20 Electric Charge, Force, in addition to Field Essential University Physics Richard Wolfso

20 Electric Charge, Force, in addition to Field Essential University Physics Richard Wolfso www.phwiki.com

20 Electric Charge, Force, in addition to Field Essential University Physics Richard Wolfso

Cody, Mary, Morning Drive On-Air Personality has reference to this Academic Journal, PHwiki organized this Journal 20 Electric Charge, Force, in addition to Field Essential University Physics Richard Wolfson In Chapter 20 you learnt How matter in addition to many of its interactions are fundamentally electrical About electric charge as a fundamental property of matter To describe the electric as long as ce between charges The concept of electric field How to calculate the fields of discrete in addition to continuous charge distributions How charges respond to electric fields Electric charge Electric charge is a fundamental property of matter. Many particles, including the electron in addition to proton, carry electric charge. Charge comes in two varieties, positive in addition to negative. Most charged particles carry exactly one elementary charge, e, either positive or negative. The proton carries exactly +e, the electron exactly –e. The quarks, which make up protons, neutrons, in addition to other particles, carry ±1/3 e or ±2/3 e. But they’re never observed in isolation. The charge in a closed system is conserved, in that the algebraic sum of charges remains unchanged. This is true even if new particles are created or destroyed. The SI unit of charge is the coulomb (C), equal to approximately 6.25 1018 elementary charges. Thus e is approximately 1.6 10–19 C.

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Coulomb’s law in addition to the electric as long as ce Like charges repel, in addition to opposite charges attract, with a as long as ce that depends on The product of the two charges The inverse square of the distance between them Mathematically, the electric as long as ce is described by Coulomb’s law: The superposition principle The electric as long as ce obeys the superposition principle. That means the as long as ce two charges exert on a third as long as ce is just the vector sum of the as long as ces from the two charges, each treated without regard to the other charge. The superposition principle makes it mathematically straight as long as ward to calculate the electric as long as ces exerted by distributions of electric charge. The net electric as long as ce is the sum of the individual as long as ces. Clicker question A charge q1 is located at , . What should you use as long as the unit vector r in Coulomb’s law if you are calculating the as long as ce that q1 exerts on charge q2 located at the point ,

Clicker question A charge q1 is located at , . What should you use as long as the unit vector r in Coulomb’s law if you are calculating the as long as ce that q1 exerts on charge q2 located at the point , The electric field The electric field at a point in space is the as long as ce per unit charge that a charge q placed at that point would experience: The as long as ce on a charge q in an electric field is The electric field is analogous to the gravitational field, which gives the as long as ce per unit mass. Fields of point charges in addition to charge distributions The field of a point charge is radial, outward as long as a positive charge in addition to inward as long as a negative charge. The superposition principle shows that the field due to a charge distribution is the vector sum of the fields of the individual charges.

The dipole: an important charge distribution An electric dipole consists of two point charges of equal magnitude but opposite signs, held a short distance apart. The dipole is electrically neutral, but the separation of its charges results in an electric field. Many charge distributions, especially molecules, behave like electric dipoles. The product of the charge in addition to separation is the dipole moment: p = qd. Far from the dipole, its electric field falls off as the inverse cube of the distance. Continuous charge distributions Charge ultimately resides on individual particles, but it’s often convenient to consider it distributed continuously on a line, over an area, or throughout space. The electric field of a charge distribution follows by summing—that is, integrating—the fields of individual charge elements dq, each treated as a point charge: Clicker question Far from a dipole, you measure an electric field strength of 800 N/C. If you double your distance from the dipole, what will the electric field strength be at your new location 400 N/C 200 N/C 100 N/C 50 N/C

Clicker question Far from a dipole, you measure an electric field strength of 800 N/C. If you double your distance from the dipole, what will the electric field strength be at your new location 400 N/C 200 N/C 100 N/C 50 N/C Two examples The electric field on the axis of a charged ring: The electric field of an infinite line of charge: The line carries charge density C/m: Matter in electric fields For a point charge q in an electric field , Newton’s law in addition to the electric as long as ce combine to give acceleration: A dipole in an electric field experiences a torque that tends to align the dipole moment with the field: If the field is not uni as long as m, the dipole also experiences a net as long as ce.

Clicker question A proton, an electron, a carbon-13 nucleus (6 protons, 7 neutrons), in addition to a helium-4 nucleus (2 protons, 2 neutrons) all find themselves in a uni as long as m electric field. Which of these particles exhibits the second-highest acceleration Assume that the mass of a proton equals the mass of a neutron. The proton The carbon-13 nucleus The electron The helium-4 nucleus Clicker question A proton, an electron, a carbon-13 nucleus, in addition to a helium-4 nucleus all find themselves in a uni as long as m electric field. Which of these particles exhibits the second-highest acceleration Assume that the mass of a proton equals the mass of a neutron. The proton The carbon-13 nucleus The electron The helium-4 nucleus Conductors, insulators, in addition to dielectrics Materials in which charge is free to move are conductors. Materials in which charge isn’t free to move are insulators. Insulators generally contain molecular dipoles, which experience torques in addition to as long as ces in electric fields. Such materials are called dielectrics. Even if molecules aren’t intrinsically dipoles, they acquire induced dipole moments as a result of electric as long as ces stretching the molecule. Alignment of molecular dipoles reduces an externally applied field.

Electric charge is a fundamental property of matter. Charge comes in two varieties, positive in addition to negative. Charge is conserved. The as long as ce between two charges is given by Coulomb’s law: The electric as long as ce obeys the superposition principle, meaning the as long as ces due to individual charges sum vectorially. The electric field describes the as long as ce per unit charge at a given point: The field of a dipole follows from Coulomb’s law: The fields of discrete charge distributions are calculated by summation. The fields of continuous charge distributions are calculated by integration. A point charge experiences a as long as ce in an electric field. A dipole experiences a torque in an electric field, in addition to a as long as ce if the field is not uni as long as m. Summary Read Problem-Solving Strategy as long as using Coulomb’s Law on p 330 Interpret: identify the source charge Develop: draw coordinates in addition to position of charges determine unit vectors (are any along the axes) Evaluate: using Coulomb’s law remembering as long as ce is a vector Assess: is the as long as ce in the direction you expect as long as the sign of the charges Problem 44 In the figure take q1 = 25 C in addition to q2 = 20 C. If the as long as ce on q1 points in the x direction, what is q3 in addition to what is the magnitude of the as long as ce on q1

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