Current Electricity Concept Notes

 Current

Current is the rate of flow of charge 

I = charge/time = Q/t= Coulomb/secs = ampere

I = Q/t = ne/t

• The flow of current is in the direction opposite to the flow of electrons
• Current is a scalar quantity
• Does not depend on the cross-section area of the wire

Conductors: large no. of free charges

            I is high

Insulators: very less or no free charge

            very less current

Semi-conductors: large or small no. of free charges

Potential Difference 

• Needed for the flow of current, for eg when there is temperature diff. heat flows, just like that, a difference is needed for the flow of current and this diff. is known as potential diff.
• Amount of work done in moving 1 C charge from A to B = Potential diff. b/w A and B
• It is the work done per charge
• PD b/w two points is the work done per unit charge 
• (delta)V= W/Q (Work/Charge) Unit= Joule/Coulomb= Volt

The amount of work done in taking 1C charge from A to B if the amount of work done is 1J then the PD is said to be 1V.

•  V(b)-V(a) = W (a to b)/Q ---PD
• Work done on 1C in taking it from infinity to A=Potential at A
• Potential at infinity=0
• V(a)=W (infinity to a)/Q ---Potential at a
• If work is -ve= work obtained
• Higher the PD more is the current

Ohm's Law 

• (delta)V is directly proportional to Current
• when temp=constant and all physical conditions remain the same then the PD at the ends of the conductor is directly proportional to the current flowing through the conductor

                  (delta)V=IR  (resistance is constant)

• resistance depends on conductor
• Slope of (delta)V-I graph gives resistance--- it doesn't change
• Ohmic conductors- conductors which obey ohm's law

Resistance

• Specific resistance is a constant which is a property of material
• Thicker the wire, less is the resistance but specific resistance remains same if material is same
• Longer the wire more is the resistance
• If you double the length area becomes 1/2.If you increase the length by 3 times area becomes 1/3 and lesser the area more is the resistance

                             R= ρl/A

                   R=(delta)V/A = Volt/Ampere = ohm

                                                           ρ= RA/l = ohm*m*m/m = ohm m

• Specific resistance depends on temperature
• In conductor on increasing temp. resistivity increases.In conductor more the temp. the random motion of molecules in the conductor increases and hence more collisions. straight graph
• In semi-conductors on increase in temp. resistivity decreases. As no. of ions increases so flow is easier. curved graph
• Alloys- in manganin on increase in temp. resistivity remains constant. almost a straight line parallel to the x-axis

Resistivity (Specific resistance)

It is the resistance of the wire of that material of length 1m and area 1sq.m.