Examples of sources of physical entropy for secure random number generation include:

Noise in the environment

Electrons emitted via the photoelectric effect

Explanation:

Physical entropy can be seen as a disorderliness that can be observed in a physical system. The noise in a physical system can be used for secure number generation because of its randomness.

Noise is known to have no defined pattern and hence, can be used to creates secure random key generations.

Electrons emitted via the photoelectric effect: Electrons are emitted in a random order whenever a beam of light is incident on a photoelectric material. These electrons emitted can be used for random key generations