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.

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

Magnetism questions

Magnetism

1. In general, what causes magnetism?

2. What is electromagnetism?

3. What is the peculiarity involving magnetic north?

4. How would you find true north?

5. What is a motor and how does it basically work?  How does it differ from an engine?

6. What is electromagnetic induction?

7. What is a generator?

8. What is a transformer?

9. How do compasses respond to magnetic fields?

Exam topics

Folks:

The topics for the final exam are:

electrical charge
proton, neutron, electron, quark - particles
atomic number and elements
charging things - what happens
voltage
current
resistance
units of the V, I and R
series circuit
parallel circuit
basics of circuits
bulb brightness predictions
V = I R
basic electrical schematics (and symbols)
magnetism
electromagnetism
electromagnetic induction
motors
engines (the very basics of a 2-stroke engine)
compasses
finding north
generators (vs. motors)
microphone

Motors are not engines - some engine info here

This depicts a "two stroke" engine, typically found in small gas-powered devices:  mopeds, chainsaws....


http://science.howstuffworks.com/transport/engines-equipment/two-stroke1.htm

http://science.howstuffworks.com/transport/engines-equipment/two-stroke2.htm

The more complicated case of the 4-stroke engine:

http://auto.howstuffworks.com/engine1.htm

http://auto.howstuffworks.com/engine2.htm

More info, FYI:

http://auto.howstuffworks.com/engine4.htm

Magnetism images

Magnetism notes

Some ideas from the Magnetism classes:

Similar to the case of charge, magnetic poles are divided into North and South poles.

A North magnetic pole is one that points toward the Earth's magnetic north pole.  This means that the Earth's magnetic north is ACTUALLY A SOUTH POLE (magnetically speaking).

Also:

- Like poles repel
- Opposite poles attract
- Each magnet must have at least one North and one South pole (though they may have more than one of each).  There is NO such thing as a magnetic monopole.
- Magnetic fields are real, but the lines are imaginary - Field lines indicate the direction that a compass needle would take in the vicinity of the magnetic field.

Magnetic north on the Earth is near Ellesmere Island in Northern Canada, several hundred miles from true (geographic) North (the North Pole).  It is moving toward Russia at several miles per year.

For gory detail:

http://en.wikipedia.org/wiki/North_Magnetic_Pole

To find True/Geographic north, it is easiest to find Polaris (the current north star).  Polaris is actually not all that bright, though in the top 50 brightest stars in the night sky.  You need to find the Big Dipper (asterism at the rear end of Ursa Major).  Follow the “pointer stars” at the end of the dipper.  These visually lead you to Polaris.  [If you were to follow the “arc” of the handle, you’d come to a bright star, Arcturus – “Follow the arc to Arcturus.”]

FYI:

http://skymaps.com/

How do we get magnetism?

Magnetic fields are related to electrons spins.  Electrons act like tiny magnetic  spinning tops.  There is a tiny magnetic element associated with each electron spin.  If the spins align, more or less, the object is said to be somewhat magnetic.  More spin alignments (domains) means more magnetism.  Materials that do this easily are generally said to be ferromagnetic.  

As it happens, metals do this best (free electrons).  In the core of the Earth, molten metal convects (rises and falls), giving the Earth a good magnetic field – measurable from the surface and beyond.  Several planets have magnetic fields.

In general, the motion of charges leads to magnetic fields.  If you have charge traveling through a wire, electrons can be thought of as moving together – this causes a magnetic field, also known as electromagnetism.  The magnetic field caused by a current passing through a wire is often small, but if you coil the wire upon itself, the magnetic fields “add up”.  Several hundred turns of wire (with current running through it) can produced quite a strong electromagnet. 

A coil with current running through it can naturally react to a permanent magnet – if this is engineered well, we have a motor.  See illustrations and demos in class.

Electromagnetic Induction

Current causes magnetism – something shown in the early 19^th century by Hans Oersted.  As it happens, the reverse is also true – magnetism can cause current, but there must be some relative CHANGE in the magnetic field or location of conductor.   There must be relative change – either coil or magnet must move, relative to the other.

This phenomenon, wherein a change in magnetic field relative to a conductor, generates electric current is called “electromagnetic induction.”  It is the secret to understanding generators.  If something, say moving water from Niagara Falls, can cause a coil of wire (in a turbine) to spin, current is generated.  More spins of wire means more current.

It’s all about moving conductors in magnetic fields

In conclusion:

Electromagnetism:

Current (moving charges) à  Magnetic Field

Electromagnetic Induction:

Change in magnetic field (through conductor), or vice versa à electric current

Circuit problem answers

Once again, I send my sincere apologies for missing class last night.  With a tree and downed power lines trapping my car onto a parking lot (accessible by only one road), I had no choice but to wait it out.  As I told one particularly upset student, it was not frivolous - as it was not when I was sick (and without voice) a few weeks ago.  Please feel free to chat with me if you have other concerns.

We'll spend the next two classes discussing magnetism, electromagnetism and electromagnetic induction.

I mentioned the forming of a study group.  To date, I have only heard from one student.

Circuit problem answers:

1.  See notes for the distinction.

2.  Voltage (in volts, V), current (in amperes, A), resistance (in ohms, W), power (in watts)

3. V = I R (though it only applies to "ohm-ic" devices, not to things like thermistors, for example).

4. 4.5 A

5. Series - constant current, voltages split.  Parallel - constant voltages, current splits.  Recall demonstrations from class.

6. A complete path for charge to travel, generally from a positive side of a voltage source to its negative side.

7. brightness goes down

8. Remains the same.

9. Parallel brighter

10. See notes.

11. Series

12. Water analogy is ok

13. Series - bulbs go out.  Parallel - bulbs remain lit.