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I'm learning about circuits in my physics class, and we've been given an assignment to create a circuit that can only use 3 identical light bulbs that should all have a different brightness. I've tried for hours to get this to work, but I've so far only found several ways to make it where one lightbulb has a different brightness but the other two are the same. I've tried many different combinations of parallel and series branches and haven't gotten anything to work so far. Any suggestions? Thank you! EDIT: Not glowing at all DOES count as a different brightness, but only as long as the current is still going through the bulb, i.e. it is not just a broken circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

schematic

simulate this circuit

These are just a few of the things I've tried, I can't remember the other ones right now but they were basically variations of these.

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  • \$\begingroup\$ A schematic would be very helpful, showing what you have tried. There's a really great built-in schematic editor available when editing your question. But for a very basic answer, having your bulbs in parallel with a different value resistor for each would yield the desired result. \$\endgroup\$ – Ryan Griggs May 10 '16 at 15:35
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    \$\begingroup\$ Can you use extra components or are you restricted to using only the 3 light bulbs and nothing else? \$\endgroup\$ – Dwayne Reid May 10 '16 at 15:37
  • \$\begingroup\$ @DwayneReid We are only allowed to use the 3 light bulbs, the battery for the voltage source, and wires. \$\endgroup\$ – carsond2704 May 10 '16 at 15:42
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    \$\begingroup\$ Any restrictions on the use of wire? If not, just connect them in parallel, but use a short wire for one, a much longer wire for the second, and a very much longer wire for the third. I don't think there's any other way given the materials you have and are allowed to use. \$\endgroup\$ – JRE May 10 '16 at 15:52
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    \$\begingroup\$ Yes JRE is right but I dont think you would be able to show the difference in brightness that way, though there will be difference in brightness but it would be very small for us to perceive. \$\endgroup\$ – Jasser May 10 '16 at 15:56
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There is no solution to this problem if you're only allowed to use ideal components.

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Limit the amount of current flowing through a bulb to reduce the bulb's brightness.

If you're using LEDs, there's a minimum voltage across the LED required in order for it to illuminate, as well as a maximum current permitted through each to avoid damage.

Example with incandescent bulbs and resistors:

schematic

simulate this circuit – Schematic created using CircuitLab

Adjust the values of R1, R2 and R3 to produce the desired brightness.

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  • \$\begingroup\$ Thank you for your response, but we are not allowed to use any resistors. We are restricted to using only the 3 light bulbs and wires. \$\endgroup\$ – carsond2704 May 10 '16 at 15:43
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    \$\begingroup\$ @carsond2704 this circuit is right, if you have different length wires which would amount different resistance values which would allow different currents to flow through each branch and hence change the brightness of bulbs. But that brightness would be difficult to perceive since the difference in resistances of wires would be very small. \$\endgroup\$ – Jasser May 10 '16 at 16:01
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Can you use switches? The brightness is related to the power dissipated by the bulb.

P = (I^2)R.

So if you have more current (less R) the bulb will be brighter.

Can you use more than 3 bulbs? You could put 2 in parallel, 3 in parallel, and 1 and the whole combination in series.

schematic

simulate this circuit – Schematic created using CircuitLab

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