Experimental evidence points to a current lifetime of at least , years. Theoretical estimates for the lifetime of a persistent current can exceed the estimated lifetime of the universe, depending on the wire geometry and the temperature.
The value of this critical temperature varies from material to material. Usually, conventional superconductors have critical temperatures ranging from around 20 K to less than 1 K. Solid mercury, for example, has a critical temperature of 4. High-temperature superconductors can have much higher critical temperatures. For example, YBa 2 Cu 3 O 7 , one of the first cuprate superconductors to be discovered, has a critical temperature of 92 K; mercury-based cuprates have been found with critical temperatures in excess of K.
It is of note that the chemical composition and crystal structure of superconducting materials can be quite complex, as seen in. Atoms are indicated with different colors. Resistance and resistivity describe the extent to which an object or material impedes the flow of electric current. Resistance is the electric property that impedes a current.
A current flowing through a wire or resistor is like water flowing through a pipe, and the voltage drop across the wire is like the pressure drop which pushes water through the pipe.
Resistance is proportional to how much pressure is required to achieve a given flow, while conductance is proportional to how much flow occurs for a given pressure. Conductance and resistance are reciprocals. The resistance of an object depends on its shape and the material of which it is composed.
The cylindrical resistor is easy to analyze, and by so doing we can gain insight into the resistance of more complicated shapes. The longer the cylinder, the more collisions charges will make with its atoms. The greater the diameter of the cylinder, the more current it can carry again, similar to the flow of fluid through a pipe. Cylindrical Resistor : A uniform cylinder of length L and cross-sectional area A. Its resistance to the flow of current is similar to the resistance posed by a pipe to fluid flow.
The longer the cylinder, the greater its resistance. The larger its cross-sectional area A, the smaller its resistance. As mentioned, for a given shape, the resistance depends on the material of which the object is composed. Different materials offer different resistance to the flow of charge. In contrast, the resistance R is an extrinsic property that does depend on the size an shape of the resistor.
Recall that an object whose resistance is proportional to the voltage and current is known as a resistor. What determines resistivity? The resistivity of different materials varies by an enormous amount. For example, the conductivity of teflon is about times lower than the conductivity of copper. Why is there such a difference? Likewise, resistors range over many orders of magnitude. The potential difference voltage seen across the network is the sum of those voltages, thus the total resistance the series equivalent resistance can be found as the sum of those resistances:.
As a special case, the resistance of N resistors connected in series, each of the same resistance R, is given by NR. Resistors in a parallel configuration are each subject to the same potential difference voltage , however the currents through them add. Thus the equivalent resistance Req of the network can be computed:.
For the case of two resistors in parallel, this can be calculated using:. A resistor network that is a combination of parallel and series connections can be broken up into smaller parts that are either one or the other, such as is shown in. Resistor Network : In this combination circuit, the circuit can be broken up into a series component and a parallel component. However, some complex networks of resistors cannot be resolved in this manner.
These require a more sophisticated circuit analysis. One practical application of these relationships is that a non-standard value of resistance can generally be synthesized by connecting a number of standard values in series or parallel.
This can also be used to obtain a resistance with a higher power rating than that of the individual resistors used. In the special case of N identical resistors all connected in series or all connected in parallel, the power rating of the individual resistors is thereby multiplied by N. Resistance, Resistors, and Resistivity : A brief overview of resistance, resistors, and resistivity.
Resistivity and resistance depend on temperature with the dependence being linear for small temperature changes and nonlinear for large. It is a description of the relationship of current to voltage for a specific class of electrical objects. Where the current is directly proportional to the applied voltage over a wide range, we say this is a device that has resistance. A resistor. Posted in : Law Commons Show details. For most applications, the resistors have no wattage value attached to their symbol.
We will speak more about watts and power dissipation later. Graphical Representation of Ohms Law Below is a circuit consisting of a voltage source and a resistor. Posted in : University Law Show details. Resistors are passive elements that introduce resistance to the flow of electric current in a circuit. When current passes through an Ohmic resistor, the voltage drop across the terminals is proportionally to the magnitude of resistance.
Posted in : Form Law Show details. The easy answer is that a ohm resistor is a common resistor included in many experiment kits. Ohm 's law is not universal.
The ideal resistor circuit element is defined by Ohm 's law but not all circuit elements obey Ohm 's law ; Ohm 's law only applies to ohmic devices.. Physical resistors and conductors approximately obey Ohm 's law but, for example, semiconductor diodes, transistors, thyristors, solar cells, vacuum tubes, batteries.
I know this sounds like a silly question, but my teacher is adamant that non-ohmic resistors still obey ohms law because although an non-ohmic resistor does not follow a linear function if one was to take two points of that curve you would still be able to find the resistance at that point using ohms law. E temperature, at fixed temperature thermistors act as metal resistors. Metallic resistors are in the definition of ohms law so they obey it.
Diodes do not obey ohms law as they produce dodgy graphs, as they only allow current to flow in one direction at something. It does not apply to other devices, such as diodes. Tungsten is an example of a conductor that does not obey Ohm 's Law. If you were to plot a graph of current against voltage, over a …. The resistance of a conductor is measured in Ohms and the Ohm is a unit named after the German physicist George Simon Ohm who was the first to show the relationship between resistance, current and voltage.
In doing so he devised his law which shows. If the EMF across the resistor is 10 V, what is the resistance of this resistor? Ask unlimited questions and get expert help right away. Practice: Two resistors are made of the same material, one twice as long as the other. If the current through the shorter resistor is 5 A, what is. In answering this question, consider the least squares fits and the graphs you have made for each resistor. Remember linear behavior of V versus I is the proof of ohmic behavior.
Evaluate the agreement between the theoretical values for the individual resis- tances and. If you were to plot a graph of current against voltage, over a range of voltages, you will find that …. Substitute different resistors in the circuit and re-take all resistance, voltage, and current measurements.
One such device is the semiconducting circuit element known as a diode. A diode is a circuit device that allows current flow in only one direction. Although we do not cover the theory of the diode in this section, the diode can be tested to see if it is an ohmic or a nonohmic device. Note that the behavior of the diode is shown as current versus voltage, whereas the resistor operation was shown as voltage versus current. A diode consists of an anode and a cathode.
When the anode is at a negative potential and the cathode is at a positive potential, as shown in part a , the diode is said to have reverse bias. With reverse bias, the diode has an extremely large resistance and there is very little current flow—essentially zero current—through the diode and the resistor. As the voltage applied to the circuit increases, the current remains essentially zero, until the voltage reaches the breakdown voltage and the diode conducts current.
When the battery and the potential across the diode are reversed, making the anode positive and the cathode negative, the diode conducts and current flows through the diode if the voltage is greater than 0.
The resistance of the diode is close to zero. This is the reason for the resistor in the circuit; if it were not there, the current would become very large. Thus, the diode is an example of a nonohmic device.
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