## Wednesday, May 2, 2012

### Fundamental Particles

Fundamental particle is one which cannot be split into smaller components.

eg: Electron , Positron (anti particle of electron)

Quarks

Quarks undergo interactions via strong nuclear forces.
eg: proton and neutron consist of quarks.

Antiparticle
paricle with same mass and opposite charge.
-proton and nutron are not fundamental particles.
-proton and nutron are constructed from smaller particles called quarks.

u - up quark : +(2/3) e
d - down quark : -(1/3)e

proton -:uud
neutron -: udd

There are six types of quarks.
* up quarks (u)
* down quarks (d)
* charm quarks (c)
* strange quarks (s)
* top quarks (t)
* bottom quarks (b)

## Wednesday, March 16, 2011

### PARALLEL CIRCUITS

Not all the current goes through every load. There is at least one fork or branch in the circuit. In the diagram below, the current, ITOT, that leaves the source is split. Some current, I1, goes through R1 and the rest, I2
goes through R2 . The sum of the currents I1 and I2 equals the total current ITOT.
In parallel circuits current is split at least one junction.

Unlike the series circuit, adding more branches reduces resistance and increases current. This is easy to understand: Suppose a large city developed on both banks of a river and there was only one bridge across the river. If the bridge was narrow, a traffic jam would probably develop at least during rush hour. One way to reduce the resistance to traffic flow would be to build another bridge. With more bridges across the river, more traffic flows. With more branches in the circuit more current can flow, and the less resistance there is to current flow.

The voltage supplied to each branch is the same as the voltage supplied to the source. A branch with low resistance carries more current than a branch with high resistance.

In summary, for parallel circuits,
ITOT = I1 + I2 + . . . .
VTOT = V1 = V2 = . . . .
1/(REFF) = 1/(R1) + 1/(R2) + . . .

PROBLEMS

4. In the diagram of a parallel circuit above, suppose R1 and R2 were light bulbs. If the light bulb at R2 burned out, what would happen to the brightness of the light bulb at R1?
5. Two resistors, 2.0 ohm and 3.0 ohm, are connected in parallel with a 12 V battery. Calculate: (a) the effective resistance of the circuit; (b) the current leaving the battery; (c) the current through each resistor; (d) the voltage drop across each resistor.
6. In the following diagram the three resistances each represent identical light bulbs. Describe what happens to the brightness of the other light bulbs when one of them goes out.
A complex circuit consists of parallel loads in series with at least one other load.

7. Sketch a diagram showing a complete circuit involving only a dry cell, one wire, and a flashlight bulb.

### SERIES CIRCUITS

In series circuits, all current goes through every load; there are no forks or branches in the circuit. So, the current, I1, through resistor, R1, and the current, I2, through resistor R2 are the same and equal to the current, I, of the circuit.
In series circuits the same current flows through all circuit components.

Just like pinching two places in the same hose results in less flow then pinching in only one place, the resistances of resistors in series add to give the total resistance of the circuit.

In accordance with the Law of Conservation of energy, the voltage drops across each resistor (the energy used by each resistor) add up to the voltage (energy) supplied by the source.

In summary, for series circuits,
ITOT = I1 = I2 = . . . .
RTOT = R1 + R2 + . . . .
VTOT = V1 + V2 + . . . .

PROBLEMS

2. In the diagram of a series circuit above, suppose R1 and R2 were light bulbs. If the light bulb at R2 burned out, what would happen to the brightness of the light bulb at R1?
3. Two resistors, 2.0 ohm and 3.0 ohm, are connected in series with a 12 V battery. Calculate: (a) the total resistance of the circuit; (b) the current leaving the battery; (c) the current through each resistor; (d) the voltage drop across each resistor.

### OHM'S LAW

The simplest electric circuit contains a source of electric energy such as a dry cell or wall outlet, a load such as a light bulb or fan, and conductors such as wires to provide a path for electric current to flow. The resistor, a load that converts electric energy to heat, will be used in all diagrams to represent the load. A dry cell will be used to represent the energy source.

When water flows through a hose friction between the water and the impedes the flow. Pinching the hose further adds more friction and reduces the flow more. Water pressure is what is needed to keep the water moving. The more pressure, the more water flows, the more friction, the less water flow.

As current flows through the conductor, the atomic structure of the conductor provides resistance to current flow in much the same way a hose impedes water flow. The more energy the source provides, the more current flows. With more resistance, less current flows. Unlike a water hose, an electric circuit has to make a complete circle from the source through the load(s) and back to the source again or no current will flow.

Resistance, R, is measured in ohms; current, I, (the amount of charge that flows with time) is measured in amperes (A), and the energy provided per coulomb of charge provided by the source is measured in volts (V). All resistance will be attributed to the load(s); the resistance of the conductors and the internal resistance of the power source will be considered negligible.

The relationship between these quantities is given by Ohm's Law:
V = IR

PROBLEMS

1. A 12 V battery causes 2.0 A of current to flow through a light bulb. What is the resistance of the bulb?

## Tuesday, March 15, 2011

### Nucear Decay

Nuclear Fission

http://library.thinkquest.org/17940/texts/fission/fission.html

Nuclear Fusion

http://library.thinkquest.org/17940/texts/fission/fission.html

## Saturday, March 12, 2011

### Waves

1. A guitar string of length, L, is fixed at both ends. Which of the following wavelengths for standing waves is not possible on this string?
a. L/2 b. 2L/3 c. L d. 2L e. 4L

2. Pipe A is 30.0 cm long and is closed at one end, open at the other. Pipe B is open at both ends. How long will pipe B have to be to have the same fundamental frequency as pipe A?

3. Two speakers receive the same 256 Hz signal from an amplifier. When one speaker is placed directly behind the other, what is the smallest distance between them that produces destructive interference at any point front of them? (speed of sound = 342 m/s)

4. Signal m is 6.8 dB louder than signal w. What is the ratio of the sound intensity of signal m to signal w?

5. A rock is dropped from a cliff is heard striking the ground 3.4s later. If the speed of sound is 340 m/s, how high is the cliff?

6. Why is the loudness directly proportional to the logarithm of intensity?

7. The noise from a grinder in a factory is 32000 times more powerful than the noise from a dishwasher. By how many decibels is the factory noise greater than the dishwasher noise?

8. Pat stands between two high walls, at a point one-third the distance between the walls. Pat fires a starter pistol, and hears one echo one-tenth of a second after the first echo. If the speed of sound is 340 m/s, how far apart are the walls?

9. A loud sound is 90 dB. When the sound is reduced to 5% of its original intensity, what is the decibel level?

### Particle Accelerators

What is a particle accelerator and why do we use them?

Just after the Big Bang, the universe was a rapidly expanding ball of fundamental particles. As the universe expanded, it cooled and the particles decayed, changing into other fundamental particles. These particles then joined together and gradually formed the matter that we see around us today.

In particle accelerators we smash beams of particles together in head-on collisions that are energetic enough to turn the clock back to just after the Big Bang. The more energetic the collisions, the more likely we are to make fundamental particles appear again. Once we've produced fundamental particles we can study their behaviour to find out why the universe is made the way it is.
What does a particle accelerator look like?

The biggest particle accelerator in the world is at CERN, the centre for particle physics research, just outside Geneva in Switzerland.

Above ground, you wouldn’t know anything about it, but if you were to go 100m underground, you’d find yourself in a circular tunnel, about the size of a London underground tube tunnel. This is where the Large Hadron Collider (LHC), the world’s most powerful particle accelerator is being built.

The LHC tunnel runs for about the same distance as the London Underground Circle Line – 27km in a ring underneath the French/Swiss border. If you were inside the accelerator tunnel you would see a tube which runs continuously in either direction. This tube is the "beam pipe" - so-called because inside here, two beams of particles fly round the tunnel. The beams are accelerated to very high energies by magnets surrounding the beam pipe. These make sure that the two particle beams circulate in opposite directions without crashing into each other. When the beams of particles reach their final top energy, the magnets alter their path and bring them into collision at pre-determined points around the accelerator ring.

By this time the particle beams are travelling so close to the speed of light that they collide forty million times a second. Inside each collision we have a snapshot of the fundamental particles that last existed billionths of a second after the Big Bang. Now all we have to do is build gigantic particle detectors at each collision point to try and work out exactl