PS304 TOPICS

Box 11

Rubbing plastic rod with fur caused a build-up of what we termed electrical charge on the rod which can be checked with the electroscope. We infer that identical charge tends to repel itself.

However, if we charged the rods with different materials, they would sometimes be attracted. Therefore, there must be at least two types of charge.

There was also an attraction between a charged object and an uncharged object. A repulsion was never observed. We concluded that all matter does contain charge and came up with the picture of the normally balanced charges in the uncharged object separating.

Magnets do not register on the electroscope. They are attracted by a charged object like any other material though.

Box 12

Is current lost after it flows through a lightbulb? No.The sum of the currents in each of the branches equals the total coming from the battery.More current flows in the circuit with two parallel lightbulbs as opposed to the circuit with one lightbulb.Less current flows in the circuit with two lightbulbs in series as opposed to the circuit with one lightbulb.Current can split, but the total remains the same.

Resistors in series: Total resistance is sum of individual resistances. Each resistor carries the total current, but they share the voltage.Resistors in parallel: Total resistance is less than either individual resistance. Each experiences the total voltage, but they share the total current.Battery's voltage is completely used up after traversing the circuit.Batteries in series give you a total voltage which is the sum of the two.Batteries in parallel give you the same voltage, but each will deliver only a share of the total current in the circuit.

Water analogy.

We observed that increase the voltage increased current, while increasing resistance decreased current. Thus we are led to the equation I = V / R. (Ohm's Law)Ohm's law works in each branch of a parallel circuit independently. It also holds for the whole circuit. More precisely, the voltage used for the V in Ohm's law is the voltage lost or consumed by the resistor in question.

Power is the rate of energy flow or consumption (units: joules per second). We measured this qualitatively by turning the hand generator. It was very easy to turn when not connected to anything, since then we are not providing any energy. We found that it was easier to turn when connected to a larger resistance. Using this observation, and matching units, we found that the formula P = V2 / R would work. By Ohm's law, we can convert this into the equivalent forms P = I V or P = I2 R.Why do these formulae make sense?

Capacitors can be charged and will hold electrical energy. As they charge, the voltage across them increases until it matches that of the battery, and the current gradually drops to zero.As they discharge, both the voltage and current gradually drop to zero as the capacitors empty.

Box 14

We observed three types of materials with respect to their magnetic interactions. Some generate magnetic fields, some merely respond to them, and some do nothing. This is somewhat similar to the electrical case.

By observing the interactions of different sides of several magnets, we deduced that similar poles attract and opposites repel.

Magnetic fields in every case that we have observed have lines which reconnect and do not extend away indefinitely. Electric fields of a single type of charge may extend indefinitely.

An electrically charged object does not (necessarily) have a magnetic field, and a object with a magnetic field does not necessarily cause a response in the electroscope, that is, is not necessarily charged electrically.

We found that a current, which is charge in motion, does generate a magnetic field. We mapped out the magnetic field of a straight conductor, and a coil.

The magnetic field generated by a current is perpendicular to the current. Magnetic fields only produce forces on currents which are perpendicular to them. We can establish the direction with the right-hand rule. The rule is reversed for specifically negative currents.

Currents in the same direction attract, and vice versa.

Flux is the amount of magnetic field passing through a loop. The most general statement we can make is that a change in flux causes a current to flow in the corresponding loop. The direction will be such that the secondary magnetic field generated by the induced current will oppose whatever you are trying to do. This is how you pay for the power you are generating.

Note that having more loops increases your ability to catch flux. Transformers are two coils which are coupled through a magnetic field. Due to the dependence of flux on the number of loops, the side with more loops will have greater voltage. Power is not created or destroyed, so that side will also have less current.

Box 15

Waves are traveling disturbances in a medium. Can be transverse or longitudinal.

Frequency, speed, and wavelength are related by the equation v = f w.

Box 16

By convention, we measure all angles with respect to a line that is perpendicular to the surface in question.A beam of light will reflect off a surface with equal incoming and outgoing angles.In general, the angle on the outside of a material will be greater than the angle on the inside. This is refraction.

Clearly the greatest possible angle is 90 for any beam. Some angle on the inside will correspond to this maximum angle on the inside. Any angle beyond this point will then fail to refract, since there is no room left on the outside to bend farther.

We observed that what was initially white light appears as a rainbow after it is refracted, indicating that white light actually contains all colors, and the each color refracts in a slightly different manner.

Light rays tend to spread out from their source. That is why you do not see a clear image of everything in the vicinity on a screen. The light rays spread out and thus appear as a blur on the screen rather than a sharp image. To see a clear image, we would need to force the light rays to converge. We found that this could be achieved with a concave mirror, or a convex lens. The image is seen if the screen is placed at the point where the rays converge. In general, we need to be able to adjust the positions of the lens or screen in be able to image an arbitrary object.

Since light is transverse, it can have various polarizations. Common light sources produce unpolarized light. A polarizer allows light to pass completely only when polarized in one direction. Else it is dimmed or totally killed. We know that light is polarized after passing though. We also saw that light was polarized at least partially after reflection.

When light passed through a very small opening, we noticed that it appears to spread out into a band or bright and dark spots. This is a phenomenon called diffraction which is observed with any kind of wave. More diffraction occurs with smaller openings and to waves with a greater wavelength.