Physics

Vandebilt Catholic High School

W. Dupre

Electrostatics – Notes

  1. Electrostatics - the study of electrical charges that can be collected and held in one place.

·        Negative Charge - an excess of electrons

·        Positive Charge - a lack of electrons or an excess of “holes

  1. An atom is the smallest particle of an element that can take place in a chemical reaction without being permanently changed.  In the atom there are protons, neutrons and electrons.

·        The atom is a neutral object - equal number of negative electrons and positive protons.

·        Objects can be charged - they can become positive or negative

·        A positive object has lost electrons

·        A negative object has gained electrons

·        Note the electrons are the only particles that are transferred to establish electric charge. When we speak of positive charge flowing into an object what is actually occurring is electrons are flowing out of the object.

  1. Specific Properties of Charges

·        Like charges (both negative or both positive) repel (they try to avoid each other)

·        opposite charges attract

  1. Classification of materials

·        Conductor – an object that allows for electrons (negative charges) to move freely. Most metals, the human body, the earth, ionic substances are good conductors. Silver is the best metallic conductor, but because of its high cost it is not often used in appliances and electrical components.

·        Insulator – an object that offers much resistance to the movement of electrons, in many cases preventing their  movement altogether. Glass, dry air, wood, plastic, pure water are insulators

·        Semi-conductor – an object that normally acts as an insulator, but can become a conductor when altered. Silicon and germanium are common semiconductors. Their ability to “turn on and off” have made them an important component for the electronics industry.

·        Superconductor – compounds that offer so little resistance to the movement of electrons that the electrons move through them at incredible rates of speed (approaching the speed of light). Most of these substances have to be super-cooled to temperatures approaching absolute zero to become superconducting. Superconductors are responsible for mag-lev trains and medical mri’s. There applications in the computer industry will make future computers thousands of times faster than today’s fastest computer.

  1. Methods of charging an object:

·        Friction – charging by rubbing. When one object is rubbed across another, one will be more likely to give up its electrons to the other; thereby causing the two objects to become charged.

·        Conduction - charging by direct contact, electrons aare transferred. - the object gets the same charge as the previously charged object. There is a limit to the number of objects that can be charged.

·        Induction – bring a charged object near a neutral object and ground the neutral object, this causes the neutral object to have a charge opposite the originally charged object. There is no direct contact between the original object and the object being charged; therefore the original object does not loose its charge and may charge an unlimited number of objects. It is possible to devise a method to charge objects to either sign using induction.

  1. Conservation of Charge

·        Ben Franklin concluded that this thing called charge was not being created. There is a finite amount of charge that is simply passed from one object to another.

·        If one objects gains electrons and becomes negatively charged, another objects had to had lost electrons and become positively charged to the same magnitude. Thereby keeping the total amount of charge constant

  1. Charge Polarization

·        Through polarization an objects can appear to be charged; if this object is a conductor this charge can be made permanent.

·        In the presence of a charged object a second object will polarize. This means that the charges in it will align according to the outside charge present.

·        If the object is a conductor, its free electrons will move toward a positively charged object and away from a negatively charged object. If the object is then grounded while the charged object is near a permanent charge of the opposite sign will result.

·        If the object is an insulator it has no free electrons, but its atoms will align in such a way that the positive nucleus will be closest to a negatively charged object and opposite a positively charged object.

·        A neutral object will always attract to any charged object because the neutral object will polarized in the presence of the charged object.

·        Some compounds, such as water, are arranged so that they are polar by nature. In other words a region of the molecule is positively charged while another region is negatively charged. These compounds are also attracted to any charged object.

  1. Charge Distribution

·        Any excess charge will arrange in a definite manner around an object.

·        If the object is an insulator the charge will remain isolated in region it was originally placed.

·        If the object is a conductor, the excess charges because they are repelling each other will equally distribute along the conductors outer surface. That is each charge will align as far away from the next charge as possible.

·        When 2 conductors come into contact any net excess charge will equally distribute throughout the two surfaces

  1. Grounding

·        If a much larger conductor is touched to a charged object, the great majority of the excess charge will wind up on the larger conductor.

·        In this case the large conductor acts as a ground (or sink) for deposit of excess charge and will basically neutralize the smaller conductor.

·        The biggest and best ground is the earth itself which is where the term “ground” is derived.

  1. Coulomb's Law

·        The magnitude of the force that a tiny sphere with a charge q exerts on a second sphere with charge q' separated by a distance d is F = k qq'/d2

·        Coulomb is the mks unit for charge

·        1 C = 6.25 x 1018 electrons

·        1 electron = -1.60-19 C (this is called the elementary charge) It is also the charge of a proton, except the sign is positive.

·        “k” is called Coulomb’s Constant it has  a value = 9.0 x 109 N-m2/C2

·        If “F” is positive the force between the two charges is repulsive; if “F” is negative it is an attractive force.