By what percentage will the illumination of the lamp decrease if the current drops by 20% ?

The illumination of a lamp is directly proportional to the square of the current passing through it, according to Joule's first law. This means that if the current decreases by a certain percentage, the illumination will decrease by the square of that percentage.

Here are the steps to calculate the percentage decrease in illumination:

1. Calculate the square of the percentage decrease in current. In this case, the current drops by 20%, so (20%)^2 = 4%.

2. The illumination of the lamp will decrease by this calculated percentage. So, the illumination of the lamp will decrease by 4%.

Please note that this is a simplified explanation. In reality, the relationship between current and illumination can be affected by other factors such as the type of lamp and its specific electrical characteristics.