E&M electrostatic 2 (urgent)

2011-05-01 7:19 pm

1. There is no electric field inside a conductor (even though it is charged). Which of the following statements are correct deductions from this fact?
a) 'No work has to be done to move a test charge inside a conductor.' Correct
b) 'A conductor must have a constant electric potential' Correct
c) 'No work has to be done to move a test charge inside a conductor.' Incorrect.
Why (c) is incorrect? The solution from the book says ' Electric potential difference between any two points inside a conductor is zero. However, electric field outside a conductor may not be zero (e.g. if it is charged). --> W.D. to bring a positive unit charge from infinity to a conductor may not be zero. --> The electric potential of a conductor may not be zero.'
What does this mean?

2. In a circuit, what is meant by 'voltage'. I know it is the electric potential difference. And it is defined as delta PE / q. But I still don't understand it. PE of what? Charged particles inside the wire or what? Can some1 further explain?

3. Why the electric potential difference of an earthed object is zero?

4. Suppose there are two parallel charged metal plates. And there is a voltage across it. Why sometimes it says the E-field is constant because the charges induced on the plates are the same, sometimes it says the voltage is the same (which is equal to the emf of the battery). How to distinguish it?

thanks

更新1:

5. I know that there is an equation E = - dV/dr. Can some1 briefly explain it to me what it is about? What does 'r' refer to? thanks

回答 (1)

天同

2011-05-02 7:35 pm

✔ 最佳答案

1. The question and answer are very unclear. Statement (c) only says to move a test charge inside a conductor. I suppose it means from one point ot another point in the conductor. But the answer refers to a point outside the conductor, it is entirely irrelevant to the question. I would regard statement C to be correct.

2. Voltage can be interprested as the "force" to drive a charge to move. In so doing, the charge gains energy. Hence, a voltage of 1 volt is defined as the energy gained by a unit charge in falling through it, the energy so gained is 1 J
This is similar to the situation in mechanics that an object of weight 1 N falling through a height of 1 m gains an energy of 1 J. The voltage (or potential difference) is just similar to the "height difference" in mechanics.
[note: the "energy" refers to the energy of the charge or (kinetic) energy of the object as the case may be].

3. Earth potential is taken as zero volt for reference. If an object is earthed, it is in contact with earth, thus acquiring the earth potential. There is no potential difference between the object and earth.
4. The voltage applied across the plates and the charges on the plates are proportional (formula: charge = capacitance x voltage).
But electric field between plates = voltage/separation = charge/(capacitance x separation). Hence, electric field is either proportional to voltage or charge. They all mean the same thing.5 "r" refers to the separation between two points.
The equation means that the electric field intensity is equal to the potential gradient, which is just the voltage between any two points divided by the separation between those two points.

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