Determine the mass of a copper bar if 45 kJ of energy is needed to melt it.

The amount of heat required to melt a certain mass of a substance taken at the melting temperature is described by the formula –

Q melt = λ * m,

where λ is the specific heat of fusion (in our case, λ of copper is 2.1 * 10⁵ J / kg), m is the body weight.

Substituting the values we have, we get –

2.1 * 10⁵ J / kg * m of a bar = 45000 J (or 45 kJ).

Hence, m bar = 45000 J / 2.1 * 10⁵ J / kg = 0.21 kg (rounding the infinite fraction to two decimal places). So, the mass of a copper bar (according to the conditions of our example) will be equal to 0.21 kg or 210 grams.