A voltmeter is the instrument used to measure a potential difference between two points in an electric circuit
Every atom has electrons. When you add new electrons to the wire, they will be passed on to an atom. The electrons keep passing from atom to atom until it reaches the light source, basically. It's kinda like that one song "100 jugs of milk" or whatever it's called. Each atom passes the atom next to it an electron.
Missing questions: "find the speed of the electron".
Solution:
the magnetic force experienced by a charged particle in a magnetic field is given by

where
q is the particle charge
v its velocity
B the magnitude of the magnetic field

the angle between the directions of v and B.
Re-arranging the formula, we find:

and by substituting the data of the problem (the charge of the electron is

), we find the velocity of the electron:
Using conservation of energy law:-
∑ work in = ∑ work out
and work= force* displacement
so when we wanted to move a 100kg a distance of 1m
we multiplied 100*1 = work out
so work in should be equal to 100*g Joules, where g is the acceleration due to gravity.
so workout = 100*g = 25*g *x (divide both sides by 25*g)
x=4m
by the same way:-
------------------------
work in = 100kg * 2m * g (m/

)= work out
so work out = 25*x*g = 200* g (divide both sides by 25*g)
x=8m