Interactive Lecture Demonstration 5 – Circuits with Two Light Bulbs

 

 

 

This demonstration involves two light bulbs: a 25 Watt bulb and a 40 Watt one. Recall that the bulb wattage is the amount of electric power that the bulb consumes. By definition, the electric power consumed by any device is P = IDV, where I is the current through the device and DV is the voltage difference between the two ends of the device. Using the definition of resistance, we can find two other formulas for electric power (only valid for resistors): P = I2R and P = (DV)2/ R.

 

  1. Calculate the resistance of the two light bulbs we will use, using one of the power formulas above (hint: you should already know another variable besides the power of the bulbs).

 

 

 

 

 

 

 

 

 

 

  1. Draw a circuit diagram showing one light bulb connected to an electrical outlet (include a symbol for the electric generator so that you have a complete, closed circuit in your diagram). Then calculate the current that will flow through each light bulb if we connect the bulbs individually to an outlet. You could use another one of the power formulas, since now you know the bulbs' resistances, but instead of doing that, use the loop rule (i.e. conservation of energy) for your circuit. That is, add up the voltage differences in making a complete trip around your closed circuit; the voltage differences must add up to zero.

 

 

 

 

 

 

 

 

 

 

 

  1. Which bulb will be brighter when we connect them one at a time to an outlet ? Why ?

 

 

 

 

 

  1. Now we will connect both bulbs at the same time in a circuit. Will the brightness of the bulbs be different now than it was when we connected them one at a time ? (Think about what happens when you turn on more than one light in a room at your home).

 

 

  1. We hope you answered "no" to the last question. Different electrical outlets in your home are wired in a way that is called "parallel wiring." Two devices that are wired in parallel automatically have the same voltage difference across them. That's why the lights in your house always keep the same brightness, regardless of how many other lights or appliances you turn on. If the voltage difference across all your bulbs is always the same, the current through them must always stay the same, therefore their power (and thus brightness) stays the same.

 

Draw a circuit diagram below that shows how the two light bulbs are connected in parallel. It should be clear from your diagram that the voltage difference across each light bulb is the same as the voltage difference across just one light bulb in the circuit for #2. Prove that the current through each light bulb in this parallel circuit is the same as the current through the light bulbs in the one-bulb circuit, by writing the two loop rules for this parallel circuit.

 

 

 

 

 

 

 

 

 

 

 

 

      6.   How much current flows out of the generator in your circuit ?

 

 

 

 

 

  1. Now we will plug the bulbs into a different set of outlets. Watch what happens, then describe and explain what you saw.

 

 

 

 

 

 

  1. The new outlets were wired in series. In order to understand what you saw, draw below the circuit diagram for the two bulbs connected in series to an outlet. Then write the loop rule for this new circuit. Using the loop rule, calculate the current, voltage difference and power of each light bulb in this circuit. Are your calculations consistent with what you saw happen ?