It is known that birds can detect the earth's magnetic field, but the mechanism by which they do this is not known. It has been suggested that perhaps they detect a motional emf as they fly north to south, but it turns out that the induced voltages are small compared to the voltages normally encountered in cells, so this is probably not the mechanism involved. To check this out, calculate the induced voltage for a wild goose with a wingspan of 1.20 m at level flight due south at v = 11.50 m/s at a point where the earth's magnetic field is 2.50E-5 T directed straight down into the earth. What would be the expected voltage difference across the goose's wingtips?