# AP Physics 2 : Electric Force in an Electric Field

## Example Questions

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### Example Question #1 : Electric Force In An Electric Field

There is a uniform electric field of  pointing north. What force will a particle of  experience?

south

north

north

south

south

south

Explanation:

We will use the following equation:

Plug in known values.

Since the particle is negatively charged, it will move opposite the electric field lines (south).

### Example Question #2 : Electric Force In An Electric Field

In the lab, you have an electric field with a strength of . If the force on a particle with an unknown charge is , what is the value of the charge on this particle?

Explanation:

The equation for force on a charge within an electric field is:

Plug in known values and solve.

### Example Question #3 : Electric Force In An Electric Field

There are two point charges in a vacuum,  and , kept  from each other. What is the force experienced by the charges?

There is no force felt by the charges

Explanation:

The equation for force given two charges is:

We're given both charges, and we know the distance between them, and we know the Coulomb's constant, so we plug in known values.

Because the answer is negative, the force experienced is attractive, which is what we expect from oppositely charged particles.

### Example Question #4 : Electric Force In An Electric Field

In a region of space there is an electric field. The field is directed straight down and has a field strength of . Into this region of space, an electron is moving north with a velocity of . What will the electron's acceleration be in this region of space? Include both magnitude and direction.

straight down

to the East

straight up

to the West

The electron experiences no force in this region of space

straight up

Explanation:

Since it's an electric field, the velocity does not matter, only the charge.

Since the electron carries a negative charge, it accelerates opposite to the field, which is straight up.

### Example Question #5 : Electric Force In An Electric Field

An electric dipole, with its positive charge above the negative charge, is in a uniform electric field that points to the right, as diagrammed above. What is the net torque and the net force on the dipole in this electric field?

Net torque = zero

Net force = right

Net torque = counterclockwise

Net force = zero

Net torque = clockwise

Net force = zero

Net torque = zero

Net force = zero

Net torque = counterclockwise

Net force = left

Net torque = clockwise

Net force = zero

Explanation:

Since the net charge of the dipole is zero, the net force will also be zero since . The force on the positive charge on top will be directed to the right since positive charge experiences force in the direction of the electric field. For the negative charge on the bottom, the force will be to the left. Both of the forces contribute to a clockwise torque.

### Example Question #6 : Electric Force In An Electric Field

In a region of space, there is a uniform electric field whose magnitude is  directed to the right as diagrammed above. There are two charged particles in the field: a positive particle at the origin with charge  and another at point (0,2) meters with charge  as shown. What is the net force on the  particle located at (0,2) meters?

Explanation:

Like all forces, electrostatic forces are vectors and must be added using a vector diagram. Fortunately, we can calculate each force separately, then combine them on the vector diagram. For the force due to the field,

This force is directed to the left since negatively charged particles experience force opposite the direction of the field.

For the force due to the charge at the origin:

This force is directed down, towards the particle at the origin because opposites attract. Now we draw the vectors:

The dashed line represents the sum. Use the Pythagorean theorem to find the vector sum.

### Example Question #7 : Electric Force In An Electric Field

Assume that the uniform electric field has a magnitude of , which points in the positive x-direction. What is the force magnitude this field exerts on a  charge?

Explanation:

Use the formula to find the force that the test charge experiences.

Substitute the values and determine the force.

### Example Question #8 : Electric Force In An Electric Field

A test charge of  is placed in an electric field of

Determine the force on the charge.

None of these

Explanation:

The definition of electric force on a charge in an electric field:

Plug in values.

### Example Question #9 : Electric Force In An Electric Field

A test charge of  is placed in an electric field of

Determine the force on the charge.

None of these

Explanation:

The definition of electric force on a charge in an electric field:

Plug in values.

### Example Question #10 : Electric Force In An Electric Field

A test charge of  is placed in an electric field of

Determine the force on the charge.

None of these