Resistance and Ohm's Law
8.1 Introduction
In previous experiments, we have investigated electric charges largely under stationary conditions. These studies were useful in order to illustrate concepts such as the electric potential and the electric field, and forms the foundation needed to further our understanding of electricity and electrical circuits. In contrast to electrostatics (charges confined to be stationary), the field of electricity deals with the flow (induced movement) of electrical charges. Due to its many uses, most individuals knowingly or unknowingly have a daily reliance on electricity. It is especially essential, in: (1) the distribution of energy, and (2) the processing of information. To enable this, electricity must be handled in circuits, a closed loop of conducting wire connecting power plant with individual homes, and businesses. To appreciate this phenomena, it is useful to investigate various aspects of simple circuits and the various laws that may govern them.
8.2 Objective
1. To verify Ohm's Law
2. To use Ohm's law to determine the resistance of a light source.
8.3 Theory
Our initial investigations will be guided by Ohm's law, which postulates that the relationship between current flow I, potential difference V, and resistance R for certain materials will observe the following mathematical relationship, given a constant temperature constraint:
V = IR 1
These materials are called Ohmic conductors, equation 1 implies that the ratio of voltage to current for these materials is constant. Manufactured resistors can be considered as such, but other components such as semiconductor diodes, filaments, and LEDs are non ohmic. In this experiment, we will verify Ohm's law by assessing whether it holds for a set resistance (typical color coded resistor). Further, we will apply this to ascertain the resistance of a light source.
8.4 Apparatus
Variable DC voltage source, color coded resistor, (2) multimeters, connecting wires, light source
8.5 Procedure
Part A Verifying Ohm's Law
1. You will be given a particular colour coded resistor from the set; use this and the other apparatus
2. Adjust DC voltage source so that a relatively small voltage reading is seen across the resistor R.
Record this voltage reading, and the electrical current reading passing through the resistor.
Your multimeter devices must be properly set to make these measurements.
3. Repeat step 2 with increasingly larger voltage values until you have 9 rows of data.
4. Plot a graph of current (yaxis) vs voltage.
5. Use your graph to determine an experimental value of the resistance used.
6. Perform an error analysis on experimental and actual resistance values to determine the
effectiveness of the theory used.
Part B Determining the resistance of a light source
1. Replace the given resistance in the circuit above with the light source provided.
2. Repeat steps 2 5 from part A above to deduce the resistance of the light source.
8.6 Report Write-up
In your discussion/conclusion, ensure to include:
? a summary of whether you believe experiment rresults agrees with theory, and why;
? possible sources of error (at least two), and precautions taken
? ways in which the experiment could be improved.