‹‹ Previous | Formulae Page 1 | Next ››
Relationship between current, charge and time:
I = Current (Ampere)
Q = Charge (Coulomb)
t = time (Second)
(Current is defined as rate of flow of charge \ I =
V = Potential difference across a conductor.
I = Current flowing through a conductor.
R = Resistance of the conductor.
R is in ohms when V is in volts and I is
1 ohm =
i.e. 1 W =
3. r =
r = Specific resistance or resistivity (ohm-m)
R = resistance of a conductor (ohm)
A = area of cross-section of a conductor (sq-metre)
l = length of a conductor (metre)
G is in Siemens or mho when R is in ohms
G is in Siemens when I is in amperes and V is
s is in siemens/metre when r is in ohm-metres.
s is in
L is in metres,
R is in ohms and A is in metre2.
Kirchhoff's 1st law:
The sum of all currents at a node is zero. i.e. S In =
Sign convention :
Currents entering a node
Currents leaving a node
I1 + I2 - I3 - I4 - I5 =
Kirchhoff's 2nd law:
The algebraic sum of the
potential. difference and e.m.f.
around any closed loop in an electrical circuit is zero.
SInRn + SEn =
In the direction of current
Opposite to the direction of current
For a cell
From negative terminal to positive terminal
From positive terminal to negative terminal
balancing condition for Wheatstone's bridge
In this condition Ig = 0 and the
points B and D are equipotential.
when ig = 0 (i.e. when bridge is balanced)
+ l2 = 1 metre = 100 cm.
l1 = length of meter bridge wire from end A
(left end) to null-point.
l2 = length of meter bridge wire from end B (right end) to
R1 = resistance in left gap (unknown resistance)
R2 = resistance in right gap.
balance point (D) is obtained,
\ G = R
G = resistance of the galvanometer
R = known resistance
lg = length of meter bridge wire
from balance point to one end of the bridge. (opposite
length of meter bridge wire from balance point to other end of the
bridge (length opposite to R).
VAP = f l1
(Principle of Potentiometer)
Potential Gradient =
VAB = potential difference between points A and B.
L = total length of potentiometer wire.
E1 = VAP when galvanometer shows
Where E1 = e.m.f. of
cell connected in the secondary circuit.
= potential gradient
l = balancing length measured from point A to point P.
VAB = IR
Rtotal = R + Rc + r0
R = resistance of the wire
r0 = internal resistance of a cell of EMF(E)
Rc = control resistance connected
in series with Potentiometer wire (in place of Rheostat)
Potentiometer : (Internal resistance of a cell)
r = R
r = internal resistance of the cell
l2 = balancing length when resistance R is connected
across the cell
l1 = initial balancing length (When R = ¥ or key in
series with R is open)
R = resistance across the cell when l2 is measured.
E2 = e.m.f.s of the
two cells which are being compared.
l1 = balancing length corresponding to E1
l2 = balancing length corresponding to E2.
and diffrence method)
l3 - l4
E1, E2 = e.m.f.s
of the two cells which are being compared. (E1 > E2)
l3 = balancing length corresponding to e.m.f.
(E1 + E2) i.e. cells are assisting.
l4 = balancing length corresponding to e.m.f.
(E1 - E2) i.e. cells are opposing.