Electricity
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Types of Electricity : There are two types of electricity
(i) Static electricity (ii) Current electricity
Static electricity causes physical effects due to the charge at rest. Current electricity causes physical effects due to the moving charge.
• Conductors: It is a material which allows the current (charge) to pass through it, e.g. Aluminium, Copper, Silver (best).
• Insulators: It is a material which does not allow the current (charge) to pass through it, e.g. Rubber, Wood, Plastic.
• Electric current and circuit : - Electric current is the rate of flow of charge.
The magnitude of electric current in a conductor is the amount of charge flowing through it in one second.
If a net charge Q flows across any cross-section of a conductor in time t, then the current I, through the cross-section is
I = Q/t
The SI unit of electric charge is coulomb (C),
• An instrument called ammeter measures electric current in a circuit. It is always connected in series in a circuit through which the current is to be measured shows the schematic diagram of a typical electric circuit comprising a cell, an electric bulb, an ammeter and a plug key.
• Electric potential and potential difference : When a unit test charge is placed outside an electric field which is produced by another charge it does not experience any force. When this test charge is brought inside the field, some work is done because the unit positive charge experiences a force which becomes more and more when the test charge moves near to the given charge. This amount of work done is known as electric potential.
Potential difference is the difference between electric potentials of two distinct points inside an electric field. A and B are two considered points inside an electric field. When a unit positive test charge moves from A to B, some work is done. This amount of work done is known as potential difference.
It is represented by V.
Therefore,
V = W/Q
• One volt: It is the potential difference when 1 joule of work is done to move a charge of one coulomb from one point to another inside an electric field.
Therefore,
1 Volt = 1 Joule/ 1 Coulomb
Or,
1V = 1J /1C
• Circuit Diagram : It is a closed conducting path containing a source of potential difference or electric energy and a device utilizing the electric energy.
Symbols of commonly used electrical components :
Ohm's law: According to Ohm's Law, “Electric current is directly proportional to the potential difference between the two ends of a conductor at constant temperature" i.e.
V ∝ I or I ∝ V
or V = RI
where R is constant of proportionality which is known as
Resistance. It is measured in ohm represented by α―
One Ohm :- We know that
R = V/I
1 α― =1V /1A = 1 V/A
Resistance of a conductor is said to be 1 ohm if a potential difference of 1 volt across the ends of the conductor makes a current of 1 ampere to flow through it.
1 kα― = 1 Kilo Ohm = 10^3 W
1 Mα― = 1 Mega Ohm = 10^6 W
• Cause of Resistance: A conductor has large number of free electrons. When a potential difference is maintained across the ends of a conductor, the free electrons drift from one end to the other end of the conductor. During their movement, they collide with each other. These collisions oppose the movement of free electrons from one end to the other end of the conductor. This opposition to the flow of free electrons due to the collisions is called resistance. More is the collision suffered more is the resistance.
• Factors Affecting Resistance of a Conductor : There are four factors on which resistance of a conductor i.e., wire depends :
(1) Length of wire or conductor.
(2) Area of cross-section.
(3) Nature of material of wire.
(4) Temperature.
• Resistivity and specific resistance : Resistance of a conductor depends on length of wire and area of cross-section i.e.
R ∝ L; R ∝ 1/A i.e.,
R∝L/A
⇒ R= ΟL/A
Ο is a constant. This constant of proportionality is known as resistivity, which is represented by Ο(rho).
Therefore, Ο = RA/L
Resistivity of a conductor is measured in ‘ohm meter’ or ‘ W m’.Resistivity, which is also known as specific resistance is also defined as the resistance offered by a conductor of length 1 m having area of cross-section 1^2 m .
Resistivity of a material does not depend on length or area of cross-section. Infact, it depends on the nature of materials.
For a particular material (conductor) resistivity is constant,e.g., resistivity of silver is 1.6×10^-8 α―m
• Resistance of alloys: It is found practically that the resistance of alloys is more than the resistance of its constituent metals.It means alloys have higher resistivity than their constituent metals e.g., nichrome which is an alloy of nickel and chromium has very high resistivity than its constituent metals i.e., Nickel and Chromium.
• Resistance of a system of resistors :
(i) Combination of resistances in series : - When two or more resistances are connected end to end, they are said to be in series. When two or more resistances are connected in series, then current flowing in the circuit remains same i.e., it is equal in each and every resistance but PD gets distributed among these resistances. It means the sum of PDs or voltage across the different resistances is equal to the total voltage given to the circuit i.e.
V = V1 + V2 + V3 + ....................
The equivalent resistance is given by,
R = R1 + R2 + R3 + ...................
(ii) Combination of resistances in parallel : - If two or more resistances are connected to the same end, they are said to be in parallel. When resistances are connected in parallel and put in a circuit with a battery of voltage ‘V’ volt which supplies a current of ‘I’ ampere, then :
(a) The PD across these resistances remain same i.e., ‘V’ volt.
(b) Current gets divided into parts in such a way that the total current in the circuit is equal to the sum of the currents flowing through each resistance.
I = I1+ I2 + I3 + .......................
The equivalent resistance is given by,
1 /R = 1/R1 + 1/R2 + 1/R3+ ..............
• Heating Effect of Electric Current : When the electric current is passed through a conductor then conductor gets heated, this effect is known as the heating effect of current. Consider a conductor AB connected to a cell of potential difference V. If W be the work done to carry the charge q from one end to another end of the conductor,
Then, V= W/q
W = Vq ..... (1)
If I be the amount of current flowing through the conductor, then
q = I.t
W = VIt ; from (1)
This work done is equal to heat (H) produced in a conductor.
| H = VIt |
But according to Ohm’s law
V = IR
Then,
| H= I^2Rt | and I = V/R
Then,
H = V^2 / R × t
i.e.,
| H=VIt= I^2Rt= V^2 /R × t |
Joule’s Law of Heating Effect: It states that the amount of heat produced in a conductor is
(i) directly proportional to the square of current passing through it,
H ∝ I2 ...... (1)
(ii) directly proportional to the resistance of conductor,
H ∝ R ....... (2)
(iii) Directly proportional to the time for which current passed ,
H ∝ t ....... (3)
Combining (1), (2) and (3).
Ha∝ I^2 Rt.
Here, constant of proportionality is 1.
| H = I^2Rt joule .|
Practical applications of heating effect of current :-
(i) Electric heater, electric iron and water heater, etc. work on the principle of the heating effect of current.
(ii) Electric bulb glows when electric current flows through the filament of the bulb.
(iii) Electric fuse in the electric circuit melts when large current flows in the circuit.
• Electric Power : The rate at which electric energy is consumed is called electric power.
Power = Work /Time
It is measured in ‘watt’.
P = W/t = I^2 R = VI = V^2 / R
where P is the power, I is current flowing, V is the potential Difference and R is the resistance.
Units of Power :
SI unit of power is watt (W)
1 watt = 1 volt × 1 ampere = 1VA
1 kW = 1 Kilo Watt = 10^3W
1 MW = 1 Mega Watt = 10^6 W
1 GW = 1 Giga Watt = 10^9 W
|1hp = 746 watt |
The commercial unit of electrical energy is Kilo Watt Hour (kWh). It is also called ‘unit’ or B.O.T. (Board of Trade Unit).
| 1kWh = 3.6 10^6 joule |
Here we are done with our complete chapter.
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