Help on some electric potential energy questions

[thursdaytbs]

Hey - just asking for some help on some electric potential energy questions.

(1) In a television picture tube, electrons strike the screen after being accelerated from rest through a potential difference of 25,000V. The speeds of electrons are quite large, and for accurate calculations of the speeds, the effects of special relativity must be taken into account. Ignoring such effects, find the electron speed just before the electron strikes the screen.

First I used the equation 1/2mv^2 = q(Va-Vb)
so v = sqrt (((2)(25000)(q)) / m )

and Va-Vb = W / q, so q = W/(Va-Vb)
but then I'm stuck and don't know where to go from there since the mass and the charge (q) isnt' given.

(2) A particle with a charge of -1.5 microColoumbs and a mass of 2.5 x 10^-6kg is released from rest at point A and acclerates toward point B, arriving there with a speed of 42m/s. (a) What is the potential difference Vb - Va between A and B? (b) Which point is at the higher potential? Give your reasoning.

So I said that mgh(of a) = 1/2mv^2 (of b)
so (9.8)h = 1/2(42^2), h = 90. That means that the potential difference between A and B is 90?, and point A is at a higher potential since the charge/mass flows TO B FROM A?

(3) An electric car accelerates for 8.0s by drawing energy from its 320-V battery pack. During this time, 1300 C of charge pass through the battery pack. Find the minimum horsepower rating of the car.

This one I'm completely lost. Leme think, so Va-Vb = 320V = W/q (and q = 1300).

So 320V = W / 1300, W = 416,000J? and that just needs to be converted into horsepower?

 

[vincentchan]

1. you need the mass and charge of an electron, check it online or your textbook...
2. it didn't say the particle is accelarate under the effect of gravity, mgh is not working in this case, instead you need E=qV=1/2mv^2 where V is your unknown
3. yes, how many joles is a horsepower?

 

[thursdaytbs]

For number 2, I thought it was two situations, one with a charge and another with a mass?

When you say qV = 1/2mv^2, that's for the charge - what would the mass equal to?

I'm a bit confused, could you explain a little bit more please?

(The other two problems I figured out with your hints, thanks for the help.)

 

[thursdaytbs]

Can anyone help me with number two please? Any other suggestions?

Potential difference between the block's A and B position is 90 correct?
since 1/2mv^2 = mgh
becomes 1/2v^2 = gh
v = 42, g = 9.8, that solves for h = 90. so potential difference = 90?

Can anyone tell me if that's correct and how the charge differs?

 

[vincentchan]

in number 2, the question is asking you a charged particle q with mass m under the influence of ELECTRIC POTENTIAL, not GRAVITITIONAL POTENTIAL.

the formulas of electric potential is qV, and the formulas of gravititional potential is mgh, so replace your mgh by qV, and you will get the answer

 

[learningphysics]

For number 2, I thought it was two situations, one with a charge and another with a mass?

When you say qV = 1/2mv^2, that's for the charge - what would the mass equal to?

I'm a bit confused, could you explain a little bit more please?

(The other two problems I figured out with your hints, thanks for the help.)

It's the same particle for which mass and charge are given. Just plug them in and solve.

The question says nothing about the particle falling from a height. As vincentchan said, the acceleration is only due to electric field... no gravitational field so you don't need mgh.