Thermodynamics Equation
What is Heat? Heat is Energy- Kinetic Energy- The motion of electrons around the nucleus of an atom. Perception as to hot and cold defined relative to out own body temperature, i.e. object is hotter or colder than oneself I feel hot He is hot
Hotter things become longer All(?) solids get bigger when they get hot A 1 meter long bar heated by 1 degree gets bigger by Steel ≈0.01 mm Glass ≈ 0.001 mm Rails expand and may buckle on a hot summer day Join two metals with different coefficient of thermal expansion to create a bimetallic strip (fire alarms and thermostats)
Thermal expansion, why? x As an atom heats up it’s electrons spin faster. Those atoms hit each other with more force and bounce further apart As an object is heated it becomes less dense. Potential energy between two atoms
Thermal expansion of water Density of ice is less than water!!! Icebergs float! It has to do with the hydrogen bonds in water. As water cools below 4°C, the hydrogen bonds adjust to hold the negatively charged oxygen atoms apart.
Conductors and Insulators Insulators prevent heat transfer. Examples: A vacuum, polystyrene, fibreglass, plastic, wood, brick Conductors transfer heat easily. Examples of poor insulators, i.e. good conductors: Most metals, Gases, liquids
Temperature and scales Temperature scales (melting & boiling of water) Degrees Celsius (MP 0°C 100°C) Degrees Kelvin (MP 273.15 K BP 373.15 K) Degree Fahrenheit (MP 32° F BP 212°F)
Converting between scales Kelvin to Celsius K = C + 273.15 C = K - 273.15 Fahrenheit to Celsius F = C x (9/5) + 32 C = (F - 32) x (5/9)
1st Law of Thermodynamics Energy can be changed from one form to another, but it cannot be created or destroyed. The total amount of energy and matter in the universe remains constant, merely changing from one form to another.
Heat transfer = energy transfer Energy measured in Joules but heat often measured in calories One cal raises one gram of water from 14.5°C to 15.5°C 1 cal = 4.186J Doing work on something usually makes it hot Splash in the bath and the water will get warm! 1st law of thermodynamics heat and work are both forms of energy
2nd Law of Thermodynamics Law of Entropy When energy transfers from one object to another some energy will always be lost to heat. No machine is 100% efficient. What is the outcome? While the amount of energy in the universe is constant (First Law), the quality of energy deteriorates gradually over time. How so? Usable energy is used for productivity, growth and repair. In the process, some energy is lost to heat. Thus, usable energy is permanently lost to unusable energy.
Specific Heat Objects heat and cool at specific rates. Some object heat up faster than others. This is an object specific heat.
How to calculate… Q = mc DT Q is energy measured in J m is mass measured in kg C is specific heat measured in J/gC Temperature is measured in degrees Celcius.
Example A pot with 5 kg of water rises in temperature from 4°C to 8°C. If water has a specific heat of 4.18 J/gC, how much energy was used to heat the pot of water?