Final exam topics / Final exam is Thursday (May 17) at 5:30.
What happens when light hits lenses and mirrors
Convex and concave lenses
Convex and concave mirrors
Plane mirrors
Real and virtual images
Focal length
Total internal reflection - fiber optics
Basics of the eye and camera
Basics of interference and holography
Static electricity - likes repel, opposites attract
Fundamental charge
Coulomb's law (and how an "inverse square law" functions)
Electrons, protons, neutrons, quarks
Current
Voltage
Resistance
Basic circuits (and schematics)
V = I R (Ohm's Law)
Series circuit
Parallel circuit
Magnetism - natural (magnetite, Earth)
Compasses
Electromagnetism
Electromagnetic induction - generators
Tuesday, May 15, 2012
Friday, May 11, 2012
final exam etc.
My apologies for missing yesterday's class - an emergency happened. It couldn't be helped.
During our final class (Tuesday) I will discuss magnetism (electromagnetic induction) for part of the class, and take some time for review.
Our final exam is Thursday, 5/17, at 5:30 PM.
During our final class (Tuesday) I will discuss magnetism (electromagnetic induction) for part of the class, and take some time for review.
Our final exam is Thursday, 5/17, at 5:30 PM.
Tuesday, May 8, 2012
Magnetism
Some ideas from the Magnetism sessions
A North magnetic pole is one that is attracted to the Earth's magnetic north pole. This means that the Earth's magnetic north is ACTUALLY A SOUTH POLE (magnetically speaking).
Like poles repel.
Opposite poles attract.
Each magnet MUST have at least one north and one south pole.
Magnetic fields are real, though the field lines are imaginary. Field lines indicate the direction that a compass needle would take in the vicinity of the magnet.
Magnetic north on the Earth is near Ellesmere Island in Northern Canada, several hundred miles from true (geographic) North - the North Pole.
For gory detail:
http://en.wikipedia.org/wiki/North_Magnetic_Pole
To find true/geographic North, it has historically been done by following Polaris (the pole-star, the lodestar, the North star). Polaris is actually not all that bright (though in the top 50). You need to find the Big Dipper (the rear end of Ursa Major) and follow the two pointer stars at the end of the scoop - these point to Polaris (which is in Ursa Minor). [If you were to follow the "arc" of the handle, it would take you to the bright star Arcturus, as in "follow the arc to Arcturus".]
FYI - "Star Hopping" is a great way to learn your way around the sky. I like the free star chart site:
http://www.skymaps.com/
Magnetic fields are related to electron spins. Electrons act like miniature (extremely miniature) spinning tops. There is a magnetic element associated with their spins. If spins align, more or less, an object can be said to be somewhat magnetic. More spin alignments (domains) means more magnetism. Materials that do this well are said to be ferromagnetic.
As it turns out, metals do this best, as they often have 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 travel through a wire, electrons can be thought of as moving together - this causes a magnetic field. The magnetic field generated 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 them) can produce quite a strong electromagnet. Understanding electromagnets allows us to understand motors.
Current causes magnetism - something shown in the earth 19th century by Hans Oersted. As it happens, the reverse is also true: magnetism can causes current, but it must be a CHANGE in the magnetic field. The magnet (or conductor/coil through which it travels) must move. There must be some relative change between coil and magnet - either the coil must move or the magnet must move.
This is referred to as electromagnetic induction, and it is the secret to understanding generators. If a coil moves in a magnetic field, a current is generated inside it. Imagine turbines at the bottom of Niagara Falls (or any waterfall) - water hits the turbine and makes it spin. Inside the turbines are large coils of wire, free to rotate. These coils spin within a permanent magnetic field inside the turbine - this generate large amounts of current. Similarly, you can burn fossil fuels to heat water and generate steam. The steam is fed into a turbine, also generating large amounts of current.
It's all about moving conductors in magnetic fields!
Monday, May 7, 2012
Circuit questions
1. What exactly is voltage, current and resistance?
2. What is Ohm's law?
3. Consider a 9-V battery connected to a 6-ohm resistor. What is the current through the resistor?
4. Consider 2 batteries (1.5 V each). They are connected in series to 2 resistors (10 and 30 ohms). Find the current through each resistor.
5. In the problem above, what would happen if one of the resistors was removed from the circuit?
2. What is Ohm's law?
3. Consider a 9-V battery connected to a 6-ohm resistor. What is the current through the resistor?
4. Consider 2 batteries (1.5 V each). They are connected in series to 2 resistors (10 and 30 ohms). Find the current through each resistor.
5. In the problem above, what would happen if one of the resistors was removed from the circuit?
Thursday, May 3, 2012
Electrical current
Thus far, we have discussed static charges. Static charges alone are useful, but not nearly as much as charges in motion. As you recall, electrons are most easily moved. However, for sake of ease in sign convention (keeping things positive, where positive), we define the following:
current (I) - the rate at which positive charge "flows"
The unit is the coulomb per second, defined as an ampere (A). One ampere (or amp) is a tremendous amount of current - more than enough to kill a person. In fact, you can feel as little as 0.01 A. Typical currents in a circuit are on the order of mA (milliamperes).
We also define other new quantities in electricity: voltage, resistance, power
voltage (V) - the amount of available energy per coulomb of charge
resistance (R) - the amount by which the voltage is "dropped" per ampere of current
You can also think of resistance of that which "resists" current. Typically, resistors are made of things that are semi-conductors (they conduct current, but less well than conductors, and better than insulators). Resistors are often made of carbon, but can also be made of silicon and other materials. The unit is a volt per ampere, defined as an ohm (Greek symbol, omega).
A convenient way to relate all the variables is embodied by Ohm's Law:
As in, "Twinkle, twinkle little star, V is equal to I R."
Well, it works for me
current (I) - the rate at which positive charge "flows"
I = Q/t
The unit is the coulomb per second, defined as an ampere (A). One ampere (or amp) is a tremendous amount of current - more than enough to kill a person. In fact, you can feel as little as 0.01 A. Typical currents in a circuit are on the order of mA (milliamperes).
We also define other new quantities in electricity: voltage, resistance, power
voltage (V) - the amount of available energy per coulomb of charge
V = E/Q
resistance (R) - the amount by which the voltage is "dropped" per ampere of current
R = V/I
You can also think of resistance of that which "resists" current. Typically, resistors are made of things that are semi-conductors (they conduct current, but less well than conductors, and better than insulators). Resistors are often made of carbon, but can also be made of silicon and other materials. The unit is a volt per ampere, defined as an ohm (Greek symbol, omega).
A convenient way to relate all the variables is embodied by Ohm's Law:
V = I R
As in, "Twinkle, twinkle little star, V is equal to I R."
Well, it works for me
What exactly *IS* a circuit?
An electrical circuit can be thought of as a complete "loop" through which charge can travel. Therefore, it actually has to be physically complete - there can be no openings. That is, the current actually has to have a full path to take.
But there is an exception:
If the supplied voltage is high enough, charge can "jump" an "open circuit." This is clearly a dangerous situation, and one way in which a person can get shocked. Think of the unfortunate situation of sticking your finger (or a paper clip, etc.) into an electrical outlet (or something like a toaster, for that matter). You would "bridge" the circuit, becoming in effect, a resistor.
That's bad.
But there is an exception:
If the supplied voltage is high enough, charge can "jump" an "open circuit." This is clearly a dangerous situation, and one way in which a person can get shocked. Think of the unfortunate situation of sticking your finger (or a paper clip, etc.) into an electrical outlet (or something like a toaster, for that matter). You would "bridge" the circuit, becoming in effect, a resistor.
That's bad.
more questions
Electrostatics questions
1. What is the difference between an electron and a proton: in terms of charge? In terms of mass? In terms of position in an atom?
2. By "charging" something, what is usually happening? If the object is becoming negatively charged? Positively charged?
3. What is the charge of a proton? An electron?
4. Explain the rotating meter stick demonstration (similar to how a balloon sticks to a wall).
5 What is a Van de Graaff generator?
6. What do you suppose occurs when lightning strikes?
7. What happens to the electrostatic force between two charged objects if the distance between them is doubled? Tripled? Halved?
8. What do we mean by "fundamental" charge? What charges are fundamental?
2. By "charging" something, what is usually happening? If the object is becoming negatively charged? Positively charged?
3. What is the charge of a proton? An electron?
4. Explain the rotating meter stick demonstration (similar to how a balloon sticks to a wall).
5 What is a Van de Graaff generator?
6. What do you suppose occurs when lightning strikes?
7. What happens to the electrostatic force between two charged objects if the distance between them is doubled? Tripled? Halved?
8. What do we mean by "fundamental" charge? What charges are fundamental?
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The last 2 images are PARALLEL CIRCUITS. Here, current has multiple paths to take, so the total resistance of the circuit is actually LESS than if the resistors were alone or in series with other resistors. Since the bulbs are connected equally to the battery, they experience the same as the battery voltage - they are, therefore, of equal brightness (and the same brightness they would have if there were only ONE bulb connected). Of course, bulbs in parallel draw more current and they cause a battery to die sooner.
What I've written above is primarily geared toward identical bulbs. In series, add up the resistances to get the total resistance. In parallel, it is more complicated. There is a formula one can use (1/Rp = 1/R1 + 1/R2 + ...), but we will only concern ourselves with the case of identical resistors in parallel. In that case, divide the value of the resistor by the number of resistors to get the total effective resistance. For example, two identical 50-ohm resistors in parallel is the same as one 25-ohm resistor. This seems strange, but it's a little like toll booths - when one toll booth is open, it can get crowded (the current is small). With multiple toll booths open, the resistance is effectively less, so the current can be greater.