# Concept of work, Heat, Internal energy, Enthalpy

Internal energy : E (or) V
The amount of energy stored in a system at constant volume and constant temperature.
U = K.E + P.E

1st law of thermodynamics :
The internal energy of an isolated system is constant. It is impossible to create a machine which is Perpetual machine (ideal machine)
ΔE = q + w (or) ΔE = q – w

For isothermal process :
ΔT = 0 , ΔE = 0
ΔE = q + w (or) ΔE = q – w
⇒ q = − w  (or)  q = w

Adiabatic process : q = 0, ΔE = q + w (1st law) ⇒ ΔE = w (or) ΔE = − w

Enthalpy : (or) Heat constant
H = E + PV
ΔH = H2 − H1
ΔE = ΔH − PΔV
ΔH = ΔE + PΔV
when volume is fixed ΔV = 0, PΔV= 0
ΔH = ΔE
ΔH = ΔE + PΔV
PΔV = ΔnRT

### Part3: View the Topic in this Video from 1:24 to 1:00:23

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1. Change in internal energy ΔU = U2 - U1

2. ΔU = q + W

where, ΔU = internal energy change

q = heat added to system

W = work added to system

3. Heat q = mcΔt

m = mass of substances,

c = specific heat

Δt = temperature difference