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Transcript
Static Electricity Physics Classroom Tutorials Textbook Chapters 32 and 33 Objectives (ch 32) • Describe the electric forces between charges as attractive or repulsive • Describe 3 ways to charge an object • State Coulomb’s Law and solve problems using this law • Distinguish between insulators and conductors • Describe the special properties of superconductors and semiconductors • Describe the process of permanently charging an electroscope by conduction and induction • Explain how the process of temporary induction causes an uncharged object may be attracted to a charged object (ex: sticking a balloon to a blackboard) Structure of Matter BASIC TERMINOLOGY AND CONCEPTS History of the atom Application of Atomic Structure • All materials are composed of atoms • An atom consists of a nucleus and a vast region of space outside the nucleus • The nucleus contains positive protons and neutral neutrons CYU 1. _____ are the charged parts of an atom a) b) c) d) e) f) Only electrons Only protons Neutrons only Electrons and neutrons Electrons and protons Protons and neutrons Neutral vs. Charged Objects and Charge Interactions BASIC TERMINOLOGY AND CONCEPTS Neutral vs Charged • The number of electrons that surround the nucleus will determine whether or not an atom is electrically charged or electrically neutral • Protons and electrons have equal but opposite charges • Protons = electrons… neutral • Protons ≠electrons… ion (more p+...cation; more e-… anion Charge as an imbalance • Electrons are always on the move • In general, for electrons to make a move from the atoms of one material to the atoms of another material, there must be an energy source, a motive, and a low-resistance pathway • For now, it is sufficient to say that objects that are charged contain unequal numbers of protons and electrons Charge Interactions CHARGE AND CHARGE INTERACTIONS Charge Interactions • Electric force is a non-contact force • Any charged object can exert this force upon other objects - both charged and uncharged objects. • Paula Abdul – Opposites attract. And likes repel. Paula Abdul and Newton’s 3rd Charged and Neutral? • Positive or negative charged objects attract neutral objects Conductors and Insulators CHARGE AND CHARGE INTERACTIONS Conductors • Conductors are materials that permit electrons to flow freely from atom to atom and molecule to molecule. – a charged object will always distribute its charge until the overall repulsive forces between excess electrons is minimized – If a charged conductor is touched to another object, the conductor can even transfer its charge to that object Insulators • insulators are materials that impede the free flow of electrons from atom to atom and molecule to molecule. – If charge is transferred to an insulator at a given location, the excess charge will remain at the initial location of charging – charge is seldom distributed evenly across the surface of an insulator The role of Insulators • Conductive objects are often mounted upon insulating objects • This prevents charge from being transferred from the conductive object to its surroundings. Calculating the Force Between Charges Coulomb’s Law Coulomb’s Law intro • Recall Newton's Law of Universal Gravitation: – F = G (m1m2/d2) – Describes that the gravitational force between two objects is proportional to the product of the masses and inversely proportional to the square of the distance between them – G is the universal gravitational constant Coulomb’s Law • Electrical force between any two objects obeys a similar inverse-square relationship with distance • States that: 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. Charge as a Quantity • Charge is measurable, just like mass. • Unit: Coulomb (C) – 1 C is very large so microC or nanoC is typically used – A total charge of -1 C would need 6.25e18 e- • The charge on a single e- is -1.6 x 10 -19 C • The charge on a single p+ is +1.6 x 10 -19 C Coulomb’s Law Equation • d is distance between the charged particles (meters) • Q1 charge on one particle • Q2 charge on 2nd particle • k is proportionality constant (Coulomb’s law constant) 9.0 x 109 SI unit of charge is the coulomb, C The force can attract or repel. The force is felt by both charges. Two charges, +3 μC and -5 μC are located 15 mm apart. Find the force exerted on each charge. (3E 6)( 5E 6) F 9.0 E 9 2 (0.015) F 600 N Does the negative sign mean attractive force or repulsive force? Diff in Gravity and Electrical Forces • G of attraction between a pair of 1 kg masses is very small, the electrical force between a pair of 1C charges is very large. • Greatest different is that while gravity only attracts, electrical forces can attract or repel. Coulomb’s Law ex) from PC • Suppose that two point charges, each with a charge of +1.00 Coulomb are separated by a distance of 1.00 meter. Determine the magnitude of the electrical force of repulsion between them. – Felect = k • Q1 • Q2 / d2 – Felect = (9.0 x 109 N•m2/C2) • (1.00 C) • (1.00 C) / (1.00 m)2 – Felect = 9.0 x 109 N • This is a gigantic force which is why objects simply do not acquire charges on the order of 1.00 Coulomb • Charge is often expressed in units of microCoulomb (µC) and nanoCoulomb (nC). – 1 Coulomb = 106 microCoulomb – 1 Coulomb = 109 nanoCoulomb Coulomb’s Law Ex) from PC • Two balloons are charged with an identical quantity and type of charge: -6.25 nC. They are held apart at a separation distance of 61.7 cm. Determine the magnitude of the electrical force of repulsion between them. – Given: • Q1 = -6.25 nC = -6.25 x 10-9 C • Q2 = -6.25 nC = -6.25 x 10-9 C • d = 61.7 cm = 0.617 m – Felect = k • Q1 • Q2 / d2 – Felect = (9.0 x 109 N•m2/C2) • (6.25 x 10-9 C) • (6.25 x 10-9 C) / (0.617 m)2 – Felect = 9.23 x 10-7 N Coulomb’s Law Ex from PC • Two balloons with charges of +3.37 µC and -8.21 µC attract each other with a force of 0.0626 Newton. Determine the separation distance between the two balloons. – Given: • Q1 = +3.37 µC = +3.37 x 10-6 C • Q2 = -8.21 µC = -8.21 x 10-6 C • Felect = -0.0626 N (use a - force value since it is repulsive) – Solve for distance • • • • Felect = k • Q1 • Q2 / d2 d2 • Felect = k • Q1 • Q2 d2 = k • Q1 • Q2 / Felect d = SQRT(k • Q1 • Q2) / Felect – Substitute givens • d = SQRT [(9.0 x 109 N•m2/C2) • (-8.21 x 10-6 C) • (+3.37 x 10-6 C) / (-0.0626 N)] • d = Sqrt [ +3.98 m2 ] • d = +1.99 m Lesson 2 METHODS OF CHARGING Charging by Friction • Results in a transfer of electrons between the two objects that are rubbed together Charging by Friction - + Electron Affinity (wanting e-) • Different materials have different affinities for electrons. • Tested materials can be ordered according to their affinity for electrons • Materials shown highest on the table tend to have a greater affinity for electrons than those below it – Which become more negative? – Which become more positive? Law of Conservation of Charge • The frictional charging process (as well as any charging process) involves a transfer of electrons between two objects • Charge cannot be created from nothing (just like mass) • net charge of the system is 0 units • When all objects involved are considered prior to and after a given process, the total amount of charge amidst the objects is the same before the process starts as it is after the process ends. Charging By Induction Charging by Induction • In the two cases before, the ultimate charge on the object is never the result of electron movement from the charged object to the originally neutral objects – The balloon never transfers electrons to or receive electrons from the spheres Importance of a Ground • A ground is simply a large object that serves as an almost infinite source of electrons or sink for electrons. Charging by Conduction • Involves the contact of a charged object to a neutral object • Van de Graaf generator • Induction does no involve contact • Because charging by conduction involves contact, it is often called charging by contact CYU 1 • A neutral metal sphere is touched by a negatively charged metal rod. As a result, the sphere will be ____ and the metal rod will be ____. Select the two answers in their respective order. – – – – – a. positively charged b. negatively charged c. neutral d. much more massive e. ... not enough information to tell • BB – This is a case of charging by conduction. When a charged object is used to charge a neutral object by conduction, the previously neutral object acquires the same type of charge as the charged object. The charge object maintains the same type of charge that it originally had. So in this case, both objects have a negative charge. CYU 2 • A metal sphere is electrically neutral. It is touched by a positively charged metal rod. As a result, the metal sphere becomes charged positively. Which of the following occur during the process? List all that apply. – – – – – – a. The metal sphere gains some protons. b. Electrons are transferred from the sphere to the rod. c. The metal sphere loses electrons. d. The overall charge of the system is conserved. e. Protons are transferred from the rod to the sphere. f. Positive electrons are moved between the two objects. • BCD – In electrostatic activities, protons are never transferred (which rules out choices a and e). Electrons are not positively charged (ruling out choice e). Choices B, C and D are all true and explain the essential nature of the conduction charging process Grounding • Grounding is the process of removing the excess charge on an object by means of the transfer of electrons between it and another object of substantial size • A ground is simply an object that serves as a seemingly infinite reservoir of electrons • the ground is capable of transferring electrons to or receiving electrons from a charged object in order to neutralize that object Polarization CHARGE AND CHARGE INTERACTIONS Polarization • polarization is the process of separating opposite charges within an object – The positive charges separate from the negative ones • Always involves the use of a charged object to induce the movement of electrons Polarization is NOT Charging • True or False: – When an object becomes polarized, it acquires a charge and becomes a charged object. • False: When an object becomes polarized, its center of positive charge becomes separated from its center of negative charge (protons still equal electrons) – There is a separation of charge, not an imbalance of charge Lesson 4 (Chapter 33) ELECTRIC FIELDS objectives • Explain how electric field lines (also called electric lines of force) can be used to represent the strength and direction of an electric field • Explain how an object can gain or lose electric potential energy • Distinguish between electric potential (volts) and electric potential energy (Joules) • Describe the function of a capacitor Force Field • The altered space around an object – Examples • Gravity • Magnetic fields • Electric fields • Action at a Distance – Example- you can “sense” the force field that surrounds a charged Van de Graaf generator • Electric field has a magnitude and a distance Electric Field Lines • Just as gravity keeps a satellite orbiting a planet, the electric field keeps an electron “orbiting” a proton • The force one electric charge exerts on another is the interaction between one charge and the electric field set up by the other Electric Fields cont. • Since there is a magnitude and direction, it’s a vector quantity. • Magnitude measured by its effect on charges in the field – A positive “test charge” is placed in the field – Where the force is greatest, the field is strongest • Direction of the field at any point, is the direction of the force on a small positive test charge Field Lines cont • Vectors point toward negative, away for positive – Always point in the direction of the force that would act on a positive test charge • Magnitude is indicated by length of the vectors Field Lines (lines of force) cont • Where the lines are farther apart, the field is weaker • For isolated charge, the lines extend to infinity • For two or more opposite charges, the lines go from a positive charge and end on a negative charge Oppositely charged plates Equal and opposite Equal like charges Electric Shielding • Charges arrange themselves to ensure a zero field within a conducting material. • Faraday cage Electric Potential Energy (p.523) • Recall that work is done when a force moves something in the direction… • A charged object can have potential energy by its location in an electric field – Work is required to push a charged particle against the electric field of a charged body – EPE is increased when work is done to push it against the electric field of something else that is charged Electric PE cont. • Like pushing down on a spring, which requires work (repulsion) • Electric Potential Energy- the energy the charge now possesses by virtue of its location Electric Potential (33.5) • Electric potential energy per charge- the total electric potential energy divided by the amount of charge • At any location the PE per charge- whatever the amount of charge- will be the same Electric potential = electric potential energy charge • Unit is the volt (V); 1 V = 1 joule/ coulomb Voltage • 1 V equals 1 joule of energy per coulomb of charge; 1000V = 1000 J per coulomb • EPE sometimes call voltage – We can speak about the voltages at different locations in an electric field whether or not any charges occupy those locations • Rub a balloon on your hair, the amt of energy there is very small- about a thousandth of a J (figure 33.13) • A high voltage requires great energy only if a great amt of charge is involved Electrical Energy Storage • Electrical energy can be stored in capacitors • Uses – Used as on-off switches on computer motherboards – Under each key on a keyboard – Found in a camera’s flash Basic capacitor (Fig 33.14) • Two plates separated by a short distance (not touching) are connected to a charging device and charge is transferred from one plate to another – Positive battery terminal pulls e- from plate connected to it – These e- pumped through battery through negative terminal to the opposite plate – The plates now have equal and opposite charges, done charging when potential difference between plates equals the battery voltage Capacitors Charged Capacitors • A charged capacitor is discharged when a conducting path is provided between the plates – Can be quite shocking – Can be fatal if voltage is high enough • The energy stored comes from the work required to charge it • The energy is in the form of the electric field between the plates