If voltage is measured at various points in a circuit, it will be seen to increase at the voltage source and decrease at the resistor. Voltage is similar to fluid pressure. The voltage source is like a pump, creating a pressure difference, causing current—the flow of charge.
The resistor is like a pipe that reduces pressure and limits flow because of its resistance. Conservation of energy has important consequences here. The voltage source supplies energy causing an electric field and a current , and the resistor converts it to another form such as thermal energy.
Thus, the energy supplied by the voltage source and the energy converted by the resistor are equal. If voltage is forced to some value V, then that voltage V divided by measured current I will equal R. Or if the current is forced to some value I, then the measured voltage V divided by that current I is also R.
We visualize the plot of I versus V as a straight line. An example is the p-n junction diode. Superconductivity is a phenomenon of zero electrical resistance and expulsion of magnetic fields in certain materials below a critical temp. Describe behaviors of a superconductor below a critical temperature and in a weak external magnetic field.
Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature.
Most of the physical properties of superconductors vary from material to material, such as the heat capacity and the critical temperature, critical field, and critical current density at which superconductivity is destroyed. On the other hand, there is a class of properties independent of the underlying material. For instance, all superconductors have exactly zero resistivity to low applied currents when there is no magnetic field present or if the applied field does not exceed a critical value.
In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature T c.
The onset of superconductivity is accompanied by abrupt changes in various physical properties—the hallmark of a phase transition. For example, the electronic heat capacity is proportional to the temperature in the normal non-superconducting regime. At the superconducting transition, it suffers a discontinuous jump and thereafter ceases to be linear, as illustrated in.
When a superconductor is placed in a weak external magnetic field H, and cooled below its transition temperature, the magnetic field is ejected. The Meissner effect does not cause the field to be completely ejected. It decays exponentially to zero within the bulk of the material.
The Meissner effect is a defining characteristic of superconductivity. For most superconductors, the London penetration depth is on the order of nm. Superconductors are also able to maintain a current with no applied voltage whatsoever—a property exploited in superconducting electromagnets such as those found in MRI machines. Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation.
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.
The examples of the ohmic conductors are metals, resistors, etc. When the current flows through the resistor , it is directly proportional to the voltage or it can be said that they have a linear relationship.
The relationship between current and voltage in non-ohmic conductors is not linear. Graphically speaking, the slope of the current and voltage for non-ohmic conductors is not a straight line but it is a curved line. The properties of non-ohmic conductors also vary according to the change in temperature. What is the resistance of the resistor? The resistance of a resistor is equal to the ratio of the potential difference across the resistor to the current through it.
The given table consists of five pairs of values, but we only need one such pair to compute the resistance of the unknown resistor. This is because the ratio of potential difference to current is the same for all five pairs. Selecting the first pair of values, we see the potential difference is 3 V and the current is 50 mA. The resistance of the resistor in this experiment is 60 ohms. A student has a resistor of unknown resistance.
She places the resistor in series with a source of variable potential difference. In the above expression, we have assumed that L and A remain unchanged within the temperature range. Intrinsic semiconductors exhibit the opposite temperature behavior, becoming better conductors as the temperature increases.
This occurs because the electrons are bumped to the conduction energy band by the thermal energy, where they can flow freely and in doing so they leave behind holes in the valence band which can also flow freely.
Extrinsic semiconductors have much more complex temperature behaviour. First the electrons or holes leave the donors or acceptors giving a decreasing resistance. Then there is a fairly flat phase in which the semiconductor is normally operated where almost all of the donors or acceptors have lost their electrons or holes but the number of electrons that have jumped right over the energy gap is negligible compared to the number of electrons or holes from the donors or acceptors.
Finally as the temperature increases further the carriers that jump the energy gap becomes the dominant figure and the material starts behaving like an intrinsic semiconductor. Just as the resistance of a conductor depends upon temperature, the resistance of a conductor depends upon strain. By placing a conductor under tension a form of strain , which means to mechanically stretch the conductor, the length of the section of conductor under tension increases and its cross-sectional area decreases.
Both these effects contribute to increasing the resistance of the strained section of conductor. Under compression the other form of strain , the resistance of the strained section of conductor decreases. See the discussion on strain gauges for details about devices constructed to take advantage of this effect. In a transmission line , the phasor form of Ohm's law above breaks down because of reflections. In a lossless transmission line, the ratio of voltage and current follows the complicated expression.
Ohm's principle predicts the flow of electrical charge i. The same equation describes both phenomena, the equation's variables taking on different meanings in the two cases. Specifically, solving a heat conduction Fourier problem with temperature the driving "force" and flux of heat the rate of flow of the driven "quantity", i.
The basis of Fourier's work was his clear conception and definition of thermal conductivity. He assumed that, all else being the same, the flux of heat is strictly proportional to the gradient of temperature. Although undoubtedly true for small temperature gradients, strictly proportional behavior will be lost when real materials e.
A similar assumption is made in the statement of Ohm's law: other things being alike, the strength of the current at each point is proportional to the gradient of electric potential. The accuracy of the assumption that flow is proportional to the gradient is more readily tested, using modern measurement methods, for the electrical case than for the heat case. Template:WikiDoc Sources. Template:Two other uses File:Ohms law voltage source.
V graphs representing ohmic blue line and non-ohmic devices red and yellow curves. Encyclopedia Britannica. Categories : Pages with broken file links Electronic engineering Circuit theorems. Cookies help us deliver our services. By using our services, you agree to our use of cookies. Namespaces Home Page Discussion.
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