4. An electric kettle produces 2000 J of energy each second. It is filled with water, weighed and switched on. After coming to the boil, it is left on for a further 120 seconds and is then switched off. It is found to be 90 g lighter. Calculate the specific latent heat of vaporisation from this data

2.67 × 10⁶ J/kg

Explanation:

Since the electric kettle is 90 g lighter, this means that 90 g of water evaporates.

Now, Q = mL where Q = quantity of heat = Pt where P = energy per second of kettle = 2000 J/s and t = time = 120 s. m = mass of water evaporated = 90 g = 0.090 g and L = specific latent heat of vaporisation of water.

L = Q/m = Pt/m = 2000 J/s × 120 s/ 0.090 kg = 2.67 × 10⁶ J/kg

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.

god i hope this !