Embedded Files
Learning Objectives
state that there are positive and negative charges and that charge is measured in coulombs
state that unlike charges attract and that like charges repel
describe an electric field as a region in which an electric charge experiences a force
represent an electric field by means of field lines
draw the field of an isolated point charge and show understanding that the direction of the field lines gives the direction of the force acting on a positive test charge
draw the electric field pattern between 2 isolated point charges
show understanding that electrostatic charging by rubbing involves a transfer of electrons
describe experiments to show electrostatic charging by friction and induction
distinguish between electrical conductors and insulators and give typical examples of each
describe examples where electrostatic charging may be a potential hazard
describe the use electrostatic charging in photocopier, spraying of paint, and electrostatic precipitator, and apply the use of electrostatic charging to new situations
Introduction
Introduction
Study of static or stationary electric charges
Some common electrostatics phenomena
*A plastic ruler is able to attract small pieces of paper after it has been rubbed with a piece of dry cloth.
* Dust sticks even harder on to a CD if you try to wipe off the dust from it.
Electric Charges
Electric Charges
There are two kinds of electric charge,
Matter is made up of electrically neutral atoms. Each atom consists of a positively charged nucleus surrounded by negatively charged electrons.
In an uncharged or neutral atom, the number of positive charges is equal to the number of negative charges.
An atom is ionised when electrons are removed or added.
A body is positively charged when it loses some of its electrons and is negatively charged when it possesses more than its normal share of electrons.
SI unit of electric charge is the Coulomb, C
The charge of an electron or proton is 1.6 x 10^-19 C.
Charges exert forces on one another:
The electrical force between two charged objects is directly proportional to the product of the quantity of charge on the objects and inversely proportional to the square of the separation distance between the two objects.
Electrostatic induction
Electrostatic induction
Electrostatic induction is the temporary separation of charges due to a charged object.
A charged object attracts neutral object due to electrostatic induction.
Neutrally charged objects (conductors or insulators) will always be attracted to a charged object. As the objects reach the charged object they end up picking up excess charge. Once they pick up excess charge they now have the same charge as the object so they are repelled by it. As they leave, they take charges with them. Given enough time, the particles in the air will remove the excess charge from the object leaving it neutrally charged. This explains why on dry days we tend to have more trouble with static electricity build-up than on humid (moist) days. On moist days there are more water molecules in the air to steal charge more rapidly. On dry days there are fewer particles in the air to steal charges so we accumulate charge until we touch something and get discharged (shocked).
Electric Field
Electric Field
A region where an electrical charge experiences an electrostatic force.
In an electric field there are electric lines of force; the lines indicate the path that an isolated positive charge, placed in the field would move. The arrows shows the direction in which a positive charge would move. The number of lines illustrate the relative field strength.
Conductors, Insulators, Semiconductors, Superconductors
Conductors, Insulators, Semiconductors, Superconductors
Conductors
Conductors
* Allow electric charges to flow through them easily.
* Examples : Metals; acids, bases ans salt solution, graphite and ionised gases.
* Solid condutors have free moving electrons as charge carriers. Liquid and gaseous conductors have ions as charge carriers.
Insulators
Insulators
* Do not allow electric charges to flow through them easily.
* Examples : Non-metals; acids, bases ans salt in solid states, diamond and non-ionised gases.
* Do not have free moving electrons or ions functioning as charge carriers.
Semiconductors
Semiconductors
* Examples : Pure germanium and silicone
* Conducting ability is between conductors and insulators
* By adding a small amount of appropriate impurities, such as phosphorus or boron would increase their conductivity ability
* For making micro-chips and transistors used in computers and other electronic appliances.
Superconductors
Superconductors
* Examples ; Mercury, tin lead.
* Has near zero resistance at very low temperature.
Method of electrostatic charging
Method of electrostatic charging
Charging by friction
Charging by friction
Insulators can transfer charge by friction.When the surface of one insulator rubs against another, electrons can be transferred. The insulator that gains electrons will get a negative charge,the insulator that loses electrons will get a positive charge. It is most important to know that it is only the negative electrons which can move. Positive charges cannot move. For example, if polythene is rubbed with a dry cloth,electrons are transferred from the cloth to the polythene.The polythene gains electrons and becomes negatively charged,the cloth loses electrons and becomes positively charged. It is not possible to predict in advance which way the electrons will be transferred for a certain material.The same cloth, when rubbed against a different type of plastic called acetate, will gain electrons and become negatively charged, leaving the acetate with a positive charge.
Charging by induction
Charging by induction
* Charging a Single Sphere by Induction :-
* Charging a Two-Sphere System by Induction :-
Charging by conduction
Charging by conduction
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