How can I make ChatGPT/OpenAI Playground write about a different topic after fine-tuning the model on a separate subject?

How can I make ChatGPT/OpenAI Playground write about a different topic after fine-tuning the model on a separate subject? For example, I’ve set it up to talk about chemistry with the fine-tuning data, but now I want the same model that I’ve trained to talk about geography, but in the same tone and sentence structure. How can I make sure that it still understands how to keep the same level of sophistication and tonality?

I’ve currently trained using 100 or so articles using Davinci but its outputs are always limited to 2-3 words max (always meaningless), when I’ve asked for 200-400 words. Not sure if I’m missing something quite significant here?