The concept of current is similar to the flow of water. To understand this concept firstly we discuss about flow of water between two water tanks placed at different height and connected through a pipe.
Place tank A is at some height and tank B is on ground. When we open the tap, then water can moves from tank A to tank B. What caused water to flow down? As we know that, tank A is at some height so it contains some gravitational potential energy. Tank B is at ground so it does not have gravitational potential energy. So there will be some potential difference. Due to that the water flow from tank A to tank B. Does flow of water depend on type of pipe, length and area of pipe?
Yes if you increase the length of pipe there will be more oppose to flow of water and loss will be more to travel long distance. If you increase the area of pipe then oppose will be reduced and water can easily move through that pipe.
Similarly, if we create some potential difference between two poles and connect them by a conducting wire then electron will move from low potential to high potential. So the motion of electron is called current. The flow of electron is same means Current is also depend on type of wire and geometrical configuration of that wire.
Definition of resistor:
The property of material to resist or oppose to flow of current in the circuit. The value of resistance is depend on material as well as geometrical configuration of the materiel i.e. shape and size of the material.
Resistor opposes to flow of current in circuit means it controls the current. If the current exceed the value then the circuit will burn out so we can avoid this by using suitable resistor.
Experimental proof:
Take some identical nichrome bars having length L and area A. Connect one nichrome barin circuit as shown in fig a and measure the value of current.
Connect another nichrome bar in circuit as shown in fig b and note down the value of current in circuit using ammeter.
Applied voltage(V) |
Current (A) | Length of wire |
24 | 12 |
L |
24 |
6 | 2L |
24 | 3 |
3L |
24 |
1.5 | 4L |
24 | 0.75 |
5L |
With the help of above data, you can calculate the value of resistance of the wire.
Resistance of the material is directly proportional to length of material
R ∝L——–1
Similarly you can perform another experiment. Arrange the equipment as shown in below. Firstly, you connect one piece of bar and measure the value in ammeter. Now you can attach second piece of nichrome baron first. In this case you increase area of wire.
Voltage (V) |
Current (A) | Area of wire |
24 | 12 |
A |
24 |
24 | 2A |
24 | 48 |
3A |
24 |
96 |
4A |
You can calculate the value of resistance of circuit using Ohms law.
From this data, you understand that resistance decreases with increase in area of the material
R ∝1/A——-2
Combine equation 1 and 2, we get
R ∝L/A
If we remove proportionality symbol then we get,
R = ρL/A
Resistance of the material is depend on length and area.
ρ = RA/L
Rho (ρ ) is called as specific resistance or resistivity of the material.
Its SI unit is Ωm.
Definition of specific resistance (Resistivity):
The resistance of material having unit area and unit length is called resistivity of that material. Resistivity is not depend on geometrical configuration of the material. It has fix value for every material.
Material |
Resistivity (10-6) |
Silver |
1.55 |
Copper |
1.7 |
Gold |
2.2 |
Aluminium |
2.7 |
Rhodium |
4.3 |
Ruthenium |
7.2 |
Osmium |
8.12 |
Palladium |
9.93 |
Platinum |
10.6 |