Electricity: Electric current, electric circuit, voltage or electric potential, resistance and (Ohm’s law).
Electric Current: The flow of electric charge is known as Electric Current, Electric current is carried by moving electrons through a conductor.
By convention, electric current flows in the opposite direction to the movement of electrons.
Electric Circuit: Electric circuit is a continuous and closed path of electric current.
Expression of Electric Current: Electric current is denoted by the letter ‘I’. Electric current is expressed by the rate of flow of electric charges. Rate of flow means, the amount of charge flowing through a particular area in unit time.
Where I is electric current, Q is a net charge and t is a time in second.
S.I. Unit of Electric Charge and Current: S.I. unit of electric charge is coulomb (C).
One coulomb is nearly equal to 6 × 1018 electrons. S.I. unit of electric current is ampere (A). Ampere is the flow of electric charge through a surface at the rate of one coulomb per second. This means, if 1 coulomb of electric charge flows through a cross section for 1 second, it would be equal to 1 ampere.
Therefore, 1 A = 1 C/1 s
Small Quantity of Electric Current: Small quantity of electric current is expressed in milliampere and microampere. Milliampere is written as mA and microampere as pA.
1 mA (milliampere) = 10-3 A
1 pA (microampere) = 10-6 A
Ammeter: An apparatus to measure electric current in a circuit.,
Charge: Like mass, the charge is the fundamental property of matter. There are two types of charge
(i) Positive charge.
(ii) Negative charge.
Positive and Negative Charge: The charge acquired by a glass rod when rubbed with silk is called a positive charge and the charge acquired by an ebonite rod when rubbed with wool is called negative charge.
Properties of Electric Charge:
(i) Unlike charges attract each other and like charges repel each other.
(ii) The.force between two charges varies directly as the product of two charges and inversely as the square of the distance (r) between both charges (q1 and q2).
S.I. unit of charge is coulomb (C).
1 coulomb = 1 ampere × 1 second.
1C = 1A × 1s
Thus, the quantity of charge which flows through a circuit when one ampere of current flows through it in one second is known as a 1-coulomb charge.
Electric Potential and Potential Difference
Electric Potential: The amount of electric potential energy at a point is called electric potential.
Potential Difference: The difference in the amount of electric potential energy between two points in an electric circuit is called electric potential difference.
Electric potential difference is known as voltage, which is equal to the amount of work done to move the unit charge between two points against static electric field.
Ohm’s Law: According to this law “Under the constant physical condition the potential difference across the conductor is directly proportional to the current flowing through the conductor.”
V ∝ I
V = IR …[Where R is proportionality constant called resistance of conductor]
⇒ I = VR
R depends upon nature, geometry and physical condition of the conductor.
The heat generated by electric current: The potential difference between two points in an electrical field is equal to the work done in moving a unit charge from one point to another.
Then, work is done, W = VQ and Q = I × t
W = V × I × t
From Ohm’s Law, we know that
V = IR
W = IR × I × t = I2.Rt
Since heat produced by the electric current is equal to work done, W
H = W
⇒ H (heat) = I2Rt Joule.
Resistance: Ratio of the applied voltage to the current flowing in the conductor is called resistance of the conductor.
⇒ R = VI
S.I. Unit of resistance is VA-1 or ohm (Ω).
Resistance is the opposition offered by the conductor in the flow of current.
Practically it is
R ∝ L (L is the length of a conductor)
R ∝ 1/A (A is the area of a conductor)
So, R ∝ L/A
R = ρL/A …[Where p is proportionality constant called specific resistance of conductor
It only depend upon nature (material) and temperature of conductor.
Specific resistance or Resistivity = ρ = RA /L
It’s S.I. Unit is Qm
Combination of resistance:
- In this combination the current across every component is same but potential across every component is different.
- If resistance R1, R2 and R3 are connected in series with a battery of Potential V, then equivalence resistance of the combination
R = R1 + R2 + R3
The parallel combination of resistance:
- In this combination the current across every component is different. But potential across every component is the same.
- If resistance R1, R2 and R3 are connected in parallel with a battery of Potential V, then equivalence resistance of combination
1/R=1/R1+1/R2+1/R3
Electric Energy is amount of work done to maintain the continuous flow of electric current in the circuit.
Its S.I. unit is joule (J).
Electric power (P): The electric work done per unit time is called electric power.
Electric Power = Electric workdone /Timetaken
or P = W/t
Electric power is also defined as the electric energy consumed per unit time.
P = E/t
S.I. unit of electric power is Watt. When one joule of energy is used for one second, electric power is equal to one watt.
Derivation of formula for electric power:
We know that electric work done, W = V × I × t or P = VIt/t
P = VI
Electric power in watts = Volts × ampere
Also V = IR …[According to Ohm’s Law]
So P = IR × I
P = I2R
We know that I = V/R
P = (V/R)2 × R = V2R Watt
The maximum value of electric current that can pass through an electric appliance without damaging electric appliance is called current rating of electric appliance.
