The heat balance equation is given by, qi = qo+q1 -Eq (2) Where, qi = Input heat rate from the heater to the copper rod (Watts). Qo = Output heat flow rate from the rod. = Heat flow rate absorbed by water in the cooling water jacket (Watts). Q1= Heat loss from the rod to the surrounding s through thermal insulation, watts (Watts), Assumed to be zero. There is a heat source at the bottom of the rod and a fixed temperature at the top. The outer surface of the rod exchanges heat with the environment because of convection. In addition, the rod itself generates heat because of radioactive decay. The goal is to find the temperature in the rod as a function of time. The 1-D Heat Equation 18.303 Linear Partial Differential Equations Matthew J. Hancock Fall 2006 1 The 1-D Heat Equation 1.1 Physical derivation Reference: Guenther & Lee §1.3-1.4, Myint-U & Debnath §2.1 and §2.5 Sept. 8, 2006 In a metal rod with non-uniform temperature, heat (thermal energy) is transferred. Manufacturer of Heat Transfer Lab - Metal Rod Thermal Conductivity, A Pin Fin Heat Transfer, Natural Convection Heat Transfer and Liquids Thermal Conductivity offered by Krishana Enterprises, Ambala, Haryana. Krishana Enterprises. Village Keshopur, Ambala, Haryana. Convective Heat Transfer - Heat transfer between a solid and a moving fluid is called convection. This is a short tutorial about convective heat transfer; Cooling Mode - Heat Flux - Heat fluxes for various cooling or heat transfer modes; Cylinders and Pipes - Conductive Heat Losses - Conductive heat losses through cylinder or pipe walls.
PrintHeat escapes (or transfers) from inside to outside (high temperature to low temperature) by three mechanisms (either individually or in combination) from a home:
- Conduction
- Convection
- Radiation
Click here to open a text description of the examples of heat transfer by conduction, convection, and radiation
- Conduction: heat moving through walls of a home from high temperature inside to low temperature outside.
- Convection: heat circulating within the rooms of a house.
- Radiation: Heat from the sun entering a home.
Conduction
Heat Transfer Through A Metal Rod Experiment
Conduction is a process by which heat is transferred from the hot area of a solid object to the cool area of a solid object by the collisions of particles.
In other words, in solids the atoms or molecules do not have the freedom to move, as liquids or gases do, so the energy is stored in the vibration of atoms. An atom or molecule with more energy transfers energy to an adjacent atom or molecule by physical contact or collision.
In the image below, heat (energy) is conducted from the end of the rod in the candle flame further down to the cooler end of the rod as the vibrations of one molecule are passed to the next; however, there is no movement of energetic atoms or molecules.
Click play to start animation.
Click here to open a text description of the Conduction Candle animation
Example of Conduction
A hand holds a metal rod above a lit candle. The molecules quickly heat up at the point where the flame touches the rod. The heat then spreads across the entire metal rod and the heat is then able to be felt by the hand.
With regard to residential heating, the heat is transferred by conduction through solids like walls, floors, and the roof.
Click here to open a text description of Conduction in Regard to Residential Heating example
Example of Conduction in Regard to Residential Heating
Picture the cross section of a wall in a house. Inside the house it is 65°F and outside it is 30°F. Two arrows point from inside the house to the outside to show how heat is transferred from the inside of the house to the outside through the wall via conduction.
Heat loss across a solid wall by conduction
Convection
Convection is a process by which heat is transferred from one part of a fluid (liquid or gas) to another by the bulk movement of the fluid itself. Hot regions of a fluid or gas are less dense than cooler regions, so they tend to rise. As the warmer fluids rise, they are replaced by cooler fluid or gases from above.
In the example below, heat (energy) coming from candle flame rises and is replaced by the cool air surrounding it.
Click here to open a text description of the Convection Candle animation
Example of Convection
A hand is held above a lit candle. As the candle heats the air, the heat rises to the hand. Eventually, it gets too hot and the hand pulls away from the candle.
In residential heating, convection is the mechanism by which heat is lost by warm air leaking to the outside when the doors are opened, or cold air leaking into the house through the cracks or openings in walls, windows, or doors. When cold air comes in contact with the heater in a room, it absorbs the heat and rises. Cold air, being heavy, sinks to the floor and gets heated, and thus slowly heats the whole room air.
Instructions: Press the play button below and observe what happens to the cold air (blue arrows) as it enters the house and encounters the warm air (red arrows) coming from the heating vent:
Click here to open a text description of the Convection in a Room animation
Example of Convection in Regard to Residential Heating
Picture a room with an open door letting in cool air on the left and a radiator creating heat on the right. As the radiator heats the air around it, the air rises and is replaced by cool air. Once the warm air hits the ceiling, it travels left towards the open door, cooling as it moves. The cool air from the open door travels to the right across the floor towards the radiator to be heated. The overall effect is a circular convection current of air within the room.
Radiation
Radiation is transfer of heat through electromagnetic waves through space. Unlike convection or conduction, where energy from gases, liquids, and solids is transferred by the molecules with or without their physical movement, radiation does not need any medium (molecules or atoms). Energy can be transferred by radiation even in a vacuum.
In the image below, sunlight travels to the earth through space, where there are no gases, solids, or liquids.
Click here to open a text description of the Radiation Example animation
Example of Radiation
Picture the Sun and the Earth with arrows traveling from the Sun to the Earth through space. The arrows represent the energy that travels to the Earth via radiation, which does not require any medium (atoms or molecules) to do so.
Test Yourself
First, identify the type of home heat loss pictured in images A-J as either: conduction, convection or radiation. Then click and drag each image down to the correct category at the bottom of the screen.
Click here to open a text description of the Test Yourself activity
Test Yourself: Types of Heat Loss
Identify the type of heat loss (conduction, convection, or radiation) for each of the following examples:
- Heat escaping through the roof of a house
- A hot stove burner
- Boiling water
- A torch halogen lamp producing light and heat
- A door hanging wide open, letting in cold air
- A fire creating heat
- Heat escaping through a wall
- A mirror reflecting sunlight
- Heat escaping through a window
- Heat escaping through a chimney
Answers:
A. Conduction
B. Radiation
C. Convection
D. Radiation
E. Convection
F. Radiation
G. Conduction
H. Radiation
I. Conduction
J. Radiation
Reducing Energy Consumption
There are two ways in which we can reduce energy consumption.
- The most cost-effective way is to improve the home’s “envelope”—the walls, windows, doors, roof, and floors that enclose the home—by improving the insulation (conduction losses) and sealing the air leaks with caulking (convection losses).
- The second way to reduce the energy consumption is by improving the efficiency of the furnace that provides the heat.
Click here to open a text description of the Conduction and Convection diagram
Line drawing of a house with arrows pointing out from the walls and roof showing conduction & arrows flowing in a circular motion inside the house showing convection.
Science >Physics >Heat Transfer > Conduction
Heat always gets transferred from the body and higher temperature to a body at lower temperature heat transfer can take place in three ways a) Conduction b) Convection and c) Radiation. In this article, we shall study the heat transfer by the conduction.
Conduction:
If one end of a metal rod is heated, the other end also gets heated up. This is due to conduction. When one end of a metal rod is heated, the kinetic energy of the molecules at that end increases. The molecules start vibrating with a higher amplitude. These molecules start vibrating with a higher amplitude. These molecules during vibration collide with the neighbouring molecules and transfer part of their energy to the neighbouring molecules. Thus the kinetic energy of the neighbouring molecules increases hence their amplitude of vibration increases and during the collision the energy transfers to the next molecule. Thus heat transfer takes place by conduction.
The modeof heat transfer between two parts of a body or between two bodies in contactwhich are at different temperatures without actual migration of particles ofthe body is called conduction.
Heat Transfer In Metal Rods
Dependingupon easiness of heat transfer by conduction the substance are classifieds intotypes a) Good Conductors and b) Bad conductors
Good Conductors:
Thesubstances which allow the heat to pass through them very easily are calledgood conductors. Examples. Aluminum, copper, Silver, Steel, Bronze, Brass, allmetals
Bad Conductors:
Thesubstances which do not allow the heat to pass through them are called badconductors. Bad conductors of heat are also called as insulators. Examples:wood, rubber, Plastic, paper, glass, air, ebonite , bakelite.
Heat Transfer In Metal Rod
Use of conduction:
- Metals are used for making utensils because the metals are good conductors of heat they allow heat to pass through them easily.
- Cooking vessels have plastic handles because plastic a bad conductor of heat it does not allow the heat to pass through from hot vessel to hands and thus danger of burning can be avoided.
- Tea-cups, Teapots, coffee jugs are made of porcelain.
- Mountaineers use sleeping bags in polar regions.
- People wear woolen cloth in winter.
- Nowadays cooking vessels are made with copper bottoms.
- In winter, the metal lock feels colder than the wooden door on touch.
Characteristics of conduction:
- In this type of heat transfer, there is no actual migration of the medium particles from one point to another.
- For conduction, there must be a material contact between the two bodies.
Concept of Steady-State and Temperature Gradient:
Heatconduction may be described quantitatively as the time rate of heat flowin a material for a given temperature difference.
Consider ametallic bar AB of length L and uniform cross-sectional area A withits two ends maintained at different temperatures. The temperaturedifference between the ends can be obtained by keeping the ends in thermalcontact with large reservoirs having temperature differences. Some holesare drilled on this rod to insert thermometers (say T1, T2,T3, and T4) in the rod. For better thermal contactbetween the rod and thermometers mercury is poured into the holes. The sides ofthe bar are fully insulated so that no heat is exchanged between thesides and the surroundings.
Let θ1, θ2, θ3, and θ4 be the temperatures recorded by the thermometers T1, T2, T3, and T4 respectively. Initially, the temperature rises and after some time every thermometer shows its own constant reading such that (θ1 > θ2 > θ3> θ4). This state is called the steady-state.
Due to the insulation of the rod, no heat is lost due to surroundings. At a steady-state, at every cross-section of the rod, the quantity of heat entering the section in one second is equal to the quantity of heat leaving the section due to conduction.
Let usconsider two sections separated by distance Δx and let Δθ be thetemperature difference between these two sections. then the quantity Δθ/ Δx is called the temperature gradient.
The temperature gradient is defined as the rate of change of temperature with the distance when the material is in steady-state.
Thermal Conductivity:
It is found experimentally that in this steady state, the rate of flow of heat (or heat current)H is proportional to the temperature difference (θ2 – θ1) and the area of cross-section A and is inversely proportional to the length L
Where K = Constant called the thermal conductivity or the coefficient of thermal conduction the material. The greater the value of K for a material, the more rapidly will it conduct heat.
The SI unitof K is J S–1 m–1 K–1 (jouleper second per metre per kelvin) or W m –1 K–1 (wattper metre per kelvin).
The value ofthermal conductivity varies slightly with temperature but can be considered tobe constant over a normal temperature range. Good thermal conductors havevery high values of thermal conductivity while thermal insulators havenegligible values of thermal conductivity.
Heat Transfer Using Metal Rod Wax And Pins
Houses made of concrete roofs get very hot during summer days because the thermal conductivity of concrete (though much smaller than that of metal) is still not small enough. Therefore, a layer of earth or foam insulation is put on the ceiling so that heat transfer is prohibited and the room remains cooler.
Searle’s Experiment:
Apparatus:
Apparatusconsists of the thermally insulated box housing a metallic bar of auniform cross-sectional area with its one end kept in contact withsteam in a steam chamber. Two holes are drilled to insert thermometers T1 and T2,in the rod separated by distance x. For better thermal contact between the rodand thermometers mercury is poured into the holes. Cooling water is circulatedaround the rod whose initial and final temperatures are measured by thethermometers T3 and T4.
Working and Calculations:
At steady state, the heat lost by rod = heat gained by the water
Where, mW = Mass of water, SW =Specific heat of water, t = time for which heat is flowing
Measuring all values on R.H.S. of the formula value of K canbe found.
Values of thermal conductivity in J S–1 m–1 K–1 fordifferent materials are given below