Dumb physics question for all you smart engineer MMMers

[Bookworm]

I know that the temperature on the inside of a car gets much hotter than the outside temperature, because the sun's rays get in and the heat gets trapped. Does this also happen in the trunk, where there's no glass to let the rays in, or is the temperature in the trunk the same as outside, or maybe even less because it's dark and shaded in there?

I've been cleaning houses as my side gig, and leaving my cleaning kit in the trunk all the time. But now it's starting to get hot (100-115F highs are common here) and I'm wondering if it's safe to leave aerosol cans (furniture polish, etc.) in there. It'll be kind of inconvenient to lug the kit back in the house after every single job, but an exploding can would obviously not be good.

[Exflyboy]

Well the answer is "maybe"..:)

If you leave the car in the shade then the max theoretical temperature inside the trunk would be the same as the temperature in the shade outside. Now it will likely be a bit lower than that because there is at least some insulation around the trunk.

Now if you leave the car in the sun, then the heating effect of the radiated heat of the Sun onto the trunk lid will in turn radiate heat into the trunk. If there is no ventilation inside the trunk (it should be minimal) then the temperature inside the trunk will rise somewhat.

But to be honest, the effect will be way less than leaving stuff inside the car.. windows are terrible of course.

As to worrying about aerosols.. personally I don't think it will be a problem.

[Exflyboy]

I did get a sense of satisfaction that you asked the engineers to answer a Physics question.. and not the Physicists...;)

[Singularity]

Leave a thermometer in your truck in sunny  conditions and see the result after at least an hour.

Sometimes just testing it is easier :)

[MDM]

+1 - I'd already written the notes below, so....

As above, theory says the trunk should stay cooler than the passenger compartment.

Think of the solar shields (often with a large picture of sunglasses) people put in their windows when parked to keep the heat down.  Blocking the sun from the trunk works similarly.

But there is a cheap way to do better than theory: stick a cheap thermometer in the trunk and measure the temperature.

Theory is good - good data is even better.

[forummm]

In addition to the radiation from the trunk metal, some of the heat from the cabin will leach into the trunk--faster if you have fold-down seats.

[Bookworm]

I did get a sense of satisfaction that you asked the engineers to answer a Physics question.. and not the Physicists...;)

Haha, I've noticed this place is full of engineers. I haven't seen the physicists. But since my STEM  prowess extends as far as pre-algebra, both impress me.

(For those suddenly terrified because I homeschool...not to worry, I outsource!)

[hope2retire]

I would say depends on the contents of the aerosol. It just takes one small defect in the aerosol can (crimp defect or other accidental damage or normal wear and tear on the can etc).  As long as the pressure built up can crack open one of the defect spots, then it is a potential projectile. If the contents have low boiling point, then easier to build pressure.

http://www.myfoxatlanta.com/story/17904158/aerosol-can-in-hot-car-explodes

http://mythbustersresults.com/episode14

use a thermometer to check the temperature as others have suggested. If under the shade, then it should be fine. If directly under the sun for longer periods of time, then it is possible to explode. You can always leave the windows slightly lowered for the air to flow through, this would lower the temperature.

If you want to do a qualitative experiment, take two identical measuring cups of water (small cups) and place it in the trunk and inside the car and leave it in the sun for couple of hours. Check how much water has evaporated after couple of hours. It may not be as accurate as a thermometer, but would give you an idea which is hotter.


H2R

[Heckler]

Instantly, I thought of this.

http://www.mrmoneymustache.com/2013/12/30/are-you-cleaning-out-your-own-wallet/

Why not ditch the aerosols and use a low cost "organic" cleaner like vinegar and water....

[Bob W]

Aerosols very bad for your lungs.   Do not use.

[GuitarStv]

Cracking a window slightly to allow a bit of the hot air to escape from the car will result in significantly lower temperatures inside . . . as will one of those foil looking sun shade things that you can put in the front and rear window.  Coupling that with keeping stuff in the trunk and I think you would be fine.

I did get a sense of satisfaction that you asked the engineers to answer a Physics question.. and not the Physicists...;)

It's a real world problem.  Physicists tend to suck at those.  :P

[nobodyspecial]

It's a real world problem.  Physicists tend to suck at those.  :P

Assuming a spherical, perfectly black body car in a vacuum....

[Exflyboy]

It's a real world problem.  Physicists tend to suck at those.  :P

Assuming a spherical, perfectly black body car in a vacuum....

And no "dark energy present"...:)

[Bookworm]

Experiment results:

I left a thermometer in the trunk for a few days (kept forgetting to check it at a hot time during the day), and it gets a LOT hotter in the trunk than the outside temperature. The thermometer reads 120 in the trunk when it is 90 outside. My car is parked in direct sun (no other option).

The thermometer I have is a candy thermometer...not sure if that matters. The markings start at 100 and go up in 10 degree increments, so it's probably not precise, but it's sitting right at the 120 mark.

[MDM]

The thermometer reads 120 in the trunk when it is 90 outside. My car is parked in direct sun (no other option).

That seems a good data point.  Just curious - how hot inside the passenger compartment?

[Exflyboy]

Good experiment.. and now we know.. for your exact car of course..:)

[forummm]

And we don't know whether his windows were up or down (and how far down) and what paint color he has and...

[Bookworm]

So I read it wrong the first time, which is very embarrassing because I do know how to read a thermometer/graph. Good grief. It is divided in increments of five, not ten. (It's also very obvious and easy to read, so I feel especially moronic.)

That means it was 110 in the trunk when it was 90 outside.

After you asked, I put the thermometer on the passenger seat and went in the house for 15 minutes. When I came back, it was 125. The passenger seat was being shaded by the door panel, so the thermometer itself wasn't getting direct sun. The outside temperature had risen to 92.

Even though I read it wrong the first time, the message is the same. If it's 90-ish degrees outside, it's 110 in the trunk and 125 in the car. I don't need to run another experiment to know that if it's 110 or 115 outside, it's really freaking hot in the trunk.

[MDM]

If it's 90-ish degrees outside, it's 110 in the trunk and 125 in the car. I don't need to run another experiment to know that if it's 110 or 115 outside, it's really freaking hot in the trunk.

Always good when quantitative measurements confirm qualitative theory.  Thanks for this nice vignette.

[marty998]

After you asked, I put the thermometer on the passenger seat and went in the house for 15 minutes. When I came back, it was 125. The passenger seat was being shaded by the door panel, so the thermometer itself wasn't getting direct sun. The outside temperature had risen to 92.

Imagine if it was getting direct sun - DIY proof of why it is so dangerous to leave kids/babies in a locked car.

[GuitarStv]

After you asked, I put the thermometer on the passenger seat and went in the house for 15 minutes. When I came back, it was 125. The passenger seat was being shaded by the door panel, so the thermometer itself wasn't getting direct sun. The outside temperature had risen to 92.

Imagine if it was getting direct sun - DIY proof of why it is so dangerous to leave kids/babies in a locked car.

Makes me wonder if you could set your car up in a particularly sunny area and slow cook a meal over the day without electricity.

[waffle]

After you asked, I put the thermometer on the passenger seat and went in the house for 15 minutes. When I came back, it was 125. The passenger seat was being shaded by the door panel, so the thermometer itself wasn't getting direct sun. The outside temperature had risen to 92.

Imagine if it was getting direct sun - DIY proof of why it is so dangerous to leave kids/babies in a locked car.

Makes me wonder if you could set your car up in a particularly sunny area and slow cook a meal over the day without electricity.

I don't know about full on cooking, but I work in construction and when I was at a project (now at the corporate office) I'd put my foil wrapped lunch on the dash of my car in the sun an hour or so before I ate it and it would be nice and hot by lunch time.

[Stash Engineer]

Makes me wonder if you could set your car up in a particularly sunny area and slow cook a meal over the day without electricity.

Now this would be worth testing!  What's the temp inside a crock pot on high?  200-250F?

[forummm]

After you asked, I put the thermometer on the passenger seat and went in the house for 15 minutes. When I came back, it was 125. The passenger seat was being shaded by the door panel, so the thermometer itself wasn't getting direct sun. The outside temperature had risen to 92.

Imagine if it was getting direct sun - DIY proof of why it is so dangerous to leave kids/babies in a locked car.

Makes me wonder if you could set your car up in a particularly sunny area and slow cook a meal over the day without electricity.

Yes. They have done this in AZ in the summer. I've heard that it makes your whole car smell like whatever you were cooking for a long time though. Back when people had CDs, the heat would melt CDs and melt CD cases permanently shut.

https://www.youtube.com/watch?v=3qlimBm87GM

You can't actually fry an egg on the sidewalk though.

[Philociraptor]

Dashboard chocolate chip cookies!

[Mirwen]

I have had my shoes melt to the sidewalk though.  Blistered my feet.  I don't wear dress shoes in the summer anymore.

[CmFtns]

Maybe you should be asking can my cleaning supplies handle temperatures in the low 100s instead of how hot will my car get. I highly doubt anything will explode or anything. It's probably gets pretty hot in the back of delivery truck that took those products across the country and to the store (or delivered it to your house).

I do some car detailing as a hobby and have left all kinds of cleaning chemicals in the back of my very dark gary car (gets very hot) and it's all been fine.

[I'm a red panda]

Dashboard chocolate chip cookies!

I tried that in Texas.  Full on sunlight, 114 degree high.
They didn't cook well enough to eat, but the car smelled good.

I HAVE seen people fry an egg using only sunlight (in a pan), but you need some reflectors to actually make it work.  A car hood might work if you want to ruin your car.