Electric potential energy problem

In summary, a charged particle is moving between two parallel charged plates with a distance of 2mm and potentials of -70.0 V and -50.0 V. It is slowing from an initial speed of 90.0 km/s at the left plate. To determine whether it is an electron or a proton, the equation V2 - V1 = -W_E / q is used. Since the electric field is negative, the force will only reduce the velocity of a positively charged particle, meaning it is a proton. To find its speed at plate 2, the equation K_f = K_i - q_e * deltaV is used, where K_f is the final kinetic energy and K_i is the initial kinetic energy.
  • #1
demonelite123
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A charged particle (either proton or electron) is moving rightward between 2 parallel charged plates separated by distance d = 2mm. The plate potentials are V1 = -70.0 V and V2 = -50.0 V. The particle is slowing from an initial speed of 90.0 km/s at the left plate.
a) Is the particle an electron or a proton?
b) What is its speed just as it reaches plate 2?

i used the equation V2 - V1 = -W_E / q (W_E is the negative of the work done by electric field). so i have 20 = -E(.002) and i got E = -10000 N/C. i don't know what to do next. how do i tell if its a proton or electron?
 
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  • #2
Note that the particle is slowing. What does this tell you about the direction of the force vector?
 
  • #3
oh so that means the direction of the Electric field vector is opposite of the displacement vector so since the particle moves to the right the electric field points to the left. and since the particle is traveling the opposite direction the electric field is pointing at, does that mean it's an electron? i checked the answer in the book but it said it was proton though.
 
  • #4
thanks for not helping me...
 
  • #5
The electric field always points in the direction of the force on a positive particle. So if E is negative the force will reduce a positive velocity only if the particle is a positively charged proton. The problem with your reasoning is that the particle is not traveling opposite the electric field BECAUSE of the electric field. It was given an initial velocity by some divine methods ; ) Ever since then the Electric field has been trying very hard to convince the particle to travel in the same direction as itself. It's a subtle difference but it will really help you understand this stuff if you think the difference through.

Now, you've got everything you need for b):

[tex]W_{E}[/tex] = [tex]\Delta[/tex]U = - [tex]\Delta[/tex]K

[tex]W_{E}[/tex] = Fd = [tex]q_{e}[/tex]Ed = [tex]q_{e}[/tex][tex]\Delta[/tex]V

[tex]K_{f}[/tex] = [tex]K_{i}[/tex] - [tex]q_{e}[/tex][tex]\Delta[/tex]V

Get the final KE and you're on your way to the finish...
 
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1. What is electric potential energy?

Electric potential energy is the energy that an object possesses due to its position in an electric field. It is the ability of an electric charge to do work. The SI unit of electric potential energy is joules (J).

2. How is electric potential energy calculated?

Electric potential energy is calculated using the equation U = qV, where U is the potential energy, q is the charge of the object, and V is the electric potential. Alternatively, it can also be calculated using the equation U = kQq/r, where k is the Coulomb's constant, Q is the source charge, q is the test charge, and r is the distance between the charges.

3. What factors affect electric potential energy?

The factors that affect electric potential energy include the amount of charge, the distance between the charges, and the strength of the electric field. As the amount of charge increases, so does the potential energy. As the distance between the charges increases, the potential energy decreases. And the stronger the electric field, the greater the potential energy.

4. How is electric potential energy related to electric potential?

Electric potential energy is directly related to electric potential. Electric potential is the electric potential energy per unit charge. In other words, it is the electric potential energy that a unit charge would have at a particular point in an electric field. The greater the electric potential, the greater the potential energy.

5. What are some real-world applications of electric potential energy?

Electric potential energy has many applications in daily life, such as in batteries, where chemical energy is converted into electrical energy. It is also used in power plants to generate electricity, and in capacitors to store electrical energy. Electric potential energy is also harnessed in devices such as electric motors and generators.

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