Investigation 5

Part 1

 What do resistors do in circuits?

 What do you think happens when two or more resistors are put in series with one another?  What is the most important rule when measuring resistance with an ohmmeter?

 Follow the procedure at the right.  Remove batteries from your board before you begin.  You have 10 minutes.

 What did you find out about resistors when they are connected in a series?  The total resistance equals the sum of the individual resistances.  What evidence do you have that this rule is always true?  Meter readings of individual resistors always add up to the meter reading of the same resistors in a series.

 The total resistance in a series circuit is equal to the sum of the individual resistances in that circuit. R t = R 1 + R 2 + R 3 … + R n

No, other components also impose resistance.

Wires, potentiometers, lamps, motors, and lots of other items.

All components impose resistance to circuits. Some components, like wires, introduce so little resistance that their resistance can be considered zero. But components like lamps do impose significant resistance and must be factored in where we calculate total resistance in a circuit.

 Calculating Resistors in a Series p. 22

Part 2

The total resistance in a series circuit is equal to the sum of the individual resistances in that circuit.

What happens to the total resistance in a circuit when 2 or more resistors are put in parallel with one another?

 Use an ohmeter to determine the resistances of the individual resistors.  Set up 2 or more resistors in parallel on your spring board.  Use the ohmeter to determine the total resistance of the set.  Record the results in schematics.  Figure out the rule for determining the total resistance of resistors in parallel.

ResistorsSchematicMeasured Total Resistance

 The more resistors you put in parallel, the less the total resistance.  Why does the resistance go down when resistors are used in parallel? (discuss with your groups for 2 minutes).  A second resistor in parallel with the first provides the electricity with an additional pathway.

Copy this in your notes 1. The total resistance through 2 resistors in parallel is less than through either resistor by itself. 2. The total resistance through 2 resistors in parallel will ALWAYS be LESS THAN the smallest resistor in the parallel array.

 Conduct an investigation to compare the total resistances imposed by: ◦ One 1000-Ω resistor ◦ Two Ω resistors in parallel ◦ Three Ω resistors in parallel  After measuring the resistances, can you: ◦ figure out how to predict the total resistance of any number of resistors in parallel ◦ Figure out how to develop an equation to calculate the total resistance of resistors in parallel.

ResistorsSchematicMeasured Total Resistance 1000 Ω

1 = …+1 R t R 1 R 2 R 3 R n

 Calculating the total resistance of a set of resistors in parallel requires that you use your math skills. You will be working with some small decimal numbers, so it will be VERY important for you to pay close attention to the handling of decimal points.

Example

 Resource Book – Using Resistors in Combinations – p  Lab book p. 23 – Resistors in Parallel

Part 3

 Lab book p. 23 – Resistors in Parallel

 Today you will assume the role of electrical engineers and use your knowledge of resistors to solve some problems.  Lab book – Making New Resistors p. 27  You will assemble combinations of resistors in series and in parallel in order to achieve a predetermined amount of resistance.  This task requires a different kind of analysis – estimation followed by trial and error.

 Number a page 1-8  Write the target resistance beside each number.  Draw a schematic showing the resistors (labeled) in series and/or parallel that equal the target resistance.

 Lab book p. 25 – this is a quiz – work on it on your own. Give an example.