Many factors, the most important being its resistivity. Resistance and resistivity are often used interchangeably, but they have slightly different meanings. Knowing the difference helps you understand how electrons flow.
Fundamental physics and Ohm's Law tell us that the resistance ( R ) of a material or device is the quotient of the voltage, also known as the electromagnetic force or electromotive force, ( E ) divided by the current through the circuit ( I ). By simply replacing Ohm's law ( I = E / R ), the resistance is
R = E/I
In general, resistance is the ability of a circuit or material to resist the flow of current, called ohms (Ω). Resistivity is a measurement of the resistance of a device. Like all units in the metric system, the resistance of pure elements is given in standard units (Ω-m) at room temperature. For example, the resistivity of pure copper is 1.68E-8 Ω-m.
Elements that are highly resistant to the flow of electrons are considered insulators. Insulators are typically tested for electrical resistance as well as for dielectric strength. The lower elements in the periodic table are conductors.
Solid elements are classified as insulators, semi-insulators or conductors according to their "resistivity" in the periodic table. Resistance in insulators, semiconductors or conductive materials is a major property to consider in any application.
The measured resistivity of a material sample depends on its size and thickness. Factors such as temperature, humidity, and energization time also affect resistivity. In general, when comparing two samples of the same other material from the same sample and all other factors being equal, the resistance of the wider sample is less than the resistance of the smaller sample, while the resistance of the longer sample is greater than the resistance of the shorter sample. Resistance depends on the size of the sample, but resistivity is not.
Common Resistivity Measurement
The three most common resistivity measurements are
· Surface resistivity
the volume resistivity or volume resistivity, and
· Contact resistivity
Surface resistivity is a measure of the electrical resistance of the surface of the material in contact with the electrodes.
To measure the surface resistivity of a flat material, use a set of equally sized electrodes that make good contact with the material surface and are separated by a space equal to the width of the electrodes. Both probes are 1" wide and separated by a 1" insulator. Because one divided by one equals one, the length divided by the width of the area cancels out the effect of measuring area size.
Regardless of electrode size, surface resistivity measurements are given in ohms/square (Ω/square):
R x (L/W)=Ω/square
A surface resistivity measurement of a conductive planar material of a homogeneous substance is also a resistance measurement of the volume between electrodes, since the path of the electrodes includes the depth or thickness of the test sample.
Volume resistivity is the measurement of resistance (R) multiplied by the cross-section of the sample (width x thickness) divided by the length of the material between the electrodes.
The electrodes are designed to measure bulk resistivity through tape or flat solid samples. In this case, the electrodes are in contact with both sides of the material at each end.
Volume resistivity units are usually measured in ohm-centimeter (Ω-cm). The length, width and thickness of the material are in centimeters. Volume resistivity is also known as volume resistivity.
R (W x T)/L = Ω-cm
Contact resistivity is a measure of the electrical resistance through a material or composite. It is not actually a measurement of the material itself, but of the quality of the electrical connection.
There are two 1" square stainless steel blocks with the test material sandwiched between them. Contact resistance is the amount of resistance to current flow in the contact surfaces or junctions of a composite material or in the leads of a device that contributes to the overall resistance of the circuit. You can calculate contact resistance by subtracting the intrinsic resistance of each conductor or insulator from a set
The total resistance of the circuit. Contact resistance is measured in milliohms (mΩ) because the resistance within a circuit is relatively small. Sometimes this measurement is also described as Z-axis resistance.
How to measure resistance?
Resistance is affected by many factors, including surface cleanliness, humidity, temperature, and applied voltage or current. Temperature and relative humidity readings are an integral part of the test. Somemeters have auto-ranging voltage and even low power settings for very low and sensitive resistivity measurements. Ohm cannot be measured directly with any meter; it is a calculated value by dividing the applied voltage by the current through the conductor. A typical ohmmeter or multimeter applies a low current through the material; measures the voltage and displays the resistance in ohms (Ω).
Many instruments can measure resistance and can be used in combination with other instruments or as stand-alone instruments. Accuracy and sensitivity depend on test conditions, sample rate and meter resolution.
Very low resistance values and very high resistances are often measured by different types of meters and probes. High resistivity measurements are often obtained using a megohmmeter or IR (insulation resistance) meter. Very low resistance values are measured by a combination of several different instruments, and in some cases, using very precise specialized instruments.
Fabrico's lab has several meters for measuring resistance, including those for very high and low resistance values. We use an Agilent Technologies 34420AN millivolt/microohmmeter, which has the resolution and accuracy required for low resistance measurements. We also use a Fluke8846A general purpose laboratory multimeter and an AEMC IRmeter/Hipot instrument for very high resistance measurements.