Tiêu đề 18.Current Electricity-F.pdf ✅

1 | P a g e NEET-2022 Ultimate Crash Course PHYSICS CURRENT ELECTRICITY
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3 | P a g e V POINTS TO REMEMBER 1. A Resistance of a conductor means the obstruction to the flow of electrons through it. While representing it diagrammatically, this obstruction is represented by hills . In case of a connecting wire made of copper which is a very good conductor, the obstruction offered to the flow of electrons is practically zero and as such a connecting wire is represented by a straight line without hills. A material is a good conductor if it has low resistance. On the other hand, a material which has a high resistance is called a bad conductor or an insulator. But it should be remembered that even a very good conductor has some resistance, though negligible. 2. We should distinguish between a resistor (an object of some conducting material) and its resistance (an electrical property) due to which it opposes electric current. Whenever we say that we have a resistance of 5 , it means that there is a resistor with offers a resistance of 5 . As , d d Ee e v v m E m     = =     e m   = ; ; e h e h e h e e m m     = = 3. We may remind ourselves, that even though non-linear circuit elements do not obey Ohm’s law, the definition of resistance R given by R = V/I still holds for any particular values of V and I. 4. If there are n resistors, each of resistance r, and if they are connected in series, then their equivalent resistance is given by R r r r s = + + + .........n times or R nr s = i.e., R s = number of resistors x resistance of each resistor 5. The current flowing through each resistor is the same and is equal to the total current in the circuit because there is no other path along which the current can flow. 6. The pd across the ends of the combination is distributed across the ends of each one of the resistors. The pd across any one of the resistors is directly proportional to its resistance. 7. The equivalent resistance ( R s ) produces the same result, i.e., gives the same current under same condition (i.e., when the same potential is applied across its ends) when connected in place of series combination. 8. To connect resistors in series is equivalent to increasing the length of a conductor. It is due to this reason that the resistance of the combination increases and is equal to the sum of the individual resistances which is obviously greater than that of one of the resistances of the combination. Thus, in series combination, the equivalent resistance is greater than the greatest resistance in the combination. 9. If there are n resistors, each of resistance r and if these are in parallel, their resultant resistance is given by 1 1 1 1 R r r r p = + + + ..... n times 10. The pd across each resistor in the same and is equal to the total pd across the combination. 11. The main current divides itself and a different current flows through each resistor. The maximum current flows through the resistor with minimum resistance and vice-versa, i.e., current in a resistor, is inversely proportional to its resistance. 12. The equivalent resistance (Rp) produces the same result (i.e., gives the same current) under the same condition (i.e., when the same pd is applied across its ends) when connected in place of the parallel combination. 13. Connecting a number of resistors in parallel is like providing an equal number of paths for the electron flow, which results in a decrease in resistance. 14. To connect a number of resistors in parallel is equivalent to increasing the cross-sectional area of a conductor and it is due to this reason that the total resistance offered by the combination decreases and is lower than the resistance of each resistor in the circuit. In other words, in parallel combination, the equivalent resistance is lesser than the least of all the resistances. 15. If two resistances 1 r and 2 r are connected in parallel, then 1 2 1 1 1 R r r p = + or 1 2 1 2 p rr R r r = + = product of two resistnaces sum of two resis tan ces 16. If there are n resistors, each of resistance r, then R nr s = and / R r n p = consequently, 2 / s p R nr n R r n = =
4 | P a g e 17. The drift velocity of electrons is small due to frequent collisions suffered by electrons. 18. Electric field is zero inside a charged conductor, but it is non zero inside a current carrying conductor and is given by = V E l = where V = potential difference across the conductor. 19. The speed of flow of current in a conductor is equal to the speed of light. Due to this reason the electric bulb glows immediately when switch is on. 20. The resistance of conductor increases with decrease density of material. 21. Emf of cell does not depend on the resistance of the circuit. It depends upon the nature of electrolyte of the cell while potential difference depends upon the resistance between the two points of the circuit and current flowing through the circuit. 22. Wheatstone bridge is most sensitive if all the four arms have equal resistances i.e. P = Q = R = S 23. A potentiometer can act as an ideal voltmeter. 24. The maximum value of current which can be passed through the fuse wire without melting it is called safe current. the safe current is independent of the length of the wire. 25. If length of wire is increased by n times then resistance will increase by n2 times i.e., 2 R n R 2 1 = . Similarly if radius of wire is reduced to 1/n times then area of cross-section will decrease 1/n 2 times so the resistance becomes n4 times i.e. R2 = n4 R1. 26. If length of a conductor increases by x% then resistance will increase by 2x % (valid only if 2x < 10%) but its resistivity remains unchanged. 27. If we have n conductor, each of equal resistance, the number of possible combinations is 2n – 1 . 28. If we have n conductors, each of different resistance then the number of possible combinations will be 2n. 29. If a wire of resistance R, is cut into n equal parts and then these parts are collected to form a bundle then resistance of combination formed will be 2 R n/ 30. If two resistance of R1 and R2 are connected first in series and then in parallel. If Rs and Rp are their equivalent resistance in series and parallel combination respectively then 2 1 1 4 2 = + −     R R R R R s s s p and 2 2 1 4 2 = + −     R R R R R s s s p 31. If in the series combination of n identical cells (each having emf E and internal resistance r) if x cells are wrongly connected then equivalent emf will be Eeq = (n – 2x)E and equivalent internal resistance will be req = nr. 32. Consider the parallel combination of two cells having emf E1 and E2 respectively, if they are connected with reversed polarity then equivalent emf is given by 1 2 2 1 1 2 − = + eq E r E r E r r 33. Thus, power consumed by a n equal resistors in parallel combination is n2 times that of power consumed in series combination if V remains same. 34. Distribution of Current: Let 1 r and 2 r be two resistors connected in parallel between the A points A and B as shown in figure. Let V be the pd between the points A and B and let I be the total current flowing through the combination. Since the pd between both the resistances 1 r and 2 r is the same, i.e., V. V I r = 1 1 and V I r = 2 2 or 1 1 2 2 I r I r = or 1 1 2 2 I r I r = As 1 1 1 1 2 1 1 2 2 , 1 I r r I I I I I I r r   = + = + = +    