Bad Conductors. Good conductors are those materials which allow electricity to pass through them easily. Bad conductors are those materials which do not allow electricity to pass through them easily. Example - Copper, Aluminium. Example - Rubber, Wood.
Lee's disc apparatus consist of a metallic disc resting on a 5 cm deep hollow cylinder (steam chamber ) of same diameter. When steam is passed through the cylindrical vessel a steady state is reached soon. At the steady state, heat conducted through the bad conductor is equal to heat radiated from the Lees disc.
Searle's bar method (named after George Frederick Charles Searle) is an experimental procedure to measure thermal conductivity of material. A bar of material is being heated by steam on one side and the other side cooled down by water while the length of the bar is thermally insulated.
The thermal conductivity of a material does not change with the thickness of the material. The thermal differential between the opposite sides of the wall will differ with thickness assuming a linear gradient through the wall where one side is held at a constant temperature.
Thermal conductivity coefficient is the quantity of heat that is transported through a unit cube of two surfaces of perpendicular distance to each other in a given unit of time when the difference in temperature of the two surfaces is 1°C and its unit is W/m∗K (Uygunoğlu et al., 2016).
Thermal conductivity refers to the amount/speed of heat transmitted through a material. Heat transfer occurs at a higher rate across materials of high thermal conductivity than those of low thermal conductivity. Metals with high thermal conductivity, e.g. copper, exhibit high electrical conductivity.
Thermal Conductivity
| Material | Thermal conductivity (cal/sec)/(cm2 C/cm) | Thermal conductivity (W/m K)* |
|---|
| Brass | | 109.0 |
| Aluminum | 0.50 | 205.0 |
| Iron | 0.163 | 79.5 |
| Steel | | 50.2 |
Materials that are good conductors of thermal energy are called thermal conductors. Metals are especially good thermal conductors because they have freely moving electrons that can transfer thermal energy quickly and easily.
The thermal conductivity is not always constant. The main factors affected the thermal conductivity are the density of material, moisture of material and ambient temperature. With increasing density, moisture and temperature the thermal conductivity increases too.
Thermal conductivity is a material property. It will not differ with the dimensions of a material, but it is dependent on the temperature, the density and the moisture content of the material. The thermal conductivity of a material depends on its temperature, density and moisture content.
Thermal properties of engineering materials comprise the following:
- Specific heat. ADVERTISEMENTS:
- Thermal conductivity.
- Thermal expansion.
- Melting point or heat resistance.
- Thermal shock. ADVERTISEMENTS:
- Thermal diffusivity.
- Thermal effect.
Calculate the R-value by taking the reciprocal of the conductivity C to get R = 1/3.93 = 0.254 kelvin-meter^2/watt. You can also calculate the R-value directly as R = L/K = 0.305/1.2 = 0.254 kelvin-meter^2/watt. Notice that the thicker the material is, the higher the R-value is.
How does thermal conductivity vary with temperature? As temperature increases, both number of free electrons and lattice vibrations increase. Thus the thermal conductivity of the metal is expected to increase. However, increased lattice vibrations obstruct the flow of free electrons through the medium.
Everyday Examples of Heat or Thermal Conduction
- After a car is turned on, the engine becomes hot.
- A radiator is a good example of conduction.
- You can warm your back muscles with a heating pad.
- Roasting wieners over a campfire is fun until the heat from the fire is conducted up the coat hanger to your hand.
Thermal conductance is the rate of heat flux through a unit area at the installed thickness and any given ΔT. Experimentally, thermal conduction is measured by placing the material in contact between two conducting plates and measuring the energy flux required to maintain a certain temperature gradient.
Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance. (Absolute) thermal resistance R in K/W is a property of a particular component.
There are several methods of experimentally determining thermal conductivity, such as the steady state or comparative method, the radial flow method, the laser-flash diffusivity method, and the pulse-power method [1].
