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 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. Thermal Conductivity of Metal Rod Heat & Mass Transfer LabAn experiment to determine the thermal conductivity of a metal rod for a given heat input. 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.
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.
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.
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.
Heat Transfer In Metal Rodgers
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.
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
Heat Transfer In Metal Rodent
Science >Physics >Heat Transfer > Conduction
Heat Transfer in a bar of steel
Heat Transfer in a bar of steel
Hello MEs.Heat Transfer In Metal Rodeo
I'm a long time member who usually stays in my own backyard - over in the structures area. But I have a situation where a relatively quick calculation (I hope) is appreciated from those of you with course work in heat transfer.I have a 10' mild steel rod that at one free end is heated by a typical oxy-acetylene torch for say a period of 5 minutes. Let's say that the temperature for the 5 minute period is about 700 degrees F. Say the bar has an area of 1 square inch. How far along the bar's length will the heat travel?
Many thanks.
Regards,
Qshake
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