What is a heat pump ?

Explanation

### Step-by-Step Solution:

1. **Definition of a Heat Pump**: A heat pump is a device that transfers heat from a colder body to a hotter body. This process requires the input of mechanical energy.

2. **Mechanism of Operation**: The heat pump operates by extracting heat from a low-temperature source (cold body) and delivering it to a high-temperature sink (hot body). This transfer is achieved using mechanical energy supplied from an external source.

3. **Diagram Representation**: To visualize the operation of a heat pump, we can draw a simple diagram:
- Draw two bodies: Body A (hot body) and Body B (cold body).
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