Lab Report Day 13 - Dim Bulbs and Bright Bulbs, Ohm's Law

Dim and Bright Bulbs

At first, we need to set up two different circuits, using two approximately bulbs, two approximately batteries and wires. One we need to have a dim light and the other one need to have a bright light. In this case, we can consider the bulbs as resistors.  

We make some predictions on how to make the bulb as bright (or dim) as possible. We use some symbols to represent the battery (power supply), bulb, which makes the drawing easier and easier for people to understand.

We find that in order to have a dim light, two light bulbs need to be in series and two batteries need to be in parallel, it will give the dimmest light. When we connect two batteries in parallels, the electric potential difference (voltage) does not change. However, the capacity is doubled. When we connect the two bulbs in parallels, the total resistance is less than any one of the two bulbs. It results in a larger current which makes the bulbs brighter.

In order to have a brightest light, two lights bulbs need to be in parallel and two batteries need to in series, it will give the brightest light. When we connect two batteries in series, the electric potential difference (voltage) is doubled, while the capacity is not changing. When two bulbs are connected in series, the total resistance is the addition of the two bulbs, which is bigger than any one of the two bulbs. It results in a smaller current which make the bulbs dimmer. 

Ohm’s Law

In this part of the lab, we are going to heat up the water in a water cup by putting a coil which is powered by a power supply. We are going to use LoggerPro to collect data. We can know the temperature and time from LoggerPro.

Using the voltage, current, and time, we can calculate the amount of energy put into water.

As seen in the following picture, the energy was calculated to be 24.1 kJ. Using the calculated energy and the energy equation Q=mcΔT, the theoretical final temperature is calculated to be about 53.2ºC.

However, the experimental final temperature is 42.1ºC. It is because the current and voltage keep changing during the 10 minutes period when data is being gathered.

From the derivation of Ohm’s law, we get that V=IRo + IRoα (VIt/mc). When α is small, the second half of the equation goes to 0 and we get the general form V=IR. By using the temperatures, voltages and currents from LoggerPro, we calculate the final resistance R to be 9.18 Ω. Relating resistance calculations with change in temperature, alpha, a constant value unique to different types of materials, is calculated to be 4.6*10^-4.

  

Derivation of the potential difference of a point charge.

Summary:

In today’s class, we learn the difference of a parallel and series circuit. We learn a more advanced version of Ohm’s Law. We know that resistance can change with temperature too.