Voltage, Current, and Resistance

Purpose:  In this lab, we investigate the behavior of current through a wire. We will determine how current flows through a wire. We will also determine what measurable variables factor in the flow of the current. A majority of the experimentation we be done using light bulbs, wires, an electroscope, and multimeters.

Lighting a Light Bulb

To start off, we are given the task of powering a light bulb using a battery, one wire, and the light bulb itself. The picture above demonstrates how we were able to light a light bulb. In a later picture, we will explain how this works.

Electroscope

Professor Mason introduces the electroscope. This device allows us to detect charge on an object by placing the charged object on the top sphere. The charge travels through the sphere, into the box, and inside the box lies two metal plates. The metals plate gain the same charge as the charged object that is placed on the metal sphere and this causes the metal plates to separate, thus detecting charge.

How Does a Light Bulb Light Up

In reference to the light bulb experiment above, we demonstrate how exactly the light bulb lights up. We see the on the top left, two examples are given for the proper way of lighting a light bulb. We connect a wire at one end of the battery and the other end of the wire to the side of the metal screw on the light bulb. Then, we connect the bottom end of the light bulb to the other end of the light bulb, as shown. This is done in this manner because there is a filament inside the light bulb. one end of the filament is touching the screw end of the light bulb, the other end of the filament is touching the bottom of the light bulb. We will discuss more about how this works when we investigate the battery.

Investigating The Battery

We examine what would happen if we put the positive end of the battery on the electroscope. Nothing happened. We refer to the light bulb experiment. The reason the light bulb lights up is because there is current that flows through the bulb. The reason current is able to flow through the light bulb is because the electrons in the wire become attracted to the positive charge in the battery. This causes the electron to flow in the same direction. The battery contains a finite amount of energy and this energy allows for work to be done on the bulb.

Our picture shows what would happen if we use two light bulbs instead of one. It turns out that the light bulb shines much brighter. The more light bulbs used to power light, the more energy is available; therefore, more work is done to light the bulb.

Using An Old Ammeter

We begin to wonder whether the current in the light bulb is the same before the current enters the light bulb and after the current enters the light bulb. It may appear to make sense to some that there would be less current, since some less energy is used up when work is done to power the bulb.

Our group, on the other hand, predicted that the current would be the same before and after because we we believed that whatever flows in, must flow out at the same rate. Well, we see that the work done on the light bulb does cause the light bulb to take energy out of the system, but this doesn't influence the current. We wrote that power equals the voltage time the current and we see that the power is measured in joules per second. We also see that the voltage is measured in joules per coulomb. These two entities are reliant on the energy in the system, but the current is measured in coulombs per second. This is another way to see why the current does not change when energy changes. 

Finding Charge

We discuss the 4 variables we need to find charge. The cross sectional area can be viewed as the area of the circular side of a tube. Current would flow through the tube and it is the cross sectional area that is important in finding charge. Of course, the tube's cross sectional area does not have to be circular, it is just for relatable visualization purposes. The drift velocity is the velocity of the current of the electrons. 

Proportionality Of Current and Voltage

We start to set up our next and final experiment. This experiment will allow us to evaluate the relationship between current and voltage. 

Our group says that current and voltage are proportional to each other. We find that this is in fact true, but we know that when it comes to proportionality, there is a variable to account for. We find that this variable is called the resistance. We conclude that the voltage is equal to the current times the resistance, but we later realize that this is not always true. Some materials do not follow these predicable behaviors and, therefore, are not ohmic.

What Factors Into The Resistance

We take a look at some different pieces of wire and measure their resistance. We look at simmilar behaviors in length and other physical properties. We begin to see that the physical properties, do factor into the resistance. We find that the resistance becomes greater when the length or the wire increases, but becomes lower when the thickness of the wire increases. This can be explained visually and symbolically. If we look at the relationship between resistance, voltage, and current, we find that the current is reliant on the thickness and the voltage is reliant on the length. We also factor in the constant that represents the material.

Conclusion:  Using light bulbs, wires, and multimeters, we found relationships between current, voltage, and the resistance. We were able to do what Harvard graduates could not, which is power a light bulb with a battery and one wire, and we were able to use what we learned to explain how the battery is lit. We, also, found that the ohm's law isn't always ideal for calculating resistance, so we found a different way to do it using its physical properties.