Calculate mass of oil droplet between parallel plates

AI Thread Summary
To calculate the mass of an oil droplet held motionless between parallel plates with a potential difference of 39.8 kV, the electric force acting on the droplet must be equal to its weight. The droplet carries an extra charge of one electron, which allows for the calculation of the electric force using the potential difference. The gravitational force can be expressed as the weight of the droplet, leading to the equation that equates the two forces. The solution involves finding the electric force and using it to determine the mass of the droplet. This approach simplifies the problem significantly.
PhysicsMan999
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Homework Statement



  1. A droplet of oil, carrying an extra charge of one electron, is held motionless between parallel plates separated by 1.89 cm, with a potential difference of 39.8 kV. What is the mass of the droplet?

2. Homework Equations
E=0.5QV
C=Q/V

The Attempt at a Solution


Really don't even know how to start here..I figured I would need kinetic energy to find the mass but the velocity is 0, so there would be none.[/B]
 
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PhysicsMan999 said:

Homework Statement



  1. A droplet of oil, carrying an extra charge of one electron, is held motionless between parallel plates separated by 1.89 cm, with a potential difference of 39.8 kV. What is the mass of the droplet?

2. Homework Equations
E=0.5QV
C=Q/V

The Attempt at a Solution


Really don't even know how to start here..I figured I would need kinetic energy to find the mass but the velocity is 0, so there would be none.[/B]

The problem should really tell you this, but you should assume that the force holding the droplet fixed is the weight of the droplet. Find the electric force on the droplet and set it equal to the gravitational force.
 
Oh, well that was easy then. Thanks!
 
Thread 'Variable mass system : water sprayed into a moving container'

Thread 'Variable mass system : water sprayed into a moving container'

Starting with the mass considerations #m(t)# is mass of water #M_{c}# mass of container and #M(t)# mass of total system $$M(t) = M_{C} + m(t)$$ $$\Rightarrow \frac{dM(t)}{dt} = \frac{dm(t)}{dt}$$ $$P_i = Mv + u \, dm$$ $$P_f = (M + dm)(v + dv)$$ $$\Delta P = M \, dv + (v - u) \, dm$$ $$F = \frac{dP}{dt} = M \frac{dv}{dt} + (v - u) \frac{dm}{dt}$$ $$F = u \frac{dm}{dt} = \rho A u^2$$ from conservation of momentum , the cannon recoils with the same force which it applies. $$\quad \frac{dm}{dt}...
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