Bi-Metallic Strip
| Physics, Chemistry: | Properties of Metals, Young's Modulus |
| Grade Range: | Elementary School, Middle School, High School |
| Format: | Hands-on, Stage |
This demonstration is easy to do and easy to explain to students. It has great versatility, with a simple explanation that younger students can understand and plenty of outside information for older students to look into and learn about.
Materials
- Bi-metallic Strip
- Blowtorch or similar
- Dry Ice or similar
Safety Precautions
Please read the Fire Safety and Cryogen Safety sections of the Demonstration Safety page before performing this demonstration.
This demonstration requires: thermal gloves, goggles.
Demonstration
- Show the Bi-Metallic Strip to the audience, and point out the two metals on it (Bronze and Nickel).
- Use the Dry Ice or similar Cryogen to cool the strip. It will start to bend over! The more you cool it, the more extreme the bend will get.
- Let the strip warm up for a minute or so. During this time, ask the audience to identify which side it bent towards. What can they deduce from this?
- Use the Blowtorch or similar heat source to warm the strip. It will now start bending, but in the opposite direction! The longer you heat it, the more it will bend.
- Compare earlier thoughts to this result. What do they think is happening?
Why This Works
Solid materials, such as metals, are not able to become "more solid". However, they can expand and contract as their temperature changes, and is known as the Tyndall Effect. With our bi-metallic strip, we can see this happen in real time as we heat or cool the strip. The two metals on the strip, brass and nickel, have very different expansion and contraction rates. Brass experiences a much larger change than nickel when its temperature changes, and so we see it having control on which way the metal strip will bend. When we heat the strip, the brass will expand more than the nickel, causing the strip to bend over in the direction of the nickel. When we cool the strip, the brass contracts more than the nickel, which is why the strip will bend towards the brass side. This is used in many home thermostats; inside the thermostat there will be a coiled flat wire, which is a bi-metallic strip. as the temperature in the house changes, the coil will expand or contract accordingly, and if it changes too much then it will set off a trigger in the thermostat. As the temperature goes back to where it was set, the coil will do the same, and will reset the trigger!
Additional Information
- This demonstration is a part of the Liquid Nitrogen Show.