# Physics 101 Lab Report – Ohms Law Experiments

Introduction

This lab experiment was conducted to test Ohm’s Law by examining a circuit with different resistance through the use of a simple resistor and a rheostat. The experiment’s main concept is to determine how voltage and frequency affect the overall resistance. The impact of voltage on the current that goes through a light bulb was determined and compared to the impact of the current that passed through a resistor. The current and voltage were measured and then graphed. The experiment was aimed at answering the question: what is the relationship between voltage and current across a resistor? The set for the lab experiment was a basic circuit connected to a 100Ωs resistor for the first experiment and another circuit set up with a light bulb rather than using a 100Ω resistor.

Procedure

Ohms law was used in the experiment to predict the graph of the relationship between the voltage and the current that flow through the light bulb and the resistor.

The graph for the current vs. voltage resistor was projected to be a straight line with a positive slope. This is because the resistance is regarded as constant. At the same time, the graph for voltage vs. current flowing through the light bulb was projected to be a relatively straight line with a positive slope slightly curving upwards. This would indicate inconsistent resistance. Before the lab experimented was started, the two assumptions were made. In the first assumption, the voltage passing through the circuit is projected to remain constant in each of the trials. But in the second assumption, some current would be lost as it moves through the light bulb to generate light and heat.

In the lab experiment, a model was developed to test the resistance of a light bulb, and resistors took the shape of Figure 1 below:

Fig. 1: Placement of the light bulb/resistor, ammeter, voltmeter, and power supply (battery pack).

A resistor of  was put on the circuit. The voltage on the power supply was adjusted in each trail and the reading recorded. The same was done for the current. However, the current supply did not exceed 0.2 Amps. When the ten trials were completed, the resistor was removed and to be replaced by the light bulb. The voltage of the power supply for the light bulb was also adjusted in each trial and result recorded. The voltage did not go above 5V.

Data

Data for Experiment 1

 Trial # Voltage (V) Amps (mA) Experimental Resistance (ohms) Accepted Resistance (Ohms) Current (I) Percent Error 1 4.93 50.0 98.60 100.0 0.050 1.40% 2 2.59 26.3 98.47 100.0 0.025 1.53% 3 2.28 23.1 98.70 100.0 0.023 1.30% 4 2.01 21.0 98.50 100.0 0.020 1.50% 5 1.73 17.5 98.86 100.0 0.017 1.14% 6 1.62 16.5 98.10 100.0 0.016 1.90% 7 1.45 14.7 98.60 100.0 0.015 1.40% 8 1.14 11.7 97.40 100.0 0.012 2.60% 9 1.00 10.2 98.00 100.0 0.010 2.00% 10 0.74 7.50 98.67 100.0 0.007 1.33%

Fig. 2: Graph of voltage and current recorded from 1o different trials for a resistor

Experiment 2 Data

 Trial # Voltage (V) Amps (mA) Experimental Resistance Current (I) 1 4.71 179.4 26.25 0.180 2 1.92 108.6 17.70 0.110 3 1.35 89.8 15.00 0.090 4 0.90 72.0 12.40 0.070 5 0.45 51.7 8.70 0.050 6 0.33 45.7 7.22 0.045 7 0.22 40.2 5.47 0.040 8 0.17 36.6 4.64 0.037 9 0.10 29.2 3.40 0.029 10 0.08 24.7 3.24 0.025

Fig 3: Graph of voltage vs. current for the Light Bulb

Analysis

Fig 2 is a graph of the voltage vs. current for the resistor. From the graph, it is evident that the resistance is constant for the resistor even when the voltage supplied to the circuit is changed.

In Fig. 3, the graph of voltage vs. current for the light bulb is displayed, and it appears that the resistance is inconsistent throughout the trials when a light bulb is used. This can be explained by a line that curves slightly upwards. The resultant graphs of voltage vs. current for a light bulb and resistor match the projections made before the experiment.

The resistor exhibits Ohmic behavior due to the linear relationship between the resistance, voltage as well as the amps, which are explained by the Ohm’s law. The data indicated by the experiment were close to the nominal 100Ω range because the values were close enough to the experimental data.

The current that flows through the light bulb when the voltage across it is zero is four amps and indicated in the graph.

To find the maximum power, this equation is used W = IV.

6.82*4.71 = 32.12 W

32.12 is considered the power when the light bulb started glowing.

The resistance of the light bulb changes throughout the experiment. This means that the light bulb is not ohmic. The maximum resistance value for its resistance would be 3.19 ohms, and the minimum resistance would be 26.14.

In the experiment, an error factor of  .01 amps existed for the current that was measured in every trial. At the same time, an error of  .01 volts existed for the voltage recorded in every trial. An increase of the voltage to a level too high may have resulted in an additional error. When the resistor was first plugged into the circuits, there were some smoke elements from the resistor since the voltage was too high. The smoking may have made the circuit inefficient because the resistor may have been slightly damaged. At the same time, it can be noted that the light bulb was kept on for very long. This may have caused an error because the longer it was kept on, the warmer it was developing. This may have affected the efficiency of the current that moved through the circuit since some of the energy was used to generate heat.

Conclusion

From the experiment, it was noted that both the resistor and the light bulb had different characteristics. For the resistor, the resistance remained relatively consistent through the trials while for the light bulb, the resistance kept changing with every trial since a different amount of energy left the circuit with every increase in voltage through the generation of light and heat. Resistors can, therefore, be considered as accurate models for electrical energy transfer. For a wide range of voltage, they are more consistent with resistance. As long as resistance is constant across a device, different values of voltage and current can be applied. It is also noted that resistance changes with temperature. This explains the inconsistent resistance in the light bulb. In an ohmic device, the current remains proportional to voltage when the temperature is constant, which is not the case in a light bulb because of its temperature changes. Therefore, Ohm’s laws and the relationship between voltage and resistance only apply to certain devices. In different devices, the relationship between resistance and voltage is different.