Electricity and Magnetism

1. Electricity and Magnetism Unit III

2. I Electrostatics • The study of electric charges at rest and their electric fields and potentials • Charges at rest if there is no net transfer of charge • Microstructure of Matter • The smallest unit of matter is an atom • Atoms are made up of protons, neutrons and electrons • The electron is the fundamental negatively charged particle of matter • The proton is the fundamental positively charged particle of matter

3. The elementary charge (e) equals • The charge on an electron (-e) • The charge on a proton (+e) • Because all atoms are electrically neutral, atoms contain equal number of protons and neutrons • Neutrons have no charge

4. B. Charged Objects • Protons and neutrons cannot be removed from an atom • Electrically charged objects are formed when neutral objects gain or lose electrons • When an atom becomes a charged particle, it is called an ion • Excess electrons creates negative ions • Loss of electrons creates positive ions • Objects with the same sign of charge are repelled • Objects with opposite signs are attracted

5. C. Transfer of Charge • If a system only contains neutral objects, the total net charge is zero • If the objects are rubbed together, electrons are transferred • The system as a whole remains neutral

6. 0e R -8e S +6e T Example 3 spheres have initial charges • When R and S touch, excess electrons (-8e) move to neutral (0e) • When separated each sphere “splits” the excess • (-8e + 0e) ÷ 2 spheres • Each sphere becomes -4e • When S and T touch, excess electrons (-4e) moves to (+6e) • When separated each sphere “splits” the excess • (-4e + 6e) ÷ 2 spheres • Each sphere becomes +1e • TOTALS R = -4e S = +1e T = +1e

7. D. Quantity of Charge • SI unit of charge is a coulomb (C) • 1C = 6.25 x 1018 elementary charges (electrons) • The charge on an electron (-e) = 1.6 x 10-19C • The net charge on a charged object is always a multiple of the charge on an electron

8. E. Coulomb’s Law • The electrostatic force between 2 charges is • Directly related to the product of the charges • Inversely related to the square of the distance between them • F…electrostatic force (newtons) • q…charge (coulombs) • r…distance between charges (meters) • k…electrostatic constant (8.99 x 109 N m2/C2) kq1q2 F = r2

9. II. Electric Fields • Region around a charged particle • Exerts a force on other charged particles • Represented by field lines • Direction a positive charge(+) would move • If lines are curved, charge would move tangent to the point on the field line • Begin (+) and end (-) • Never cross • Energy varies inversely squared with distance

10. E = F q • Strength Where E = electric field strength (N/C) F = force the charge experiences (N) q = the charge (C)

11. E = F q What is the magnitude of the electric field at a point in a field where an electron experiences a 1.0 N force? E = 6.3 x 1018 N/C E = 1.0 N 1.60 x 10-19C

12. V = W q A. Potential Difference • Work done as charge is moved against a field • Where W is work in joules • q is charge in coulombs • V is potential difference • Joules/coulomb is the derived unit volt • 1 J/C = 1 volt • If the charge is an elementary charge (ex. electron) thepotential difference is an electron volt (eV) • 1 eV = 1.60 x 10-19J

13. Moving a point charge of 3.2 x 10-19 coulombs between two points in an electric field requires 4.8 x 10-18 joules. What is the potential difference between these two points? V = W q V = 4.8 x 10-18 J 3.2 x 10-19C V = 15 V

14. III Electric Current A. Electric current • The rate that a charge passes a point in a circuit • Represented using the symbol, I • Unit is an ampere (A) • Formula I = q t • I is current in amperes • q = charge in coulombs • t = time in seconds • Current is measured using an ammeter

15. B. Electric circuit • The closed path that a charged particle moves along • Potential difference is also needed for an electric current • Supplied by • Cell • Battery • Measured using a voltmeter • Electron flow determines the direction of current • A switch is used to make, break or change the connections in a circuit

16. C. Resistance • Electrical resistance (R) is the opposition of electron flow • Ratio of potential difference to current flow • R = V I • Where • V = potential difference in volts • I = current in amperes • R = resistance in volts per ampere • Ohm (Ω)represents 1 volt per ampere • Resistance is affected by temperature

17. A current of 0.10 amperes flow through a lamp connected to a 12.0-volt source. What is the resistance of the lamp? I = 0.10 A V = 12.0 V R = V I R = 12.0 V 0.10 A R = 120 Ω

18. Factors Affecting Resistance Length of Wire • Increasing length, increases the collisions of electrons with atoms in the wire • Directly related Thickness of Wire • Increasing thickness creates more spaces for electrons to travel through, decreasing the resistance • Inverse relationship with cross-sectional area Resistivity • Characteristic of the material • Good conductors have low resistivities • As temperature increases, resistivity increases • Values found in reference tables

19. Resistance of a Wire R = ρL A Where • R is resistance in ohms (Ω) • ρ is the resistivity in ohm-meters (Ω-m) • L is length in meters (m) • A is cross-sectional area in square meters (m2)

20. Determine the resistance of a 4.00-meter piece of copper wire that has a diameter of 2.00 mm at 20°C. ρ copper = 1.72 x 10-2 L = 4.00 m Diameter = 0.002 m Area = πr2 Area = π(.001)2 Area = 3.14 x 10-6 m2 R = (1.72 x 10-2 x 4.00) 3.14 x 10-6 R =2.19 x 10-2Ω

21. Resistor • Device to have a definite amount of resistance • Used to limit current flow • Provide a potential drop • Symbol

22. Electric Power Power (P) = Work/time Units Work,,,joules Time sec Power watts Electrical power is the product of VI Since V=IR then P = I2R