Embedded Files
Play-Doh Resistors:
Play-Doh Resistors:
Electrical Meters:
Electrical Meters:
Ammeter:
Ammeter:
An ammeter measures current through a circuit element; therefore it must be connected in series with it. An ammeter should have a much lower resistance than the other elements in the circuit; otherwise current may change significantly.
Voltmeter:
Voltmeter:
A voltmeter measures voltage across a circuit element; therefore it must be connected in parallel with it. A voltmeter should have a much higher resistance than the element in parallel; otherwise voltage will change significantly.
Resistance v. Length:
Resistance v. Length:
Create a long cylinder of play-doh. Be sure it is of consistent diameter.
Similar to the image to the right we will construct a circuit to measure both Voltage (V) and Current (I) in a circuit while varying the length of the resistor.
On a shared Google Sheet, create the following table.
Use a formula to calculate the resistance for each given length.
4. Create a graph of your data relating the length of the resistor to the total resistance.
How would you describe the relationship between the length of the resistor and the resistance?
Resistance v. Diameter
Resistance v. Diameter
Roll a 10 cm long cylinder of play-doh. Cooperate with the other groups THAT HAVE THE SAME COLOR PLAY-DOH (RED or BLUE) to aide in data collection.
By making your largest cylinder 10 cm long (this will be the constant length of your resistor), you will then roll the cylinder to smaller and smaller diameters to create new 'resistors'
Using a ruler, measure the diameter of the cylinder. Record this in the data table on your shared Google Sheet.
Place the cylinder into your circuit and measure the Voltage (parallel) and Current (series) as shown above. Record these values in your spreadsheet. The two measurements above should be made simultaneously!
Roll the cylinder to a smaller diameter.
Cut the cylinder to a length of 10 cm. The quality of the experimental results will depend strongly on the uniformity of the cylinders.
Same diameter over the entire length
No cracks or voids, knead thoroughly before rolling
For each cylinder:
Calculate its resistance using Ohm's law.
Calculate its cross-sectional area.
Record the color of the play-doh
Create a Resistance v. Cross-sectional Area graph of your calculated data.
If necessary (HINT), linearize your data.
Write an algebraic expression for resistance in terms of Length, Area and the resistivity constant (ρ).
Write an algebraic expression for resistivity (ρ) in terms of resistance, length and area. Check your IB Data booklet for verification.
Determine the Resistivity Constant
Determine the Resistivity Constant
The potentiometer at the right has a resistance (R) of 8.5 ohms, using the equation below, determine the resistivity constant (ρ).
Please:
outline the steps you took to take the appropriate measurements,
describe the materials you used to take appropriate measurements,
be prepared to share your process on a whiteboard with your peers,
determine the 'known/accepted value' of the resistivity constant of the rheostat, and
compare your findings to the known.
Commercial Resistors
Commercial Resistors
Introduction
In this activity you will investigate the effect of combining resistors in series and parallel.
Apparatus
breadboard/circuit board, various resistors, ohmmeter, connecting leads
Background
A breadboard is the name given to a board used to make an experimental model of an electric circuit without the need to solder components permanently to a circuit board. A typical breadboard is shown below. Circuit components can be inserted into the sockets. The five sockets in every column are connected to each other.
Fixed resistors sometimes look like the ones shown below.
Beneath the ceramic coating is a 'ribbon' of resistive wiring. As you can see in the photo, the ribbon varies in length and cross sectional area.
The coloured bands on the resistor indicate its resistance and uncertainty (tolerance). The resistor colour chart below can be used to determine the magnitude of the resistance from the coloured bands. The magnitudes are given in either 2 or 3 significant digits based upon the number of coloured bands. Below is a table for 5-band resistors. The patterns hold true for 4-band resistors as well, the 3rd digit however is removed.
For example, the picture below shows a single resistor inserted into the board. The coloured bands are brown, green and brown. This code represents a resistance of 150 Ω.
Using the PhET Simulation, you may have noticed the bands changing color as you changed the value of the resistor.
You should begin by measuring the resistance of a single resistor using a multimeter set up as an ohmmeter. The circuit diagram and a photograph for this are shown below.
The stated value of the resistor is 150 Ω, although as can be seen from the photograph below the measured value with an ohmmeter in this case is 149.1 Ω. This however is still within the tolerance of the resistor.
ACS - The stated value of the resistor is 10Ω, the photo shows that the measure value is the same as the stated value.
Energy in a Circuit
Energy in a Circuit
Energy In = Energy Out
A power source, battery, cell, or capacitor all provide an electric potential in terms of Energy per charge [Joules/Coulomb]. It is considered a power source due to the measurement of how quickly it can provide this energy [Watts]
Energy into a circuit must equal energy out [Kirchhoff's 2nd Rule]. There are areas of high potential (potential energy per charge) and low potential (can be read as zero) energy per charge.
Resistors in Series or Parallel
Resistors in Series or Parallel
Instructions
Measure the actual resistance of a range of resistors using an ohmmeter as shown above, then measure the combined resistance of resistors in series and parallel.
0. Resistor Values
Determine the values of the RED, GREEN and BLUE resistors.
1. Resistors in Parallel
Combine two of the resistors in parallel and measure the combined resistance, as shown in the circuit diagram and photograph below.
Repeat for other combinations of resistors.
Calculate the reciprocals of the resistances and enter these values in the table below.
The change in voltage per branch of a parallel circuit is constant. Therefore, the relationship can be written as above. The current (I) then can cancel, resulting in the generalized equation for resistance in a parallel circuit.
Show that three resistors in parallel of 20Ω, 40Ω, and 60Ω have a combined resistance of 11Ω.
Check that your relationship also applies to three resistors in parallel.
2. Resistors in Series
Combine two of the resistors in series and measure the combined resistance as shown in the circuit diagram and photgraph below.
Record the readings in a table like the one below.
Repeat for other combinations of resistors. Try to avoid using resistors that vary greatly in their magnitude. Can you deduce a relationship for resistors in series?
Check that your relationship also applies to three resistors in series.
3. Mixed Series and Parallel circuits
There are 9 different circuits shown in the photos below. Calculate the combined resistance using the relationships for series and parallel circuits. Check your predictions by measuring the combined resistance with an ohmmeter.
Combine parallel resistors into series.
For circuit A:
The two resistors in parallel (each 5Ω) combine to 2.5Ω.
Add total resistors in series.
For circuit A:
Can now be treated like a series circuit. 5Ω + 2.5Ω = 7.5 Ω.
Determine the total resistance of each of the circuits shown above.
Green = 5Ω, Blue = 10Ω and Red = 20Ω
Relate series and parallel circuits to the equation at the top of the page.
Please be prepared to:
share your results with your peers using the whiteboard rubric.
create an analogy comparing the total resistance of the circuit to a 'common' activity.
Some Additional Resources:
Some Additional Resources:
0345_lecture_notes_-_resistivity.pdf0346_lecture_notes_-_graphing_resistivity.pdfCircuit TSPs
Circuit TSPs
(a) An aluminum power transmission line has a resistance of 0.0580Ω/km. What is its mass per kilometer? (b) What is the mass per kilometer of a copper line having the same resistance? A lower resistance would shorten the heating time. Discuss the practical limits to speeding the heating by lowering the resistance.
An old lightbulb draws only 50.0 W, rather than its original 60.0 W, due to evaporative thinning of its filament. By what factor is its diameter reduced, assuming uniform thinning along its length? Neglect any effects caused by temperature differences.
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