1. The direction of motion of the electron
2. Horizontal component.
There is a rule of thumb that you may come across several times while solving physics problems; it is that a negetive particle will always ravel or accelarte in the direction opposite to that of the electric field and a positive particle in the direction of the field. Now, since elcetron is a negetively charged particle, it travels in a direction opposite to the direction of the field.
If there is a horiontal component, there will be a vertical component too. Here the vertical component is gravitation accelaration (assuming that we are carrying this experiment on Earth :P ) . The question only concerns the horizontal component which means that the accelaration caused by the electric field on the electron.
Now, we move to the question. a= f/m where f= force on the paticle and m= mass of the particle. Now we know that mass of electron is 9.10938291 * 10^-31 kilograms i.e m=9.10938291 * 10^-31 kilograms. We now need to determine the value of force acting on the particle.
Force f on a particle is the product of the charge q and electric field E acting on the particle. i.e f=q*E. The value of q= -1.60217657 * 10-19 C and E= -2.55*10^4 N/C(as the electric field points towards the negative x direction).
Hence the value of f becomes q*E which is -1.60217657 * 10-19 *(- 2.55*10^4) =4.0855*10^-15 N.
The positive sign suggests that the force acting on the electron is in the direction opposite to the direction of the electric field and hence towards the positive x axis. So , the value of accelaration a(horizontal component) will be.
a=f/m =4.0855* 10^-15 N/ (9.10938291 * 10^-31)= 4.485 * 10^15 m/s^2
The acceleration remains constant between the plates because the electric field is uniform.