Electric chargeCopy

Learning Objectives

• State that there are positive and negative charges.
• State that charge is measured in coulombs.
• State that unlike charges attract and that like charges repel.
• State that the direction of an electric field at a point is the direction of the force on a positive charge at that point.
• Describe an electric field as a region in which an electric charge experiences a force.
• Describe simple field patterns, including the field around a point charge, the field around a charged conducting sphere and the field between two parallel plates (not including end effects).
• Give an account of charging by induction.
• Describe simple experiments to show the production and detection of electrostatic charges.
• State that charging a body involves the addition or removal of electrons.
• Distinguish between electrical conductors and insulators and give typical examples.
• Recall and use a simple electron model to distinguish between conductors and insulators.

Contents to learn

Electric charge

All atoms are made up of three kind of particles electrons , protons, and neutrons. Electrons are the tiniest of these, and have a negative charge (1.6 X 10^-19 C). Protons and neutrons have about the same mass, but protons are positively charged , while neutrons have no charge.

The amount of charge on an object is measured in Coulombs.

In neutral atom, the number of electrons and protons are equal, so having no net charge. If there are more electrons than protons in an atom, then the atom carries an overall negative charge. If there are fewer electrons than protons, then the atom carries an overall positive charge.

One Coulomb Charge
A charge of 1C is a charge on 6.2 X 10¹⁸ electrons, so an object with a charge of +1.0 C has these many too few electrons. This is an enormous charge, and objects would explode long before they could be given so much.

Typical electrostatic charges are less than 1micro coulomb ( 1.0 X 10^-6 C)

Production of electric charge
When you charge an object you are giving or taking away negatively charged electrons, so that the charge on the object overall is unbalanced.

For example,

1 – when you rub a glass or acetate rod with a cloth, electrons from the rod get rubbed on to the cloth. So the cloth become negatively charged overall and the rod is left with an overall positive charge.

2 – When you rub a polythene rod with a cloth, electrons from the cloth get transferred to the rod, so the polythene carries a negative charge overall, and the cloth carries a positive charge.

Material like glass, acetate and polythene can only become charged because they are insulators. Electrons don’t move easily through insulating materials so when extra electrons are added, they stay on the surface instead of flowing away and the surface stays negatively charged.

Similarly if electrons are removed, electrons from other part of material do not flow in to replace them, so the surface stays positively charged.

A material through which electrons flow easily is called a conductor. Conductors, such as metal can not be charged by rubbing.

Detection of electric charge by using gold-leaf electroscope
A common type of gold-leaf electroscope has been shown in the diagram. It is used for the detection and testing of small electrical charges.

It consists of a brass rod surmounted by a brass disc or cap and having at its lower end a small rectangular brass plate with a leaf of thin gold or aluminium attached.

The leaf is protected from draughts by enclosing it in an earthed metal case with glass windows. The brass rod is supported by passing it through a plug of some good insulating material such as alkathene at the top of the case.

The three horizontal parallel lines shown at E in the figure is the conventional symbol for an earth connection.

Experiment to detect the electric charge
(1) To detect the presence of charge on a body: If a rod of some suitable material is charged by friction and then brought near to the cap of gold-leaf electroscope, the leaf is seen to diverge from the plate. A charge has been induced on the leaf and plate, and consequently repulsion occurs between them. On removing the charged rod, the leaf collapses, showing that the induced charge on the electroscope is only temporary. (Very small charges may be detected by this method)

(2) To charge a gold-leaf electroscope by contact: Generally speaking, it is not always easy to charge an electroscope by contact with a charged rod but usually it can be done after a few attempts. An ebonite rod is given a small charge by rubbing with fur, and is then rolled over the cap of an electroscope. The leaf will be seen to diverge, and then the rod is removed. If the leaf does not stay diverged the process is repeated until it does. we may now assume that the electroscope is charged with negative electricity by conduction from the ebonite rod.

If the cap of the elecrtoscope is touched with the fingure, the charge flows to earth through the experimenter’s body, and the leaf collapses. This is called “eathing the electroscope”. Before proceeding any further it must be pointed out that harging by contact is not a good methos and often gives a charge opposite to that epected. it is better to use the method of induction.

(3) To test for the sign of the charge on a body: Having charged the electroscope negatively as described above, the ebonite rod should be rechared and brought near to the cap. An increase in the leaf divergenece is noted.

A glass rod rubbed with silk (positive charge) is now coutiously brought own towards the cap from a hight of about 50 cm. This time, a decrease in divergence is noticed.

The electroscope is discharged by touching it with the fingure and afterwards charged positively by contact, using a glass rod rubbed with silk. We shall now find that an increase divergence is caused by bringing a charged glass rod near the cap and a decreased divergence by charged igonite rod.

Conclussion of the above experiments: An icrease in divergence occurs when the charge on the electroscope and the test charge are of the same kind.

(4) To test the insulating properties of various materials: The insulating or, conversely, the conducting property of a given substance may be tested by holding a sample of the substance in the hand and then bringing it in to contact with the cap of a charged electroacope. If the substance is a good insulator, there will be no leakage of a charge through it anf the leaf divergence will not alter. If, however the leaf colapses instantly it shows that the substance is a good conductor.

Between these two extremes there are certain subatances, which produce a slow collaps of the leaf. Thease are classed as poor insulators or poor conductors. Examples of this type of material are paper, wool, cotton and wood. Nevertheless, if these substances are dried throughly, they become quite good insulators. This suggests that there ability to conduct electricity comes from there moisture contents. Among the good conductors are all the metal and carbon. The good insulators are sulphure, quartz, paraffin wax, polyvinyl chloride (P.V.C.) shellac, polythene and silk.

Electric field
Every electron and proton produces an electric field. So around any object in which charges are not balanced, there is an electric field.

Electric field can be defined as ” the region around the charge where the force of attraction or repulsion can be observed by another positive test charge”.

The strength of the electric force depends upon the following two factors.

1- how close the particles are: the closer they are, the larger the force

2- how much electric charge they carry: the more charge, the larger the force

Direction of electric field:

In case of positive charge, the direction of electric field is always radially outward, while in case of negative charge, the direction of electric field is always radially inward.

If two opposite static charges are close to each other, then electric field between between them will be positive charge to the negative charge.