Ohm's law
ohm's law state that in a closed DC Circuit current directly praportional to voltage and inversely praportional to resistance in the circuit and tempreture remain constant .
There is a fundamental relationship between the value of resistance R, measured in ohms ( Ω ) the current I, measured in amperes (A), and voltage V, measured in volts (V).
Mathematical operation in ohm's law is :
Resistance = Voltage/ current
R = V/I
Current = voltage/ resistance
I = V/ R
Voltage= current× resistance
V = I×R
EXAMPLE : how much current flow in circuit
Given:
Voltage (V) = 1.5 kilo volt
Resistance(R) = 1kilo ohm
Find :
Current(I) = ?
Current = voltage / resistance
I = 1.5 v /1000 ohm = 0.0015 Amper
Example : What voltage is required to send a current of 2 amps. through a resistance of 60 ohms?
Solution: Given,
R = 60 ohms
I = 2 amps
V =
V=I×R= 2 × 60 = 120 volts.
...The voltage required is 120 volts.
Connection of resistance
Resistance are generally connected in three way
1- series connection
2- parallel connection
3- series and parallel combined connection
1- series connection : in a Circuit if two or more resistances are connected in such a way that there is only one path for the current to flow, it is called series connection.in a Circuit If three resistances are connected as per diagram, the one end of the first resistance is connected with the start of second and end of the second with the start of the third resistance and so on, and connect the two outside free ends to a supply these connections are called series connections.
This connection is called a series connection , in which the same current flow in the lamp .
The lamps are replaced by resistors in the Fig . Fig shows two resistors connected in series between point A and point B. Fig shows three in series, and Fig hows four in series. Of course, there can be any number of resistors in a series connection. Such connection provides only one path for the current to flow.
.jpg)
In an actual circuit diagram, a series connection may not always be as easy to identify as those in the figure. For example, shows series resistors drawn in other ways.
Parallel Connection: When in a Circuit two or more than two resistances are connected in such a way that all the starting ends are connected to one terminal and all the ending ends are connected Together at another terminal and two points are taken for supply as shown in figure, these types of connections are known as parallel connection .
These connection is called a parallel connection .
In this Circuit resistors in the Fig . Fig shows three resistors connected in parallel between point of three resistors . Fig shows three in parallel , and Fig hows four in parallel . Of course, there can be any number of resistors in a series connection. Such connection provides different path for the current to flow.
Series Parallel Connection: in a Circuit when three or more than three load are joined in such a way that the circuit has the characteristics of both the series as well as the parallel. This type of connection is known as series-parallel connection as shown in figure.
Some important question and answer :
Q1. According to Ohm's law
(a) I= V.R
(b) V=I.R
(c) V=I+R
(d) R=IV Ans=B
Q2.Factors affecting the resistance of a metal are as follows
(a) Length of conductor
(b) Temperature
(e) Thickness
(d) All of these Ans=D
Q3. SI unit of resistivity
(a) Ohm/metre
(b) Ohm-Ampere
(c)Ohm-metre
(d) Ohm/Volt Ans=C
Q4. Ratio of intensity of electric field and current density is called
(a) Resistance
(b) Conductance
(c) Resistivity
(d) Voltage. Ans= C
Q5. Reciprocal of resistivity is called
(a) Current density
(b) Electric conductance
(c) Electric conductivity
(d) Conductivity density Ans= B
Q6. On increasing the temperature, resitivity of metals
(a) Decreases
(b) Increases
(c) Does not change
(d) Becomes zero Ans= B
Q7. Siemen is the unit of
(a) Resistivity
(b) Conductivity
(c) Conductance
(d) Drift velocity Ans=C
Q8. On increasing the temperature, resistance of a metal increases because
(a) Mean relaxation time decreases
(b) Mass of electron increase
(c) Electron density decreases
(d) Charge of electron increases. Ans=A
Q9. If a conductor of 5Ω is stretched in such way that its length becomes double, then resistance becomes
(a) 5Ω
(b) 10 Ω
(c) 20 Ω
(d) 25 Ω Ans= C
Q10.The resistance of a wire of uniform diameter d and length L is R. The resistance of another wire of the same material but diameter 2d
and length 4L will be
(a) 2R
(b ) R
(c) R/2
(D) R/4 Ans=B
Q11.A certain piece of silver of given mass is to be made like a wire. Which of the following combination of length (L) and the area of
cross-sectional (4) will lead to the smallest resistance
(a) Land A
(b) 2L and 4/2
(C) L/2 and 24
(d) Any of the above, because volume of silver remains same
Ans=C
Q12.For a metallic wire, the ratio Vi (v= the applied potential difference, i current flowing is
(a) Independent of temperature
(b) Increases as the temperature rises
(c) Decreases as the temperature rises
(d) Increases or decreases as temperature rises, depending upon the metal
Ans=B
Q13.Which of the following has a negative temperature coefficient
(a) c
(b) Fe
(c) Mn
(d) Ag Ans=A
14.All of the following statements are true except
(a) Conductance is the reciprocal of resistance and is measured in Siemens
(b) Ohm's law is not applicable at very low and very high temperatures
(c) Ohm's law is applicable to semiconductors
(d) Ohm's law is not applicable to electron tubes, discharge tubes and electrolytes
Ans=C
Q15.If the resistance of a conductor is 5 ohm at 50°C and 7 at 100°C then the mean temperature coefficient of resistance of the material is
(a) 0.008/°C
(b) 0.006/°C
(c) 0.004/°C
(d) 0.001/°C Ans=A
16.The resistivity of a wire depends on its
(a) Length
(b) Area of cross-section
(c) Shape
(d)Material. Ans=D
Q17. A strip of copper and another of germanium are cooled from room temperature to 80 K. The resistance of
(a) Each of these increases
(b) Each of these decreases
(c) Copper strip increases and that of germanium decreases
(d) Copper strip decreases and that of germanium increases.
Ans=D
.jpg)
Post a Comment