Convection in Action: Make a paper cup spin

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We live in a small town, and most mornings when the sun is rising, 2-3 hot air balloons are heading up into the sky right outside of our door. My kids get absolutely JAZZED about those hot air balloons and run out to see them (yes, they we wake up that early).

One big question is: how do those huge hot air balloons fly?

We used that as the perfect opportunity to talk about convection with a fun, indoor STEM experiment!

The paper cup spin experiment demonstrates convection by showing how hot air rises. When we light a candle below a freely moving paper cup, the cup begins to spin, showing how convection works.

Let’s put on our scientist caps and get to creating!

How to make the Convection in Action Spinning Cup experiment

Supplies you will need

For this experiment, you’ll need:

• Small paper cups (2)
• Box cutter
• Paper clip
• Lighter
• Optional: Small candle

Before you start

Please watch your children’s fingers around the flame.

Instructions

Here is how to do this experiment with your child:

Step 1: Cut a hole in one of the paper cups

In one of the paper cups, use your box cutter to cut out a hole in the side of the cup.

That hole should be big enough to fit your optional candle through and/or the lighter.

Step 2: Cut flaps into the second paper cup

In the second cup, cut vertical slits a little less than an inch apart and a smaller horizontal slit at the top of each vertical slit. These will be the flaps in the upper cup.

Bend each slit so it makes an angled flap (see picture).

Step 3: Create the support

Next, we need to bend the paper clip to support the upper cup.

Unbend the paper clip so you have a straight line (well, as straight as it can be).

Then, bend two points at 90-degree angles along the paper clip. You should have a vertical piece, a 90-degree bend into a horizontal piece, and a final 90-degree bend into a vertical piece.

The longer vertical piece should be slightly longer than the top cup (see picture).

The bottom vertical piece will be taped alongside the lower cup, the horizontal piece will sit over the rim of the bottom cup, and the final vertical piece of the paper clip will support the upper cup.

See pictures below to help!

Step 4: Place candle and light

All that’s left now is to heat and witness convection!

If you’re using a candle, place it inside the lower cup and light it. It will need to be near the middle of the cup.

If you’re just using the lighter, just ensure the flame is near the middle of the opening of the bottom cup.

The STEM behind the Convection in Action Spinning Cup experiment

This experiment teaches:

• Convection
• Energy conversion
• Measurement

How it works

The Convection in Action Spinning Cup Experiment is a classic demonstration of convection, a method of heat transfer that involves the movement of a fluid (like air).

The candle in the bottom cup heats the air directly above it. As the air is heated up, it becomes less dense. That less dense air rises due to buoyancy.

The rising warm air creates a low-pressure area below in the bottom cup, and the pressure difference between the cooler, higher-pressure air and the warmer, lower-pressure air creates a lifting force.

Those pressure differences essentially generate a circulation within the cups, creating a convection current. That current drives the top cup to start spinning!

Convection

The heat from the candle causes the air around it to warm up. Warm air is less dense than cold air, so it rises.

Convection is the transfer of heat through the movement of a fluid (like air or water).

In this experiment, the heat from the candle warms the air directly above it. This warm air becomes less dense than the cooler air around it, causing it to rise. As the warm air rises, it pushes against the bottom paper cup and the top cup, creating a lifting force that causes the top cup to spin.

This demonstrates the basic principles of convection:

• Heat transfer: Heat is transferred from the candle to the air.
• Density difference: Warm air is less dense than cold air.
• Fluid movement: The less dense warm air rises, while the denser cold air sinks.

Energy conversion

This experiment shows how heat energy can be used to create motion (mechanical energy).

The heat from the candle causes the air to rise, creating a pressure difference that drives the cup to spin. This process is similar to how steam engines and internal combustion engines work, where heat from burning fuel is used to produce mechanical energy.

In essence, the spinning paper cup experiment is a simple but effective way to illustrate the principle of energy conversion.

Measurement

You can also use this experiment to practice measurement!

What exactly can you measure?

You can measure what distance between the two cups provides the greatest spin (does more distance or less distance make the top cup spin faster?).

Students can measure how long it takes the top cup to complete one rotation, calculating its speed. To help, you could color one of the slits in the top cup a bright color to help identify a rotation more easily.

Since measurement is a fundamental math skill, these activities will help sharpen that skill.

More experiments about energy to try out with your child

• Rubber Band Regatta: Build a working paddle-wheel boat!
• Swinging Science: Creating mesmerizing pendulum art
• Build a Solar Oven Snack Shack!