The electric field direction is defined by the direction of the force felt by

By a positive charge

Explanation:

The electric field is a vector consisting of a magnitude and a direction.

- The magnitude of the field depends on the configuration of the field: for example, for the electric field produced by a single point charge, it is

where k is the Coulomb's constant, q is the magnitude of the charge, r is the distance from the charge.

- The direction of the field corresponds to the force that a positive charge would feel in that field. For example, the electric field produced by a single-point positive charge points away from the charge itself: this is because a positive test charge placed in this field would feel a repulsive force, so it  would move away from the source of the field. Vice-versa, the electric field produced by a single-point negative charge points towards the charge itself: this is because a positive test charge placed in this field would feel an attractive force, so it  would move towards the source of the field.

b.transmitters send radio waves, and receivers capture radio waves.