Electrons in a magnetic/electric field

[gokuls]

How could you tell whether moving electrons in a certain region of space are being deflected by an electric field or by a magnetic field (or both)?

The answer from the textbook is:
If the moving electrons are changing speed as they are being deflected, then an electric field is present. This will be the case whether or not a magnetic field exists in the same region of space. If the moving electrons are being deflected but they are not changing speed, then only a magnetic field is present.

I don't understand this explanation, why would the speed only change in an electric field, but not in a magnetic field?

Thanks,

 

[gneill]

How could you tell whether moving electrons in a certain region of space are being deflected by an electric field or by a magnetic field (or both)?

The answer from the textbook is:
If the moving electrons are changing speed as they are being deflected, then an electric field is present. This will be the case whether or not a magnetic field exists in the same region of space. If the moving electrons are being deflected but they are not changing speed, then only a magnetic field is present.

I don't understand this explanation, why would the speed only change in an electric field, but not in a magnetic field?

Thanks,

Consider, separately, how the magnetic and electric forces are applied to the electron. What directions do they act? What do the resulting trajectories look like?