Electricity: electricity facts
Charge:
- The physical property which determine the deficit or excess of electrons in body is called charge.
- charge is an intrinsic property of matter.
- Its SI unit is coulomb.
- It is scalar quantity.
Type of Charge:
Positive Charge:
- The charge on body due to deficit of electrons is called positive charge. and body is called positively charged body.
Negative Charge:
- The charge on body due to excess of electrons is called negative charge. and body is called negatively charged body.
Properties of Charge:
- A stationary charge produces electric field around it.
- A moving charge produces magnetic field around it motion.
- Like charges exert repulsion force to each other.
- Unlike charges exert attraction force to each other.
- Charge is quantized in nature. i.e value of charge never lie between ( n – 1 ). e and n.e. where ‘n’ is integer and ‘e’ is charge on electron.
- Charge is conservative . this is because it is scalar quantity.
Q.
Define +1 Coulomb:
Ans:
- If two equal positively charged body are kept in air at 1 m distance and they exert force of 9 x 109 N to each other , then amount of charge on each body will be +1 C.
OR,
- If a body has deficit of 6.25 x 1018 electrons, then charge on body will be +1 C.
Q.
Define -1 Coulomb:
Ans:
- If two equal negatively charged body are kept in air at 1 m distance and they exert force of 9 x 109 N to each other , then amount of charge on both body will be -1 C.
OR,
- If a body has excess of 6.25 x 1018 electrons, then charge on body will be +1 C.
Calculate Amount of charge on body:
- If a body has deficit of ‘n ‘ electrons, Then charge on body , Q = + ne C
- If a body has excess of ‘n ‘ electrons,Then charge on body , Q = - ne C
- +ve sign show deficit of electrons in body.
- -ve sign show excess of electrons in body.
Calculate Number of deficit or excess electrons : (n)
n= Q / e
or, n=
6.25 x
1018 x Q
Electricity:
The flow of free electrons ( charge) in
conductor is called electricity.
Electric Current:
- Amount of charge passing through any cross sectional area of conductor in one second is called electric current.
In time ‘t’ second, flowing charge = Q
In time
1 second , charge
= Q / t
i.e
i = Q / t
in
other words,
- the rate at which electricity passes through any cross sectional area of conductor is called electric current.
Properties of electric current:
- It is scalar quantity.
- Current is measured by ammeter and galvanometer .
- Its SI unit is ampere.
- Conventional direction of electric current is opposite direction of electron’s motion.
- In circuit , direction of current is taken from positive terminal of battery.
Q. Define
1 ampere .
Ans:
We know that , I = Q
/ t
If Q = 1 C
and t = 1 sec.
Then,
I = 1 C / 1
s =
1 A
- When 1 coulomb charge passes through any cross section of conductor in one second , then current in conductor will be 1 ampere.
Mechanism of electric current:
- When voltage is applied across conductor , the free electrons from one end ‘A’ of conductor begin to move towards positive terminal of battery due to attraction.
- Hence end ‘A’ becomes positive.
- Free electrons near the end ‘A’ move towards it to neutralize .
- There occur deficit of electrons at other end ‘B’ of conductor.
- The end ‘B’ attract electrons from negative terminal of battery.
- Thus flow of electrons is maintained from negative terminal to positive terminal.
- Direction of conventional current is taken opposite direction of electron’s motion. So direction of current is positive to negative.
Potential:
- Amount of energy containing per coulomb charge at any point in electric circuit is called potential at that point.
Potential difference:
- Energy required to move per coulomb charge (electrons) from one point to other point in circuit is called potential difference between that points.
- Electrons gain energy from battery to move in circuit.
Energy gain by Q coulomb = work done by battery on Q coulomb (W)
Potential difference = work done by battery on one coulomb electrons
i.e VA - VB = W /
Q
Properties:
- Its SI unit is Volt.
- It is a scalar quantity.
- Positive test charge always move from high potential to low potential.
- Negative test charge always move from low potential to high potential.
- Thus electric current always flow from high potential to low potential.
Electro motive force ( E.M.F ) :
- Energy given by battery to each coulomb charge to flow in circuit is called e.m.f of battery.
- e.g. if a battery supplies 4 J energy to each coulomb, it means emf of battery is 4 V.
Resistance:
- The opposition offered by a conductor to the flow of free electrons is called resistance.
- Resistance is also called electric friction.
- Its SI unit is ohm ( Ω )
Factors on which resistance depend :
Resistance of conductor depends on following
factors.
- It is directly proportional to length of conductor.
i.e R
α L
- it is inversely proportional to cross sectional area of conductor.
i.e R α 1 /
A
- it depends on nature of conductor.
- It depends on temperature of conductor.
Derive resistance formula: R = ρ . L / A
At constant temperature and for same material, resistance depend on only two factors.
- It is directly proportional to length of conductor.
i.e R
α L ---(1)
- it is inversely proportional to cross sectional area of conductor.
i.e
R α 1 / A ----- (2)
combine
(1) and (2)
R
α L / A
i.e
, R = ρ . L / A
- where , ρ is constant and known as specific resistance or resistivity of materal.
Specific resistance :
We have ,
R = ρ . L / A
When
L= 1 m, A= 1 m2
Then ,
R = ρ
- Thus, resistance of conductor having length 1 m and area of cross section 1 m2 is known as resistivity.
Or,
- Resistance between opposite face of cubic material having dimension 1 m is called resistivity.
- It depends on nature of material and its temperature.
- Resistivity increases on increasing with temperature.
- It does not depend on shape and size of material.
- Its unit is Ω-m .
Ohm’s law:
- At constant temperature and same physical condition, Current flowing through the conductor is directly proportional to the voltage applied across it .
i.e
I α V
or,
V α I
V = IR
- Where , R is constant and called resistance of conductor.
- Its SI unit is V / A or Ohm .
I
- V graph:
( For Conductor )
- Graph drawn between current and voltage is straight line . it is because the ratio of voltage and current is always consatant.
i.e. V
/ I =
R
- Here , R is slope of graph and R = tanθ, Where, θ is angle of inclination .
Define 1 ohm :
- Resistance of conductor through which 1 A current flow when 1 V is applied across it is called 1 ohm.
