VIEWS: 15 PAGES: 12 POSTED ON: 4/21/2012 Public Domain
Chapter 23 Electric Fields 23.1 Properties of Electric Charges 23.3 Coulomb’s Law 23.4 The Electric Field 23.6 Electric Field Lines 23.7 Motion of Charged Particles in a Uniform Electric Field Nadiah Alenazi 1 23.1 Properties of Electric Charges • There are two kinds of electric charges in nature: – Positive – Negative • Like charges repel one another and Unlike charges attract one another. • Electric charge is conserved. • Charge is quantized q=Ne e = 1.6 x 10-19 C N is some integer Nadiah Alenazi 2 23.3 Coulomb’s Law From Coulomb’s experiments, we can generalize the following properties of the electric force between two stationary charged particles. The electric force • is inversely proportional to the square of the separation r between the particles and directed along the line joining them. • is proportional to the product of the charges q1 and q2 on the two particles. • is attractive if the charges are of opposite sign and repulsive if the charges have the same sign. Nadiah Alenazi 3 • Consider two electric charges: q1 and q2 • The electric force F between these two charges separated by a distance r is given by Coulomb’s Law • The constant ke is called Coulomb’s constant • is the permittivity of 2 1 12 C 0 k where 0 8.85 10 free space 4 0 Nm2 • The smallest unit of charge e is the charge on an electron (-e) or a proton (+e) and has a magnitude e = 1.6 x 10-19 C Nadiah Alenazi 4 Example 23.1 The Hydrogen Atom The electron and proton of a hydrogen atom are separated (on the average) by a distance of approximately 5.3 x 10-11 m. Find the magnitudes of the electric force. Nadiah Alenazi 5 • When dealing with Coulomb’s law, you must remember that force is a vector quantity • The law expressed in vector form for the electric force exerted by a charge q1 on a second charge q2, written F12, is • where rˆ is a unit vector directed from q1 toward q2 • The electric force exerted by q2 on q1 is equal in magnitude to the force exerted by q1 on q2 and in the opposite direction; that is, F21= -F12. Nadiah Alenazi 6 • When more than two charges are present, the force between any pair of them is given by Equation • Therefore, the resultant force on any one of them equals the vector sum of the forces exerted by the various individual charges. • For example, if four charges are present, then the resultant force exerted by particles 2, 3, and 4 on particle 1 is Nadiah Alenazi 7 • Double one of the charges – force doubles • Change sign of one of the charges – force changes direction • Change sign of both charges – force stays the same • Double the distance between charges – force four times weaker • Double both charges – force four times stronger Nadiah Alenazi 8 Example: • Three point charges are aligned along the x axis as shown. Find the electric force at the charge 3nC. Nadiah Alenazi 9 Example 23.2 Find the Resultant Force • Consider three point charges located at the corners of a right triangle, where q1=q3= 5.0μC, q2= 2.0 μC, and a= 0.10 m. Find the resultant force exerted on q3. Nadiah Alenazi 10 Nadiah Alenazi 11 Nadiah Alenazi 12