Circuit constructor (to play with)

https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

Go to this site, hit download, and then run the program.  Build some practice circuits and use the voltmeter tool (in parallel with any resistor(s) and the ammeter tool (in series) to "measure" currents and voltages.  It's fun - try it!

I have found that this applet is easier to use with a mouse than a trackpad.

HW for E for Wednesday

Use the newly derived formula for Resistors in parallel to hep solve these problems.

1.  What is the total resistance of:

A.  Two 10-ohm resistors in parallel

B.  Three 12-ohm resistors in parallel

(Notice anything convenient about these answers?)

C.  One 4-ohm and one 12-ohm resistor in parallel

2.  A 4-ohm resistor is in series with a 14-ohm resistor.  These are in series with a pair of resistors (3 and 6 ohms) that are in parallel with each other, and a 60-V battery.  Find:

A.  The total resistance of the circuit

B.  All currents and voltages - draw a table to make this a bit easier

E block - for HW Friday / A-block - for HW Monday

Please try the following question:  3 resistors (10, 20, and 30 ohms) are in parallel with a 50-V battery.  Find the following:

- all currents
- all voltages
- total current
- total resistance

Also, try the last problem (combination circuit, #3) on the page handed out in class.  This will be tricky.  See how far you can get with it.

HW for E (Wednesday) and A (Thursday)

Graph I versus R.  Does this graph make sense?  Does it fit an expected mathematical model or equation?

Also, answer these:

Consider a 20-ohm resistor in series with a 12-V battery.  Draw a schematic to represent this and determine the current through the resistor.

Lastly, find definitions and/or pictures of series and parallel circuits.

HW for E and A (Tuesday)

Define the following:

Current

Resistance

HW (for Thursday E and Friday A classes)

Electrostatics homework – to play around with; NOT to be collected

1.  Define charge.

2.  Explain why a charged balloon will stick to a (neutral) wall.

3.  What is the charge (in coulombs) of a electron?

4.  How many protons does it take to make 15 coulomb of charge?

5.  In any atom, which particle(s) are fundamental and which are composite (made of smaller particles)?

6.  You have two clusters of charge:  10 C and 20 C, separated by 1-m of distance.

a.  Use Coulomb’s law to calculate the force that exists between the charges.

b.  Is this force attractive or repulsive?

c.  If you were to quadruple the distance between the charges, what exactly would happen to the amount of force between the charges?

7.  Carbon is element number 6.

a.  What does the 6 represent?

b.  What do you suppose is the difference between Carbon-12 and Carbon-14?

8.  Why do you think that electrons orbit protons (and not the other way around)?

>

Sites from class:

http://www.falstad.com/vector3de/

http://www.falstad.com/emstatic/index.html

Introduction to Electricity

Charge!

Charge

- as fundamental to electricity & magnetism as mass is to mechanics

Charge is a concept used to quantatively related "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 particles interact with other particles. 

In general, the terms are negative and positive, with differing amounts of each, quantified as some multiple of the fundamental charge value (e):

e = 1.6 x 10^-19 C

That's hard to visualize, since a coulomb (c) is a huge amount of charge.  One coulomb, for example, is the charge due to:

1 coulomb = charge due to 6.3 x 10^18 protons

A typical cloud prior to lightning may have a few hundred coulombs of charge - that's an enormous amount of excess charge.

If the charge is negative (-), the excess charge is electrons.

If the charge is positive (+), the excess charge is protons - however, we can NOT easily move protons.  That usually takes a particle accelerator.  Typically, things are charged positively by REMOVING electrons, leaving a net charge of positive.

Other things to remember:

Neutral matter contains an equal number of protons and electrons.

The nucleus of any atom contains protons and (usually) neutrons (which carry no charge).  The number of protons in the nucleus is called the atomic number, and it defines the element (H = 1, He = 2, Li = 3).

Electrons "travel" around the nucleus in "orbitals."  See chemistry for details.  The bulk of the atom is empty space.

Like types of charge repel.  Opposite types of charge attract.

The proton is around 2000 times the mass of the electron and makes up (with the neutrons) the bulk of the atom.  This mass difference also explains why the electron orbits the proton, and not the other way around.

Protons in the nucleus of an atom should, one would imagine, repel each other greatly.  As it happens, the nucleus of an atom is held together by the strong nuclear force (particles which are spring-like, called gluons, keep it together).  This also provides what chemists called binding energy, which can be released in nuclear reactions.

COULOMB'S LAW

How particles interact with each other is governed by a physical relationship called Coulomb's Law:

F = k Q1 Q2 / d^2

Or, the force (of attraction or repulsion) is given by a physical constant times the product of the charges, divided by their distance of separation squared.  The proportionality constant (k) is used to make the units work out to measurable amounts.

Note that this is an inverse square relationship, just like gravity.

The "big 3" particles you've heard of are:

proton
neutron
electron

However, only 1 of these (the electron) is "fundamental".  The others are made of fundamental particles called "quarks""

proton = 2 "up quarks" + 1 "down quark"
neutron = 2 "down quarks" + 1 "up quark"

There are actually 6 types of quarks:  up, down, charm, strange, top, & bottom.  The names mean nothing.

Many particles exist, but few are fundamental - incapable of being broken up further (so far as we know).

In addition, "force-carrying" particles called "bosons" exist -- photons, gluons, W and Z particles.

The Standard Model of Particles and Interactions:

http://www.pha.jhu.edu/~dfehling/particle.gif

practice problem answers

1.

a.  convex (+f)
b.  75 cm
c.  real
d.  -1.5
e.  larger
f.  upside-down
g.  within f

2.

a.  5.2 E 14 Hz
b.  1.7 E 8 m/s
c.  21 degrees
d.  same as a
e.  3.2 E -7 m (or 320 nm)
f.  33.7 degrees

Interference, diffraction, and holography

http://www.falstad.com/ripple/


Interference

Consider 2 waves meeting each other in the same space.  Their energies (amplitudes) can add or subtract.  This phenomenon is called interference.  If you've ever added sine waves on a calculator before, the effect is similar.

Crests can add to other crests, or cancel with troughs.  However, it is usually some combination (depending on the waves in question).  And often, beautiful "interference patterns" can result.

Diffraction is the phenomenon wherein light waves pass through small openings - the openings cause "new" waves to form, and these "new" waves interfere with each other.

Diffraction and Holography

Holography

Holography is a direct application of interference patterns - indeed, it is the recording of an interference pattern on film, reconstructed with a laser.

Holography is an interference phenomenon caused by two beams - a reference beam (coming from a laser), and an object beam (which reflects off the object).  This interference pattern is burned into the film emulsion of the holographic film.  It can be reconstructed when light passes through it again.

Test practice

1.  Consider a lens with a +30 cm focal length.  An object is 50 cm from the center of lens.  Find the following:

a.  type of lens
b.  location of image (di)
c.  type of image (real/virtual)
d.  magnification of image
e.  whether image is larger or smaller than original object
f.  whether image is upside-down or rightside-up
g.  where should you place the object so that you ONLY get virtual images

2.  Review from the older optics stuff:

Consider a rectangular plastic box (n = 1.8).  Light of wavelength 575 nm is sent into the block at a 40-degree angle (with respect to a normal line).  Find the following:

a.  frequency of light outside box
b.  speed of light inside box
c.  angle of light inside box
d.  frequency of light inside box
e.  wavelength of light inside box
f.  critical angle of this plastic

Upcoming deadlines

E block:

5/8 (Monday):  lab draft due

5/10 (Wednesday):  Optics test

5/12 (Friday):  Final lab due


A block:

5/9 (Tuesday):  lab draft due\

5/11 (Thursday):  Optics test

5/15 (Monday):  Final lab due

Lens simulations

https://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html

https://graphics.stanford.edu/courses/cs178-10/applets/thinlens.html

http://www.physicsclassroom.com/Physics-Interactives/Refraction-and-Lenses/Optics-Bench/Optics-Bench-Refraction-Interactive

https://simbucket.com/lensesandmirrors/