A heating coil carries an electrical current of 2 A for 100 s at a voltage of 20 V. If this is sufficient to boil away 20 g of liquid nitrogen at its…

Question

A heating coil carries an electrical current of 2 A for 100 s at a voltage of 20 V. If this is sufficient to boil away 20 g of liquid nitrogen at its boiling point, what is the specific latent heat of vaporisation of nitrogen?

Answer 1

A heating coil carries an electrical current of 2 A for 100 s at a voltage of 20 V. If this is sufficient to boil away 20 g of liquid nitrogen at its boiling point, what is the specific latent heat of vaporisation of nitrogen?

3.0 × 10⁶ Jkg⁻¹
3.0 × 10⁵ Jkg⁻¹
2.0 × 10⁶ Jkg⁻¹
2.0 × 10⁵ Jkg⁻¹ ✓

Explanation

Electrical energy supplied = VIt = 20 × 2 × 100 = 4000 J.

This energy changes 20 g (0.02 kg) of liquid nitrogen to gas. Use Q = mL.

L = Q ÷ m = 4000 ÷ 0.02 = 2.0 × 10⁵ J kg⁻¹.

Remember to convert grams to kilograms. Using 20 g directly gives a wrong power of ten.

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