Electric forces and electric fields

1. Electric forces and electric fields

2. 1Proprieties of electric charges • (Give an ex. of electrically charged) • Electric charge can be + or – • Like charges repel one another; and unlike charges attract one another • Electric charge is always conserved • The object become charged because – charge is transffered from one object to another • An object may have charge of ±e, ±2e, ±3e • e = 1.60219x10-19C • SI unit: C (Coulomb)

3. 2 Insulators and conductors • In conductors, electric charges move freely in response to an electric force. All other materials are called insulators (give an ex. of each) • Semiconductors are between conductors and insulators. • An object connected to a conducting wire buried in the Earth is said to be grounded. • Induction – charging of a conductor

4. Charging an object by induction requires no contact with the object inducing the charge. • 3. Coulomb’s Law • An electric force has the following properties: • It is directing along a line joining the two particles and is inversely proportional to the square of the separation distance r, between them

5. It is proportional to the product of the magnitudes of the charges, |q1|and |q2|, of the 2 particles • It is attractive if the charges are of the opposite sign, and repulsive if the charges have the same sign • The magnitude of the electric force: • F=ke (|q1||q2|/r2) • ke – Coulomb constant ke = 8.9875x109N m2/C2

6. 4. Electric Field • The electric field E produced by a charge Q at the location of a small “test” charge qo is defined as the electric force F exerted by Q and qo divided by the charge qo . • E=F/qo E=ke (|q|/r2) • Si unit : N/C

7. Pb. Strategies: • 1. Draw a diagram of the charges • 2. Identify the charge of interest • 3. Convert all units in SI • 4. Apply Coulomb’s Law • 5. Sum all the x- components of the resulting electric force • 6. Sum all the y-components of the resulting electric force • 7. Use Pythagorean theorem to find the magnitude and the direction of the force

8. 5. Electric field lines • 1. The electric field E is tangent to the electric field lines at each point • 2. The number of lines per unit area through a surface perpendicular to the lines is proportional to the strength of the electric field in a given region • Rules for drawing electric field lines: • -The lines for a group of point charges must begin on + charge and end on – charge • - The number of lines drawn leaving a + charge or ending a – charge is proportional to the magnitude of the charge • - No two field lines can cross each other

9. 6. Conductors in electrostatic equilibrium • When no net motion of chartge pccurs within a conductor, the conductor is in electrostatic equilibrum • 1. the electric field is zero inside of the material • 2. any excess charge on an isolated conductor resides entirely on its surface • 3. the electric field just outside a charge conductor is perpendicular to the conductor’s surface • 4. On an irregularly shaped conductor , the charge accumulates at sharp points, where the radius of curvature of the surface is smallest

10. 9. electric flux and Gauss’s Law • The electric flux ( the number of the field lines) is proportional to the product of the electric field and surface of the area • ΦE =EA • ΦE =EA cosθ • For a close surface, the flux line passing into the interior of the volume are negative, and those passing out of the interior of the volume are positive

11. E= ke q|/r2 • A= 4πr2 • ΦE =EA=4πke q • Permittivity of free space: εo=1/4πke =8.85x10-12C2/N m2 • Gauss’s Law: • ΦE =q/ εo • The electric flux through any closed surface is equal to the net charge inside the surface divided by the permittivity