Download 7TH CLASSES PHYSICS DAILY PLAN

10TH CLASSES
PHYSICS
DAILY PLAN
DATE:
SUBJECT: Electromotive force and internal resistance:
AIM:
DURATION: 80 min
REAL LIFE:
PRESENTATION:
A real battery does two things:
a) It produces electric energy from chemical energy
b) It gets hot when it is working
Therefore part of the energy produced by the battery is lost as heat in the battery.
We can model this as a small resistance in the battery:

+
+
r
Electromotive force (EMF, ): of a battery is the energy given to unit charge by the battery.
Therefore:
EMF is NOT force, but the ENERGY that the battery gives to 1 Coulomb charge.
.. ε
=
W
..
q
Unit:
Joule
Coul
 Volt
The Combination Of Batteries
Batteries connected in series
R
The emf. that the combination supplies to an
external circuit is equal to the sum of the individual
emf’s. This can be shown easily by applying the
1
2
3
principle of energy conservation.
eqIt = 1It + 2It + 3It + ......
eq = 1 + 2 + 3 +..... and
eq

I=
=
R+r1+r2+r3+....
R + r
Batteries connected in parallel:
The equivalent emf must supply a current of magnitude equal to the sum of the currents supplied by the individual
emf’s. It is essential that the parallel connected batteries have equal emf’s. Otherwise current may flow through some
of the batteries in opposite directions to their emf’s and this occurs even when the external circuit is open. This
situation is not desired and therefore, always equal emf are connected in parallel. When nb. of such batteries are
connected ,we have,
eq ( nIt ) = It + It + It + It = nIt
eq =  then
Req = R + req = R + r/n so, I = eq / Req =  / ( R + r / n )
Circuit Connected With Back EMF:
When unit current passes through the circuit,the value of electric energy converted into
energy other than heat energy (dissipated on the motor) is called back emf of the motor.
From the conservation of the energy principle ,
 I t = 1i t + i2 I t + i2 R t + i2 r1 t
 = 1 + I(R + r + r1)
The Efficiency Of A Motor: Efficiency is the ratio of the energy supplied from the device to energy received by
the device.

,
 ,  ir ,
4) Electrical energy and power:
Energy turned into heat on a resistor:

Energy in
one second
 

Number of Coulombs
passing in one second


Energy lost by
one Coulomb

P=VI
Unit: Watt = Ampere x Volt
Ex: Show that Ampere x Volt = Watt
Formulas:
Since ..W= P t..
Power (P)
P=IV
P = I2 R
P = V2 / R
and
..V=I R..
Energy (W)
W= I V t
W = I2 R t
W= (V2/R) t
Ex: How much heat does a 40-W light bulb generate in 20 min?
Kirchhoffs Rules
10th classes direct current lecture note 1.pdf(page 9)
KIRCHHOFF’S RULES.
1.Kirchhoff’s loop rule:The algebriac sum of the changes in potential encountered in going around any circuit loop is zero.
2.Kirchhoff’s junction rule:The sum of the currents entering any junction must equal the sum of currents leaving that junction.
HOMEWORK:
MULTIMEDIA:
EXPERIMENT:
DEMONSTRATION:
TEACHER:
DIRECTOR: