Electrical resistance and conductance - Wikipedia
Electronics Tutorial about Electrical Resistivity and the Conductivity of a them encouraged by the presence of the potential difference between these two points. . So if its resistance is doubled, the conductance halves, and vice-versa as. Molar, specific and equivalent conductance. Specific Conductance or Conductivity. The resistance. Conductance is the reciprocal of resistance. The relationship between them is G = (A x sigma) ÷ L, and since cross-sectional area is πr2, this.
Conductance and resistance are reciprocals.
Conductivity of a solution – Andy Connelly
The voltage drop i. In hydraulics, it is similar: The pressure difference between two sides of a pipe, not the pressure itself, determines the flow through it. For example, there may be a large water pressure above the pipe, which tries to push water down through the pipe. But there may be an equally large water pressure below the pipe, which tries to push water back up through the pipe.
If these pressures are equal, no water flows.
In the image at right, the water pressure below the pipe is zero. The resistance and conductance of a wire, resistor, or other element is mostly determined by two properties: In the same way, a long, thin copper wire has higher resistance lower conductance than a short, thick copper wire. Materials are important as well.
A pipe filled with hair restricts the flow of water more than a clean pipe of the same shape and size. Similarly, electrons can flow freely and easily through a copper wire, but cannot flow as easily through a steel wire of the same shape and size, and they essentially cannot flow at all through an insulator like rubberregardless of its shape.
The difference between copper, steel, and rubber is related to their microscopic structure and electron configurationand is quantified by a property called resistivity. In addition to geometry and material, there are various other factors that influence resistance and conductance, such as temperature; see below.
Conductors and resistors[ edit ] A 6. An ohmmeter could be used to verify this value. Substances in which electricity can flow are called conductors. A piece of conducting material of a particular resistance meant for use in a circuit is called a resistor. Conductors are made of high- conductivity materials such as metals, in particular copper and aluminium. Resistors, on the other hand, are made of a wide variety of materials depending on factors such as the desired resistance, amount of energy that it needs to dissipate, precision, and costs.
Ohm's law The current-voltage characteristics of four devices: Two resistorsa diodeand a battery. The horizontal axis is voltage dropthe vertical axis is current. See  for more information. Conductivity and temperature Conductivity increases with temperature. This increase is significant, between 1. So, if you measure the same solution at different temperatures you will get a different conductivity unless a temperature correction is used.
Conductivity of a solution
For this reason, conductivity readings must be temperature compensated. Conductivity probes should have an integrated temperature sensor to aid this compensation.
There are two main temperature correction algorithms in common use [4,5,6]: However, that is clearly not always possible. Errors in temperature measurement will also contribute to issues in compensation and so it is vital to let your conductivity probe and solution come into thermal equilibrium.
Temperature coefficient The change in conductivity with temperature is expressed as the change in conductivity as a percentage per degree celsius. Examples of conductivity temperature coefficients for various solutions . Temperature dependence of conductivity temperature coefficient . The linear temperature correction is widely used. The most accurate way is to calculate a coefficient specifically for the sample temperature for your solution.
Tabulated values can vary for the same solution depending on source. To calculate a coefficient one method is described below see  or  for more details: This temperature is generally the temperature your calibration standard is quoted at.
Using the same sample, find the conductivity at another temperature. Generally this is the temperature you will be measuring other samples at. Measuring conductivity Measuring conductivity can be achieved in a variety of ways. The most common method is using a conductivity probe as seen in Figure 1. These use two or more platinum electrodes and measure the conductivity directly. However, a DC voltage would soon deplete the ions near the plates, causing polarization, and a higher than actual resistance to be measured.
As such, an AC voltage is generally used to avoid this problem see ref. A more advanced conductivity cell uses four electrodes see Figure 2. These probes use an alternating current through the outer electrodes and measures the voltage across the inner electrodes . The four electrode system gives a lower current and so has less charge transfer at the metal-liquid interface. This allows a much wider dynamic range to be measured than a two-electrode sensor. Such measurements are volume dependent and the outer sheath of the probe ensures that the volume of sample solution remains constant for all analysis.
Diagram of conductivity probe with 4 electrodes. Conductivity cell Figure 3 shows the set up of a basic conductivity probe with two square platinum electrodes. The specific design of the probe will vary depending on the range of conductivities to be measured. The area of the electrodes and the distance between them defines this range. These values are encapsulated in the Cell Constant, K. The conductivity, C, between electrodes in given by the expression: Diagram showing the operation of a conductivity probe.
Cell constant The cell constant K is defined as the ratio of the distance between the electrodes d to the electrode area A. However, the fringe-field effect AF alters the electrode area, therefore: For solutions with low conductivity the electrodes can be placed closer together or made larger so that the cell constant is less than one. This has the effect of raising the conductance to produce a value more easily interpreted by the meter.
The reverse also applies, in high conductivity solutions, the electrodes are placed farther apart or made smaller to reduce the conductance of the sample. The ideal K value for a probe varies with the range of conductances being measured.
In theory, the cell constant can be applied directly out of the box without calibration.