In Belgium, we’ll soon install the first ever battery-based system for replacing generators at a hyperscale data center. In the event of a power disruption, the system will help keep our users’ searches, e-mails, and videos on the move—without the pollution associated with burning diesel. But even more important is what will happen when Google doesn’t need emergency power. Whereas diesel generators sit idle most of the year, batteries are multi-talented team players: when we’re not using them, they’ll be available as an asset that strengthens the broader electric grid.

In physics, fields describe the distribution of forces in space. Their description thus requires the knowledge of magnitude and direction. Both are expressed through field contours; field forces act in the direction of the contours, and the distance between the contours indicates their magnitude. (The closer the contours are together, the larger the force.) In an electric field the direction of the field is given by the electric charge of the charged sources. If the charge is negative, the field is directed toward the charge. All electric fields begin on a positive charge and end on a negative charge. If two positive charges interact, their forces are directed against each other. This creates a repellent force as shown in the illustration. (The same occurs with two negative charges, because their respective forces also act in opposite directions.)