To estimate the specific latent heat of vaporisation of water

Apparatus

Plastic electric jug kettle with power rating marked on it, electronic balance and timer.

Procedure

• Read the power rating (wattage) P of the kettle.
• Half fill the kettle with water and place it on the electronic balance.
• Switch on the kettle, leaving the lid off. This prevents it from switching off when it starts to boil.
• Allow the kettle to boil.
• When steam is coming freely from it take the reading m1 on the balance and start the timer.
• Allow the kettle to boil until the mass has decreased by about 50 g.
• Read the mass $m_2$.
• Stop the timer and note the time $t$ taken.

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To estimate the specific latent heat of vaporisation of water

Calculations

Mass of water converted into steam $m = m_1 - m_2 =$

Energy required to do this is $ml_v$, where $l_v$ is the specific latent heat of vaporisation of water.

Electrical energy used to produce the steam equals $Pt$.

If it is assumed that all electrical energy went into producing steam

$ml_v = Pt$

$\Rightarrow l_v = \frac{Pt}{m}$

This gives a value for $l_v$ of the correct order of magnitude and helps to make the concept of specific latent heat of vaporisation more realistic.

Notes