Conduction is the process by which something, such as heat or an electric current, moves from one substance to another substance. Conduction occurs in solids, liquids, and gases. However, solids transfer energy most efficiently since the molecules in solids are most tightly packed, and the molecules are closer together.
Conduction of heat occurs when molecules increase in temperature; they vibrate, and this vibration and movement pass the heat energy to the surrounding molecules.
When two bodies at different temperatures are kept in contact, heat flows from a body at high temperature to the body at low temperature. The average kinetic energy of the substance is a measure of the temperature of the body. When there is a rise in the average kinetic energy of molecules of a substance, its temperature increase, and if there is a fall in the average kinetic energy of molecules of a substance, its temperature decreases.
Keep a frying pan on a flame. The frying pan soon gets hot, because the heat passes from the flame to the pan. Now remove the pan from the flame. Gradually the pan will cool down because the heat is transferred from the pan to the surroundings. In both cases, heat flows from a hotter object to a colder object.
You would have experienced that if you touch a hot cup of tea, your hand feels the hotness of the cup. The reason is some of the heat energy is transferred from the cup to your hand. Heat transfers from a hot object to a cold object if there is contact between them. In physics, we say that transfer of heat requires a medium. Thermal conduction is the movement of heat from one object to another one that has different temperatures when they are touching each other. In solids, generally, the heat is transferred by the process of conduction. Thermal conductivity describes how efficiently a material can pass heat through it. It is defined by the rate of energy flow per unit area when compared to a temperature gradient.
Fourier’s Law Of Heat Conduction: Fourier's Law shows that thermal energy moves from warmer materials to cooler materials. Fourier's Law can be written as
q = kAdT∕s
In this equation, q refers to the rate of heat conduction, A is the heat transfer area, k is the material's thermal conductivity, dT is the difference in temperature across the material, and s refers to how thick the material is.
Examples:
Conduction of electricity occurs due to the movement of electrically charged particles through a medium. This movement can result in an electric current, which may be carried by electrons or ions. An example of electrical conduction is when you accidentally get electrocuted when you touch a live wire because your body contains water, which is a conductor of electricity. Another example is when electricity passes through wires, which are conductors, so we can watch TV or use a computer.
Electrical conductivity is a measure of how well a material accommodates the movement of an electrical charge. In solids such as metals, the electrons are loosely bound to the atoms because of which electrons can freely move from atom to atom in a metal object. This electron mobility allows us to pass an electric current through it. If we can easily pass an electric current through objects, we call them Good electricity conductors. Materials that do not allow electricity to flow through them are known as insulators. The conductivity of semiconductors is intermediate between that of an insulator and a conductor. A "perfect vacuum," contains no charged particles; vacuums normally behave as very good insulators.
Conduction in metals is well described by Ohm's Law, which states that the current is proportional to the applied electric field. The ease with which current density (current per area) j appears in a material is measured by the conductivity σ, defined as:
j = σ E,
E is the electric field at that location and σ is the conductivity of the material, a measure of how easily charges move through it.
The electrical conductivity or resistivity of a material is an immutable property that does not change with respect to the size or shape of the material.
Charging by conduction:
Bodies can be charged by the method of conduction, that is by contact. By conduction, the body acquires the same charge as on the charging body.
Experiment: Make a paper cylinder by rolling a strip of paper on a pencil and then gently pulling out the pencil. Suspend the paper cylinder by a thread tied to its center. Take a glass rod and rub it with silk so it has a positive charge. Touch the paper cylinder with this glass rod. Remove the glass rod and then again bring the glass rod near the paper cylinder.
You will notice that the paper cylinder is repelled by the glass rod, This means that the paper cylinder has acquired a positive charge that is the same charge as on the glass rod due to conduction.
Examples:
Photoconductivity occurs when a material absorbs electromagnetic radiation, resulting in a change in the substance's electrical conductivity. The electromagnetic radiation can be caused by something as simple as a light shining on a semiconductor or something as complex as a material being exposed to gamma radiation. When the electromagnetic event occurs, the number of free electrons increases, as does the number of electron holes, thus increasing the object's electrical conductivity. Certain crystalline semiconductors, such as silicon, germanium, lead sulfide, and cadmium sulfide, and the related semimetal selenium, are strongly photoconductive
Examples:
Any object that transfers thermal or electrical energy, or both, efficiently is a conductor. Materials that do not permit heat and electricity to pass through them is an insulator.
| Materials with good thermal conduction | Materials with good electrical conduction |
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Metals generally move heat efficiently and are good heat conductors. Fabrics and wood tend to provide poor heat conduction. Generally, if a substance is a good thermal energy conductor, it will also be a good electrical conductor. This is not always true; for example, mica is a heat conductor but an electrical insulator. Saltwater is a poor heat conductor but a good electrical conductor. Still, in general, the same close packing of atoms and relatively free movement of their electrons that makes thermal energy move about in the substance also makes the electrons’ electrical energy move about.
Pure water, with no dissolved solids, is not electrically conductive. Electric current flows more easily when the water has more dissolved minerals in it. Air, a mixture of gases, is generally not a good conductor of either heat or electricity. Air, like water, is considered an insulator. Yet when the particles in air receive a strong electrical charge from, say, built-up static (from an electrical field when lightning is about to strike or from a power line’s electrical field), the air can conduct electricity.
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Saltwater is a poor heat conductor but a good electrical conductor: Freshwater holds heat longer than saltwater because the addition of salt lowers the heat capacity of the solution relative to pure water. The lower heat capacity means that saltwater both heats up and cools down more quickly than fresh water under the same conditions. The thermal conductivity decreases with increasing salinity and increases with increasing temperature. The electrical conductivity of water depends on the concentration of dissolved ions in the solution. The Sodium Chloride salt dissociates into ions. Hence seawater is about a million times more conductive than freshwater. |
