Why is bubble wrap good at preventing convection?

Answer 1

Bubble wrap functions as a good insulator for two reasons. The first reason is related to the plastic making the bubble wrap, and the second reason is related to the air bubbles.

Firstly, the plastic of the bubble wrap is probably either poly(vinyl chloride) (PVC) or low-density poly(ethylene) (LDPE). Both of these plastics have a very low value for a quantity called the heat transfer coefficient and a fairly high value for the not completely unrelated quantity called specific heat capacity. The heat capacity in this case is not too important to consider since it plays more of a role on perception of relative temperature, but we'll discuss it anyway.

The heat transfer value essentially describes the ability of a material to pass energy in the form of heat from one place to another by direct contact, and the specific heat capacity describe the amount of energy required to change the temperature of an object by one Kelvin (K). The higher the value of the heat transfer coefficient, the faster the object will cool or heat, and the lower the value of the coefficient, the slower the object will cool or heat. The higher the value of the specific heat, the more energy required to increase the temperature of the object.

Additionally, air tends to have a low heat transfer coefficient and high specific heat.

The observable result of these two factors is that even when it's hot on one side of the bubble wrap, the other side of the bubble wrap will stay cool. This happens specifically for a few different reasons.

The differential equation described by Newton's Law of Cooling basically explains that an object changes temperature at a rate dependant on the difference between its temperature and the temperature of its surroundings. For example, if a hot cup of coffee is placed in a cold room, it will cool more quickly than a hot cup of coffee in a warm room. Additionally, the rate at which any material at any given temperature cools depends on the coefficients of heat transfer and specific heats of the materials which are changing in temperature. So, take for example an aluminum plate and a bucket of water. Aluminum and water both have high coefficients of heat transfer and water has a very high specific heat relative to aluminum. Now, consider an aluminum plate at 500 K when placed in 300 K water will quickly drop since both water and the plate have high coefficients of heat transfer. However, since the water has a very high specific heat compared to aluminum, the temperature of the water will not rise nearly as much as the temperature of the aluminum fell.

Applying this idea to the plastic. Since both air and the plastic have comparable specific heats, the change in the temperature of the same masses of air and plastic will occur in relatively similar manners (If the air temperature changes by about 1, the plastic temperature will change by about 1 as well.), however, since the coefficients of heat transfer are so low, they will occur very slowly.

Consider a room with a very big sheet of plastic wrap with a mass of 1 kg. In the room room there is a quantity of air with mass 1 kg. The air is hot, the bubble wrap is cold. Since the air and bubble wrap have such low coefficients of heat transfer, even though the air and bubble wrap change temperature by the same amount (one degree degrease in hot air implies one degree increase in the bubble wrap) the rate at which this occurs may be very long depending on how hot it actually. So, it might take hours for the bubble wrap and room to equilibrate in temperature.

Now divide the room in two with 1 kg air on both sides; one side is hot while the other is cold. With the room completely divided, the air cannot mix change temperature by convection, so the only method left to it is heat transfer through the bubble wrap. Air temperature through the bubble wrap is extremely slow and as a result the the cool side of the room stays fairly cool and the hot side stays fairly hot. If the bubble wrap were to be replaced with aluminum, we would see heat transfer occur much more quickly (not as quickly as allowing the sides to mix, but still much more quickly that bubble wrap).

In addition to this action, consider the bubbles of air in the bubble wrap. They each act as individual steps of heat transfer with in the wall. It's almost as if one were to put up two walls of plastic to divide the room into three sections, slowing heat transfer even more. Just imagine how effective many layers of bubble wrap could be.

(Specific heats: Air ~1.00, PVC ~0.9, Aluminum ~0.90)

(Coefficients of Heat Transfer: Air ~10, PVC ~0.16, Aluminum ~220)

Answer 2

I don't no what convection is but it's a lot of to play with!!! it also killed my dog. poor foofy ><>oO

Answer 3

There is air pockets within the bubble wrap so it travels through these air pockets to get to your ears.