QotW: Why can the fridge door be hard to open?
James Tytko helps Josie ahead of the weekly shop. "Why can't I immediately open my fridge door after closing it? It feels like air pressure is keeping it closed. Is this the case?"
James - With the summer sun heating up the outdoors, I’m relying on my fridge a lot for cool beverages. Which is why it’s annoying when the door won’t budge open after someone else closed it shortly before. Fortunately, Cambridge University’s Engineering Professor Vikram Deshpande is here to explain the science behind this.
Vikram - Opening of the fridge door will be expected to equalize the pressure inside the fridge with that outside so at first it seems counterintuitive that the outside air pressure is keeping the fridge door shut. However, closing the door quickly or slamming it shut forces air out of the fridge and lowers the mass of air inside the fridge. This in turn lowers the pressure inside the fridge compared to the outside and hence you are correct that the air pressure is forcing the fridge door shut immediately after closing it.
James - So the difference in pressure is what prevents it from opening. But why then does it become easier to open after some time instead of forever keeping it shut?
Vikram - It takes several minutes for the vacuum/low pressure created by slamming the fridge door to be released. This naturally occurs due to small leaks in the fridge seals and after that vacuum is released the door is back to normal.
James - Thank goodness. Does this also explain why my freezer door is harder to open that my fridge door?
Vikram - This is slightly different and actually a great example of what in physics is called ‘Gay-Lussac’s law’. The law suggests that the pressure of a gas is related to its temperature when the mass and volume are kept the same. When the freezer door is open, the air temperature and pressure inside the fridge are the same as the outside. But when you shut the door, the freezer cools the air inside so the molecules in the air vibrate less and exert a lower pressure. Now you have a low pressure inside the freezer and a higher pressure outside which forces the freezer door shut and makes it difficult to open. This effect is present in the fridge too but since the temperature of the freezer is lower than that of the fridge the pressure difference between the inside and outside is higher for the freezer compared to the fridge.
James - So Josie. The reason why fridge doors are difficult to open after immediately shutting them is indeed because of air pressure. Forcing the fridge door shut causes a small vacuum. Thank you to Professor Vikram Deshpande for helping us find the answer. Next week, we’ll be serving you the solution to this brain blast from listener Neil.
Neil - Would an explosion on the moon be more powerful than on the surface of the earth?
James - If you at home have a question, submit it to our forum, nakedscientists.com/forum. Pop it on our website nakedscientists.com. Or email us at Chris@nakedscientists.com. I’m James Tytko, thank you for listening, and I’ll speak to you next time.