وحدة Electrostatics الفيزياء الصف الثاني عشر

 وحدة Electrostatics الفيزياء الصف الثاني عشر

1.1 Electromagnetism

Perhaps no mystery puzzled ancient civilizations more than electricity, which they observed primarily in the form of lightning strikes (Figure 1.2). The destructive force inherent in lightning, which could set objects on fire and kill people and animals, puzzled people because they did not understand what caused it or where the lightening came from

The ancient Greeks knew that if you rubbed a piece of amber with a piece ofb cloth, you could attract small, light objects with the amber. We now know that rubbing amber with a cloth transfers negatively charged particles called electrons from the cloth to the amber. (The words electron and electricity derive from the Greek word for amber.) Lightning also consists of a flow of electrons. The early
Greeks and others also knew about naturally occurring magnetic objects called lodestones, which were found in deposits of magnetite, a mineral consisting of iron oxide. These objects were used to construct compasses as early as 300 BC

The relationship between electricity and magnetism was not understood until the middle of the 19th century. The following chapters will reveal how electricity and magnetism can be unified into a common framework called elec- tromagnetism. However, unification of forces does not stop there. During the early part of the 20th century, two more fundamental forces were discovered: the weak force, which operates in beta decay (in which an electron and a neu- trino are spontaneously emitted from certain types of nuclei), and the strong force, which acts inside the atomic nucleus. Currently, the electromagnetic and weak forces are viewed as two aspects of the electroweak force (Figure 1.3). For the phenomena discussed in this and the following chapters, this electroweak unification has no influence; it becomes important in the highest-energy particle collisions. Because the energy scale for the electroweak unification is so high, most continue to speak of four fundamental forces: gravitational, elec- tromagnetic, weak, and strong

Today. a large number of physicists that the electroweak force and the strong force can also unified, that is, described in a common framework. Sev- eral theories propose ways to accomplish this, but so far experimental evidence is missing. Interestingly, the force that has been known longer than any of the other fundamental forces, gravity, seems to be hardest to shoehorn into a unified frame- work with the other fundamental forces. Quantum gravity, supersymmetry, and string theory are current foci of cutting-edge physics research in which theorists are attempting to construct this grand unification and discover the (hubristically

 

Flukis and tistæ can abo serve as coruitrtcrs Pure distilled water is a very gcxxi condtrtor. disolving table salt (NXI), exampk, in water improves its conductWity trenH1dotßly. the vxsitively charged scxiium (Na+) and chlcrine ims (Cl¯) can move within the water to cmdLKt el«tric- ity. In liquicE, unlike solkis. as well as nqative charge carriers ue mobile. Organk tisstæ is not a very gcxxi ccmductor. but it coruh1Cts ektrk-ity well enough to make large currents dangerous to us

 

Semiconductors

A class of materials called semiconductors can change from an insulator to a conductor and back to an insulator agairu Semiconductors were only a little more than 50 years ago but are the backbone of the entire computer and consumer electronics industries. The first widespread use of semiconductors was in transistors (Figure 1.7ay, modern computer chips (Figure 1.7b) ERrform the functions of millions of transistors. Computers and basically all modern consumer electronics prcxiucts and devices (televisions, cameras, video game players, cell phones, etc.) would irnB)ssible without semiconductors. Gordon Mcx•re. cofounder of Intel, famously stated that due to advancing technology. the B’wer of the average com- puter’s CPU (central prcxessing unit) doubles every 18 months which is an empirical average over the last 5 decades. This doubling phenomenon is known as l.nw

Physicists have and will continue to the driving force this prcxess of scientific discovery, invention, and improvement are of two kilXis: intri1BK and extrinsic. of confirtors are cirrnkally F.A1re crystak of gallium arsenide. germanium or. e*rcially. sili- con Engineers extri’bic by which the aldition of minute ammrnts (typically I part in 106) of other rnaterials that can act electrcm c%mrs ek- trcm receptors. with ektron domrs are calkd n-ttJB (n stan& K.)r •negative charge•) If the doping sulstarre acts as an electron the hole left t.Rhind by an that attac}B to a can travel and Ets vxÉtive charge carrier. are called
rwgative charges move. have movernem rrgative or BSitive charges (which are really ekctron hoks, that is, mising electrons)
 

Superconductors

Superconductors are materials that have zero resistance to the conduction Of elec- tricity. as opposed to normal conductors. which conduct electricity well but with losses. Materials are only at very low temvrratures. A typical is a niobium-titanium alloy that must kept near the ternErrature of liquid helium (4.2 K) to retain its suvrrconducting During the last 20 years. new materials called stgvmomitrtors (Tc stands for •critical tempera- ture,• which is the maximum that allows suvrrconductivity) have These are at the at which nitrogen can exist as a liquid (77.3 K). Materials that are superconductors at rcx)tn (3(X) K) have not yet tRen found. but they would be extremely useful. Research at &veloping such materials and explaining what physical phenomena cause high -Tc is currently in progress 

named) Theory of Everything. They are mainly guided by symmetry principles and the conviction that nature must be elegant and simple. In this chapter, we consider electric charge, how materials react to electric charge, static electricity, and the forces resulting from electric charges. Electrostatics covers situations where charges stay in place and do not move

 

1.2 ‘Electric Charge

Let’s look a little into the cause of the electric sparks that you cxcasionally receive on a dry winter day if you walk across a and then touch a metal doorknob. (Elec- trostatic sparks have even ignited gas fumes while someone is filling the tank at a gas station. This is not an urban legend; a few of these cases have caught on gas station surveillance cameras.) The prcxess that causes this sparking is called charging. Charging consists of the transfer of negatively charged particles, called electrons, from the atoms and molecules of the material of the to the soles of your This charge can move relatively easily through your body, including your hands. The built-up electric charge discharges through the metal of the dcx)rknob, creating a spark

The two types of electric charge found in nature are positive charge and nega- tive charge. Normally, objects around us do not seem to be charged; instead, they are electrically neutral. Neutral objects contain roughly equal numbers of and negative charges that largely cancel each other. Only when and negative charges are not balanced do we observe the effects of electric charge

If you rub a glass with a cloth, the glass rod becomes charged and the cloth acquires a charge of the sign. If you rub a plastic rod with fur, the rod and fur also become charged. If you bring two charged glass rods together, they repel each other. Similarly, if you bring two charged plastic rods together. they
also repel each other. However, a charged glass rod and a charged plastic rod will attract each other. This difference arises because the glass rod and the plastic rod have opposite charge. These observations led to the following law

Law of Electric Charges

Like charges and opposite charges attract