Hello there! My name is Ken and I am a Chinese senior high A Level student in Guangdong Country Garden School. Welcome to Ken's Study Journey website with Physics knowledge explanations!
Temperature change of water is very slow and we usually get cold after swimming. It is related to physics knowledge like specific heat capacity and evaporation.
Recently, I travelled to Xunliao Bay, played on the beach and discovered some physics knowledge, including specific heat capacity and evaporation. I always discover and share the knowledge I learned in real life.
I also discovered some other knowledge such as tidal energy, accelerated motion and projectile motion. This knowledge is all covered in CAIE (IGCSE and A Level) Physics syllabuses.
Today, I am going to describe and explain some physics knowledge I discovered while travelling, let's go!
Specific Heat Capacity
Specific heat capacity is the energy needed to heat a substance per kilogram for 1 degree Celsius (or Kelvin temperature). The unit is J kg-1 ºC-1 or J kg-1 K-1 (K=Kelvin temperature).
The s.h.c. of water is larger than most of other substances. It is 4.2 × 103 J kg-1 ºC-1.
In IGCSE and A Level physics, we usually use powers in units. We usually use m s-1 rather than m/s in speed or velocity.
The power of -1 means dividing it. For example, 2-1 means 1/2, 2-2 means (1/2)2.
In AS and A Level physics, we need to work on units, including finding SI base units, so we usually use this notation.
The energy needed is calculated from a formula: E = c m ▵T. E is energy in Joules, c is specific heat capacity in J kg-1 ºC-1 or J kg-1 K-1, m is mass in kg and ▵T is the temperature change (▵ symbol means change).
Because the energy is proportional to specific heat capacity, the higher the s.h.c., the grater the energy is needed when the mass and temperature change are the same.
When we heat both 1 kg water and sand for 1 ºC, the energy absorbed from water is far more than sand.
We need to have some control varables while doing physics or chemistry experiments. While energy and s.h.c. are changing, mass of the substances and temperature changes are unchanged when we use different substances.
When we make comparisons, some other amounts are unchanged to ensure that the experiments are fair.
Evaporation
The second knowledge I want to mention is evaporation.
Before we start, let's talk about some basic knowledge: three states of substances.
Three States of Substances
There are 3 states in substances, solid, liquid and gas.
In solid, the molecules are packed together because they have strong intermolecular attractive forces. They can only vibrate and they cannot move.
Liquid molecules can move and slide over each other, but they are still close together. The liquid takes the shape of the container.
Gas molecules are far apart and they have low intermolecular forces and higher energy. They can move freely.
Now, I am going to talk about a relevant AS chemistry knowledge: Intermolecular Forces.
There are 3 types of intermolecular forces: induced dipole - induced dipole, permanent dipole - permanent dipole and hydrogen bonding.
Hydrogen bonding is the strongest type of intermolecular forces. It requires a hydrogen (H) atom and a fluorine (F), oxygen (O) or nitrogen (N). For example, H2O has δ+ and δ- charges. The 2 long pairs with 4 electrons on the oxygen atom can attract a hydrogen atom in an H2O molecule.
Like charges repel and unlike charges attract. If there are opposite charges on any two opposite sides in a molecule, it is polar with pd-pd forces or hydrogen bonds. Otherwise, it is non-polar with id-id forces.
For example, methane (CH4) is non-polar while ammonia (NH3) is polar.
Transformation of States
There are some transformations between these three states.
Solid and Liquid
When the temperature is very high beside the ice, the ice melts and the water is formed. Melting is involved in this process. When a heat is applied, the solid absorbs energy and it melts when the temperature arrives at its melting point.
During the melting process, the temperature remains constant because the energy absorbed is used to overcome intermolecular forces at the melting point instead of increasing temperature.
When the liquid is cooled down, the solid will be formed again at its melting point.
For example, here is a temperature-time graph of water when heating:
Solid and Gas
In fact, solids do have a method to form gases directly without forming liquids first. Also, gases can directly form solids.
These two processes are called: sublimation.
Liquid and Gas
However, the process of change between liquids and gases is slightly different from the above.
There are 2 ways for liquids to form gases: evaporation and boiling.
Similarly, the temperature is unchanged while the liquid is boiling. Also, water vapour can condense when it meets cold substances (e.g. cold Cocacola bottle taken from the refrigerator).
Evaporation vs. Boiling
So, what are the differences between evaporation and boiling? Here are the comparisons.
| Evaporation | Boiling |
|---|---|
| At the surface of liquid | Throughout the liquid |
| At any temperature | At a fixed temperature (boiling point) |
| Slow process | Fast process |
When we go out from the sea or swimming pool, we usually get cold because the water on the body evaporates. The water molecules on the surface have enough energy to escape from the liquid and form water vapour, so the remaining water has less energy and lower temperature.
Conclusion
The temperature change of water is very slow because it has a very high specific heat capacity and it absorbs more heat than other substances. We usually get cold after swimming because the water on the body evaporates and the temperature of the remaining water becomes lower.
Alright! This is the end of the physics knowledge explanation of specific heat capacity and evaporation. Thanks for reading and I hope you can understand the physics knowledge while travelling and playing on the beach.
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