How Things Work - Fall 2018

  Recall the definition of Voltage: Voltage  (V) - the amount of available energy per coulomb of charge.  The unit is the joule per coulomb, called a volt (V, in honor of Allesandro Volta, inventer of the battery). V = E/Q Batteries and other sources (such as wall sockets) "provide" voltage, which is really a difference between TWO points (marked + and - on a battery).   Or in schematic form: We also need to quantify the rate at which charge can flow.  For mathematical simplicity, we assume that positive charge is "flowing".  We call this flow rate "current". Current  (I) - the rate at which positive charge "flows" I = Q/t The unit is the coulomb per second, defined as an  ampere  (A).  Just as one coulomb is a huge amount of charge (nearly 6.3 billion billion protons), one ampere (or amp) is a tremendous amount of current - more than enough to kill a person.

Thus far, we have only discussed "static" (stationary) charges.  Static charges alone are useful, but not nearly as much as charges in motion.  As you recall, electrons are the most easily moved particles.  Recall the Galvani experiments discussed in class: https://www.youtube.com/watch?v=o8zNSzbjRLI https://www.youtube.com/watch?v=sJifWqUa2pY So, electrons are moved by the chemistry of the electrochemical/voltaic cell (originally called the "electric pile" and now simply, battery).  Here are pictures/diagrams of Volta's original batteries: There is a chemical reaction between the electrolyte paste (often an acid) and the two metals (often zinc and carbon).  Electrons are given up by one metal (zinc) and accepted by another (carbon or copper).  The motion of electrons is called "current", but we usually imagine that it is positive charge moving (to keep the numbers positive). We can't see electrons, but we can cer

1.  What exactly *is* charge?  How do we think of it?  How does this relate to protons and electrons, etc.? 2.  Why is it that electrons are the easiest particles to manipulate? 3.  What does atomic number (Hydrogen = 1, Helium = 2, etc.) mean? 4.  What are quarks? 5.  Coulomb's law is an "inverse square law" - what does this mean exactly? 6.  Why can a charged balloon stick to a wall? 7.  What is "grounding"? 8.  Recall the demonstration where I charged up the small suspended piece - what was I showing?

Charge.... - as fundamental to electricity & magnetism as mass is to mechanics Charge is a concept used to quantitatively relate "particles" to other particles, in terms of how they affect each other - do they attract or repel?  If so, with what force? Charge is represented by letter Q. The basic idea - likes charges repel (- and -, or + and +) and opposite charges attract (+ and -). Charge is measured in units called coulombs (C).  A coulomb is a huge amount of charge, but a typical particle has a tiny amount of charge: - the charge of a proton is 1.6 x 10^-19 C.  Similarly, the charge of an electron is the same number, but negative, by definition (-1.6 x 10^-19 C).  The negative sign distinguishes particles from each other, in terms of whether or not they will attract or repel.  The actual sign is arbitrarily chosen. The charge of a neutron is 0 C, or neutral. But what IS charge? Charge is difficult to define.  It is property of particles that describes how partic

Answers will appear at the bottom.  Thanks for playing today. 1.        What is the difference between mechanical waves and EM waves? Mechanical waves require a medium Mechanical waves travel at the speed of light EM waves travel at the speed of sound EM waves cannot travel through a medium 2.        What is the frequency of a note that is one octave above 50 Hz? 51, 75, 200, 100 3.        Which is faster, the speed of light or the speed of sound? Speed of sound Depends on the type of light Speed of light They are equal 4.        WTMD is found at 89.7 on the FM dial. What does this number represent? Speed Frequency Wavelength It has no meaning 5.        To find the frequency of the immediately adjacent key on the piano, multiply the frequency by: 2, 4, 1, 1.059 6.        What does this image represent?   (Flashlight hitting mirror) Refraction Reflection Diffraction Polarization 7.        Which region is more optically