Physics of Technology PHYS 1800

1. Physics of TechnologyPHYS 1800 Lecture 28 Electricity and Charge

2. No area of physics has had a greater impact on the way we live than the study of electricity and magnetism. UNIT THREEElectricity and Magnetism

3. PHYSICS OF TECHNOLOGYSpring 2009 Assignment Sheet *Homework Handout

4. Physics of TechnologyPHYS 1800 Lecture 28 Electricity and Charge Charge

5. Describing Motion and Interactions Position—where you are in space (L or meter) Velocity—how fast position is changing with time (LT-1 or m/s) Acceleration—how fast velocity is changing with time (LT-2 or m/s2) Force— what is required to change to motion of a body (MLT-2 or kg-m/s2 or N) Inertia (mass)— a measure of the force needed to change the motion of a body (M) Energy—the potential for an object to do work.(ML2T-2 or kg m2/s2 or N-m or J) Work is equal to the force applied times the distance moved. W = F d Kinetic Energy is the energy associated with an object’s motion. KE=½ mv2 Potential Energy is the energy associated with an objects position. Gravitational potential energy PEgravity=mgh Spring potential energy PEapring= -kx Momentum— the potential of an object to induce motion in another object (MLT-1 or kg-m/s) Angular Momentum and Rotational Energy— the equivalent constants of motion for rotation (MT-1 or kg/s) and (MLT-2 or kg m/s2 or N) Pressure— force divided by the area over which the force is applied (ML-1T-1 or kg/m-s or N/m2 or Pa)

6. What are the major subfields in Physics? • Classical Physics(pre 20th century) • Mechanics → forces, motion • Thermodynamics → heat, temperature • Electricity and magnetism → charge, currents • Optics → light, lenses, telescopes • Modern Physics (20th century) • Atomic and nuclear → radioactivity, atomic power • Quantum mechanics }→ basic structure matter • Particle physics • Condensed matter → solids and liquids, computers, lasers • Relativity, Cosmology → universe, life!

7. State of Physics cira 1895 • Conservation Laws • Energy • Linear & Angular Momentum • Statistical Mechanics • 3 Laws of Thermodynamics • Kinetic Theory

8. Chapter 12Electrostatic Phenomena

9. What does lightning have in common... ... with hair on a dry winter day?

10. Effects of Electric Charge • Hair seems to have a mind of its own when combed on a dry winter day. • What causes the hairs to repel one another? • Why does a piece of plastic refuse to leave your hand after you peeled it off a package? • Why do you get a slight shock after walking across carpet and touching a light switch?

11. Effects of Electric Charge • All these phenomena involve different materials rubbing against one another. • Electrostatic effects can be demonstrated by rubbing plastic or glass rods with different furs or fabrics. • Small wads of dry, paperlike material called pith balls are light enough to be strongly influenced by electrostatic forces. • When a plastic rod, vigorously rubbed with cat fur, is brought near the pith balls, at first the pith balls are attracted to the rod like bits of iron to a magnet. • After contacting the rod, the pith balls dance away from the rod. • They are now repelled by the rod and also by each other.

12. Effects of Electric Charge • A repulsive force must be acting between the two pith balls after they have been in contact with the rod. • Perhaps the balls have received something (call it electric charge) from the rod that is responsible for the force we observe. • This charge was somehow generated by rubbing the rod with the cat fur. • The force that is exerted by one stationary charge on another is called the electrostatic force.

13. Experiments with different materials indicate that there are two types of charge. • An electroscope consists of two metallic-foil leaves suspended from a metal post inside a glass-walled container. • If the foil leaves are uncharged, they will hang straight down. • If a charged rod is brought in contact with the metal ball on top, the leaves immediately spread apart and stay apart, even if the rod is removed. • If an object of the same charge as the original rod is later brought near the metal ball, the leaves will spread farther apart. • An object with the opposite charge will make the leaves come closer together. • A larger charge produces a larger effect. Like charges repel each other, and unlike charges attract each other.

14. Benjamin Franklin introduced the names positive and negative for the two types of charge. • He also proposed that a single fluid was being transferred from one object to another during charging. • A positive charge resulted from a surplus of the fluid, and a negative charge resulted from a shortage of the fluid. • Franklin arbitrarily proposed that the charge on a glass rod when rubbed with silk be called positive. Like charges repel each other, and unlike charges attract each other.

15. Franklin’s model comes surprisingly close to our modern view. • When objects are rubbed together, electrons may be transferred from one object to the other. • Electrons are small, negatively charged particles present in all atoms and, therefore, in all materials. • A negatively charged object has a surplus of electrons, and a positively charged object has a shortage of electrons. • The atomic or chemical properties of materials dictate which way the electrons flow when objects are rubbed together. Like charges repel each other, and unlike charges attract each other.

16. Conductors and Insulators • Different materials behave differently in the presence of electrostatic forces. • Charge can readily flow through conductors: • metals, like copper, silver, iron, gold; our bodies • Materials that do not ordinarily permit charge to flow are insulators: • plastic; glass; ceramics; other nonmetallic materials • Charge flows much more readily through several miles of copper wire than through the few inches of insulating ceramic material. • Semiconductors are intermediate between a good conductor and a good insulator. • Their importance to modern technology is enormous.

17. Can you charge an object without actually touching it with another charged object? • Charging by induction involves the conducting property of metals: • Charge a plastic rod with cat fur and bring the rod near a metal ball mounted on an insulating post. • The electrons in the metal ball are repelled by the negative rod. • There is a negative charge buildup on the side opposite the rod, and a positive charge on the near side.

18. Can you charge an object without actually touching it with another charged object? • To charge the ball by induction, now touch the ball with your finger on the side opposite the rod. • The negative charge flows from the ball to your body, since it is still repelled by the negative rod. • If you now remove your finger and then the rod, a net positive charge is left on the ball.

19. Charging By Induction • Charging by induction illustrates the mobility of charges on a conducting object such as the metal ball. • The process will not work with a glass ball. • Charging by induction is an important process in machines used for generating electrostatic charges, and in many other practical devices. • It also explains some of the phenomena associated with lightning storms.

20. Why are insulators attracted to charged objects? Charging By Induction • Recall that the pith balls were attracted to the charged rod before they were charged themselves. • Electrons are not free to move in the insulating material of the pith balls. • However, within each atom or molecule, charges can move. • Each atom becomes an electric dipole: the center of the negative charge is slightly displaced from the center of the positive charge. • The material is polarized.

21. Charging By Induction • Since the negatively charged surface is closer to the rod than the positively charged surface, it experiences a stronger electrostatic force. • The overall effect is that the pith ball is attracted to the charged rod, even though the net (total) charge on the pith ball is zero. • After the ball comes in contact with the charged rod, some of the charge on the rod is transferred to the pith ball. • The pith ball is then positively charged like the rod, and so is repelled by the rod.

22. Polarization • Polarization explains why small bits of paper or styrofoam are attracted to a charged object such as a sweater rubbed against some other material. • Electrostatic precipitators used to remove particles from smoke in industrial smoke stacks use this property. • Polarized particles are attracted to charged plates in the precipitator, removing them from the emitted gases.

23. Charging on the James Webb Space Telescope

24. Physics of Technology Next Lab/Demo: Electric Charge Electric Circuits Thursday 1:30-2:45 ESLC 46 Ch 12 and 13 Next Class: Wednesday 10:30-11:20 BUS 318 room Read Ch 13