Electric cars a 12 volt car battery has a capacity of 100 ampere-hours, supplying 2 a of current for 50 hours. how much energy does it store when fully charged? how does this compare to the contents of a 9 gallon petrol tank (1350 mj) (express as percent)?

a) 4.32 MJ ; b)  0.32%

Explanation: In order to solve this question we have to consider that power consume to full charge the battery is I*V

so we have 100 A-hours* 12 V = 1200 W*hours ( J*hour/s)

To convert to Joules

we have to multiply by 3600 ( seconds per hour) the Total energy stored in  the battery is:

E= 1200 J/s* 3600 s= 4.32 MJ

To compare with the energy of 9 gallon petrol tank  ( 1350 MJ)

%= E electric/ Epetrol= (4.32 MJ/(1350 MJ))*100%=0.32 %