Joule Heating: Resistance vs. Temperature
For some reason, your laptop charger gets super hot when you plug it in.
It had recently stopped working, and so you left it with a friend who claimed to know how to fix it. He said that all he’d need to do would be to swap out a broken resistor with a replacement—but he also mentioned that he replaced the charger’s resistor with a resistor with a lower value of resistance. His reasoning was that it would make your laptop charge faster. For this same reason, he also cranked up the voltage that travels through the charger, and now you’re sitting home with a laptop charger that feels like it could burn a hole through your couch.
Let’s do an experiment to get to the bottom of how your friend’s executive decisions probably contributed to your dangerously hot laptop charger!
Problem: Explore joule heating and explain resistance vs. temperature.
- Ice cube tray
- Cutting board
- 2 10-ohm, 10 watt resistors
- 2 1000-ohm resistors
- 9-volt battery
- AA battery
- Ammeter (optional)
- 2 wires with insulation removed from both ends
- On the day before you perform your experiment, place a resistor in the bottom of each individual ice cube compartment in your ice cube tray. Bend the wires so that they poke out over the top of the tray like the prongs of a fork: they should lean against one side of the slot and stick out of the tray by at least ¼ inch. Fill the tray with water and leave it in your freezer overnight.
- Tape two wires to your cutting board. The wires should be bent 90 degrees at one end, laying flat and not quite touching. Don’t tape over the ends—leave them exposed so we can hook them up to our resistors. The other ends should stick over the edge of the cutting board by about 2 inches. Bend these ends inward so that they can clamp around your battery when you place it in between them.
- Repeat step 2 to set up another test station (you can use the same cutting board if you want, just make sure you space them out). This way, you’ll have two sets of wires ready for resistors.
- Hook each battery up to the wires in a test station. Make sure that the wires aren’t touching—we don’t want to complete a circuit yet!
- Gather all your materials and remove your ice tray from the freezer. Remove the cubes with the 1000-ohm resistors first. Place each so that the prongs sit under the bent wires on the cutting board, completing each circuit. What do you think will happen?
- After one minute, remove the 1000-ohm resistors and replace them with the 10-ohm resistors. Observe. Remove the 10-ohm resistors after one minute. What do you think will happen this time? Why?
Extra: Attach the ammeter to your circuit and look at how much current flows when the resistors are in the ice vs. when they aren’t. What do you observe?
Your 10-ohm resistor melted the ice! However, the AA battery didn’t melt it nearly as quickly as the 9-Volt battery did.