# Investigate the relationship between the variables V, R, and I in an electric circuit.

Experiment 3: Ohm’s Law                                                                    Date: 22.03.2004

Objective: 1) To investigate the relationship between the variables V, R, and I in an

electric circuit.

2) To analyze series and parallel connection in terms of V, R, I

3) To determine the relationship between current and voltage in a circuit that

contain a filament bulb

Theory:

It is known that electrons flow through a conductor metal wires with an ease without any resistance. This means that nothing would prevent the flow of the electrons. In a normal case however, under normal conditions, a resistance in the wire would affect the flow of the electrons in much the same way that friction slows down a sliding box on a platform. To allow more electrons to move against the resistance of a wire, it is necessary to apply a potential difference between the two ends. One can calculate the potential difference needed to create a current, I if the constant resistance, R of the wire is known. The relationship between these three variables (R, I, V) is given through Ohm’s Law which stated that the potential difference, V in a wire is proportional to the current, I if the temperature and all other physical quantities are fixed. From the law itself, we have an expression:

V=IR

If we solve the law for resistance we would find that,

R=V

I

From the expression above, it is very clear that the unit of resistance is volt per ampere or V/I. In particular, we define 1 volt per ampere to be 1 ohm which is represented by the Greek letter omega, Ω. Thus, we have an expression

1Ω = 1V

1A

Ohm’s Law is not a law on nature but more on the order of a useful rule of relationship like Hooke’s law for spring and or the ideal gas law that approximate the behaviours of real gases.

Resistors in series

When two or more resistors are connected end to end as in the figure below, they are said to be connected in series. Any charges that pass through R1 will also pass through R2 and R3, so the same current, I pass through each resistor.

If V , V2  and V3 are the potential difference across each of the resistors, R1 , R2 and R3, applying Ohm’s law to each resistor would give:

Note that, the total voltage across all resistors is equivalent to the voltage of the battery. Thus we have:

The equivalent single resistance, Req that would draw the same current, I, would be related to V by:

Since V= IR then we would have:

Since the potential differences must be the same as the ...