Electrical Influence – Charge distribution on the conductor

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Static electricity is the result of an imbalance between negative and positive charges in an object. These charges can build up on the surface of an object until they find a way to be released or discharged.

 

 

 

There are three ways to charge an object:

  1. Charge by Friction
  2. Charge by Conduction
  3. Charge by Induction

 

 

 

 

 

1) Friction

 

How to remove electrons from one material and put them in another? By rubbing them together (as long as one material have the electrons more loose then the other).

Example: Electrons are held more firmly in plastic than in hair. That means that when you comb your hair, electrons pass on from hair to comb. As a result, hair becomes positively charged and the comb becomes negatively charged (it can pick up little pieces of paper). If the plastic comb is rubbed into silk, it will become positively charged, because the silk has a higher affinity for electrons.

 

2) Contact

If a neutral object is touched by a charged object, charging of the neutral object will take place. For example, if an object has an excess of electrons (so that it is negatively charged) and touch a neutral object, electrons will pass on to the neutral object. The result is that the neutral object acquires a negative charge and the initially charged object becomes less negative. Now we will examine the behavior of leaves when we touch charged object to the charged electroscope. There are different situations in this case, let’s look them one by one (we assume that capacities of electroscope and rod are same.

 

 

 

3) Induction

In this case, the charged object doesn’t touch the neutral object, it only gets close. That causes a separation of charges. Then, another object touches the neutral object and charging takes place.


 

Electrostatic induction, also known as “electrostatic influence”.

Example:  In charging by induction, a charged body A imparts to another body B, some charge of opposite sign without any contact between A and B. The process of charging of body by without making any contact with other  of body is called charging by induction. Body A shall not lose any charge as it is not in contact with B. Charging by induction is explained as below. Bring two metal spheres, A and B, supported on insulating stands, in contact as shown in picture.

Bring a positively charged rod near one of the spheres; say A. The free electrons in the spheres are attracted towards the rod. This leaves an excess of positive charge on the rear surface of sphere B. Both kinds of charges are resided on the surfacesThe left surface of sphere A, has an excess of negative charge and the right surface of sphere B, has an excess of positive charge. However, not all of the electrons in the spheres have accumulated on the left surface of A.

As the negative charge starts building up at the left surface of A, other electrons are repelled by these. In a short time, equilibrium is reached under the action of force of attraction of the rod and the force of repulsion due to the accumulated charges.

The process is called induction of charge and happens almost instantly. The accumulated charges remain on the surface, as shown, till the glass rod is held near the sphere. If the rod is removed, the charges are not acted by any outside force and they redistribute to their original neutral state.  Separate the spheres by a small distance while the glass rod is still held near sphere A. the two spheres are found to be oppositely charged and attract each other.

Remove the rod. The charges on spheres rearrange themselves as shown in pisture below.

Now, separate the spheres quite apart. The charges on them get uniformly distributed over them, as shown in picture below.

The animation below shows the procedure that we have described.

 

 

Suppose that a negatively charged rubber balloon is brought near a single sphere as shown below. The presence of the negative charge will induce electron movement in the sphere. Since like charges repel, negative electrons within the metal sphere will be repelled by the negatively charged balloon. There will be a mass migration of electrons from the left side of the sphere to the right side of the sphere causing charge within the sphere to become polarized. Once charge within the sphere has become polarized, the sphere is touched. The touching of the sphere allows electrons to exit the sphere and move through the hand to “the ground”. It is at this point that the sphere acquires a charge. With electrons having left the sphere, the sphere acquires a positive charge . Once the balloon is moved away from the sphere, the excess positive charge redistributes itself (by the movement of remaining electrons) such that the positive charge is uniformly distributed about the sphere’s surface.

 

 

Distribution of charge over surfaces

Since the charges repel, the charge on a hollow or solid metal sphere will move away from the centre as far as it will go.

 

 

 

Thus the charge on a conductor of any shape is always found on the surface and never inside the conductor. It has been found that a charged metal sphere has a uniform charge density all over its surface.

 

 

 

In the presence of an external charge q, the charges in a metal redistribute. The electric field at any point has three contributions. Electric charges on a conductor migrate to the outside surface no matter where you put them initially. Field intensity inside the conductor must be zero All charge on a conductor gathers on the outside. If a charge is contained inside a conductor the electric field is zero. If a charge is outside a conductor the inside of the container will not be affected by the outside charge.

 

Example Faraday Cage: Faraday cages are named after the English scientist Michael Faraday, who invented them in 1836 but Benjamin Franklin also made a great contribution to “Faraday Cage” development and application.  A Faraday shield may be formed by a continuous covering of conductive material or in the case of a Faraday cage, by a mesh of such materials

 

 

A very pointed conductor has a large charge concentration at the point. The electric field is very strong at the point and can exert a force large enough to transfer charge on or off the conductor.

 

 

Lightning rods are used to prevent the buildup of large excess charges on structures and, thus, are pointed.

 

 

 

 

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