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# 4.2: Activities

## Things You Will Need

All you will need for this lab is this PhET simulator. Go ahead and run this simulator now in another window for use in the activities below.

## Light Bulbs

Let's start with a few basic settings. The default settings in the right panel will be fine, but you may want to uncheck the distracting "Show Current" box, or at the very least, set it for "Conventional" so that the direction of the current is animated, rather than the motions of the electrons. Make no changes to the "Wire Resistivity" (which should be set at its minimum level of "tiny"), or the "Battery Resistance" (which should be set at its minimum level of 0.0 ohms).

By dragging components from the left panel, construct the following circuit with wires, a battery, a switch, and 5 light bulbs:

Figure 4.2.1 – Light Bulb Circuit #1

By clicking on each component, a slider bar appears that allows you to set its specifications. Set these as follows:

• battery: 24.0 volts
• every light bulb: 12.0 ohms

Close the switch to get the current flowing.

1. The brightness of a light bulb is measured by the rate of energy conversion from electrical to light, i.e. the power dissipated by that bulb. Compute the brightness (power) that comes from all the bulbs in the circuit combined, by using measurements made with the voltmeter.
2. Use the ammeter to confirm that the power emitted by the light bulbs calculated above equals the power supplied by the battery. [Note that unlike the case of the voltmeter, which can be used as an external probe without disturbing the circuit, you will need to alter the circuit itself to use an ammeter.

Construct the following circuit, and once again set the voltage of the battery to 24.0 volts.

Figure 4.2.2 – Light Bulb Circuit #2

1. You are given two light bulbs, one of which has twice the resistance as the other. When arranged as they are in the circuit above, their combined brightness is the same as the 5-light bulb arrangement above. Do the necessary computation to determine the resistances of these light two bulbs, and then test your answer with the simulator by setting the light bulb resistances to what you computed and showing the power usage is the same as you computed above.

## Kirchhoff's Rules

Our next exercise involves using Kirchhoff's rules to analyze a network of batteries and resistors. Start once again by constructing the following dc network of 2 switches, 2 batteries, 3 ammeters, and 4 resistors (note that not all the wires in the diagram are necessary – many were included here for the sake of clarity):

Figure 4.2.3 – A DC Network

Click on the following components, and use the slider bar to set them at the following values:

• left battery: 60.0 volts
• top resistor: 12.5 ohms
• bottom-right resistor: 15.0 ohms
• bottom-center resistor: 45.0 ohms
1. When the switches are closed, the left ammeter measures a current of $$1.00A$$ (upward), and the center ammeter measures a current of $$0.50A$$ (upward).
1. Draw a circuit diagram, labeling the components and currents in the various branches.
2. Use Kirchhoff's rules to find the resistance of the bottom-left resistor, the emf of the right battery, and the current through the right ammeter. Verify your answers with the simulator.
2. Find the power supplied by each of the batteries, and use the voltmeter to show that this equals the power dissipated by the four resistors.

## Lab Report

Download, print, and complete this document, then upload your lab report to Canvas. [If you don't have a printer, then two other options are to edit the pdf directly on a computer, or create a facsimile of the lab report format by hand.]