The most important part in studying: Participate in class with Clickers Interactive lectures – should keep you alert and thinking.
“Electricity” – from the Greek word electron ( - meaning “amber”. The ancients knew that if you rub an amber rod with a piece of cloth, it attracts small pieces of leaves or dust. “amber effect”– the object becomes electrically charged All physics to date has led to one primary conclusion: There are four fundamental forces: There are four fundamental forces: 1)Gravitational 2)Electromagnetic 3)Strong nuclear 4)Weak nuclear all based on the Electromagnetic Theory ~250 yrs or so since we first learned what electricity is grand unified theory GUT
Electricity & Magnetism static electricity (Electrostatics)static electricity (Electrostatics) –Why do I get a shock when I walk across the rug and touch the door knob? –Why do socks stick to my pants in the dryer? –Why does my hair stick to my comb, and I hear a crackling sound ? –Why does a piece of plastic refuse to leave my hand when I peel it off a package? –What is lightning? What is that all about? It’s the CHARGE
Electric charge is defined by the effect (force) it produces. effect (force) it produces. –positive charge –negative charge Benjamin Franklin Benjamin Franklin (1706 - 1790, American statesman, philosopher and scientist) Benjamin Franklin No one has ever seen electric charge; it has no weight, color, smell, flavor, length, or width. Charge is an intrinsic property of matter electron has it, proton has it, neutron doesn’t have it – and that’s all electron has it, proton has it, neutron doesn’t have it – and that’s all
Electricity has origin within the atom itself. Name NameSymbolChargeMassElectron e - e - e 9.11 x 10 -31 kg Proton p e 1.67 x 10 -27 kg Neutron n none none 1.67 x 10 -27 kg Atom is electrically neutral = has no net charge, since it contains equal numbers of protons and electrons. 10 -10 m 10 -15 m r atom ≈ 100000 x r nucleus m nucleon ≈ 2000 x m electron
Electric forces charges exert electric forces on other charges The repulsive electric force between 2 protons is 1,000,000,000,000,000,000,000,000,000,000,000,000 times stronger than the attractive gravitational force!+ + + – –two positive charges repel each other – –two negative charges repel each other – –a positive and negative charge attract each other
charge is measured in Coulombs [C] charge is measured in Coulombs [C] French physicist Charles A. de Coulomb 1736 - 1806 Every electron has charge -1.6 x 10 -19 C, Every electron has charge -1.6 x 10 -19 C, and every proton 1.6 x 10 -19 C and every proton 1.6 x 10 -19 C 1C represents the charge of 6.25 billion billion ( 6.25 x 10 18 ) electrons ! 1C represents the charge of 6.25 billion billion ( 6.25 x 10 18 ) electrons ! Yet 1C is the amount of charge passing through a 100-W light Yet 1C is the amount of charge passing through a 100-W light bulb in just over a second. A lot of electrons! bulb in just over a second. A lot of electrons! Attractive force between protons and electrons cause them to form atoms. Electrical force is behind all of how atoms ond…chemistry…
The smallest amount of the free positive charge is the charge on the proton. The smallest amount of the free negative charge is the charge on the electron. Charge of the single proton is q proton = e. Charge of the single electron is q electron = - e Charge of the single electron is q electron = - e Charge is quantized: cannot divide up charge into smaller units than that of electron (or proton) i.e. all objects have a charge that is a whole-number multiple of charge of the smallest amount (a single e).Charge is quantized: cannot divide up charge into smaller units than that of electron (or proton) i.e. all objects have a charge that is a whole-number multiple of charge of the smallest amount (a single e). The net charge is the algebraic sum of the individual charges (+ 5 - 3 = 2).The net charge is the algebraic sum of the individual charges (+ 5 - 3 = 2). quarks have 1/3, but they come in triplets let e = 1.6 x 10 -19 C
Objects can be charged – there can be net charge on an object. How? Everyday objects - electronically neutral – balance of charge – no net charge. The only type of charge that can move around is the negative charge, or electrons. The positive charge stays in the nuclei. So, we can put a NET CHARGE on different objects in two ways Add electrons and make the object negatively charged. Remove electrons and make the object positively charged.
Some materials have atoms that have outer electrons (farthest from nucleus) loosely bound. They can be attracted and can actually move into an outer orbit of another type of atom. The atom that has lost an electron has a net charge +e (positive ion). An atom that gains an extra electron has a net charge of – e (negative ion). This type of charge transfer often occurs when two different materials (different types of atoms) come into contact. Which object gains the electrons depends on their electron affinity: Which object gains the electrons depends on their electron affinity:
During that process, the net charge produced is zero. The charges are separated, but the sum is zero. The amount of charge in the universe remains constant (we think!!) It is CONSERVED! electrons can be transferred from one object to another Another Law of Conservation: Charge is always conserved: charge cannot be created or destroyed, but can be transferred from one object to another. Conclusion:
When objects are charged by rubbing, they don’t stay charged for ever. They eventually return to neutral state – very often the charge will “leak off” onto water (polar) molecules in the air. Sometimes they will be neutralized by charged ions in the air (formed, for example, by collisions with charged particles known as cosmic rays). 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).
Any material that allow charges to move about more or less freely is called conductor. So, if you transfer some electrons to the metal rod, that excess of charge will distribute itself all around rod. Tap water, human body and metals are generally good conductors. That’s all very nice, but why is that so? Electrical conductors, insulators, semiconductors and superconductors - distinction based on their ability to conduct electric charge.
What makes conductors conduct? Atoms have equal numbers of positive and negative charges, so that a chunk of stuff usually has no net charge the plusses and minuses cancel each other. However, in metal atoms the valence electrons – the electrons in the outermost orbits - are loosely bound, so when you put a bunch of metal atoms together (to form a metal) an amazing thing happens valence electrons from each atom get confused and forget which atom they belong to. They now belong to the metal as the whole. As the result, positive ions which are tightly bound and can only oscillate around their equilibrium positions, form a positive background. All the homeless electrons - “Free electrons” wander around freely keeping ions from falling apart – metallic bond!!
Electrons in insulators are tightly bound to atomic nuclei and so cannot be easily made to drift from one atom to the next. Only if a very strong electric field is applied, the breakthrough (molecules become ionized resulting in a flow of freed electrons) could result in destruction of the material. The markings caused by electrical breakdown in this material – look similar to the lightening bolts produced when air undergoes electrical breakdown. Materials like amber, pure water, plastic, glass, rubber, wood… are called insulators. They do not let electricity flow through them. Electrons are tightly bound to nuclei, so it is hard to make them flow. Hence, poor conductors of current and of heat.
Semiconductors Materials that can be made to behave sometimes as insulators, sometimes as conductors. Eg. Silicon, germanium. In pure crystalline form, are insulators. But if replace even one atom in 10 million with an impurity atom (ie a different type of atom that has a different # of electrons in their outer shell), it becomes an excellent conductor. Transistors: thin layers of semiconducting materials joined together. Used to control flow of currents, detect and amplify radio signals, act as digital switches…An integrated circuit contains many transistors.
The movement of electrons in semiconductors is impossible to describe without the aid of quantum mechanics. As the conductivity of semiconductors can be adjusted by adding certain types of atomic impurities in varying concentrations, you can control how much resistance the product will have. ADVANTAGE – A HUGE ONE
Superconductors Have zero resistance, infinite conductivity Not common! Have to cool to very very low temperatures. Current passes without losing energy, no heat loss. Discovered in 1911 in metals near absolute zero (recall this is 0 o K, -273 o C) Discovered in 1987 in non-metallic compound (ceramic) at “high” temperature around 100 K, (-173 o C) Under intense research! Many useful applications eg transmission of power without loss, magnetically-levitated trains…
Conductors and Insulators – and how to charge them Most things are in between perfect conductor/ insulator Electrons are free to move in a conductor Electrons stay with their atom in an insulator REMEMBER:
ELECRTOSTATIC CHARGING 1.Charging by Friction: The transfer of charge is due the rubbing - friction between two previously neutral materials. The transfer of charge is due the rubbing - friction between two previously neutral materials. When you move your comb through your hair, the friction between the comb and hair can pull some of the electrons out of your hair and onto the comb. As a result your comb ends up with a net negative charge and attracts your hair which is now positive. Rubbing: rubber rod with fur or cloth, glass rod with silk, hair with balloon, shuffling across a carpeted floor.
Insulated stand 2. Charging by Conduction (Contact): 2.1 Conductors: When a charge is placed on a conductor, the mutual repulsion of the individual charges causes them to move as far away from each other as possible. Thus, a charge deposited on a conductor quickly spreads out over its surface. 2. 2 Insulators: When a charge is placed on an insulator, it remains where it is deposited and surrounding molecules become polarized. An external (negative) charge distorts the shape of an atom by forcing its negatively-charged electron clouds to shift away from the charge and the positively charged nuclei to shift toward the charge. Such a distorted atom is said to be polarized. Glass sphere Insulated stand Metal sphere
Consider a negatively charged rod touching a conductor versus touching an insulator. What is the difference between how the electrons are arranged on the conductor and insulator? Would spread out evenly on a good conductor, because the transferred e’s repel each other. But on insulator, or poor conductor, would be more localized at where the rod touched. But on insulator, or poor conductor, would be more localized at where the rod touched. Question: charges can be transferred from/to conductors or non- conductors but they can only move through conductors.charges can be transferred from/to conductors or non- conductors but they can only move through conductors.
3. Charging by Induction a. Neutral conductor with free electrons b. free electrons in the metal are repelled as far as possible from the charged object. d. Object is left positively charged Charge has been separated, but metal sphere is still neutral. Or you can touch it with finger, electrons flow from your finger, through you, to the ground. 3.1 Conductors: c. The Earth is reservoir of any charge. It can easily accept or give up electrons. Connect conductor with a conducting wire to the ground - many of free electrons in metal are able to move even further from charged object down the wire into the Earth. e. cut the wire, remove the rod and the metal sphere has evenly distributed positive charge.
Insulators: When insulator is charged by induction, there will be no change of charge on that object. Instead of that charge is moved within the molecule/atom (the net charge is kept zero) Therefore we call it rather: Positive surface charge Even though sphere is neutral there is attraction force acting between the rod and sphere. Charging by Polarization A charge placed near an insulator polarizes its atoms. While the insulator’s interior remains electrically neutral, a net charge appears on the surface, and can produce force on other charges near the insulator. 3.2 Insulators:
Charge polarization When bring a charged object near an insulator, electrons are not free to migrate throughout material. Instead, they redistribute within the atoms/molecules themselves: their “centers of charge” move Here, usual atom, with center of electron cloud at positive nucleus When a – charge is brought near the right, electron cloud shifts to the left. Centers of + and – charges no longer coincide. Atom is electrically polarized Surfaces of material look like this. A – charge induced on right, and + on the right. (Zero net charge on whole object)
EXAMPLE - QUESTION Charging by induction Bring a charged object near a conducting surface, electrons will move in conductor even though no physical contact: Due to attraction or repulsion of electrons in conductor to the charged object – since free to move, they will! Note, the charged rod never touched them, and retains its original charge. Once separated from each other with rod still close they’ll remain charged. Charge is conserved, so charges on spheres A and B are equal and opposite.
QUESTION: A metal ring receives a positive charge by contact. What happens to the mass of the ring? Does it increase, stay the same, or decrease? Will the object have deficiency or excess of electrons? When the positively charged ball touches the ring, electrons inside it are attracted to the ball. Some will leave the ring trying to neutralize the ball. Only a tiny fraction leaves the ring. The mass of the electrons is so small compared to the atoms, so although the mass of the ring decreases, measuring it would not be possible. (By the way, both will be positively charged, but the ball will be less then before)
Van de Graaff The sphere gives the girl a large negative charge. Each strand of hair is trying to: 1) 1)Get away from the charged sphere. 2) 2)Get away from the ground. 3) 3)Get near the ceiling. 4) 4)Get away from the other strands of hair. 5) 5)Get near the wall outlet. Like charges attached to the hair strands repel, causing them to get away from each other. Example:
Seeing the effects of charge: the electroscope the electroscope is a simple device for observing the presence of electric charge it consists of a small piece of metal foil (gold if possible) suspended from a rod with a metal ball at its top If a negatively charged rod is placed near the ball, the electrons move away because of the repulsion. The two sides of the metal foil then separate. ++
Charging by Induction Bring a charged rod in near Positive charged rod results in positive leaves.
Attracting uncharged objects uncharged metal sphere + + + + A negatively charged rod will push the electrons to the far side leaving the near side positive. The force is attractive because the positive charges are closer to the rod than the negative charges
Why? Balloon becomes charged by friction when rub on hair, picking up electrons. It then polarize molecules on the surface, induces + charge layer on the wall’s surface closest to it, and next negative furthest away. So balloon is attracted to + charges and repelled by – charges in wall, but the – charges are further away so repulsive force is weaker and attraction wins. Charge polarization is why a charged object can attract a neutral one : Charge a comb by rubbing it through your hair, and then see it attracts bits of paper and fluff… DEMO: Rub balloon on your hair – it will then stick to the wall !
You can bend water with charge! charged rod The water molecule has a positive end and a negative end. When a negative rod is brought near the stream of water, all the positive ends of the water mole- cules turn to the right and are attracted to the negative rod. stream of water What happens if the rod is charged positively?
As we said Like charges repel, and opposite charges attract. This is the fundamental cause of almost ALL electromagnetic behavior. But how much? How Strong is the Electric Force between two charges?
ELECTROSTATIC – ELECTRIC - COULOMB FORCE The force between two point charges is proportional to the product of the amount of the charge on each one, and inversely proportional to the square of the distance between them. Force is a vector, therefore it must always have a direction.
SHE accumulates a charge q 1 of 2.0 x 10 -5 C (sliding out of the seat of a car). HE has accumulated a charge q 2 of -8.0 x 10 -5 C while waiting in the wind. a) They exert equal forces on each other only in opposite direction b) r’ = 0.5 r (“-“ = attractive force) At very small separation - spark How many electrons is 2.0 x 10 -5 C ? What is the force between them a) when she opens the door 6.0 m from him and b) when their separation is reduced by a factor of 0.5?
When you comb your hair with a plastic comb, some electrons from your hair can jump onto it making it negatively charged. Suppose that you could borrow all the electrons from a friend’s body and put them into your pocket. The mass of electrons would be about 10 grams (a small sweet). With no electrons your friend would have a huge positive charge. You, on the other hand, would have a huge negative charge in your pocket. If you stood 10 m from your friend the attractive force would be equal to the force that 10 23 tons would exert sitting on your shoulders – more 100,000 times greater than the gravitational force between the earth and the Sun. Luckily only smaller charge imbalances occur, so huge electrical forces like the one described simply do not occur. Your body contains more than 10 28 electrons.
Three point charges : q 1 = +8.00 mC; q 2 = -5.00 mC and q 3 = +5.00 mC. (a) (a)Determine the net force (magnitude and direction) exerted on q 1 by the other two charges. (b) (b)If q 1 had a mass of 1.50 g and it were free to move, what would be its acceleration? 1.30 m 23 0 q1q1 q2q2 q3q3 Force diagram F2F2 F3F3 q1q1
F2F2F2F2 F3F3F3F3 q1q1q1q1 x-components will cancel, because of the symmetry F = 0.166 N ma = F a = 111 m/s 2 in y - direction ma = F a = 111 m/s 2 in y - direction electric force is very-very strong force, and resulting acceleration can be huge
+- - Positive charge is attracted (force to left) Negative charge is repelled (force to right) Positive charge is closer so force to left is larger. A positive and negative charge with equal magnitude are connected by a rigid rod, and placed near a large negative charge. In which direction is the net force on the two connected charges? 1) Left 2) Zero 3) Right
Three Charges Q=-3.5 C Q=+7.0 C Q=+2.0 C 6 m 4 m Calculate force on +2 C charge due to other two charges – –Calculate force from +7 C charge – –Calculate force from –3.5 C charge – –Add (VECTORS!) F7F7 5 m F3F3
Q=-3.5 C Q=+7.0 C Q=+2.0 C 6 m 4 m F7F7 5 m F3F3 F Adding Vectors F 7 +F 3 Decompose into x and y components. F 7x = F 7 cos = F 7 (3/5) = 3x10 -3 N F 7y = F 7 sin = F 7 (4/5) = 4x10 -3 N F 3x = F 3 cos = F 3 (3/5) = 1.5x10 -3 N F 3y = F 3 sin = F 3 (4/5) = -2x10 -3 N F x = 3 10 -3 N + 1.5 10 -3 N F y = 4 10 -3 N – 2.0 10 -3 N F x = 4.5 10 -3 N F y = 2.0 10 -3 N