Circuit Lab
Author: Adapted from a lesson by Herbert Anderson (http://www.iit.edu/~smile/ph8902.html), by Mark Roland
Time: / 3 class periodsPreparation Time: / 30 Minutes
Materials: / Garden hose with nozzle
Per Group:
Circuit board or plywood
3 - 1.5 Volt DC batteries
6 battery clips & 3 battery holders
2- 4.5 V bulbs
2 variable resistors (one could be non-variable)
Bell wire, screw driver
2 lamp sockets and bulbs
Multimeter (if available).
Abstract
A circuit lab is an essential part of any unit on electricity. The laws governing the behavior of electricity in simple circuits are, well, simple. They lend themselves well to inquiry labs because the relationships are easy to see. This lab will show students the need for a circuit in order to have a flow of electricity. It will also demonstrate Ohm’s law, which relates voltage, current, and resistance.
Purpose – This is an exploratory lesson for the unit of electricity. It will require a little bit of a jump from static electricity, which was covered in the engage lesson, to circuitry.
Objectives
Students will be able to:
1. Recognize that the force seen between two charges, and its ability to do work, is what makes electrical power possible.
2. Know that electricity is a flow of electrons, and therefore understand the importance of a circuit in electrical systems.
3. Use Ohm’s Law to solve simple circuit problems.
4. Use Ohm’s Law, Kirchoff’s Law, and the equivalent resistance formula to analyze parallel and series-parallel circuits.
National Science Education Standard:
Content Standard B-Physical Science
· CONSERVATION OF ENERGY AND THE INCREASE IN DISORDER- All energy can be considered to be either kinetic energy, which is the energy of motion; potential energy, which depends on relative position; or energy contained by a field, such as electromagnetic waves.
Content Standard A-Science as Inquiry
· DESIGN AND CONDUCT SCIENTIFIC INVESTIGATIONS. Designing and conducting a scientific investigation requires introduction to the major concepts in the area being investigated, proper equipment, safety precautions, assistance with methodological problems, recommendations for use of technologies, clarification of ideas that guide the inquiry, and scientific knowledge obtained from sources other than the actual investigation. The investigation may also require student clarification of the question, method, controls, and variables; student organization and display of data; student revision of methods and explanations; and a public presentation of the results with a critical response from peers. Regardless of the scientific investigation performed, students must use evidence, apply logic, and construct an argument for their proposed explanations.
Related and Resource Websites
http://www.iit.edu/~smile/ph8902.html
Activity
Day 1
1. As students enter the classroom, have these questions posted, which students should respond to:
a. What does static electricity have to do with electricity?
b. What basic parts of an atom contain charge? There are two—of those two, which one has to do with electricity?
2. Use the garden hose to demonstrate the analogy of water pressure with voltage, drops of water with electrons, flow of water with electrical current, resistance of hose and nozzle with resistance in a circuit etc. During discussion, introduce key vocabulary terms: AMPERE, VOLTS, OHMS, RESISTANCE, CURRENT, and CIRCUIT.
In groups, have students perform the following:
3. Connect one, then two, and finally three 1.5 volt dry cells in a series circuit to a socket with a 4.5 volt bulb. The pupils will observe and explain the varying degree of brightness of the bulb with respect to the increased voltage and the increased current. Use the multi-meter to measure the current for each circuit.
4. Connect 3 - 1.5 volt dry cells in series to both sockets, using 4.5 volt bulbs. The pupils will notice the bulbs' brightness as compared to using one or two dry cells. The pupils will explain why, and then use the multi-meter to confirm by measuring the current.
5. Insert the variable resistor into the circuit [You will have to connect the negative terminal to the center terminal with a clip], using 3 dry cells to one socket with a 4.5 volt bulb. The pupils will vary the resistance from the least resistance to where the bulb will not light. The pupils will observe and explain. Again, use the multi-meter, if available.
6. Through discussion, the class will realize the relationship between current, voltage, and resistance (Noting that this is for D.C. current only).
7. Introduce the pupils to OHM'S LAW : CURRENT = VOLTAGE
RESISTANCE
VOLTAGE = CURRENT X RESISTANCE RESISTANCE = VOLTAGE
CURRENT
Current is measured in Amperes; Voltage is measured in Volts; Resistance is measured in Ohms.
8. Pupils will use math skills to solve selected problems using Ohm's Law.
Day 2
1. Use circuit boards to set up parallel circuits with a lamp socket on one branch, and the variable resistor in the other branch. Introduce the term NODE.
2. Attach varying voltage to the circuit, while keeping the variable resistor constant, and measure the current through both branches using the multimeter (two multimeters per group would be ideal). Higher voltage should result in greater current through both branches, similar to a series circuit. Have students collect their results on a table similar to the following:
Voltage Current through resistor Current through bulb
1.5
3.0
4.5
etc.
3. Now attach one 4.5 V battery to the circuit. Vary the resistance in branch with the variable resistor in it, and measure the currents through both branches. Students should get the general idea that the current will take the path of least resistance. As the variable resistor is cranked up, the current will increase in the other branch, making the bulb light brighter. If the variable resistor is lowered, or even removed, the current will want to go through that branch, so the bulb will get dimmer. Again, students should gather their data on a table like the following:
Resistance Current through resistor Current through bulb
10
20
etc.
4. Finish the class with a discussion of the results that they obtained. Attempt to lead students to the realization that the current takes the path of least resistance. Use the garden hose analogy to help students with this idea. If a hose splits, and only one end has a nozzle or stopper, it’s going to go the other way!
Discussion questions:
a. Which path did the current go through each time? Why?
b. What should you do with the variable resistor in order to make the bulb burn the brightest? For it to be the dimmest?
c. What is the resistance of the light bulb? (Hint--when are the currents in the two branches equal?)
Day 3
1. The last type of circuit to analyze is a series and parallel together. Have each group keep the circuits the same as yesterday, but add the other resistor past the point where the other two branches come together.
2. Allow the students more flexibility in examining this circuit. Discuss first as a class what students think will happen. Here are some questions to help their thinking, if necessary:
a. Think about the hose analogy. What might that tell us about today’s circuit?
b. What currents might we want to measure and record?
c. Which amounts should we vary in order to test our ideas? How should we set up a table to record data?
3. Monitor each group closely. Ask what hypothesis they have regarding the circuit’s behavior. What are they doing to test these hypotheses? Decisions on how much to lead them might be best-made considering time restrictions and how they’re progressing.
4. Depending on time, if there are groups who are struggling to reach any conclusions, you could allow groups to double up and share their ideas. Ask them to discuss what their hypotheses were, how they were testing them, and what the results were.
5. Discuss as a class what the students discovered. The first principle that you should lead them to should be Kirchoff’s Law. Kirchoff’s Law simply states that the sum of the current coming into a node must equal the sum of the current leaving a node. The formula for how resistors act in parallel is a little tricky to “discover.” However, the discussion can get them there. Once they understand Kirchoff’s Law, use Ohm’s law along with voltage and the current of the entire circuit to figure out what the equivalent resistance is. Since they know what the resistance is of the series resistor, they can subtract to find the equivalent resistance of the parallel portion of the circuit. Do this as a class for a few different data values, looking at the resistance of the bulb (estimated yesterday), the value of the variable resistor placed in parallel, and the equivalent resistance that we know those two must be. The rule the students are looking for is the following:
Closure
Ask each student to summarize in their notes all the laws of circuit behavior that they have learned the past three days. Allow them the freedom to write them in their own words, or in ways that they will remember them. Discuss these findings as a class, or have them fill these out as an exit card instead of their notes. The exit card would be a good idea if you think you need to check for misconceptions.
Embedded Assessment
Assessment suggestions:
· Participation grade during discussions
· Homework assignments
· Exit card on the last day
· A formal lab write-up in a format that they are familiar with for your class.
Homework Suggestions
Day 1—Ohm’s Law problems
Day 3—Circuit problems containing Ohm’s Law and Kirchoff’s Law principles.