You have 2 moles of a mono-atomic ideal gas in a container – fitted with a piston. V, P, T are the volume, pressure, and temperature. Compress the gas very fast adiabatically to half the initial volume Vf=V/2. Then let the gas expand isothermally (constant temperature) back to the original volume. The final pressure is … • Pfinal = P • Pfinal = 2P • Pfinal = ½ P • Pfinal > P • Pfinal < P • other QUIZ
Electric charge • First we look at some of the phenomenon, later processes/descriptions etc. • Charges – attractive-repulsive phenomenon • Insulators and conductors • Charging something • Grounding • Induction • Polarization
Read a textbook • Understanding Physics is on Blackboard • Knight/Tipler The charged phenomena result from the movement of charged particles (No, it’s not magic). The descriptions of the historical experiments and figuring out the mechanism behind them helps us develop intuition about how these charged particles interact with each other.
Electric Charge Electrostatic Rules 1. Opposite charges attract. Like charges repel. 2. The greater the charges, the stronger the attractive or repulsive force between them. 3. The closer the charges, the stronger the attractive or repulsive force between them.
Insulators and Conductors Metals are materials in which some or all of the valence electrons become unbound from their nuclei. These free electrons then form an electron sea—a reservoir of electrons that are easily able to flow through the metal. So, metals are conductors. An insulator, by contrast, is a material through which charge does not flow easily. A chunk of carbon (such as coal) is an example of an insulator.
While metals are typically used to conduct electricity in electrical devices and transmission lines, many other materials are conductors as well. • each neuron in your brain contains conductive membranes, so that ions can flow easily across them. • And tap water typically contains impurities (such as salts) that are easily ionized, so is also able to conduct electricity.
Charge versus charged particles • The net charge is typically the mismatch between the number of positive and negative charged particles. • For an uncharged rod, it still physically has charged particles in it; just that the number of positively charged particles perfectly matches the negatively charged particles.
Consider a electroscope that already has some excess electrons on it. Now if I bring a negatively charged rod near the top of the electroscope – but do not touch it to the electroscope, the “hands” of the electroscope would …. • Deflect less (collapse) • Deflect more • No change
Consider a electroscope that already has some excess electrons on it. Now if I bring a negatively charged rod near the top of the electroscope – but do not touch it to the electroscope. Now I take it away – again, no touch anywhere. After removing the rod, the hands of the electroscope …. • Remain more deflected • Go back to original deflection • Are less deflected than original • Deflect even more
Consider a electroscope that already has some excess electrons on it. Now if I bring a negatively charged rod near the top of the electroscope – and accidentally touch it to the scope. Now after removing the rod, the hands of the electroscope …. • Remain more deflected • Go back to original deflection • Go back, but are less deflected than original • Increase the deflection even more
Conservation of charge • Charge cannot just disappear – because charged particles cannot just disappear • In a closed system, i.e. if you are not adding or removing electrons/protons, then the net charge in the system cannot change • In order to change the net charge, you must be adding or removing some of the charged particles; specifically, adding/removing more of one kind than the other. • It could happen that the positive and negative charge together produce a net charge of zero.
Science and everyday reasoning • Duke University Mechanical Engineering researchers – explain how the lotus leaf keeps off the water from its surface – designing experiments taking inspiration from how he saw the leaf shake in the wind …. http://www.nytimes.com/2009/10/27/science/27lotus.html?_r=1&scp=1&sq=%2btechnology+%2bengineering&st=nyt
Pickup your 1st exam regrade at the end of lecture • David Blumenstyk • Scott Beverly
Pick up your HW from me at the end of lecture • Avi Prince • PreethaGautam • Jeffrey Braun
Grounding • The earth is a very large object – sort of can take in any amount of excess charge … so if I connect something with a wire/conductor to the earth/ground, all the unbound/free charge would go into the ground – this is called grounding.
Coulomb’s Law The electrostatic constant k has been experimentally determined to be, in S.I. units, k = 9.0x109N•m2/C2.
A dust particle of charge +1 pC sits near a small bead of charge +100 pC. Which of the following is true? • The force on the dust particle is more than the force on the bead of charge • The force on the bead is more than that on the dust particles • The bead exerts a force on the dust particle; There is no force on the bead itself • The force on the bead equals that on the dust particle
Due to their electrostatic repulsion, the dust particle and bead move apart. Intuitively, whichobject (if either) gains speed more quickly? • The dust particle • The bead • They gain speed at the same rate