Anneke Levelt Sengers and the Story of Heat

Anneke Levelt Sengers is a Dutch physicist who is known for her work on thermodynamics, heat and critical states of fluids. Through the story of Anneke Levelt Sengers, we’ll explore the physics of heat and show it can be created and converted to other forms of energy.

Anneke Levelt was born in Amsterdam, the Netherlands, in 1929.  Her father was a Ph.D. Chemist and her mother had a Master’s degree in physics; an emphasis on education and science was clearly part of the family culture.  However, Anneke was the oldest of the family’s ten children, and some family members and friends expected her to stay home after finishing high school and take care of her younger siblings.  But Anneke had more ambitious plans!  She applied for and received an interest-free government loan and enrolled at the University of Amsterdam, home of Nobel Laureates Van der Waals and Zeeman. Her parents supported her choice.

Anneke spent her career working on critical phenomena in fluids, fluid mixtures and ionic fluids, from theory to experiment and databases for practical applications. They developed critical-region scaling concepts for fluids and fluid mixtures, and for solubility behavior near a solvent’s critical point. They also performed measurements of density, phase behavior and other properties of industrially important fluids such as carbon dioxide, ethylene, water and geothermal fluids.
Her outstanding research also attracted many collaborators from all parts of the world.  Anneke said: “I was particularly proud of the gifted postdoctoral women to whom I could give a boost at a time when hurdles were high for women aspiring careers in science and engineering.”

Levelt Sengers retired from the National Institute of Standards and Technology (NIST) in 1994, after a 31 year career there. She currently co-chairs the InterAmerican Network of Academies of Sciences women for science program.

What is heat?

When you think of heat, you might be thinking of a being out on a hot summer’s day, but in physics there is a big difference between heat and temperature. Heat is a form of energy and like all other forms of energy, it can be converted into other forms. We see can see an example of this in a mercury thermometer. When you put the thermometer into a hot liquid, the heat energy gets transferred to the mercury in the thermometer. This causes the mercury to heat up and rise to indicate a certain temperature. In this case, heat energy has been converted to kinetic energy. When heat energy is released from one object into another, we call this process heat transfer.

Heat transfer

Have you ever left a tea spoon in a cup of tea for too long and burned yourself when you picked it up? Or passed your hand over a boiling kettle and gotten a nasty burn from the steam? Have you ever made popcorn in a microwave? These are all examples of heat transfer, and each of them are a different way of transferring heat. The three ways that heat can be transferred are:

  1. Conduction: Heat is transferred through a solid, via the vibration of molecules
  2. Convection: Heat is transferred through a liquid or gas, via the movement of molecules
  3. Radiation: Heat is transferred without the need of a medium, via electromagnetic radiation

Expansion of Solids, Liquids and Gases

The vast majority of materials will expand (get bigger) when heated and contract (get smaller) when cooled. When heat is transferred to a solid, liquid or a gas they will expand. This is a very important property that needs to be needs to be taken into account by engineers in sunny countries who need to make sure a bridge or building won’t collapse when the metal they are made from expands in the heat. One of the most common exceptions to this rule is water. When you freeze water (i.e. make it colder) it will expand – it is the only liquid to behave like this. You can see this if you fill a plastic bottle with water and put it in your freezer. After a day or so, the ice will burst through the bottle.  

Temperature

So we now know that heat is a form of energy, but what is temperature? Temperature has nothing to do with energy at all, it is a measure of how hot or cold something is. To measure the hotness or coldness of something we have to use a thermometer. Thermometers give a numerical value for the temperature of an object but first must be calibrated so we can trust their measurements. The best way to do this is to use ice cold water (which we know is at 0 oC) and boiling water (which we know is at 100 oC). From getting those two points, we can get the temperature of any material. If we want to raise the temperature of an object, we need to apply energy (in the form of heat). Every material has its own specific heat capacity, which is the amount of energy required to change the temperature of 1 kg of a material by 1 degree Celsius.