Chapter 15: Electric Charge, Force, and Field 15.1 Electric Charge 15.2 Electrostatic Charging 15.3 Electric Force 15.4 Electric Field 15.5 Conductors and Electric Fields
Homework for Chapter 15 • Read Chapter 15 • HW 15.A: p.506-507: 7-10, 13, 14, 22,23,25-28, 30. • HW 15.B: p.508-509: 43-47, 64-67.
Warmup: Electric Force Physics Warmup #102 An electric force is the force that exists between any two charged particles due to their charge. Objects with opposite charges will attract each other, while objects with the same charge will repel each other. ********************************************************************************************* Complete the table for each pair of objects listed. yes repel no no no yes repel attract yes
electrostatics - the study of electric forces between charged objects at rest. electric charge - the property of an object that determines its electrical behavior: the electric force it can exert, and the electric force it can experience. There are two types of charges: positive (+) – associated with the proton negative (-) – associated with the electron • The positive and negative signs are arbitrary conventions. • Both protons and electrons have the same charge, but with opposite signs. Neutrons have no charge. law of charges or charge-force law - Like charges repel each other, and unlike charges attract each other.
• The fundamental unit of charge is the magnitude of the charge of an electron: e = 1.60 x 10-19 C coulomb (C) - the SI unit of charge • Charge is “quantized”; it can only occur in integral multiples of e. The charge of an object is q = n e (n = 1,2,3…) net charge - an object has an excess of either positive or negative charges. law of conservation of charge - The net charge of an isolated system remains constant.
Example 15.1: A piece of glass has a net charge of -2.00 C. a) Are there more protons or electrons? b) What happens if an identically charged piece of glass is placed near the first one? c) How many fundamental units of excess charge does one piece of the glass contain?
conductor - a material which has the ability to conduct or transmit electric charges ex: metals valence electrons are loosely bound, can move insulator - poor electrical conductors ex: glass, rubber, plastic valence electrons are tightly bound, can’t readily move semiconductor - electron conductivity is intermediate between a conductor and insulator ex: transistors, computer chips
electroscope - a device that can be used to demonstrate characteristics of electric charge. • When a charged object is brought near the bulb of a neutral electroscope, the leaves separate.
electrostatic charging - the process by which an insulator or an insulated conductor receives a net charge. • • Electrostatic charging can be done by four processes. • 1) charging by friction - insulators are rubbed with different materials and the insulators and the materials acquire equal but opposite charges. • ex: Rubbing a rubber rod with fur will transfer electrons from the fur to the rubber, and leave the rod with a net negative charge. • ex: After walking across a carpet, you get zapped when touching a metal doorknob.
2) charging by contact or by conduction - a charged object makes a contact with a uncharged object and some of the charge on the charged object is transferred to the uncharged object.
3) charging by induction - a charged object is brought near (not touching) an uncharged object, the uncharged object is then “grounded”, and the uncharged object acquires an opposite charge than the charged object.
4) charging by polarization - the positive and negative charges are simply separated or realigned within the object and the net charge of the object is still zero. Charges are merely separated so a portion of the object has excess positive charges and another has excess negative charges. When the balloons are charged by friction and placed in contact with the wall, an opposite charge is induced on the wall’s surface, to which the balloons then stick by the force of electrostatic attraction.
Water molecules are polar in nature, but some molecules that are normally not polar can become induced molecular dipoles in the presence of a nearby charged object. A charged balloon easily attracts polar water molecules.
Check for Understanding • A combination of two electrons and three protons would have a net charge of • +1 • -1 • +1.6 x 10-19 C • -1.6 x 10-19 C Answer: c
Check for Understanding • 2. The directions of the electric forces on two interacting charges are given by • the conservation of charge • the charge-force law • the magnitude of the charges • none of these Answer: b “Like charges repel each other, and unlike charges attract each other.
Check for Understanding • 3. A rubber rod is rubbed with fur (fur will lose electrons to a rubber rod). The fur is then quickly brought near the bulb of an uncharged electroscope. What is the sign of the charge on the leaves of the electroscope? • positive • negative • neutral • unable to tell Answer: a. When positively charged fur is brought near an electroscope, the leaves are charged by induction, so the charges on the leaves are positive.
Check for Understanding • 4. When a positively charged rod is brought near the bulb of a negatively charged electroscope, the leaves • separate further • collapse • remain unchanged • unable to tell Answer: b. Negative charge is drawn to the top, and the leaves collapse.
Warmup: Just Charge It Physics Warmup #100 Most objects are electrically neutral. They contain equal amounts of positive and negative charges. However, many objects often become charged as they gain or lose electrons. One way this may happen is through charging by friction, in which two neutral objects rub against each other. The resulting transfer of electrons leaves one object negatively charged and the other positively charged. ********************************************************************************************* Describe a common event in which objects become charged by friction. Answer: example – walking across a carpet; clothes tumbling in a dryer.
• On your gold sheet, Coulomb’s law constant k is expanded to: k = 1 where 0 = 8.85 x 10-12 C2/N·m2 (vacuum permittivity) 40 0is a physical constant given on your blue sheet. • The mutual electrostatic forces on two point charges are equal and opposite.
• For a configuration of two or more point charges, the force on a particular charge is the vector sum of the forces on it due to all the other charges. • Like the universal law of gravitation, Fg = Gm1m2 , r2 Coulomb’s law in a inverse square law because force is proportional to 1 r2 • Electrical forces are MUCH stronger than gravitational forces. • Electric charge comes in two types, positive and negative. Mass does not; it only attracts.
Example 15.2: Two charges are separated by a distance d and exert mutual attractive forces F1 on each other. What are the mutual attractive forces if the charges are separated by a distance of 3d?
Example 15.3: A +4.0 coulomb charge is at the origin and a +9.0 coulomb charge is at x = 4.0 m. Where can a third charge q3 be placed on the x axis so the net force on it is zero?
Example 15.4: Consider three point charges located at the corners of a triangle as shown in the figure on the right. If q1 = 6.0 nC, q2 = -1.0 nC, and q3 = 5.0 nC, what is the net force on q3?
Check for Understanding: • The magnitude of the electric force between two point charges is given by • a) the charge-force law • b) conservation of charge • c) Coulomb’s law • d) both a and b Answer: c
Check for Understanding: 2. Compared to that of the electric force, the strength of the gravitational force between two protons is a) about the same b) somewhat large c) very much larger d) very much smaller Answer: d
Check for Understanding: 3. An electron is a certain distance from a proton. How would the electric force be affected if the electron were moved twice the distance away from the proton? a) 2 times greater b) 4 times greater c) half as much d) one quarter as much Answer: d
Check for Understanding: 4. Two identical point charges are at a fixed distance from one another. How would the electric force be affected if one of the charges was doubled and the other halved? a) 2 times greater b) same c) half as much d) one quarter as much Answer: b HW 15.A: p.506: 7-10, 13, 14, 22,23,25-28, 30.
Warmup: It’s a Wrap Physics Warmup #101 Clear food wraps cling to whatever surface they are pressed against (even themselves). When stretched tightly over a container and pressed against the sides, they keep their tension and secure the container. The secret behind these plastic wraps is static electricity. Some of the static charge is created when the roll is manufactured. Since plastic is not a good conductor, the charge remains for a long time. More static charge is created when the plastic is pulled off the roll. Pull faster and it will cling even better. ********************************************************************************************* You might have noticed that these clear plastic wraps do not work as well in humid conditions and hardly work at all when the container has moisture on it. Explain why this is so. Answer: Moisture drains the static charge.
test charge - a very small charge that doesn’t affect the field producing charge arrangement.
• The SI Units of the electric Field is N/C. View an Applet: http://www.falstad.com/emstatic/
Electric Field • Electric field direction is determined by using a positive test charge. • Note that the magnitude of the field (lengths of vectors) becomes smaller as the distance from the source charge increased, reflecting the inverse-square law. • The vectors are connected to give electric field lines, or lines of force. The closer the lines are together, the stronger the electric field. • The direction of the electric field is tangent to the lines of the force. • The electric field lines start at positive charges and end at negative charges. • The number of lines leaving or entering a charge is proportional to the magnitude of that charge. • Electric field lines never cross.
electric dipole - two separate electric charges ex: water molecule • Even though the net charge on a dipole is zero, it creates an electric field because the charges are separated. • Observe the electric field line pattern produced by a dipole. • On the x-axis, the E-field points downward. x
Electric Field: Very Large, Oppositely Charged, Parallel Plates • Above a positively charged plate, the net electric field points up. The horizontal components of the field cancel out. Below the plate, the field points down (not shown). • For a negatively charged plate, the E-field direction is reversed. • Superimposing the fields from both plates results in cancellation outside the plates and an approximately uniform field between them. • E = 4 k Q (between parallel plates) • A • where Q is the total charge on one plate • k is Coulomb’s constant • and A is the area of one plate.
Electric Fields for Various Configurations b) The field between the plates is relatively uniform. c) The electric field outside the sphere is as though all the charge on the sphere were concentrated at its center. The electric field inside the sphere is zero.
Example 15.5: A 2.0 coulomb charge is 10 m from a small test charge of 0.10 nC. • What is the electric field at the location of the test charge? • What is the direction of the electric field at the location of the test charge? • What is the magnitude of the force experienced by the test charge?
Example 15.6: A charge of 5.0 C is placed at the 0 cm mark of a meterstick and a charge of -4.0 C is placed at the 50 cm mark. • What is the electric field at the 30 cm mark? • At what point along a line connecting the two charges is the electric field zero?
Example 15.7: Four charges occupy the corners of a square as shown in the figure on the right. Each charge has a magnitude of 3.0 C and the side of the square is 0.50 m. Find the electric field at the geometrical center of the square.
Check for Understanding: • The electric field due to a negative point charge • varies as 1/r • points toward the charge • has a finite range • is the same as that of a positive charge Answer: b
Check for Understanding: • 2. The units of electric field are • C • N/C • N • J Answer: b