Date: 17/7/05

Aim:

The aim of this experiment is to compare the predicted and actual resistance in a circuit of resistor combinations in series and in parallel.

Background:

A resistor is an electrical component/device that has electrical resistance. Resistors can be used in electric circuits for protection of components, voltage division or current control. In an ideal resistor the resistance remains constant regardless of the applied voltage or current, or the rate of change in the current (Resistor, 2005, Wikipedia).

Electrical resistance is a measure of the ability of an object to oppose the passage of an electric current. The electrical resistance of an electrical component can be found by using Ohm's law. Ohm's law states that the potential difference (voltage) between the ends of a conductor (e.g. a resistor) and the current flowing through the conductor are proportional at a given temperature (Storen & Martine, 2000, p221-226). This law can be written as: R=V .

I

The SI unit used for electrical resistance is an ohm. An electrical device that has an electrical resistance of 1 ohm will cause a current of 1 amp to flow through it if a voltage of 1V is passed through it.

From previous scientific research it has been determined that the general law for resistors in series is: Rseries = R1 + R2 + R3+Rn

It has also been determined that the general law for resistors in parallel is

Rparallel =

Hypothesis:

From the formulas stated in the Background of this report it can be seen that the total resistance of resistors in series can be found by adding together the individual resistances of each component. It can also be seen that the total resistance of resistors in parallel can be found by adding together the individual reciprocals of each component's resistance and then taking the reciprocal.

Predictions:

By knowing the individual resistor values we can accurately determine the total resistance of resistor combinations. Please refer to the Results section of experiment for more detail.

Materials:

-Seven resistors

-12v power pack

-Multimeter

-2 alligator clips

-Small lengths of thin copper wire

-Circuit test (bread) board

Method:

1.The circuit shown in Diagram 1 (see Results) was set up using the appropriate electrical components.

2.The 12v power supply was turned on and set to 6v.

3.The amperage, voltage and resistance of the three resistor combinations (see results) within the circuit were found, as well as the total amounts for the whole circuit by using the multimeter.

4.The voltage and amperage of each resistor was then found by using the multimeter.

Results and Calculations:

Diagram 1: Circuit Setup

Table 1: Resistor Values

All actual resistances obtained were within the relevant upper and lower tolerance values stated above except for the resistance obtained for R4.

Prediction calculations:

Total Resistance (Equivalent to Resistor Combination 3) (See Image 1)

R total lower:

= 1847.3 ohms

R total upper:

= 2319.20 ohms

(The actual value should therefore be between 2319.20 and 1847.3 ohms.)

Resistor Combination 1 (See Image 2)

R lower:

= 245.51 ohms

R upper:

= 271.36 ohms

(Actual value should therefore be between 271.36 and 245.51 ohms)

Resistor Combination 2 (see Image 3)

Rlower

R lower: 1664.24 ohms

Rupper

R upper: 1846.36 ohms

(Actual value should therefore between 1664.24-1846.36 ohms)

Resistor Combination 3 (Equivalent to Total Resistance)

Refer to Total Resistance

Experimental measurements:

Data for Total Circuit (see diagram 1):

Amperage= 0.01 amps Resistance= 2538 ohms

Voltage from power supply = 6.04V

Diagram 2: Resistor Combination 1

Data for Resistor Combination 1 (see diagram 2):

Voltage= 0.72V Amperage= 0.01 amps Resistance= 270 ohms

Diagram 3: Resistor Combination 2

Data for Resistor...

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