Lab Report Day 12 - Lighting a Bulb, Charge Detector, Ammeter, Ohm's Law

1. Light a Bulb
In the beginning of the class, we watch a video of MIT engineering students being asked to make a bulb light using a battery, a wire, and a bulb. They all cannot do it. We then use a battery, a bulb, and a piece of wire to try to make the light bulb. We were then asked to do the same. 

We have four different situations, which two of them work and the other two do not. We get a conclusion that in order to make a bulb to light, we need a power supply/battery, having current flow, a closed circuit, not too high resistance, and a conductor . The bulb lights up when the bottom of the bulb touches the battery or the wire, like the left two pictures. In the right two pictures, the bottom of the bulb is not touching. 

We are then asked to use two batteries and we can see that it produced a light that was twice as bright. We just need to double the power supply. We put two battery in series which makes the voltage of it doubled. The two time difference in current flow results in the bulb twice brighter.

We analyze how the energy works when lighting up a bulb.

We define the different in electric potential to be voltage, and the flow rate to be the current. 

We now have the equation for power. 

2. Charge Detector

This is what a charge detector looks like. Using the charge detector and a couple other materials, Professor Mason show us how the charge detector works.

[VIdeo 184]

First, Professor Mason rubs a balloon with his head and brings it near the charge detector and the light goes to yellow. What the red light tells us is that there is a negative potential near by and when the positively charged balloon is brought in, it can be seen in the video below that the light changes from red to yellow.

Afterwards, Professor Mason takes a battery and brings in the positive end of the battery to the charge detector and the light remains red. Because the light did not change, it is clear that the charge detector was not detecting any charge at all. This tells us that the overall charge of the battery is zero.

3. Ammeter

At first, we find that the current in a series circuit is the same for everywhere.

An ammeter can measure the current in a circuit. It measures the current flowing through different parts of a simple circuit . First we created a simple circuit and the current is above 200mA. After measuring each part of the circuit, we noticed that the current remained roughly the same, around 200mA. No matter where each object was, the current remained the same. This proves that in a series circuit, the current is the same for everywhere.

We derive the unit for charges. 

4. Ohm's Law

In order to do Ohm’s Law Lab, we need power supply, current meter, coil, power supply for set up.

In this part of the lab, the circuit has a voltmeter and current meter and a resistor. We then apply varying currents and changing the voltage across the coiled resistor.

By using LoggerPro, from graph of V vs. I, it shows a perfect linear relationship between current and voltage. It makes sense because when we have a constant , V=IR. V is proportional to I. It proves the direct relationship between volt and current with equation V=IR.


With the linear relationship from voltage and current, we get an equation y=mx+b which y is the voltage (electric potential). b is initial potential, x is the current. m, which is the slope is the ratio of voltage over current, is the resistance.  From the graph, we can know that the Resistance of this wire is about 7.26 Ω.

Since we find the relation between V, I, R to be R=V/I, we are going to determine how the thickness of a wire could affect the resistance. 

The set up to see how thickness of a wire affect the resistance of a wire.

We get 4 different wires.

The circuit is hooked up to different wires with different thicknesses. The slopes of the graphs (V against I) become smaller when the wire becomes thicker. The smaller slope means that the resistance is less. Since a thicker wire allow more electrons to pass through with a bigger cross section area, the resistance of the wire will be smaller.

From the equation, R=ρ*L/A.  Resistance is related to the resistivity of different materials, the length of the wire, the area of the cross area. From the table above, we can tell that the resistivity varies with different materials.

Summary:

In today’s class,  we learn how to build a close circuit which a bulb can light up with only one wire, one battery, and one light bulb. We also know that when the electric potential (voltage) is doubled, the bulb will be twice lighter. We learn how to use a charge detector and how to use an ammeter to detect the current. We learn the relationship between P(power), I(current), and V(voltage) to be P=VI. At last, we learn Ohm’s Law, V=IR.