CBSE Class 12-science Answered

wheat stone bridge

Asked by ANAGHA AV | 14 May, 2012, 07:56: PM

Expert Answer

A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of abridge circuit, one leg of which includes the unknown component.

Wheat stone bridge is an electrical circuit . In wheat-stone bridge four resistance R₁, R₂, R₃ and R₄ are connected end to end with each other to form a closed loop. A sensitive galvanometer "G" is connected between their junctions as shown . The circuit is provided with two keys K₁ and K₂. Generally wheat-stone bridge is used to determine unknown resistances.

	BALANCED WHEAT-STONE BRIDGE
	When K₁is connected, no current passes through the galvanometer because K₂ is disconnected. When K₁ and K₂ are connected and no current flows through the galvanometer, the condition of bridge is called "Balanced condition". It is possible only when the potential difference between the terminals of galvanometer is zero or potential of point B = potential of point D i.e.
	V_B V_D= 0 or V_B= V_D

	RELATION AMONG RESISTANCES IN BALANCED CONDITION
	R₁/R₂= R₃/R₄
	EXPRESSION FOR BALANCED BRIDGE
	FROM FIGURE:
	R₁ & R₂ are connected in series. Reason: (only one path for the flow of current) R₃& R₄are connected in series. R₁& R₃ are connected in parallel. Reason: (two paths for the flow of current) R₂& R₄ are connected in parallel. Let current I₁ flows through R₁& R₂ and I₂through R₃& R₄. When bridge is balanced,
	potential of point B = potential of point D
	Therefore,
	V_AB= V_AD but[V = IR]
	IR₁= I₂R₃_{--------------(I)}
	Similarly,
	V_BC= V_CD _or I₁R₂= I₂R₄_{------------(II)}
	Dividing equation (i) by equation (ii)

	APPLICATIONS OF WHEATSTONE BRIDGE
	A number of resistance measuring devices have been devised on the principle of wheatstone bridge.For example : Meter bridge Post office box Potentiometer Carey Foster's bridge Callendar bridge Griffith's bridge