# Heat and Energy Transfer

## Presentation on theme: "Heat and Energy Transfer"— Presentation transcript:

Heat and Energy Transfer
How does energy move around? How can you make energy do what you want it to do? Images from Microsoft Office Clipart

Energy Energy is the ability to do work
Massless and does not take up space It can be measured only by its effects on matter Comes in many forms such as energy of motion, stored (potential) energy, chemical energy, or energy associated with nuclear forces Energy is defined as the ability to do work or to put matter into motion. Sometimes that motion is microscopic, resulting in heat, and other times it is macroscopic. It can be obvious, like the energy associated with a moving car, or hidden, such as that stored in a stretched rubber band or a rock high on a mountain. NASA Image from

Heat Heat is transferred energy that arises from the random motion of molecules The ability to transfer and store heat is related to the structure of a substance Heat transfer involves at least one substance losing energy and another gaining energy Heat is energy that is being transferred by motion of molecules. Once the energy has been transferred, it is no longer “heat” per se, but instead is energy. One way to measure the amount of energy contained internally in a substance is through a temperature measurement. NASA Image from

Transfer of Energy through Heat
Conduction (solids) Convection (fluids – gases and liquids) Radiation (light, heat, radio waves - can pass through a vacuum) Conduction involves moving molecules bumping into each other because they are physically close. Convection moves molecules from one place to another, allowing these molecules to provide or receive energy. Radiation is considered heat because the vibration of molecules forms electromagnetic radiation NASA images from

Conduction Transfer of heat within and between substances that are in direct contact with each other The better the conductor, the more rapid the heat transfer A better conductor has a higher “conductivity”. Conductivity is typically related to how close the molecules of a substance are together (so they bump into each other more frequently) and the heat capacity (lower heat capacity substances “rather” move the heat along instead of keeping it form themselves.) Heat capacity increase for substances with more ways (more degrees of freedom) to store energy. For example, an atom in a monotonic gas can just move about while a molecule in a diatomic gas can move about and spin, so the diatomic gas will likely have a higher heat capacity (and a lower conductivity.) Images from Microsoft Office Clipart

Convection Convection is the transfer of heat by the bulk motion of the substance containing the heat. Natural convection happens when warm (less dense) substances rise Natural convection plays a roll in plate tectonics, global wind and ocean currents. Forced convection is a result of moving the substance intentionally, such as with a fan heating systems and computer cooling fans are examples of forced convection In natural convection, hot air, water, molten rock, rises as it expands because of buoyancy. As it moves farther away from the heat source it cools and becomes denser and so falls. Plate tectonics – molten rock in the mantle is heated by the hot core of the Earth and then rises to just below the Earth’s crust where it cools and slowly drops. Ocean currents – Warm water heated at the surface moves poleward where it cools and slowly drops Wind – Hadley cells. Air is heated at the equator and rises creating low pressure. It then moves to cooler upper parts of the atmosphere where It slowly descends and since it is cooling and denser creates high pressure. Wind is really air moving from high pressure to low pressure across the surface of the Earth. In forced convection we intentionally move air from one place to another, typically with a fan. Air coming from a heating vent in a home or car is one example, as is a fan blowing air through a computer to cool it off. Image from NASA: Image from EPA:

Radiation, or electromagnetic energy comes many forms depending on the amount of energy. Energy indicated by temperature scale at the bottom of the image. Different than other energy types in that MATTER IS NOT NECESSARY for radiation transfer. Example: Suns energy in the form of visible light passes from the Sun, through the vacuum of space to us on Earth. Visible light Microwaves Radiowaves X-rays Radiation heat transfer can be highly efficient as the rate is proportional to the difference in the fourth power of the temperatures. It is also affected by the type of surface (shiny versus dark) and the “shape factor”, which accounts for how one object “sees” the other. Electromagnetic radiation is formed when the molecules in a substance vibrate at high frequencies. The more energy in the system, the stronger the vibration, and the higher the radiation frequency. Wikimedia Commons from NASA

Key Terms Energy – ability to do work
Heat – Heat is transferred energy that arises from the random motion of molecules Temperature – a relative term reflecting how vigorously the atoms of a substance are moving and colliding. Work: measured in Joules (SI) or Nm Heat flows from a warmer object to a cooler object Efficiency: ratio of work output to work input

Units of Heat Calorie BTU (British Thermal Unit) Joule
The amount of heat required to raise the temperature of one gram of water 1oC. BTU (British Thermal Unit) The amount of energy needed to heat one pound of water one degree Fahrenheit Joule The international unit of energy, not defined by a temperature change  4.184 Joules in one calorie 1, Joules in one BTU The calorie has been commonly replaced by the Joule. The unit of heat in the SI-system is the Joule BTU: English System natural gas is measured in BTUs

Cooling a Hot Glass of Liquid
Heat is transferred in many ways from a hot glass of liquid. Shown in this diagram: Conduction from the liquid to the air just above it Natural convection of the air above the liquid up and away from the glass Conduction through the glass itself The surface of the glass radiating heat away from it (perhaps to a cooler wall) The same surface is conducting heat to the air next to it (not shown) That air then convects away as it heats up Not shown is that radiation can also happen from the top of the liquid. There can be internal convection currents moving the cooler liquid from the sides down to the bottom. You can accelerate the cooling of the liquid by blowing on it (forced convection). Original Image, Joe LoBuglio, North Carolina School of Science and mathematics

Thermos A thermos largely prevents heat transfer by blocking the various paths. Conduction from the liquid to the air just above it is stopped with a stopper near the surface of the liquid Natural convection of the air above the liquid up and away from the glass is stopped because it is closed Conduction through the glass itself is prevented by having a vacuum (no material) in the walls The surface of the glass radiating heat away from it (perhaps to a cooler wall) is reduced by having shiny surfaces There are still some areas where heat can escape, for example conduction through the cap and the bridge of material at the very top separating the inner and outer layer of the glass. Original Image, Joe LoBuglio, North Carolina School of Science and mathematics

Heat Capacity A temperature difference causes heat to transfer from one place to another Upon gaining or losing energy, an object will increase or decrease its temperature The heat capacity is a constant that tells how much heat needs to be added per unit temperature rise The greater the heat capacity, the more energy can be stored at a given temperature difference. Image courtesy of NASA:

Example of Heat Capacity
1 kg or iron has a heat capacity of 450 J/K 1 kg of water has a heat capacity of 4181 J/K 𝑄= 𝑐 ∆𝑇 ∆𝑇 = 𝑄 𝑐 450 joules of energy is needed to increase the temperature of 1kg of iron by 1 degree kelvin That same 450 joules would change the temperature of 1 kg of water by about 0.11 degrees K. 1 kg of water has a greater volume than 1 kg of iron, which is illustrated by increasing each dimension of the square by a factor of 2. If each were a cube, the water would be 8 times the volume. Original image: Joseph LoBuglio, North Carolina School of Science and Mathematics

Heating Losses in a Home
In the winter homes lose heat to the outside through conduction, convection, radiation, and infiltration These losses can be reduced by good home design, but there is always some loss of heat To keep the inside of a home warm the lost heat needs to be replaced Consider a home like a thermos. Where is conduction, convection, and radiation occurring and what can be done to reduce these? Conduction: insulated walls, windows, ceiling Convection: sealing up leaks, tight fitting doors and windows, sealed heating unit Radiation: reflective barrier internal to walls and below the roof. Varying roof and wall color and also landscaping Image from energy.gov:

Temperature Swings in a Home
Homes lose and gain energy, resulting in temperature changes Temperature changes can be lessened if a home contains materials with a high heat capacity Solar homes gain energy during the day. Very high heat capacity allow the energy to be stored in the house without overheating Solar homes have a more extreme variation in heat because of the large input during part of the day and the desire to use that heat long after the sun has peaked. Raising the heat capacity of a home allows heat to be stored without a dramatic temperature swings. Measures are taken to increase the heat capacity (increasing thermal mass), such as internal brick walls and concrete floors. The home in this picture has a 4” thick concrete floor that absorbs heat from the large southern-facing window. Original photograph by Joseph LoBuglio, North Carolina School of Science and Mathematics