Current Electricity
Current
Current is the rate of flow of charge across a cross-section normal to the direction of flow of current.
Charge Q
Current I
Time t
It is a scalar quantity. Its SI unit is ampere (A).
Current is one ampere if the rate of flow of charge is one coulomb per second.
If n electrons pass through the cross-section of a conductor in time t, then the total charge passed
through the conductor is given as Q = ne, and current in the conductor is given as
Q ne
I
t t
Concept of Potential and Potential Difference
The conductor with higher concentration of electrons is said to be at a lower potential, and the
conductor with lower concentration of electrons is said to be at a higher potential.
The electrons flow from a body at a lower potential to a body at a higher potential.
Potential is the electrical state of a conductor which determines the direction of flow of charge when
the two conductors are either kept in contact or joined by a metallic wire.
The electric current flows from a body at a higher potential to a body at a lower potential, and this is
called conventional current. This direction is opposite to the direction of flow of electrons which is
called electronic current.
Measurement of Potential as Work Done Per Unit Charge
The potential at a point is defined as the work done in bringing a unit positive charge from infinity to
that point.
Work done
Potential difference
Charge
W
V
Q
The potential difference between two points is said to be 1 volt if the work done in bringing 1 coulomb
of charge from infinity to the point is 1 joule.
The potential difference between two points is equal to the work done in moving a unit positive charge
from one point to the other.
W
VA VB
Q
It is a scalar quantity. Its SI unit is also volt (V).
, Concept of Resistance
There is always some obstruction in the current which flows through a conductor like a metal wire, and
this obstruction is called its electrical resistance.
The current in the circuit flows due to the drift of electrons. The metal wire has free electrons which
move in a random manner.
When the ends of a wire are connected to a cell, the electrons start moving from the negative terminal
to the positive terminal. In this process, they collide with the positive ions, and due to this, the speed of
electrons decreases. Thus, the metal offers resistance to the flow of electrons because of these
collisions.
The resistance of a conductor depends on four factors:
1. Length of the conductor:
Resistance R length l
2. Thickness of the conductor:
1
Resistance R
Area of cross-section A
3. Nature of the conductor: The resistance depends on the material of the wire as there is different
concentration and different arrangement of atoms in different materials.
4. Temperature of the wire: A higher temperature of the wire causes the ions in it to vibrate more
rapidly. As a result, the number of collisions increases, and hence, the resistance increases.
Current
Current is the rate of flow of charge across a cross-section normal to the direction of flow of current.
Charge Q
Current I
Time t
It is a scalar quantity. Its SI unit is ampere (A).
Current is one ampere if the rate of flow of charge is one coulomb per second.
If n electrons pass through the cross-section of a conductor in time t, then the total charge passed
through the conductor is given as Q = ne, and current in the conductor is given as
Q ne
I
t t
Concept of Potential and Potential Difference
The conductor with higher concentration of electrons is said to be at a lower potential, and the
conductor with lower concentration of electrons is said to be at a higher potential.
The electrons flow from a body at a lower potential to a body at a higher potential.
Potential is the electrical state of a conductor which determines the direction of flow of charge when
the two conductors are either kept in contact or joined by a metallic wire.
The electric current flows from a body at a higher potential to a body at a lower potential, and this is
called conventional current. This direction is opposite to the direction of flow of electrons which is
called electronic current.
Measurement of Potential as Work Done Per Unit Charge
The potential at a point is defined as the work done in bringing a unit positive charge from infinity to
that point.
Work done
Potential difference
Charge
W
V
Q
The potential difference between two points is said to be 1 volt if the work done in bringing 1 coulomb
of charge from infinity to the point is 1 joule.
The potential difference between two points is equal to the work done in moving a unit positive charge
from one point to the other.
W
VA VB
Q
It is a scalar quantity. Its SI unit is also volt (V).
, Concept of Resistance
There is always some obstruction in the current which flows through a conductor like a metal wire, and
this obstruction is called its electrical resistance.
The current in the circuit flows due to the drift of electrons. The metal wire has free electrons which
move in a random manner.
When the ends of a wire are connected to a cell, the electrons start moving from the negative terminal
to the positive terminal. In this process, they collide with the positive ions, and due to this, the speed of
electrons decreases. Thus, the metal offers resistance to the flow of electrons because of these
collisions.
The resistance of a conductor depends on four factors:
1. Length of the conductor:
Resistance R length l
2. Thickness of the conductor:
1
Resistance R
Area of cross-section A
3. Nature of the conductor: The resistance depends on the material of the wire as there is different
concentration and different arrangement of atoms in different materials.
4. Temperature of the wire: A higher temperature of the wire causes the ions in it to vibrate more
rapidly. As a result, the number of collisions increases, and hence, the resistance increases.
