# AP Physics 2 : Electric Fields

## Example Questions

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### Example Question #1 : Electric Fields

What is the value of the electric field at point C?

Points A and B are point charges.

in the  direction

in the  direction

in the  direction

in the  direction

in the  direction

in the  direction

Explanation:

First, let's calculate the electric field at C due to point A.

We can tell that the net electric field will be in the  direction.

in the  direction.

### Example Question #2 : Electric Fields

What is the electric field  away from a particle with a charge of ?

away from the charged particle

away from the charged particle

towards the charged particle

towards the charged particle

away from the charged particle

away from the charged particle

Explanation:

Use the equation to find the magnitude of an electric field at a point.

Solve.

Since it is a positive charge, the electric field lines will be pointing away from the charged particle.

### Example Question #3 : Electric Fields

You are at point (0,5). A charge of  is placed at the origin. What charge would you need to place at (0,-3) to cause there to be no net electric field at your location.

None of these

Explanation:

We will need to use the electric field equation, twice. Because we are given coordinates, we will need to use vector notation.

Combine the two equations.

Plug in known values.

Note that the charge is positive. This is because the electric field lines point towards the negative charge at the origin, and in order to balance this at your location, the electric field lines of the charge at (0,-3) must be pointing away from the charge.

### Example Question #4 : Electric Fields

In the diagram above where along the line connecting the two charges is the electric potential  due to the two charges zero?

to the left from

to the left of

to the right of

to the right of

There is no point on the line where the electric potential is zero

to the right of

Explanation:

Potential is not a vector, so we just add up the two potentials and set them to each other. The equation for electric potential is:

If the point we are looking for is distance  from , it's  from . Cancel all the common terms, then cross-multiply:

Since we had  associated with , it's from that charge toward the weaker charge.

### Example Question #5 : Electric Fields

In the diagram above, where is the electric field due to the two charges zero?

to the left of

to the right of

to the left of

There is no point where the electric field is zero

to the right of

to the right of

Explanation:

Electric field is a vector. In between the charges is where 's field points right and 's field points left, so somewhere in between, the two vectors will add to zero. It will be closer to the weaker charge, , but since field depends on the inverse-square of the distance, it will not be linear, and we'll have to do some math.

First set the magnitudes of the two fields equal to each other. The vectors point in opposite directions, so when their magnitudes are equal, the vector sum is zero.

Many of the terms cancel, making it a bit easier. Now cross multiply and solve the quadratic:

### Example Question #6 : Electric Fields

If charge  has a value of , charge  has a value of, and  is equal to , what will be the magnitude of the force experienced by charge ?

None of these

Explanation:

Using coulombs law to solve

Where:

it the first charge, in coulombs.

is the second charge, in coulombs.

is the distance between them, in meters

is the constant of

Converting  into

Plugging values into coulombs law

Magnitude will be the absolute value

### Example Question #7 : Electric Fields

Charge has a charge of

Charge  has a charge of

The distance between their centers, is .

What is the magnitude of the electric field at the center of due to

None of these

Explanation:

Using the electric field equation:

Where is

is the charge, in

is the distance, in .

Convert  to and plug in values:

Magnitude is equivalent to absolute value:

### Example Question #8 : Electric Fields

Charge has a charge of

Charge  has a charge of

The distance between their centers, is .

What is the magnitude of the electric field at the center of due to ?

None of these

Explanation:

Using the electric field equation:

Where is

is the charge, in

is the distance, in .

Convert  to and plug in values:

Magnitude is equivalent to absolute value:

### Example Question #9 : Electric Fields

Charge has a charge of

Charge  has a charge of

The distance between their centers, is .

What is the magnitude of the electric field at the center of due to

None of these

Explanation:

Use the electric field equation:

Where is

is the charge, in

is the distance, in .

Convert  to and plug in values:

Magnitude is equivalent to absolute value:

### Example Question #10 : Electric Fields

What is the electric field strength of a stationary 30C charge at a distance of 80cm away?

pointing towards the source charge

pointing towards the source charge

pointing away from the source charge

pointing away from the source charge

pointing away from the source charge

Explanation:

To solve this question, we need to recall the equation for electric field strength.

Notice that the equation above represents an inverse square relationship between the electric field and the distance between the source charge and the point of space that we are interested in.

Plug in the values given in the question stem to calculate the magnitude of the electric field.

Now that we have determined the magnitude of the electric field, we need to identify which direction it is pointing with respect to the source charge. To do this, we'll need to remember that electric fields point away from positive charges and towards negative charges. Therefore, since our source charge is positive, the electric field will be pointing away from the source charge.

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