Lab #6 – Ohm, Kirchhoff, Watt, and Joule (no need for a formal report) Purpose of today’s lab: • To use Ohm and Kirchhoff rules to solve circuits. • To calculate the energy and wattage used by a circuit.
Ohm’s and Kirchhoff ‘s , Watt’s and Joule’s words. Exactly how large the current is in a wire depends not only on the voltage between its ends, but also on the resistance the wire offers to the flow of electrons. Kirchhoff’s first rule (junction/node rule): At any junction point, the sum of all currents entering the junction must equal the sum of all currents leaving the junction. Kirchhoff’s second rule (loop rule): The sum of the changes in potential around any closed loop of a circuit must be zero. The total energy used by any device is simply its power consumption multiplied by the time it is on. If the power is in watts and the time is in seconds, the energy will be in joules. Power is defined as the rate at which work is done. In SI units, power is measured in joules per second, and this unit is given a special name, the watt. Lab specific questions Try to express the above statements in equation form to use of in this lab. Report: Instead of a formal report , return a document where you show a snapshot of each circuit you construct, you show all the work of solving the circuit, and answers all the questions briefly. Experimental Set-up. Circuit 1. Single loop with two resistors and a battery.
Construct the circuit shown below using the usual Phet simulation applet: 1.1. use https://phet.colorado.edu/sims/html/circuit-construction-kit-dc-virtual-lab/latest/circuitconstruction-kit-dc-virtual-lab_en.html 1.2. Choose the voltage for the power supply Va to be somewhere between 8-10 volts. 1.3. Use R1 = 10 Ohm and R2=20 Ohm. 1.4. Add a simulated amp meter in each “branch” of the circuit. 1.5. Switch the view to show the traditional current flow.
Measure and record the voltage differences across the power supply and all the resistors. (use the simulated voltmeter). 2.1. Make notes about the polarity of the voltage difference you measure for each element. 2.1.1.Why is that important?
Calculate the current through each resistor using Ohm’s Law. 3.1. Do the values and the directions you calculated agree with the readings of the simulated amp meter?
Start from the positive end of the battery. “Walk” the circuit either clockwise or counterclockwise and add all potential (voltage) gains and drops. 4.1. What is the total potential gain/drop around the loop? 4.2. Does Kirchhoff’s rule work here?
Calculate the power consumed on R1 and R2. Which is more and why? Circuit 2. More resistors, twice the loop.
Construct the circuit 2 in another instance of the Phet applet. 1.1. Choose the voltage for one power supply to be somewhere between 8-10 volts, and choose the other power supply to be somewhere between 5-7 volts. 1.2. Chose various values for the resistors and record them. 1.2.1.Embed one amp-meter next to each resistor.
Measure and record the voltage difference (including its polarity) across each power supply and each resistor, as you did in the previous circuit.
Use the 2 Kirchhoff’s rules to create a linear system to solve the circuit. 3.1. Show all your work when you calculate the magnitude of the current through each resistor. 3.2. Compare the result and direction with the embedded amp meters.
What can you tell about the currents through R1 and R2 ?
What is the power consumed on R2?
What is the sum of all currents that go into the junction A?
What is the sum of all currents that go out of the junction A? 7.1. What conclusion can you make about the sum of all currents at the junction? Figure 1 Circuit 1
Choose one loop and calculate the total potential (voltage) gain/drop around the loop as in previous exercise. What conclusion can you make? Figure 2 circuit 2 Challenge Circuit. (Optional) Even more resistors for a triple loop. Construct the following circuit using the same instruction as the circuit-2 and some random choices for the values and set up the linear system to solve the circuit. Find the power consumed on R4. Figure 3 circuit 3, challenge References Theoretical definitions from PHYSICS PRINCIPLES WITH APPLICATIONS by DOUGLAS C. GIANCOLI You can find the values of resistance of commercially available components from their markings. https://demonstrations.wolfram.com/4BandColorCodeForResistors/ The circuit simulation tool choice for this lab is a playful applet. It may give the right answers as you have noticed but it has very limited features. You have access to more featureful and professional packages. The school has the licenses for you to use for free, you can get the activation keys from your department.
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