Find the efficiency e of the oven. you will need to use the fact that 4.2 j of energy is required to raise the temperature of 1.0 ml of coffee by 1.0∘c.

Note that

V = 1.0 mL = 1 cm³ = 10⁻⁶ m³, the volume of water (coffee)

ρ = 1000 kg/m³, the density of water

c = 4.18 kJ / (kg-K), the specific heat of water

The mass of 1 mL of water (coffee) is

m = ρV = (1000 kg/m³) * (10⁻⁶ m³) = 10⁻³ kg

The heat required to raise the temperature by ΔT = 1.0 °C (1 K) is

Q₁ = m*c*ΔT

= (10⁻³ kg) * (4.18 kJ / (kg-K)) * (1 K)

= 4.18 x 10⁻³ kJ

= 4.18 J

The actual heat used is Q₂ = 4.2J.

The efficiency is

e = Q₁/Q₂ = 4.18/4.2 = 0.995 or 99.5%

Note that this efficiency does not include the efficiency of the electrical circuitry and the magnetron.

Answer: 99.5%