530 likes | 662 Views
Ch. 19 Electric Charges, Forces, and Fields. The atom. The atom has positive charge in the nucleus , located in the protons. The positive charge cannot move from the atom unless there is a nuclear reaction.
E N D
The atom The atom has positive charge in the nucleus, located in the protons. The positive charge cannot move from the atom unless there is a nuclear reaction. The atom has negative charge in the electron cloud on the outside of the atom. Electrons can move from atom to atom without all that much difficulty.
Question • You charge the balloon by rubbing it on hair or on a sweater, and the balloon becomes negative. How can it pick up a neutral tissue?
Charge Charge comes in two forms, which Ben Franklin designated as positive (+) and negative(–). Charge is quantized. • The smallest possible stable charge, which we designate as e, is the magnitude of the charge on 1 electron or 1 proton. • We say a proton has charge of e, and an electron has a charge of –e. • e is referred to as the “elementary” charge. • e = 1.602 × 10-19 Coulombs. • The coulomb is the SI unit of charge.
Sample Problem: A certain static discharge delivers -0.5 Coulombs of electrical charge. How many electrons are in this discharge?
Sample Problem: How much positive charge resides in two moles of hydrogen gas (H2)? How much negative charge? How much net charge?
Sample Problem: The total charge of a system composed of 1800 particles, all of which are protons or electrons, is 31x10-18C. How many protons are in the system? How many electrons are in the system?
Sample Problem: The total charge of a system composed of 1800 particles, all of which are protons or electrons, is 31x10-18C. How many protons are in the system? How many electrons are in the system?
Electric Force • Charges exert forces on each other. • Like charges (two positives, or two negatives) repel each other, resulting in a repulsive force. • Opposite charges (a positive and a negative) attract each other, resulting in an attractive force.
Coulomb’s Law – form 1 • Coulomb’s law tells us how the magnitude of the force between two particles varies with their charge and with the distance between them. • Coulomb’s law applies directly only to spherically symmetric charges.
k = 8.99 × 109 N m2 / C2 • q1, q2 are charges (C) • r is distance between the charges (m) • F is force (N)
Electric Force The electric force between 2 objects is symbolic of the gravitational force between 2 objects. RECALL:
Coulomb’s Law – form 2 • Sometimes you see Coulomb’s Law written in a slightly different form • eo= 8.85 × 10-12 C2/ N m2 • q1, q2 are charges (C) • r is distance between the charges (m) • F is force (N) • This version is theoretically derived and less • practical that form 1
Sample Problem: A point charge of positive 12.0 μC experiences an attractive force of 51 mN when it is placed 15 cm from another point charge. What is the other charge?
Sample Problem: Calculate the mass of ball B, which is suspended in midair. A qA = 1.50 nC R = 1.3 m B qB = -0.50 nC
Superposition • Electrical force, like all forces, is a vector quantity. • If a charge is subjected to forces from more than one other charge, vector addition must be performed. • Vector addition to find the resultant vector is sometimes called superposition.
The Electric Field • The presence of + or – charge modifies empty space. • This enables the electrical force to act on charged particles without actually touching them. • We say that an “electric field” is created in the space around a charged particle or a configuration of charges. • If a charged particle is placed in an electric field created by other charges, it will experience a force as a result of the field. • Sometimes we know about the electric field without knowing much about the charge configuration that created it. • We can easily calculate the electric force from the electric field.
Why use fields? • Forces exist only when two or more particles are present. • Fields exist even if no force is present. • The field of one particle only can be calculated.
Field between charged plates ++++++++++++++++++++++++++++ ----------------------------------------------
Field Vectors from Field Lines • The electric field at a given point is not the field line itself, but can be determined from the field line. • The electric field vectors is always tangent to the line of force at that point. • Vectors of any kind are never curvy!
Force from Electric Field • The force on a charged particle placed in an electric field is easily calculated. • F = E q • F: Force (N) • E: Electric Field (N/C) • q: Charge (C)
Sample Problem: The electric field in a given region is 4000 N/C pointed toward the north. What is the force exerted on a 400 μg Styrofoam bead bearing 600 excess electrons when placed in the field?
Sample Problem: The electric field in a given region is 4000 N/C pointed toward the north. What is the force exerted on a 400 μg Styrofoam bead bearing 600 excess electrons when placed in the field?
Sample Problem: A proton traveling at 440 m/s in the +x direction enters an electric field of magnitude 5400 N/C directed in the +y direction. Find the acceleration.
Sample Problem: A proton traveling at 440 m/s in the +x direction enters an electric field of magnitude 5400 N/C directed in the +y direction. Find the acceleration.
For Spherical Electric Fields • The Electric Field surrounding a point charge or a spherical charge can be calculated by: • E = k q / r2 • E: Electric Field (N/C) • k: 8.99 x 109 N m2/C2 • q: Charge (C) • r: distance from center of charge q (m) • Remember that k = 1/4peo
Sample Problem: A particle bearing -5.0 μC is placed at -2.0 cm, and a particle bearing 5.0 μC is placed at 2.0 cm. What is the field at the origin?
Sample Problem: A particle bearing -5.0 μC is placed at -2.0 cm, and a particle bearing 5.0 μC is placed at 2.0 cm. What is the field at the origin?
Sample Problem: A particle bearing 10.0 mC is placed at the origin, and a particle bearing 5.0 mC is placed at 1.0 m. Where is the field zero?
Sample Problem: A particle bearing 10.0 mC is placed at the origin, and a particle bearing 5.0 mC is placed at 1.0 m. Where is the field zero?
Sample Problem: A particle bearing 10.0 mC is placed at the origin, and a particle bearing 5.0 mC is placed at 1.0 m. Where is the field zero?
Sample Problem: A particle bearing 10.0 mC is placed at the origin, and a particle bearing 5.0 mC is placed at 1.0 m. Where is the field zero?
Sample Problem: A particle bearing 10.0 mC is placed at the origin, and a particle bearing 5.0 mC is placed at 1.0 m. Where is the field zero?
Excess Charges on Conductors • Where does the excess charge reside on a charged conductor? (Van de Graf Generator)
Excess Charges on Conductors • Where does the excess charge reside on a charged conductor? (Van de Graf Generator)
Field within a Conductor • When the electric charges are at rest, the electric field within the conductor is zero.
Electric Fields at Conductor Surfaces Electric field lines contact conductor surfaces a right angles.