An electric food mixer is tested before and after the bearing supporting its rotating beaters is lubricated. In each test, the mixer operates at the same speed for the same time and receives 2.4 kJ of electrical energy.
| Condition of bearing | Useful mechanical energy transferred by the beaters to the material being mixed | Temperature rise of the bearing |
|---|---|---|
| Before lubrication | 1.44 kJ | 18 ∘C18\ ^\circ\text{C}18 ∘C |
| After lubrication | 1.68 kJ | 10 ∘C10\ ^\circ\text{C}10 ∘C |
Calculate the efficiency of the mixer after the bearing has been lubricated.
Explain how lubricating the bearing has affected the energy transfers in the mixer. Use the data to support your answer.