The work done by the electric force during a lightning flash with a potential difference of 2.7 x 10^9V and a transfer of − 21 C of charge is − 56.7 x 10^9 joules.
The student is asking how much work is done by the electric force during a lightning flash when a charge is transferred between the cloud and the ground. The relevant concept here is that work done by an electric force can be calculated as the product of the charge and the potential difference through which the charge is moved.
In the given scenario, the potential difference Vcloud − Vground is 2.7 x 109V and the charge Q transferred is − 21 C. Using the formula W = Q × V, we can calculate the work Wground to be:
Wground = ( − 21 C) × (2.7 x 109V) = − 56.7 x 109 joules.
The negative sign indicates that work is done by the electric force when the charge moves against the potential difference.