因什麼才有反電動勢

反電動勢係當你施加電壓係一啲電感元件時，元件儲存電能，當輸入電壓斷開時，元件可以反向輸出電能，常見電感元件有電容同線圈，電阻如用線繞都有電感可能，但純碳電阻則冇電感。

In theory, there should not be any induced emf in a non-inductive circuit. If you observed such effect, it might probably because of induction in the connecting wires, especially when these wires are long and coiled.