Electrostatics
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AP Physics 2 › Electrostatics
You place a box with square faces of side length 4m in a uniform electric field of strength 
Explanation
A way to think of flux is to count the number of electric field lines exiting a shape and subtract from it the number of field lines entering the shape. The only way for there to be more lines exiting the shape than entering the shape (and the only way to have any flux) is when the shape is enclosing charge.
In our problem, the box has no charge in it. Therefore, it has the same amount of field lines leaving it as it does entering it. This means that there is 0 net flux through the box.
You place a box with square faces of side length 4m in a uniform electric field of strength
Explanation
A way to think of flux is to count the number of electric field lines exiting a shape and subtract from it the number of field lines entering the shape. The only way for there to be more lines exiting the shape than entering the shape (and the only way to have any flux) is when the shape is enclosing charge.
In our problem, the box has no charge in it. Therefore, it has the same amount of field lines leaving it as it does entering it. This means that there is 0 net flux through the box.
Which of the following will cause a superconductor to have 0 resistance?
Cooling it to below its critical temperature
Heating it to above its critical temperature
Increasing the applied voltage to beyond the critical voltage
Stretching the wire to a sufficiently small diameter
Increasing the applied pressure to above the critical pressure.
Explanation
A superconductor is a material that has 0 electrical resistance when cooled to below a certain temperature.
In general, materials have a decreasing resistance as they are cooled. With a superconductor, once the critical temperature is reached, the resistance abruptly goes to 0. Superconductivity is a quantum mechanical phenomenon.
Which of the following will cause a superconductor to have 0 resistance?
Cooling it to below its critical temperature
Heating it to above its critical temperature
Increasing the applied voltage to beyond the critical voltage
Stretching the wire to a sufficiently small diameter
Increasing the applied pressure to above the critical pressure.
Explanation
A superconductor is a material that has 0 electrical resistance when cooled to below a certain temperature.
In general, materials have a decreasing resistance as they are cooled. With a superconductor, once the critical temperature is reached, the resistance abruptly goes to 0. Superconductivity is a quantum mechanical phenomenon.
A sphere of radius 
Explanation
Use the equation for electric flux:
Plug in values:
A sphere of radius
Explanation
Use the equation for electric flux:
Plug in values:
Explanation
We need to find the voltage, and we have the resistance, so if we can find the current, then we can use Ohm's Law to find the voltage.
The definition of current is amount of charge that flows through a point in 
We're told how many electrons have passed through a resistor, and we know how much charge a single electron has. If we convert the number of electrons into total amount of charge, we can divide that number by the amount of time in seconds to find the current.
Now we have the amount of charge in 
Now that we have the current, we can use Ohm's Law to find the voltage.
Therefore, the potential difference is 2.96V.
Suppose that a point charge of 1 Coulomb undergoes a change in which it is moved from point A to point B while in the presence of an external electric field. During this transposition, it undergoes a voltage change of 
Explanation
In order to solve for electrical potential energy, we'll need to remember the equation for it.
In the above expression, 
Furthermore, we can remember the equation for voltage:
With both these equations in mind, we can combine the two:
This above expression tells us that the electrical potential energy of a system is directly proportional to the voltage change and to the charge of the test charge that is undergoing the voltage change.
Plug in the values and solve for electrical potential energy:
Explanation
We need to find the voltage, and we have the resistance, so if we can find the current, then we can use Ohm's Law to find the voltage.
The definition of current is amount of charge that flows through a point in
We're told how many electrons have passed through a resistor, and we know how much charge a single electron has. If we convert the number of electrons into total amount of charge, we can divide that number by the amount of time in seconds to find the current.
Now we have the amount of charge in
Now that we have the current, we can use Ohm's Law to find the voltage.
Therefore, the potential difference is 2.96V.
Suppose that a point charge of 1 Coulomb undergoes a change in which it is moved from point A to point B while in the presence of an external electric field. During this transposition, it undergoes a voltage change of
Explanation
In order to solve for electrical potential energy, we'll need to remember the equation for it.
In the above expression,
Furthermore, we can remember the equation for voltage:
With both these equations in mind, we can combine the two:
This above expression tells us that the electrical potential energy of a system is directly proportional to the voltage change and to the charge of the test charge that is undergoing the voltage change.
Plug in the values and solve for electrical potential energy: