1. Phys141 Principles of Physical Science Chapter 8 Electricity and Magnetism Instructor: Li Ma Office: NBC 126 Phone: (713) 313-7028 Email: malx@tsu.edumalx@tsu.edu Webpage: http://itscience.tsu.edu/ma Department of Computer Science & Physics Texas Southern University, Houston Oct. 6, 2004

2. Topics To Be Discussed Electric Charge and Current Voltage and Electric Power Magnetism Electromagnetism Skip –§8.3 –The Earth’s Magnetic Field in § 8.4 –Motors and Generators in §8.5

3. Electric Charge Electric charge is a fundamental quantity (mentioned in Chapter 1) The property of electric charge is associated with certain subatomic particles Two types of charges: –positive (+) –negative (-)

4. Electric Charge (cont) All matter is made up of small particles called atoms An atom is composed of electrons, protons and neutrons: –Electrons: negatively charged particles –Protons: positively charged particles –Neutrons: neutral particle –Table 8.1 on page 167

5. Electric Charge (cont) All three particles have certain mass Electrons and protons possess electric charges –Magnitudes are equal –Natures are different When we have same number of electrons and protons –Total charge is zero – neutral situation

6. Electric Charge (cont) Unit of electric charge: coulomb (C) Electric charge is usually designated by the letter q –An excess of positive charges: +q –An excess of negative charges: -q When charge flows, or is in motion, we have electric current

7. Electric Current Electric current is defined as the time rate of flow of electric charge Unit: ampere (A), 1 A = 1 C/s current = charge time I = qtqt or

8. Electric Current (cont) Electrical conductors: materials in which electric charge flows readily –The outer, loosely bound electrons of the atoms: ex. metals Electrical insulators: materials that do not conduct electricity very well –Electrons are more tightly bound –ex. Wood, glass, plastic, rubber, etc

9. Electric Current (cont) Semiconductors: materials that are neither good conductors nor good insulators –ex. graphite (carbon) Net charge q: –Electrons move randomly and chaotically => no net charge flows –More electrons move in one direction => net charges flow => current

10. Electric Force An electric force exists between any two charged particles This kind of mutual forces may be either attractive or repulsive, depending on the types of charges (+ or -) Law of charges: –Like charges repel, and unlike charges attract

11. Electric Force (cont) The law of charges gives the direction of an electric force, what about the magnitude? Coulomb’s law: –The force of attraction or repulsion between two charged bodies is directly proportional to the production of two charges and inversely proportional to the square of the distance between them

12. Electric Force (cont) Where F is magnitude of the electric force q 1 is magnitude of the first charge q 2 is magnitude of the second charge r is distance between charges k is a proportionality constant: k = 9.0 x 10 9 N-m 2 /C 2 F= kq 1 q 2 r 2

13. Electric Force (cont) Compare Coulomb’s law and Newton’s law of universal gravitation: –Both forces depend on the square of the separation distance –Coulomb’s law depends on charge, whereas Newton’s law depends mass –Coulomb’s law can give rise to either an attractive or a repulsive force; on the other hand, the force of gravitation is always attractive –The electric forces are comparatively much stronger than the gravitational forces

14. Voltage It takes work to separate charges The more charges to separate, the more work it takes => electric potential energy Freely moving charge will move toward the charge of opposite sign: electric potential energy => kinetic energy

15. Voltage (cont) Voltage, or potential difference, is defined as the amount of work it would take to move a charge between two points, divided by the value of the charge In other word, voltage (V) is the work (W) per unit charge (q) or the electric potential energy per unit charge

16. Voltage (cont) Unit: volt (V), 1 V = 1 J/C Voltage is caused by a separation of charge voltage = work charge V = WqWq or

17. Voltage (cont) Electric potential energy may be used to set up a current When there is current, it meets with some opposition because of collisions within the conducting material Resistance (R): the opposition to the flow of charge –Unit: ohm (Ω)

18. Voltage (cont) Ohm’s law: voltage = current x resistance orV = I·R It is a simple relationship involving voltage, current and resistance One ohm is one volt per ampere: R = V / I

19. Electric Power When current exists in a circuit, work is done to overcome the resistance of the circuit, and power is expended Electric power is defined in terms of current and voltage P = I·V orP = I 2 ·R

20. Electric Power (cont) The power that is dissipated in an electric circuit is frequently in the form of heat This heat is called joule heat or I 2 R losses This heating effect is used in electric stoves, heaters, hair dryers, light bulb, and so on

21. Magnetism Two poles of the magnet: one at each end –The north pole (N): the north-seeking pole –The south pole (S): the south-seeking pole Law of poles: –Like poles repel, and unlike poles attract –N-S poles attract, and N-N poles and S-S poles repel –The strength of the magnetic force is inversely proportional to the square of the distance between the poles

22. Magnetism (cont) All magnets have two poles: i.e. magnets are always dipoles Every magnet produces a force on every other magnet Magnetic field (B): a set of imaginary lines that indicates the direction in which a small compass needle would point if it were placed near a magnet

23. Magnetism (cont) Magnetic field is a force field: the field lines are indications of the magnetic force In a magnetic field pattern: –The arrows in the field lines indicate the direction in which the north pole of a compass would point –The closer together the field lines, the stronger the magnetic force

24. Electromagnetism There is an electric field around charges Electric field is the electric force per unit charge The electric and magnetic fields are vector quantities The interaction of electrical and magnetic effects is known as electromagnetism

25. Electromagnetism (cont) Two basic principles of this interaction: –Moving electric charges (current) give rise to magnetic fields –A magnetic field may deflect a moving electric charge The source of magnetism is moving and “spinning” electrons – the 1st principle There may be magnetic force on a moving electric charge – the 2nd principle