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ELECTRICITY
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ApNIKAKSHA_
Afmgrwetarwd
Not included in CBSE 2020-21
, CURRENT ELECTRICITY
"
'
D
'
⇐
Electric current (I)
"
⎯ Current is defined as the rate of flow of charge |Scalar quantity | SI Unit- ampere (A)
⎯ Direction of current through a cross section is the direction of flow of positive charge.
⎯ If the net charge that flows through a cross-section is negative, it implies a current in the backward direction.
Instantaneous current Average current
Iinst = dq/dt Iavg = ∆q/∆t
⇐
Electric current in conductors
"
Conductors: Materials which allow current to pass through them easily. The charge carriers inside metallic conductors
are free electrons. When an electric field is applied, the free electrons experience a force and an electric current is setup
inside the conductor.
Note: Charge carriers in various conductors-
Metals- free electrons | Electrolytes- +ve and –ve ions | semiconductors- free electrons and holes.
Inside a conductor-
A) In absence of electric field – In the absence of electric field, the free electrons move due to thermal energy. During
this thermal motion the electrons collide with each other and the fixed positive ions. After a collision with a +ve ion,
the electron emerges with the same speed as before. However, the direction of its velocity will be completely
random. Thus, on average, the number of electrons moving in a particular direction will be equal to the number of
electrons moving in the opposite direction. So, there will be no net electric current in the conductor in the absence
of an electric field.
B) In presence of electric field- When a constant electric field is applied to the ends of a conductor, the free electrons
experience a force in a direction opposite to that of electric field and hence start moving in and therefore a net
electric current is setup in the direction of electric field (flow of positive charge). This electric current will remain as
long as the field is applied.
⇐
Ohm’s Law
"
Ohm’s law states that at constant physical conditions (temp etc), the current through a conductor is directly
proportional to the potential difference applied across its ends.
Where R is a constant of proportionality and is called the Resistance of the conductor | SI Unit- ohm (Ω)
R is dependent on both the material and dimensions of the conductor.
Also,
I
Where ρ is called the resistivity of the material
, .
Current density (j)
Current density is defined as the current per unit area (taken normal to the current) | SI Unit = A/m2
Consider an electric field E in a conductor of length l, then potential difference across it, V = E.l. Then from ohm’s law-
Where σ is called the conductivity of the material and σ = 1/ ρ|σ depends on nature of the material and not dimensions.
Vector form of current density-
According to definition,
Where is the angle between direction of current and normal to the area.
Note: j is directed along I(current) and is also parallel to E
.
Drift velocity (vd) (PYQ 2016, 2012)
It is defined as the average velocity with which the free electrons move towards the +ve end of a conductor under the
influence of applied electric field.
Let electric field be E, the force F on each electron and hence its acceleration a will be-
(Where m is mass of each electron)
→
→
Let initial velocity of electron be u and velocity after time τ be v. From Newton’s first equation of motion-
to
,
(The average initial velocity is zero as before
electric field is applied, there is no specified
direction in which electrons move.)
(The -ve sign indicates that Velocity is opp to field)
ELECTRICITY
•
BEST
NCERT NOTES
• Guaranteed
Full Marks
• •
*
•
All Previous
Year Questions
covered
ApNIKAKSHA_
Afmgrwetarwd
Not included in CBSE 2020-21
, CURRENT ELECTRICITY
"
'
D
'
⇐
Electric current (I)
"
⎯ Current is defined as the rate of flow of charge |Scalar quantity | SI Unit- ampere (A)
⎯ Direction of current through a cross section is the direction of flow of positive charge.
⎯ If the net charge that flows through a cross-section is negative, it implies a current in the backward direction.
Instantaneous current Average current
Iinst = dq/dt Iavg = ∆q/∆t
⇐
Electric current in conductors
"
Conductors: Materials which allow current to pass through them easily. The charge carriers inside metallic conductors
are free electrons. When an electric field is applied, the free electrons experience a force and an electric current is setup
inside the conductor.
Note: Charge carriers in various conductors-
Metals- free electrons | Electrolytes- +ve and –ve ions | semiconductors- free electrons and holes.
Inside a conductor-
A) In absence of electric field – In the absence of electric field, the free electrons move due to thermal energy. During
this thermal motion the electrons collide with each other and the fixed positive ions. After a collision with a +ve ion,
the electron emerges with the same speed as before. However, the direction of its velocity will be completely
random. Thus, on average, the number of electrons moving in a particular direction will be equal to the number of
electrons moving in the opposite direction. So, there will be no net electric current in the conductor in the absence
of an electric field.
B) In presence of electric field- When a constant electric field is applied to the ends of a conductor, the free electrons
experience a force in a direction opposite to that of electric field and hence start moving in and therefore a net
electric current is setup in the direction of electric field (flow of positive charge). This electric current will remain as
long as the field is applied.
⇐
Ohm’s Law
"
Ohm’s law states that at constant physical conditions (temp etc), the current through a conductor is directly
proportional to the potential difference applied across its ends.
Where R is a constant of proportionality and is called the Resistance of the conductor | SI Unit- ohm (Ω)
R is dependent on both the material and dimensions of the conductor.
Also,
I
Where ρ is called the resistivity of the material
, .
Current density (j)
Current density is defined as the current per unit area (taken normal to the current) | SI Unit = A/m2
Consider an electric field E in a conductor of length l, then potential difference across it, V = E.l. Then from ohm’s law-
Where σ is called the conductivity of the material and σ = 1/ ρ|σ depends on nature of the material and not dimensions.
Vector form of current density-
According to definition,
Where is the angle between direction of current and normal to the area.
Note: j is directed along I(current) and is also parallel to E
.
Drift velocity (vd) (PYQ 2016, 2012)
It is defined as the average velocity with which the free electrons move towards the +ve end of a conductor under the
influence of applied electric field.
Let electric field be E, the force F on each electron and hence its acceleration a will be-
(Where m is mass of each electron)
→
→
Let initial velocity of electron be u and velocity after time τ be v. From Newton’s first equation of motion-
to
,
(The average initial velocity is zero as before
electric field is applied, there is no specified
direction in which electrons move.)
(The -ve sign indicates that Velocity is opp to field)
