Current Electricity 3 Flashcards
what is kirchhoff’s first law/ junction rule
At any junction, the sum of the currents entering
the junction is equal to the sum of currents leaving the junction.
It is based on law of conservation of charge
what is kirchhoff’s second law or loop rule
The algebraic sum of changes in
potential around any closed loop involving
resistors and cells in the loop is zero.
it is based on law of conservation of energy.
sign convention for loop rule
-When the current is in the same direction as that of loop, it is taken as +ve.
-when the direction of loop is from the -ve of the cell to the +ve of the cell then it is positive.
sign convention for junction rule
current entering the looop = current leaving the looop
what is power loss or ohmic loss? how to reduce ohmic loss in transmission cables.
Electrical power is transmitted
from power stations to homes and factories, which
may be hundreds of miles away, via transmission
cables. One obviously wants to minimise the power
loss in the transmission cables connecting the power stations to homes and factories. We shall see now how this can be achieved. Consider a device R, to which a power P is to be delivered via transmission cables having a resistance Rc to be dissipated by it finally. If V is the voltage across R and I the current through it, then
P=VI (3.34)
The connecting wires from the power station to the device has a finite resistance Rc. The power dissipated in the connecting wires, which is wasted is Pc with
Pc =I2 Rc
= P2 Rc /V2
(3.35)
from Eq. (3.32). Thus, to drive a device of power P, the power wasted in the connecting wires is inversely proportional to V 2. The transmission cables from power stations are hundreds of miles long and their resistance Rc is considerable. To reduce Pc, these wires carry current at enormous values of V and this is the reason for the high voltage danger signs on transmission lines — a common sight as we move away from populated areas. Using electricity at such voltages is not safe and hence at the other end, a device called a transformer lowers the voltage to a value suitable for use.
What do you understand by ‘sensitivity of Wheatstone bridge’ ? How the sensitivity of wheatstone
bridge can be increased?
A Wheatstone bridge is most sensitive when its four arms have resistances that are of the same order of
magnitude. This means that all four resistors provide the same output resistance. A Wheatstone bridge is in
a balanced state when its galvanomater shows zero deflection
