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### Author Topic: Why does it take longer to heat two of the EXACT same items in a conventional oven?  (Read 5969 times)

#### Isaac_Smith

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« on: 18/06/2010 22:30:03 »
Isaac_Smith asked the Naked Scientists:

A similar question is answered on your website for microwaves but I think that is entirely different than a conventional oven.  Literally, no one on the internet has answered this question:

Why does it take longer to heat two of the EXACT same item in a conventional oven?  The size and the amount of "work" the oven needs to do is the exact same on each item individually and if the oven only knows it needs to be "400 degrees," why is it taking longer to cook each item when they are together than just one on its own?

Isaac Smith

What do you think?
« Last Edit: 18/06/2010 22:30:03 by _system »

#### Geezer

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #1 on: 19/06/2010 03:53:05 »
Isaac_Smith asked the Naked Scientists:

A similar question is answered on your website for microwaves but I think that is entirely different than a conventional oven.  Literally, no one on the internet has answered this question:

Why does it take longer to heat two of the EXACT same item in a conventional oven?  The size and the amount of "work" the oven needs to do is the exact same on each item individually and if the oven only knows it needs to be "400 degrees," why is it taking longer to cook each item when they are together than just one on its own?

Isaac Smith

What do you think?

Hi Isaac,

I would think it's because the air movement (convection) inside the oven is altered when there are two. The average temperature in the oven will be the same in both cases, but the alteration of the air convection paths will reduce the rate of heat transfer into the items being cooked. It would also have a lot to do with where they were placed in the oven relative to each other.

The effect should be less noticeable with a fan assisted convection oven.

#### LeeE

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #2 on: 19/06/2010 16:58:27 »
Items inside an oven are heated by a combination of Infra Red (IR) and conduction.  Without anything inside the oven, the heating elements will emit IR, which will then be absorbed by the other interior walls of the oven, making them IR emitters too, and the air which, as it comes into contact with the interior walls of the oven, will be heated by conduction.  As the air becomes heated it will start convection currents, leading to a degree of air circulation within the oven.

When you put something inside the oven some of the IR that is heating the interior walls of the oven will be absorbed by the item you're cooking instead, with the result that the interior walls that were being heated will receive less IR and start to cool.  Similarly, the item will also absorb some of the energy from the heated air, as the air comes into contact with it, cooling the air down too.

So when you put something cold into an oven it will cool the oven as it absorbs heat energy from it.  An oven though, can only produce a limited amount of 'heat' at any particular temperature, so the more stuff you put in it the more quickly the heat that the oven can produce is absorbed and so the longer it will take to heat them.

#### Geezer

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #3 on: 19/06/2010 17:16:46 »
So when you put something cold into an oven it will cool the oven as it absorbs heat energy from it.  An oven though, can only produce a limited amount of 'heat' at any particular temperature, so the more stuff you put in it the more quickly the heat that the oven can produce is absorbed and so the longer it will take to heat them.

Doesn't the thermostat compensate for that effect? With more "stuff" in the oven, the duty cycle will simply increase. The heating element is usually on for relatively short intervals, so the oven has plenty of reserve heat generating capacity.

(I'm referring to an electric oven BTW.)

#### tommya300

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #4 on: 19/06/2010 23:45:28 »
So when you put something cold into an oven it will cool the oven as it absorbs heat energy from it.  An oven though, can only produce a limited amount of 'heat' at any particular temperature, so the more stuff you put in it the more quickly the heat that the oven can produce is absorbed and so the longer it will take to heat them.

Doesn't the thermostat compensate for that effect? With more "stuff" in the oven, the duty cycle will simply increase. The heating element is usually on for relatively short intervals, so the oven has plenty of reserve heat generating capacity.

(I'm referring to an electric oven BTW.)
.
I can see both of your posts make sense!
The heating process inside any oven is not an instantaneous energy transfer.
It is more exponential, temperature vs time.
That is why preheating an oven is important.
Anything you put in the oven to cook is essentially a heat sink.
.

#### LeeE

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #5 on: 20/06/2010 12:46:21 »
Although electric ovens may have a thermostat they still have a limited heating capacity: it seems that ~4kW is typical for a home oven.  I don't know if any gas ovens are made with thermostats, or not, but none of the ones of which I'm aware have them and are just set to a particular level or 'Mark'.  With these, you're clearly just supplying a constant rate of energy, regardless of how much you've put in them.

#### tommya300

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #6 on: 20/06/2010 13:57:52 »
I seen only thermostatic controls on all indoor ovens.
That is how one can maintain a specific temperature given by the cook books.

Outdoor grilling are not thermostatically controlled since the cook always watching, and usually haing a visual on the cooked surface of the food.

I have never seen a gas driven convection oven.

#### Bored chemist

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #7 on: 20/06/2010 17:24:13 »
The simple answer is contained in this part of the question. "Why does it take longer to heat two of the EXACT same item in a conventional oven?  The size and the amount of "work" the oven needs to do is the exact same on each item individually ".
It's true that the oven has to deliver the same amount of energy to each item. So in total it has to deliver twice as much. It only has a finite amount of power available and so it takes longer.

#### Geezer

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #8 on: 20/06/2010 17:50:58 »
Although electric ovens may have a thermostat they still have a limited heating capacity: it seems that ~4kW is typical for a home oven.  I don't know if any gas ovens are made with thermostats, or not, but none of the ones of which I'm aware have them and are just set to a particular level or 'Mark'.  With these, you're clearly just supplying a constant rate of energy, regardless of how much you've put in them.

Gas ovens do have a thermostat. When you turn them on, the flame goes at full blast for a while until the desired temperature is achieved, then the flame drops down. If you leave the door open for a bit, the flame increases again.

Also, 4kW is a huge amount of power relative to anything you are likely to be trying to cook. Imagine what would happen if you embedded a 4kW heating element inside a chicken and turned it on.

The purpose of an oven is to supply heat slowly enough to allow the item be thoroughly cooked. It's really quite easy to burn things in an oven if the temperature is set too high, so there must be plenty of power available.

#### Geezer

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##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #9 on: 20/06/2010 18:23:00 »
The simple answer is contained in this part of the question. "Why does it take longer to heat two of the EXACT same item in a conventional oven?  The size and the amount of "work" the oven needs to do is the exact same on each item individually ".
It's true that the oven has to deliver the same amount of energy to each item. So in total it has to deliver twice as much. It only has a finite amount of power available and so it takes longer.

I think I have established that the oven has more than enough power available. The difference is much more to do with the energy coupling method than anything else. Most of the heat transfer is by conduction between the item being baked and the air in the oven.

Clue: Fan assisted ovens cook food faster.

I don't know for sure, but I suspect fan assisted ovens are also much less likely to be influenced by the position of the food and number of items in the oven.

Without fan assistance, two items will significantly alter the convection pattern in the oven compared to one item.

#### The Naked Scientists Forum

##### Why does it take longer to heat two of the EXACT same items in a conventional oven?
« Reply #9 on: 20/06/2010 18:23:00 »