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1. PHYSICS CLASSES
BY ER. PRAKASH JHA

2. CURRENT ELECTRICITY
CLASS X

3. ELECTRIC CHARGE:
• When glass rod is rubbed with a silk cloth the charge acquired
by the glass rod is positive and silk is negative.
• The matter is made up of Atoms which consists of Proton,
Electron and Neutron.
• Proton is positively charged and electron is negatively
charged.
• Charge on 1 proton is 1.6*10^-19 Coulombs and 1 electron is
-1.6*10^-19 Coulombs.
• When a glass rod is rubbed with silk cloth, some electrons get
transferred from glass rod to silk cloth this causes positive
charge on glass rod and negative charge on silk .
• Like charges attract each other while unlike charges repel
each other by the virtue of electrostatic force.

4. POTENTIAL DIFFERENCE:
A B
q1
q2
q3
q4 q5
q6
q
Suppose there is a system of many
charges present in a space. Considering a
charge q which is located at point B, the
electrostatic forces of all the charges acts
on charge q.
Under such conditions, to bring that
charge from point B to A, some work done
is required
This Work done per unit charge is defined
as potential difference.
Potential difference V =
Work done/charge
Potential difference between two points is
required for Electric Current to flow
between them.

5. FLOW OF CURRENT :
• Current flows from the point of higher electrical
potential to lower electrical potential.
• It is similar to the case of water flow from container of
higher water level to lower water level.
• Potential difference between two points is required for
flow of electrons between two points.
• The potential of a positive charged body is considered
higher than negative.
• The flow of electric current is considered as opposite
to the flow of electron.
• The rate of flow of charge is known as current.
• Current i=q/t here q is charge passing at t interval of
time
• The SI unit of current is Ampere.
• The SI unit of potential difference is Volt.

6. Constant Potential Difference:
• A cell is considered as a
source of constant potential
difference.
• The different chemical
reaction taking place within
a cell causes separate
positive and negative
terminals and a constant
potential difference between
them.

7. ELECTRICAL CIRCUIT :
The closed path of flow of current is known as electrical Circuit.
Electrical circuit and its component can be represented by its symbols.

8. OHMS LAW:
• The current passing through a metallic Element is proportional to the
potential difference applied between it ends, provided temperature
remains Constant.
A
V
Voltmeter
Ammeter
A B
At constant temperature, if voltage is increased, Current increased linearly.

9. RESISTANCE:
• By ohms law Voltage is directly proportional to the
the current passing through it.
• This proportionality constant is known as
resistance.
• It is basically the hindrance offered to electric
current passing through the conductor.
• Resistance is directly proportional to the length of
conductor and inversely proportional to the Cross
sectional Area.
• Its SI Unit is Ohm. 1 ohm resistance is the
resistance offered by a circuit having potential
difference across it 1v and 1A of current passing
through it.

10. RESISTIVITY:
• As we know that resistance is directly
proportional to the length and
inversely proportional to the cross
sectional Area.
• Resistivity can be defined as specific
resistance. It is resistance offered by
a conductor of unit length and unit
cross sectional area at a specific
temperature.
• Resistivity varies with temperature. It
increase with increase in
temperature.

11. Resistance in Series:
• Resistance are said to be in series if same
current is flowing through them.
• one resistance is removed from connection no
current will flow through the circuit.
• The potential difference is different for
different resistance.

12. Resistance in Parallel:
• The resistances are said to be in
parallel if the same potential
difference has been applied across
them
• The current flowing through them
are different.
• The wiring done at household is in
parallel that is tube light, fan,
television all are connected in
parallel to a constant potential
difference

13. Equivalent Resistance:
• If a combination of resistances
is removed from a circuit and a
is replaced by a single
resistance which draws a equal
current from the voltage source
as that combination of
resistances, then that single
resistance is Equivalent
Resistance for that
combination.
For Series Combination
Since current through R1, R2, R3 is same.
V=iR1+iR2+iR3=i(R1+R2+R3)=iRT
It can be replaced by a resistance
RT= R1+R2+R3

15. MEASURING INSRTUMENTS:
• Ammeter is device used
to measure current.
• It is connected in series
with the component
whose current have to be
measured.
• Ideal Resistance is Zero
• Voltmeter is device used to measure
potential difference across any
component.
• It is connected in Parallel to the
component.
• Ideal Resistance is infinite.
• Galvanometer is a device used to detect the
presence of current.
• It also indicates direction of current flow.

16. Potentiometer:
• Potentiometer is the device used to
measure potential difference across a cell.
• It has high level of accuracy.
• In a circuit, if Galvanometer deflection is
null, It indicates no current passing
through it and potential difference across
is zero.
• Thus, by comparing values, we can easily
calculate required parameters like
potential difference across cell as well as
internal resistance of Cell.
• As we know resistance is directly
proportional to the length of conductor,
this principal along with null deflection is
used in potentiometer.

17. Potentiometer Applications:
• We have to get the potential difference across cell E. In order to do so, we use one cell whose
value is known to us E1.
• The value of resistance per unit length of meter resistance is known. Let it be r.
• Potentiometer connected to cell E1Suppose at point x1, we get a null deflection, so at this
point we know, potential difference across AC is equal to E1.
• Now we connect E to the potentiometer, so at length AB we get null deflection.
• Now we have E1=irx1, and across E= irx .
• Dividing both E1E = x1/x. Value of E1,x1,x is known., so we can easily get the value of E.
x1
x
c
Circuit 1
Potentiometer
Circuit 2
Test Circuit
Current I
Flowing
through
circuit 1 in
null
deflection.

18. Heating Effects of Current:
• The heat is generated in a resistor or a device
when current flows through it.
• The real time example is when we switch on a
bulb, current passes through It, and it becomes
hot.
• The heating effect of current has its application in
electric heater, geyser, press etc..

19. • V=iR
• V=work done/charge
• Work done=vq
• i=q/t
• q=it
• Putting all equation we get
• Work done=(i^2)Rt
• Work done =(V^2)/R
• This work done is converted into Heat Energy.
• ‘If for two resistor current is constant we consider equation first equation for work done
and if potential difference is constant we consider equation second to be the equation
for work done
• For constant current system, heat produced is directly proportional to the resistance.
• For constant voltage system, heat produced is inversely proportional to the resistance.

20. ELECTRIC POWER:
• Power is defined as rate of doing Work.
• Electric Power of a device is defined as rate by which electric power is
consumed by it.
• Power=work done/time=( i^2)rt/t= (i^2)R= V^2/R=Vi.
• It’s SI unit is Watt.
• Generally the electrical energy is measured in terms of Kilowatt Hour (Kwh)
• Most of the electrical devices are ratted as Kwh. That is how many kw
power it will posses during 1 hour of use.

21. FUSE:
• An electrical fuse is a simple device used to interrupt an electrical circuit during over current condition
due to short circuit and/or overload.
• An electrical fuse operates on the principle of heating effect of electric current. During normal
operating condition, the current flowing through the fusing element is such that the rate of heat
production in the fuse element is nearly equal to the rate of heat dissipation from the fuse element
and thereby they are will be no considerable increase of temperature of that fuse element.
• But if current through the fuse element crosses certain predetermined value, the rate of production of
heat is no longer equal to the rate of dissipation of heat and as a result there will be a rapid increase of
temperature of the fuse element and very soon it reaches to the melting point of the fuse element.
• Then the fuse element melts and disconnects the supply from the load and safes the circuit connected
through this fuse element.
• In this way an electrical fuse protects an electrical installation from permanent damage due to
overheating.

22. Wheatstone Bridge:
• In this case, If R1/R2=R3/R4 then potential
difference across A and B is Zero and current
through RL resistance is Zero.
A
B