hello friends this video on electricity part 5 is brought to you by exam 4 calm no move here from exam please make sure that you have watched all the videos to platform before going ahead read part 5 so what does ohm slot all who was a scientist who performed an experiment and from his experimental observation he found that as the potential difference between the ends of a conductor increases the current flowing through the conductor also increases so he said that the current flowing through a conductor is directly proportional to the potential difference applied between the ends of the conductor provided temperature remains constant so what he actually did he set up a he had set up a circuit and he kept on increasing the potential difference between the ends of the conductor and he kept measuring currents for different values then he plotted a graph where he took voltage along the x-axis and current along the y-axis and he found that the line which was obtained was something like this which showed that current is directly proportional to the potential difference so this means if you want to convert this proportionality into an equality you should in bring in picture of constant so this current was equal to 1 by I into V this constant was introduced so from this it was written as V is equal to I at so V is equal to I I became the mathematical expression of Ohm's law where R is the resistance of the conductor so resistance was a new property which was introduced from here so what is resistance let us look into that knob resistance as the name suggests it resists the flow of current it opposes the flow of current so if you have any if you eat for example we say that metals are good conductors of electricity that itself means that the resistance of metals are very less so if the resistance is less the amount of current flowing will be more if the resistance is more than the current flow will be less for example in case of insulators there assistances mode therefore the amount of current flowing fluency lattices less right let us look at this picture here if you see these are the free electrons which are carrying current now since there are so many na atoms present inside a so this electrons are facing obstructions several number of times so therefore in this case this is Lauren's resistance so look at this picture which will give you an idea of high resistance and low resistance so if you look at this figure here you see that the movement of this free electron is little smooth because it doesn't it is not colliding or it does not hit getting hit by any other atoms so here relatively the resistance is less now if the resistance is less the amount of current flowing will be more whereas in this case the the free electron is getting hit or getting collided by so many atoms as a result the resistance in this cases more so whenever you have whole resistance the current flow will be less and vice versa right so now let us see what are the factors on which resistance of a material dependent the first one is length of a conductor so greater the length of a conductor more will be the collision because as as you saw in our last slide if the length of the conductor is more if you see if this is my conductor so if the length of the conductor is more that means the free electron will experience more and more cohesion so more cohesion means more resistance right so that means resistance will increase with increasing length so we can say that resistance is directly proportional to the length of the conductor next one is cross-sectional area of the conductor so similarly for cross-sectional area also now if you increase the cross-sectional area of the conductor what will happen again let us look at the slide let us suppose now if I increase the cross-sectional area that means this area if I increase I mean if this width is increased what will happen the number of coalition-building is because the number of atoms will remain the same if you increase the cross sectional area that will not increase the number of atoms the number of atoms will remain the same but we are adding up some extra space therefore the collision be reduced because the free electrons will get some extra space to move about freely so the collision will reduce so we can say that resistance is inversely proportional to area if you increase the area the resistance will decrease it is something like this let us suppose if this is the length of a conductor in this length you have these many atoms so that means the free electron will collide with these many atoms now if you increase the length of the conductor so when you increase the length of the conductor the number of atoms will also increase right therefore the number of collisions will increase therefore resistance will increase now let us suppose instead of increasing the length if you increase the cross-sectional area now increasing the cross sectional area will not increase the number of atoms so the number of atoms are still the same but now this free electron will experience lesser condition so therefore the resistance will decrease with increase in area the third one is the nature of material of the conductor so value of resistance is also dependent on the nature of material of a conductor next is temperature of the conductor so how does it change with temperature when the temperature increases what happens for any object if you increase the temperature that the particles of that object will start moving randomly so they will be set in the motion so whenever there is random motion there will be more collisions so wherever you have more collision the frequency of collision will also increase now if the frequency of collision increases what will happen to your resistance the resistance will also increase with the increase in temperature right so these are some of the things on which the resistance of the resistance depends so from this we can see that the expression for resistance can be written as Rho is directly proportional to n by a because it is directly proportional to length on the inversely proportional to a again to remove this proportionality we will introduce a constant called Rho and what is this row this row is known as specific resistance what is Rho it is known as specific resistance for sometimes it is also known as resistivity this specific resistance is specific of a material that means the value of Rho depends on the material of the conductor right so the value of the resistivity is specific to every conductor so if we are talking about a metal the value of resistivity will be very low if we are talking about an insulator the value of resistivity builting very high so resistivity is a constant which is often defined as resistance of unit length and unit area so that is how we define specific resistance so it is something it is constant for a specific material okay so this is how we I mean we express resistance of a conductor a resistance is given as Rho into L by a now you will you might ask that how does the dependence of resistance on temperature taken into account in this case well the dependence of resistance on temperature is not exactly taken into account in this case however that has been observed experimentally that in case of metals or insulators or semi ters the resistance changes with change in temperature so you will study about the dependence of resistance on temperature now in your higher classes because that involves some other concepts as well so we will talk about that in in class 12 so for now you should know that this is how we express resistance and these are the factors on which resistance of a substance depends now we studied Ohm's law but do you think that Ohm's law is a foolproof law or there are certain limitations of Ohm's law as well yes there are certain limitations of Ohm's law Ohm's law is not universally true for every object when we look at the behavior of diodes and not sure if you are aware of water diodes they are also a kind of a electronic element which allows flow of current in one direction so diodes are those devices which allow unidirectional flow of current you will study about diodes also in class 12 but for now you should I mean this is just for an information that when you plot the VI characteristic that is the when you plot the voltage current graph for diodes you see that instead of a straight line you get a curve like this so this shows that Ohm's law is not true for values similar entities of this compound called gallium arsenide it is seen that the Ohm's law Ohm's law does not hold true again for water voltammeter it is seen that the when the voltage this this graph is often known as VI characteristic this is all often known as the name of this plot is VI characteristic so when when the VI characteristic of water Volta method is also plotted it is seen that it doesn't follow Ohm's law so in case of water voltammeter it is seen that when you plot this voltage versus current you get a graph like this instead of a graph this so the Ohm's law is not true in this case also so that these are these by some of the limitations of Ohm's law that it was not true for everything thank you please visit exam few calm to watch free educational videos try three online tests get the best quality study materials study from the best tutors and mentors and much more thank you once again
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