triboelectric series.

Most of us have probably all generated a static electrical charge at some point in time. The shock you receive when you walk across a carpet and touch a metal surface or when you feel the cling of statically charged clothing are evidence of a static electrical charge.

Hair and rabbit’s fur give up electrons easily, whereas polyethylene and Teflon are good at attracting electrons. When two materials in different positions on the triboelectric series are rubbed together, the materials towards the positive end give up electrons and become positively charged, and those towards the negative end accept electrons and become negatively charged.

How do you create an electrical charge?

Consider a practical example of creating an electrical charge. When a balloon is rubbed on hair, electrons are moved from the hair to the surface of the balloon. The electrons ‘stick’ to the balloon where the balloon was rubbed and do not move across the surface of the balloon. The balloon becomes negatively charged (red electrons), and the person’s hair becomes positively charged (fewer electrons than before).

Now let’s use the newly charged balloon to charge another balloon. When we touch the charged balloon to a neutral (balanced – no extra electrons) balloon, some of the added electrons are transferred to the neutral balloon. Now both balloons have a negative charge, and we notice something – the balloons push apart. The charge on the balloons causes a force that pushes the balloons apart. The force is much like the repulsive force you feel when you bring two magnets together north to north or south to south.

Insulators and conductors

Insulators are materials such as glass, rubber, wood and most plastics where the electrons are held quite tightly and are not free to move easily from place to place. Conductors are materials such as copper, silver, gold and iron where electrons are free to move from place to place.

For example, a charged balloon (insulator) is brought near a neutral aluminium can (conductor). While the can is far away from the balloon, the negative charge on the balloon has little or no effect on the can, and the electrons on the can are evenly dispersed. When the balloon is brought near the can, something interesting happens – the electrons move to the side of the can to get as far away from the negatively charged balloon as possible. This leaves the side of the can nearest the balloon positively charged. The can as a whole is still neutral, but because the positively charged side of the can is close to the balloon, the negative charges on the balloon attract the positive side of the can and the force pulls the balloon and can together. If the can is lying on its side, it will roll towards the balloon.

Something quite different happens if the conducting can is allowed to touch the charged balloon. When the balloon and can touch, some of the electrons on the balloon will flow onto the can, leaving the can with extra electrons and therefore negatively charged. Now both the balloon and can are negatively charged, and the like charges cause a force pushing the can and balloon apart. If the can is lying on its side, it will roll away the balloon.

Induction

In the above example, we used a negatively charged balloon to impart a negative charge to the can by a process of direct contact. It is also possible to use a negatively charged balloon to impart a positive charge on a can through a process called induction.

If the balloon is brought near the can, the electrons move to the far side of the can, as described earlier. If another object, like a finger, now touches the negative side of the can, some of the overcrowded electrons will flow onto the object, leaving the can positively charged.

Thus it is possible to use a negatively charged balloon to impart either a negative charge (by contact with the balloon) or a positive charge (by induction).

Nature of science

In science, a model is a representation of an idea, an object or even a process or a system that is used to describe and explain phenomena that cannot be experienced directly. Models are central to what scientists do, both in their research as well as when communicating their explanations.

Related content

This article will help understand atomic structure and provides a simple model to explain the movement of electrons in conductors and insulators.

Find out how to make a simple electromagnet in this activity.

This activity supports students to construct simple electrical circuits and test a variety of materials to identify those that are good conductors and those that do not conduct electricity.

This recorded professional learning session will build your confidence to teach Physical World.

Useful link

Use this University of Colorado simulation to experiment with electrical charges on balloons and other objects.

Why does friction cause a negative charge?

Why do objects get charged when rubbed together?

Why does a balloon becomes negatively charged when rubbed?

What happens when an object is charged by rubbing?