Much of physics concerns itself with how one form of energy is changed into another. This experiment explores how heat can cause an electrical current to flow. Although this is not yet efficient enough to be used as a significant source of electrical power, it is widely used in the form of thermocouples to measure temperature. This is known as the Seebeck effect.
The reverse—where electrical current flowing through certain materials results in one part of the circuit getting hot and the other part getting cold—is known as the Peltier effect. Unwanted heat is dissipated by electronic components. These components must be cooled to function correctly. Peltier coolers have no moving parts and have been used to cool high-speed computer microprocessors. They are also used instead of dry ice in cloud chambers. (See Project 125.)
What You Need
- voltmeter (or multimeter configured as an voltmeter)
- ammeter (or multimeter configured as an ammeter)
- variable DC power supply
- jumpers with alligator clips
- 1000 ohm resistor
- 2 4-inch lengths of various types of (uninsulated) metal wire, including iron, copper, constantan, and aluminum
- heat source, such as a candle, Bunsen burner, or a soldering iron
- ice cubes
- optional: 2 thermocouples to be used as temperature sensors
- Select two different wire materials. Take two pieces of the first material and one piece of the second material. Let's say we start with two pieces of iron and one piece of copper.
- Attach each end of the copper wire to each of the two pieces of iron wire by twisting about a one-half inch length of the wire together.
- Connect the two unattached ends of the iron wire to the positive and negative terminals of the voltmeter. See Figure 95-1. Set the voltmeter on the most sensitive setting. The 250 mV (0.250 V) range is a good place to start.
- Measure the voltage at room temperature. (Momentarily disconnect one of the voltmeter connections to verify that the voltage you are reading is the result of the circuit you set up, rather than a small stray voltage reading.)
- Touch one junction (twisted wire connection) to the ice, leaving the second junction at room temperature. How does that affect the voltage reading?
- Place the second junction in the heat source and see what happens. Be careful because the various metal wires conduct heat and can burn anything that comes in contact with it.
- Try this with as many material combinations as you can.
- Place a temperature sensor on each of the twisted metal junctions. Vary the junction temperatures. Plot the voltage as a function of the difference between the two junction temperatures.
- As before, connect each end of one piece of wire to a piece of a second type of metal wire.
- Connect the components as a series circuit consisting of the wires, the ammeter, a 1000 ohm resistor, and an ammeter. This circuit is shown in Figure 95-2.
- Adjust the DC power supply, so about 10mA (10 milliamps or 0.01 amps) is flowing through the circuit. (You can use a 9-volt battery instead of an adjustable DC power supply, which results in slightly less than this current with the 1000 ohm resistor.)
- Put water on each of the junctions. What happens? Reverse the direction of the current flow by exchanging the wire connected to the DC power supply. How does that affect what you find?
- Monitor the temperature of each of the junctions as you vary the current flowing through the circuit. Plot the temperatures of each junction and the difference versus the current.
In the Seebeck effect, a temperature difference at the two junctions results in a voltage generated through the circuit.
The Peltier effect results in temperature differences at the junctions when a current flows through the circuit.
Why It Works
A temperature difference between two dissimilar metals results in an electrical potential that drives a current through a circuit. The reverse effect causes a temperature difference when a current flows.
Other Things to Try
Commercial thermocouples that employ dissimilar metals are based on the Peltier effect and can be used to study this principle. Some temperature control devices that serve as a means of studying the Seebeck effect are available commercially.
The Seebeck effect and Peltier effect describe a set of interactions between the thermal properties and the electrical properties of matter.