Electricity and Circuits

Chapter 17Jan. 13 - 14

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Properties of Electric Charge

• There are two kinds of electric charge: +, -• Like charges repel and unlike charges attract.• Electric charge is conserved. Explain.– Ions – atoms that are positively or negatively

charged

Transfer of Electric Charge• The fundamental unit of charge, e, is the charge

of a single electron or proton.• Conductors and insulators can be charged by

contact. • Semiconductors are a third class of materials

characterized by electrical properties that are somewhere between those of insulators and conductors.

• Conductors can also be charged by induction.• A surface charge can be induced on an insulator

by polarization.

Table 17.1 Charge and mass of atomic particles

Particle Charge (C) Mass (kg)electron - 1.60 x 10-19 9.109 x 10-31

proton +1.60 x 10-19 1.63 x 10-27

neutron 0 1.65 x 10-27

Electric Force

• The closer two charges are, the greater the force between them.

• Coulomb’s Law: F electric = k C

electric force = Coulomb constant x

k C = 8.99 x 109 N. m2 / C2

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Coulomb’s Law Sample Problem• The electron and proton of a hydrogen atom are separated, on average, by a

distance of about 5.3 x 10-11 m. Find the magnitude (scalar quantity) for the electric force and the gravitational force that each particle exerts on the other.

• Given:r, K c, me, mp, q e, q p, G

• Unknowns:F electric F g

• Equations: F electric = k C F g = G

• Make substitutions.• Solutions: F electric = 8.2 x 10-8 N F g = 3.6 x 10-47 N• Which quantity is larger?

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Practice Problems1. A balloon rubbed against denim gains a charge of -8.0 μC. What is

the electric force between the balloon and the denim when the two are separated by a distance of 5.0 cm? (Assume that the charges are located at a point.)

( 230 N attractive)2. Two identical conducting spheres are placed with their centers

0.30 m apart. One is given a charge of +12 x 10-9 C and the other is given a charge of -18 x 10-9C.a. Find the electric force exerted on one sphere by the

other. (2.2 x 10-5N attractive)b. The spheres are connected by a conducting wire. After

equilibrium has occurred, find the electric force between the two spheres. (9.0 x 10-7N repulsive)

2. Two identical conducting spheres are placed with their centers 0.30 m apart. One is given a charge of +12 x 10-9 C and the other is given a charge of -18 x 10-9C.a. Find the electric force exerted on one sphere by the

other. (2.2 x 10-5N attractive)b. The spheres are connected by a conducting wire. After

equilibrium has occurred, find the electric force between the two spheres. (9.0 x 10-7N repulsive)

3. A small cork with an excess charge of +6.0 μC is placed 0.12 m from another cork, which carries a charge of -4.3 μC.a. What is the magnitude of the electric force

between the corks? (16 N)b. Is this force attractive or repulsive? c. How many excess electrons are on the negative

cork? (2.7 x 1013 electrons)d. How many electrons has he positive cork lost?

(3.8 x 1013 electrons)

3. A small cork with an excess charge of +6.0 μC is placed 0.12 m from another cork, which carries a charge of -4.3 μC.a. What is the magnitude of the electric force

between the corks? (16 N)b. Is this force attractive or repulsive? c. How many excess electrons are on the negative

cork? (2.7 x 1013 electrons)d. How many electrons has he positive cork lost?

(3.8 x 1013 electrons)

4. Two electrostatic point charges of +60.0 μC and + 50.0 μC exert a repulsive force on each other of 175 N. What is the distance between the two charges? (39.3 cm)

Electric force is a field force

• Compare and contrast the electrical force and the gravitational force.

• a) equation?• b) field strength?• c) attractive vs. repulsive?

Challenge Problem

• Due at the end of class.

The resultant force on a charge is the vector sum of the individual forces on that charge.

• Consider 3 point charges at the corners of a triangle, where q1 = 6.00 x 10-9 C, q2 = -2.00 x 10-9 C, and q3 = 5.00 x 10-9 C. Find the magnitude and direction of the resultant force on q3.

• q2 4.00 m q33.00 m θ = 37.0o

5.00 m q1

• (According to the superposition principle, the resultant force on the charge q3 is the vector sum of the forces by q1 and q2 on q3. First find the force exerted on q3 by each, and then add these forces together vectorially to get the resultant force on q3.) (7.16 x 10-9 N , 65.2o)

Exit ticket• What is electrostatic charge? • How do electric charges interact with each other? • How does an object become electrically charged or

discharged? • How are materials classified as conductors and

insulators based on their electrical properties? • How is the electric force between two objects affected

by charge and distance between the objects? • How is Coulomb’s law algebraically manipulated to

calculate force, charge, or separation distance?

Next class

• Meet in B110• Research assignment (Hand in brochure.)• If absent, then get the assignment from

teacher web. Due upon your return.