Write an initial response to the following three discussion prompts. Post your r

Question

Write an initial response to the following three discussion prompts. Post your response to the discussion board.

With respect to Ohm's Law, explain in your own words what is meant when it is said that there is "a linear response between voltage and current."

Name a specific device in which Ohm's Law is not a good description of the relationship between current and voltage. Do your best to explain this failure in terms of the basic properties of electrons flowing through a conducting material.

Describe the temperature response of resistors in a circuit. Are there any devices which take advantage of this property? If so, explain how they work in terms of the properties of electrons.

Explanation / Answer

Ohm's law is most commonly known as V = IR

What it actually means is that the current passing through the wire is directly proportional to the applied voltage difference, That is, more the voltage difference, more will be the current. V I (V is proportional to I)

To remove the proportionality and make it an equation, we put a constant R and write  V = IR

where R is the constant (material dependent property) called Resistance.

Another way of saying is that if you draw graph between V and I, you will get a straight line ( linear response) with slope R.

Note that Ohm's law holds good for conductors (like the wires we use in circuits). But, many things do not follow Ohm's law. Water doesn't. Neither does air. Because they are not conductors. Or in other words, I can say they are not resistors because they don't obey Ohm's law. Also, there are many standard electrical components like filament lamps, diodes (say, LEDs which are Light Emitting Diodes used in your LED TV Screens) and thermistors (it is a type of resistor whose resistance is dependent on temperature).

Ohm's law has sometimes been stated as, "for a conductor in a given state, the electromotive force is proportional to the current produced." That is, that the resistance, the ratio of the applied electromotive force (or voltage) to the current, "does not vary with the current strength ." The qualifier "in a given state" is usually interpreted as meaning "at a constant temperature," since the resistivity of materials is usually temperature dependent. This resistivity () gives the relation between material properties and the electrical Resistance offered by that material.

The electrical resisitance of a uniform conductor is given in terms of Resistivity by the equation :

R = l / A , where l is the length of the conductor and A is the area of cross-section.

Since, is temperature dependent and also taking into account the themal expansion or contraction which can change the dimensions (l and A) with the temperature, we can see through the above formula, how Temperature can affect the resistance.

Thermistors are the best example of devices which tak advantage of this change in resistance with temperature.

Thermistors are used as current limiters and self-resetting overcurrent protectors. When excess current passes through them, they get heated raising the temperature and henced raising the resistance and hence limit the current flow.

Alternatively, they are also used as temperature sensors, where the change in resistance is detected (by measuring change in the current) to find the change in the temperature.

Also, there are some non-linear circuit elements like capacitors and inductors which do not obey ohm's law. Althought they are made of a conducting material, their special shaping gives them this non-linearity. For examplr inductor is nothing but a coiled wire. When current passes thorough an inductor, a magnetic field is induced inside the coil and hence the indictor stores the energy as a magnetic field. This magnetic field hinders the movement of electrons in the coil (our conducting wire) giving rise to the non - linearity. Ohm's law - linearity between V and I. This non-linearity implies violation of Ohm's law. By the way, inductors are used in many electric circuits as electro magnets to create very high magnetic fields.

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