Chapter 4: Preparing for the test

• Natuurkunde
• Middelbare school
• havo, vwo
• Leerjaar 2

HEAT SOURCES

A heatsource converts energy into heat.

Some examples:

•  the sun
• a gasstove
• a hairdryer

Heat transfer

The movement of heat energy from one warmer place to another colder place through conduction, convection or radiation

Heat transfer 

by conduction

Metals can conduct heat very well, that's why for cooking we mainly use metal pans. 

But some metals conduct better than others, therefore you see a lot of copper pans in a professional kitchen.

Keep in mind that if a material heats up very quickly, it will cool down quickly too.

Heat transfer 

by convection

Heat can move around freely in a liquid. Therefore cooking is a great way to heat up your food. 

The liquid will make sure that all parts of the food come into contact with the heat, while at the same time it prevents the food from burning because it will not get too hot at any one point.

 

Heat transfer 

by radiation

Heat radiation is one way for the heat to move around. Every object with a lot of heat in it, can emit this heat again as heat radiation. 

Matt black objects are very good at absorbing heat, but can also radiate a lot of heat as well.

Heat radiation is also called infrared radiation. This type of radiation cannot pass through glass and is reflected by shiny objects.

HOT

COLD

Heat will only move between places if there is a temperature difference! 

Heat will flow 

from high to low temperatures.

Heat radiation is one way for the heat to move around. Every object with a lot of heat can emit heat as heat radiation. Matt black objects are very good at absorbing heat, but can also radiate a lot of heat as well.

Heat radiation is also called infrared radiation. This type of radiation cannot pass through glass and is reflected by shiny objects.

Summary heat transfer

Conduction:

The flow of heat through physical contact,

Some materials conduct better than others

Convection:

Heat transfer due to motion of a fluid or gas.

If the temperature rises, the density drops making the fluid or gas go up

Radiation:

Heat transfer by electromagnetic waves.

When these electromagnetic waves are absorbed by an object the object gets warm.

A material which can expand very quickly will contract quickly as well.

Gasses can expand more than liquids. Liquids can expand more than fluids.

INSULATION

This has all to do with preventing heat from moving from one place to another. We take a look at the different methods to do this for every type of heat transfer (i.e. conduction, convection & radiation).

Heat radiation

When an object gets hot, 

it can radiate this heat again.

You can make this heat radiation visible using an infrared camera

Absorption & reflection

(heat radiation)

Heat can be absorbed by dark, matt objects. If you live in a warm country you can do several things to prevent your home from heating up, like painting the walls white, use small windows or thick walls.

Don't wear black

There is a reason why fireman have special (shiny) suits.

But that's not all there is to it. Their suits will often be build up out of different layers of materials to combat the heat as best as possible.

Stagnant air

• One way to prevent heat conduction is to make use of materials  (like foam) in which the air is prevented from moving around. Stagnant air is a very good insulater because it is a bad conductor by itself and cant transfer heat through convection.
• To keep a hot drink hot or a cold drink cold you can therefore make use of a cup with a double wall. Inside, between the two layers of glass, the air is prevented from moving around making convection impossible.

A thermos flask

is a great example 

of how all the ways 

of preventing heat transfer come together

Summary heat insulation

Always try to think about the three ways heat can transfer from one place to another. Think about what each method needs and how you can prevent this.

MEASURING HEAT

Heat is not the same as temperature.

Heat is an energy source and energy can be used to make things work.

But this energy never gets lost. The energy is just being converted into other forms of energy.

For instance you can use heat to make something move and then produce electricity.

Temperature is something you van measure. You need to agree on which method you use to measure the temperature to be able to compare it with other.

This temperature does give you an indication about the amount of heat.

difference

Heat is an indication of how much energy an object can give.

Temperature is an indication of how hot or cold an object is.

CALCULATIONS

The way to solve a question

1. write down what is given
2. write down what is asked
3. think about the formula's you might need
4. fill all the given information into the formula
5. convert some of the units if needed
6. if there is only 1 unknown vairable then you can make a calculation
7. check if your anwer is reasonable and if it has the right unit

So, work in little steps.

Heat 

Heat = specific heat x mass x temperature difference

Efficiency

In this example the efficiency will be calculated as follows:

​40J​​8J​​

x 100 % = 20 %

Efficiency is about the amount of energy which is used usefully compared to the total amount of energy you put into everything.