Twelfth Day

Spring 2015
9th of April's Class

The class started with the basic concepts of electricity, demonstrated through wire, battery and a light bulb. We were to experiment and draw out two possible arrangements of the set up that would allow the bulb to light up, and also two that does not.

The reason for the failure of a bulb to light up would be that the charge does not flow through the filament of the bulb. Since it received no energy, it did not heat up, and therefore unable to generate light.

We were introduced to an apparatus named electroscope. The image below shows the apparatus up close:

Professor Mason rubbed an iron rod with a pelt so that it may build up charges. He then placed it in contact with the conductor of the electroscope, circled in green. The two films of conductor within the electroscope then repelled each other.

Professor Mason then placed batteries to be in contact with the conductor, but nothing happened. This is as predicted as there is not circuit in the system, where there is an inlet and an outlet for the charges to flow through.

The way charges move around to produce electricity is very much like the way water flows. The potential energy stored within water when it is at a certain height is analogous to the voltage of electricity, and the rate at which it flows is analogous to current. These two systems also need a circular path which allows a one-directional motion for the charge or water to flow, to produce work.

We were then taught the concept of Drift Velocity, which is the flow velocity of a particle.

Professor Mason, then experimentally determined a graph of Voltage vs. Time and Current vs. Time. With them, he formulated the Voltage vs. Current graph, which was found to be linear. Although we were given a warning that in real life, materials tend to not conform to that linear relationship.

After a material is heated up beyond a certain temperature, the heat will cause the chemical properties of the material to change, causing the graph to become erratic and oddly shaped.

Professor Mason then taught us the relationship between Resistance and Length. He charged a copper wire of length 160 cm and 200 cm. The 160 cm yield 17.4 Ohm, while the 200 cm yield 21.4 Ohm.

Calculating their ratio of resistance to length, both of them yield nearly the same ratio, therefore we can conclude that the length of material do affect the resistance.

We were then asked the question: What if diameter was increased? How would it affect resistance?

Using the formula circled in pink, we can see that the area affects the Current. As the radius gets larger, the area gets larger too; and as the area increases, the current increases too. Using that in conjunction with Ohm's Law, depicted in the image below, as the current increases, resistance would decrease. Therefore, larger cross-sectional area actually leads to smaller resistance.

As Resistivity rises, Temperature also rise as the particles would collide more frequently.