CONTENT LIST
ELECTRICITY
ELECTRIC CURRENT AND CIRCUIT
Flow of electric charges through a conductor (for example, metallic wire),is called electric current in the conductor.
Current flow in a closed circuit.Electrons constitute the flow of charges which is termed as electricity.
In an electric circuit the direction of electric current (+ve to -ve terminals of battery) is considered as opposite to the direction of the flow of electrons, which are negative charges.
A closed and continuous path of an electric current is called an electric circuit.
A circuit constitute a source of power , mettalic wire, switch and load (bulb).
Electric current is measured by the amount of charge flowing through a particular area in unit time.
If a net charge Q, is flowing 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 taken as coulomb (C), which is equivalent to the charge contained in nearly 6 × 10^18 electrons.
The unit of electric current is ampere (A).
If any circuit is passing one coulomb of charge per second through a cross-section of conducting wire,then value of current is called one ampere.
Small quantities of current are expressed in milliampere
(1 mA = 10^–3 A) or in microampere (1 µA = 10^–6 A).
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.
ELECTRIC POTENTIAL AND POTENTIAL DIFFERENCE
The electrons move inside the conductor only if there is a difference of electric potential or electric pressure , called the potential difference along the conductor.
This difference of potential is be produced by a Cell or battery , consisting of one or more electric cells.
The chemical action happened within a cell .The cell generates the potential difference across the terminals of the cell.
When the cell is connected to a closed circuit element, the potential difference sets the charges in motion across the conductor and produces an electric current.
In order to maintain the current in a closed electric circuit, the cell has to expend its chemical energy stored in it.
The SI unit of electric potential difference is taken as volt (V).
The electric potential difference between two points in a closed circuit is equal to work done to move a unit charge from one point to other point .
Potential difference (V) between any two points = Work done (W)/Charge (Q)
V = W/Q
When 1 joule of work is done to move a charge of 1 coulomb from one point to the other in closed circuit ,then potential difference of these two points is called 1 volt.
Therefore, 1 volt =1 joule/1 coulomb
1 V = 1 J C^–1
CIRCUIT DIAGRAM
A schematic diagram of an electric circuit in which different components of the circuit are represented by the symbols is called circuit diagram.
OHM’S LAW
There a relationship between the potential difference across a conductor and the current passing through it.
A German physicist Georg Simon Ohm found out the relationship between the current (I), flowing in a wire and the potential difference (V) across its terminals.
The potential difference(V) across the ends of a given wire in an electric circuit is directly proportional to the current (I) flowing through it, provided its temperature remains the same.
This is called Ohm’s law.
V ∝ I
or V/I = constant = R
or V = IR
Where R is a constant for the given wire at a given temperature and is called its resistance.
FACTORS ON WHICH THE RESISTANCE OF A CONDUCTOR DEPENDS
The resistance of the conductor depends
(i) on its length,
(ii) on its area of cross-section, and
(iii) on the nature of its material.
The resistance of a metallic wire is directly proportional to its length (l) and inversely proportional to the area of cross-section (A). That is,
R ∝ l ------(eqn 1)
and R ∝ 1/A. -----(eqn 2)
Combining Eqs. (1) and (2) we get
R ∝l/A
or, R = ρl/A
Where ρ (rho) is a constant of proportionality . It is called the resistivity of the material of the conductor.
The SI unit of resistivity is Ohm(Ω).
Resistivity is a characteristic property of the material.
The metals and alloys have very low resistivity in the range of 10^–8 Ω m to 10^–6 Ω m. They are good conductors of electricity.
Insulators such as rubber and glass have resistivity of the order of 10^12 to 10^17 Ω m. Both the resistance and resistivity of a material vary with temperature.
RESISTANCE OF A SYSTEM OF RESISTORS
There are two methods of joining the resistors together.
In Series combination
If two or more rasistences are joined end to end in circuit and each has a different current passing through them . They are called in series combination.
In Parallel combination
If two or more resistances are connected between two common points in circuit and each has a different current passing through them .They are called in parallel combination.
Resultant Resistance of Two Resistances Connected in Series
Consider two resistances and connected in series R1 and R2. A battery of V volts has been applied to the ends of this series combination.
Now, Let the potential difference across the resistance R1 is V1 and the potential difference across the resistance R2 is V2.
Since rasistences are connected in series and same current (I) is passing through them.
Circuit contains a battery of voltage V.
Net potential difference across the two resistances is equal to the voltage of the battery.
V = V1 + V2
.... (1)So, applying Ohm’s law to the whole circuit, we get:
VI = R
V = I × R ...(2)
V1 = I × R1 ...(3)
V2 = I × R2 ...(4)
Putting value of V,V1 and V2 in equation 1
VR =VR1 + VR2
or R =R1 + R2
Resultant Resistance of Two Resistances Connected in Paralel
Consider two resistances and connected in parallel R1 and R2. A battery of V volts has been applied across points of parallel combination.Which generating current (I) in the circuit.
Since rasistences are connected in parallel so different current (I1) and (I2) is passing through rasistences R1 and R2 respectively.
Net potential difference across the two resistances is equal to the voltage of the battery.
I = I1 + I2 … (1)
So, applying Ohm’s law to the whole circuit, we get:
VI = R
I = V/ R ...(2)
I1 = V/ R 1 ...(3)
I2 = V/ R2 ...(4)
Putting value of V ,V1 and V2 in equation 1
V/R =V/R1 + V/R2
1/R =1/R1 + 1/R2
HEATING EFFECT OF ELECTRIC CURRENT
When current flows through a metallic conductor, cuurent has to work against the rasistance,this work converted into heat .thus heat energy is produced in the conductor. This is called heating effect of an electric current .
The heat generated depends on three factors:
The resistance, R of the conductor.
A higher resistance of conductor produces more heat and lower the resistance of conductor produces less current
The time, t for which current flows.
The longer the time the larger the amount of heat produced and vice versa.
The amount of current, I. the higher the current the larger the amount of heat generated.
Joule's law of heating effect :
This law states that the heat produced in a wire is directly proportional to the square of the current, resistance of wire and time for which the current is passed.
H∝i^2RT
Practical Applications of Heating Effect of Electric Current
(i) This effect is used in selection of fuse wire for protecting household wiring.
(ii) It is also used in water heaters,electric iron,oven etc.
ELECTRIC POWERElectric power means amount of work done by electrical device in unit time .
Suppose you have a heater which produced W Joule of heat in time t seconds then
power =W/t
SI Unit of power is Watt (W).
Bigger unit of power is KW or MW
1KW=1000 Watt or 10^3W
1MW=1000000 Watt or 10^6W