كتاب الطالب Physics الصف 12

كتاب الطالب Physics الصف 12

1.1  Electromagnetis

Perhaps no mystery mizzled 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 Erople and animals. puzzled 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 of cloth. you could attract small. light objects with the amber. We now know that rubbing amber with a transfers negatively charged particles called electrons from the cloth to the (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 naturally cycurring magnetic objects called luiestones. which were found in devxysits of magnetite. a mineral consisting of iron oxide. These objects were used to construct as early as .3Ck) W
The relationship electricity and magnetism was not understcxxi 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- tronragnetism. However. unification of forces not stop there. During the early part of the 20th century. two more fundamental forces were discovered: the weak force. which in decay (in which an electron and a new trino are svx)ntaneously emitted from certain of nuclei). and the strong force. which acts inside the atomic nucleus. Currently. the electromagnetic and weak forces are viewed as two asvrcts of the electroweak force (Figure 1.3). For the phenomena discussed in this and the following chapters. this electroweak unification has no influence; it important in the highest-energy particle collisions. the energy scale for the electroweak unification is so high, most texttX)0ks continue to speak Of four fundamental forces: gravitational. elec- tromagnetic. weak. and strong

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

Fluids and organk tÉstæ can alq) serve cmductors. Pure distilled water is a very gcxxi cmdtrtor. However, common table salt (N*ZII example, in water irnproves its conductivity trenrru%usly, trcau* the BBitively charged scxiium icms (Na 4) and negatively charged chlorine (Cl¯) can move within the water to electric-
ity. In liquicE unlike solicE, vxBitive well as negative charge are is a very condtrtor, it well emugh to nuke currents dangerotß to us

Semiconductors

A class of materials called semiconductors can change from being an insulator to being a conductor and back to an insulator again. Semiconductors were discovered only a little more than SO years ago but are the backtx•ne of the entire computer and consumer electronics industries The first widespread of semiconductors was in transistors (Figure 1.7a): rncxiern computer chips (Figure 1.7b) the functions of millions of transistors. Computers and basically all modem consumer electronics prcxiucts and devices (televisions. cameras video garne players. cell phones. etc.) would imvx»ssible without semiconductors. Mcx»re. cofounder of Intel. famously stated that due to advancing technology. the of the average com- puter•s CPU (central prcxessing unit) doubles every 18 months, which is an empirical average over the last 5 decades. •nus doubling phenomenon is known as ,McxyreS Lat Physicists have and will undoubtedly continue to the driving force trhind this process of discovery. invention. and improvement are of two kinds intrilbic and Examplß imnrsic conirtcvs are cirrnkally pure crystals of gallium arseni&. germanium sili- con Ervirwers prrxiuce extrinsic by ‘i’pby. which aMition minute anxnmts (typically I part in 100) of ot}rr nuterials that can Xt electron chnors elec- receptors dovrd with electron donors are called n-ttwr (n stan& •negative charge-i If the substance acts an electrcm receptor. the IE’le left tphind by an el«tron that attaches to a can also travel through the semiconductcr and acts as an effective vxysitive charge carrier. semkonductors are calkd p•-twr (p starui ‘BSitive charge-). Thus. unlike rormal condtrtors in whk-h cmly negative chargß rnove. ærniccmductors have n»vernent of rrgative or Bhitive charges (which are really holß, that is missing ektrorb)

Superconductors

Superconductors are materials that have zero resistance to the conduction Of tricity, as opvx)sed to normal conductors, which conduct electricity well but with some losses Materials are only at very low A typical suvrrconductor is a niobium-titanium alloy that must kept near the ternvrrature of liquid helium (42 K) to retain its During the last 20 years. new materials called high-Tc (Tc stands for •critica ture,• which is the maximum that allows superconductivity) have been These are at the temperature at which nitrogen can exist as a liquid (773 K). Materials that are st4rrconductors at rcx•rn temperature (3(X) K) have not been found. but they would extremely useful. Research at developing such materials and theoretically explaining what physical phenornena high- T c superconductivity is currently in progress

1.2 ‘Electric Charge

Let’s lcx»k a little into the of the electric sparks that you «casionally receive on a dry winter day if ycll walk acrss a and then touch a metal doorknob. (El«-trostatic sparks have even ignited gas furnes while MNneone is filling the tank at a gas statim. This is not an urban legend; a of cases have been caught station surveillance cameras.) The proces that causes this sparking is called charging. Charging consists the transfrr of negatively charged particles, called el«trons, frorn the atoms and rnolecules of the material the caqrt to the soles your stxrs- This charge can nwve relatively easily through your including your hands. The builtup electric. charge discharges through the metal of the dcxrkm»b, creating a spark The two tyvrs of electric charge found in nature are charge and nega- tive charge. Normally, objects around us do not seem to charged; instead, they are electrically neutral. Neutral objects contain roughly equal numbers of positive and negative charges that largely cancel each other. Only when positive and negative charges are not balanced do we the effects Of electric charge- If you rub a glass with a cloth. the glass becomes charged and the cloth acquires a charge Of the sign- If you rub a plastic rod with fur, the rod and fur also t.rcome oppositely charged If you bring two charged glass rcxis together

they each other. Similarly, if you bring two charged plastic rcx.is together, they also reTRl each other. However, a charged glass rod and a charged plastic rod will attract each other- This difference arises tx•cause the glass rod and the plastic have opB)site charge- These observations led to the following law. Law of Electric Charges Like charges revel and opposite charges attract