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The normal density of gold is ρ and its bulk modulus is K .The Increase in the density of a lump of gold, when pressure P is applied uniformly on all sides is
A. ρPK
B. ρKP
C. PρK
D. KρP

Answer
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Hint: We first write the bulk modulus formula, then we know in bulk modules there is no change in mass, applying this statement in the problem we get a relation between volume and density of the gold. Use this relation in the bulk modulates formula.Now after rearrangement we will get the increase in the density of the lump gold.

Formula used:
K=dPdVV
Where, Bulk modulus = K, Pressure applied = dP and Change in its volume =dVV.
ρ=mV
Where, Density = ρ, Mass = m and Volume = V.

Complete step by step answer:
As given in the problem, there is a lump of gold whose normal density is ρ , bulk modulus is K and the pressure applied is P.Due to this applied pressure there is an increase in the density of the lump of gold.Bulk modulus is defined as the pressure that we applied divided by the fractional change in its volume due to application of this pressure.Mathematically we can write,
K=dPdVV

Now by applying this bulk formula we will get,
K=dPdVV
K=VdPdV(1)
We can define density as mass divided by volume of the lump of gold.
Mathematically,
ρ=mV
ρ=mV1
Now differentiating the above expression we will get,
Δρ=V1ΔmmV2ΔV
Divind both side by ρ we will get,
Δρρ=V1ΔmρmV2ΔVρ

Putting ρ value in RHS we will get,
Δρρ=V1ΔmmV1mV2ΔVmV1
Cancelling the common terms we will get,
Δρρ=ΔmmΔVV
We know that in bulk modulus change in mass is equal to zero.
Hence the above equation will become,
Δρρ=ΔVV
Small change in the term,
dρρ=dVV(2)

Replacing equation (1) with equation (2) we will get,
K=VdPdV
K=dPdVV
By putting,
K=dPdρρ
K=ρdPdρ
As given in the problem here dP=P
Now replacing the value in the above equation we will get,
K=ρPdρ
Rearranging the above equation we will get,
dρ=ρPK
Where, change in density = dρ

Therefore the correct option is (A).

Note: Be careful while differentiating the density term. Differentiate the RHS term into two different parts: first differentiate with respect to mass by keeping volume constant and then again differentiate with respect to volume by keeping mass constant. This eases the calcul;ations in the problem.
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