Question about electrial potential

[leolaw]

The work done by an external force to move a -8.50{\mu}C charge from a point a to point b is 15.0*10^{-4}J. If the charge was started from rest and had 4.82 * 10^{-4}J of kinetic energy when it reached point b, what must be the potential difference between a and b.

I am not sure if this is suppose to be a trick question or not, but if the final kinetic energy is 4.82 * 10^ -4, then the initial potential difference has to be 4.82 * 10^-4 right?

[vsage]

Consider the units of electric potention (V or J/C) and work (J)

[dextercioby]

What is the unit of potential difference...?

Daniel.

[leolaw]

so
{\Delta} KE = -{\Delta}PE}
\frac{1}{2}mv^2 = -qV_{ba}
4.82*10^{-4}J=8.5*10^{-6} V_{ba}
So V_ba = 5.6 * 10 V

But when I use
W = -qV_{ba}
15 * 10^{-4} = 8.5 * 10^{-6} V_{ba}
V_{ba} = 1.76 * 10 ^2

I am kind of lost here, can somebody help

[xanthym]

You're not properly using the energy information provided. What's the relationship between the 2 energy values given in the problem?

~~

[leolaw]

4.82 * 10^{-4}J, which is the kinetic energy

[xanthym]

What about:

Work_{ext} = \Delta K.E. + \Delta P.E.


~~

[xanthym]

HINT:

:(1): \ \ \ \ Work_{ext} = \Delta K.E. + \Delta P.E.

:(2): \ \ \ \ (15x10^{-4} Joules) \ \ = \ \ (4.82x10^{-4} Joules) \ \ + \ \ (Charge)*(PotentialDifference)

Do you see the technique?


~~

[leolaw]

I have the answer -1.19 x 10 ^ -2 V

[dextercioby]

I've said that b4.It's the OP-s job to do the "dirty" work.Namely the simple calculations...We assume that the person coming in for advice/help has minimum knowledge of arithmetics...

Daniel.

P.S.Your result is 10.000 times smaler...
P.P.S.xanthym,the latex code for multiplication is either "\times" or "\cdot" or leaving a space...

[leolaw]

What about:

Work_{ext} = \Delta K.E. + \Delta P.E.


~~

I didnt know this forumla before because I started from the middle of the book (since that i am taking second half of the course), so i skipped all the Mechnical energy stuff

But once i have this, it makes everything easier

[leolaw]

I've said that b4.It's the OP-s job to do the "dirty" work.Namely the simple calculations...We assume that the person coming in for advice/help has minimum knowledge of arithmetics...

Daniel.

P.S.Your result is 10.000 times smaler...
P.P.S.xanthym,the latex code for multiplication is either "\times" or "\cdot" or leaving a space...
you mean it is -1.2 x 10 ^3?

[xanthym]

LEOLAW --
Don't guess. Repeat the calculation. (10^3 is still wrong.)

[leolaw]

HINT:

:(1): \ \ \ \ Work_{ext} = \Delta K.E. + \Delta P.E.

:(2): \ \ \ \ (15x10^{-4} Joules) \ \ = \ \ (4.82x10^{-4} Joules) \ \ + \ \ (Charge)*(PotentialDifference)

Do you see the technique?


~~
(15*10^{-4} Joules) = (4.82x10^{-4} Joules) + (Charge)*(PotentialDifference)

1.018 * 10 ^{-3} = 8.5*10^{-6} * (PotentialDifference)

and i still get the result -1.19 x 10 ^2

now I get the idea.... but not the answer?/

[xanthym]

CORRECT!! :smile:

[leolaw]

CORRECT!! :smile:

finally..wholala