Magnitude of electric charge on a glass rod

In summary, the conversation discusses the relationship between electric charges and gravitational forces, using an example of a hollow metal sphere and a piece of tinfoil with different charges. The question is posed about how much charge can be obtained by rubbing a glass rod with fur and whether it would become too heavy to hold. The conversation also mentions using a capacitor to estimate the charge and relates electrical force to gravitational force.
  • #1
barryj
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Homework Statement


I am a physics tutor. I want to show the relationship between electric charges and forces and gravitational forces. I am relating an example as follows.

If we have a hollow metal sphere the size of the Earth with a charge of 1.63E15 C and a piece of tinfoil the surface with a charge of only 2.72E-9 C , the tinfoil would weigh the same as a 250 pound (986 N) person. So I am wondering..

How much charge can you get if you rubbed a glass rod with fur, i.e. how many coulombs. If I rubbed the glass rod with fur, would it be to heavy to hold?

Just wondering.

Homework Equations

The Attempt at a Solution

 
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  • #2
A glass rod charged by rubbing with fur is capable of lifting small bits of paper against gravity over a few centimeters of distance. So you might be able to estimate the charge from the force it can produce.

Can you think of anything that might limit the amount of charge that can be deposited on or removed from the surface of the rod?

By the way, 250 lbs is about 113 kg, weighing about 1112 N at the Earth's surface.
 
  • #3
Estimating charge by lifting paper might be hard since you don't know the induced charge on the paper. I am now thinking that if I conceptually made a capacitor from two 0.1 X 0.1m plates 1 mm apart and charged the plates with 30 volts I would have the correct charge on the plates. Then I could cut the wire on one of them to isolate the charge. Now a plate would have 2.72E-9 C of charge and weigh about 946 N = 211 pounds.

Anyway, just trying to relate electrical force to gravitational force.
 
  • #4
I'm not sure where you're getting this "weight" from. If it's an electric force, you'd need a heck of a big charge nearby to attract (or repel) a plate by that amount with that tiny charge.
 
  • #5


The magnitude of electric charge on a glass rod after rubbing it with fur would depend on various factors such as the material of the fur, the humidity of the surrounding air, and the amount of friction applied during rubbing. It is difficult to determine the exact amount of charge that could be generated in this scenario without specific details.

However, it is important to note that the magnitude of electric charge is not directly related to weight or mass. The weight of the tinfoil in the given example is due to the interaction between its charged surface and the charged metal sphere, not the actual amount of charge it possesses.

In terms of Coulombs, the charge on the glass rod would likely be in the range of microcoulombs (1E-6 C) to millcoulombs (1E-3 C) depending on the factors mentioned above. This amount of charge would not be enough to make the rod too heavy to hold. The weight of the rod would still be determined by its mass and the force of gravity, not the electric charge.

Furthermore, it is important to note that the electric force between two charged objects is much stronger than the gravitational force between them. In the given example, the tinfoil would experience a much stronger electric force from the charged metal sphere than the gravitational force from a 250-pound person. This highlights the significance of electric charges and forces in comparison to gravitational forces.
 

1. What is the magnitude of electric charge on a glass rod?

The magnitude of electric charge on a glass rod depends on various factors such as the type of glass, the material it is rubbed against, and the environmental conditions. Generally, it can range from a few microcoulombs to a few hundred microcoulombs.

2. How is the magnitude of electric charge on a glass rod determined?

The magnitude of electric charge on a glass rod can be determined using an electroscope or an electrometer. These devices measure the amount of charge on the rod by detecting the deflection of a charged object placed near it.

3. Can the magnitude of electric charge on a glass rod be negative?

Yes, the magnitude of electric charge on a glass rod can be negative if it is rubbed against a material that has a higher affinity for electrons. This results in the transfer of electrons from the glass rod to the other material, leaving the rod with a negative charge.

4. How does the magnitude of electric charge on a glass rod affect its interactions with other objects?

The magnitude of electric charge on a glass rod determines the strength of its interactions with other charged objects. A larger magnitude of charge will result in a stronger repulsion or attraction between the glass rod and another charged object.

5. Is the magnitude of electric charge on a glass rod constant?

No, the magnitude of electric charge on a glass rod can change depending on its environment. For example, if the rod is exposed to high humidity, it may collect more water molecules on its surface, altering its charge. Additionally, rubbing the rod against different materials can also change its charge.

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