Sci I - Electric Fields

What is a field in physics?

A way to assign a numerical value to every point in space due to a phenomenon.

What are the two commonly used types of fields?

Scalar fields and vector fields.

What is a vector?

A quantity with both magnitude and direction.

What is a scalar?

A quantity with magnitude but no direction.

Give an example of a vector.

Velocity

Give an example showing why velocity is a vector.

Two cars going at the same speed in opposite directions have opposite velocities.

Give an example of a scalar.

Mass

Why is mass a scalar?

An object has the same mass regardless of direction.

Give an example of a scalar field.

Temperature in a room

Why is temperature a scalar field?

Each point in the room has a temperature; it varies slightly across positions.

How does room temperature vary?

It is slightly warmer near the ceiling and near a human body.

Give an example of a vector field.

Wind over the surface of the Earth

Why is wind a vector field?

Wind has different speeds and directions at different positions.

How can gravity be thought of as a field?

As a vector field pointing toward the center of the Earth.

How does a gravitational field affect objects with mass?

It accelerates them in the direction of the field.

What is another way to model gravity?

As a force pulling objects toward the Earth.

What is the formula for gravitational force using the field g?

FG = m * g

What does m represent in FG = m * g?

The mass of the object in question.

What does g represent in FG = m * g?

The strength of the Earth’s gravitational field at that location.

What is the approximate value of g near the ground?

9.8 N/kg

How does g change as you move further from the Earth?

It decreases with the square of the distance.

What is the value of g for satellites in orbit?

About 90% of the surface value.

What is the value of g at the Moon’s orbit?

Less than 1% of the surface value, but enough to keep the Moon bound.

What is the formula for electric force using electric field?

FE = q * E

What does q represent in FE = q * E?

The charge of the object.

What does E represent in FE = q * E?

The electric field at that location.

How does a proton move relative to the electric field?

In the same direction as E.

How does an electron move relative to the electric field?

In the opposite direction of E.

Do charged objects create their own electric field?

Yes, every charged particle generates an electric field around it.

Which way does the electric field of a positive charge point?

Away from the charge.

Which way does the electric field of a negative charge point?

Toward the charge.

What happens when another proton is placed near the first proton?

It feels a repulsive force away from the first proton.

What happens when an electron is placed near a proton?

It moves toward the proton, opposite the field of the proton.

What is the equation for the strength of a proton’s electric field?

E = k * q / r²

What happens if we multiply E = k * q / r² by another charge q?

We get Coulomb’s law for the electric force.

Why do we use electric fields for large objects instead of Coulomb’s law for each particle?

Because a metal plate has an unfathomable number of charges, and calculating individually is a huge hassle.

Can electric fields from multiple objects be added together?

Yes, vector addition applies to electric fields.

What happens to the electric field far from two nearby protons?

It looks like a field from a single particle with twice the charge of a proton.

What happens at a point equidistant between two identical positive charges?

The electric fields cancel, so the net field is zero.

How does a proton feel at a zero-field point between two protons?

Equally repelled by both, feels zero net force.

How does an electron feel at a zero-field point between two protons?

Equally attracted in both directions, feels zero net force.

What happens in a proton-electron pair far away?

The electric field is very weak, appearing as if the total charge is zero.

What happens directly between a proton and an electron?

Both fields point in the same direction, reinforcing each other.

What is an electric dipole?

A configuration with one positive and one negative charge creating an electric field.

How do electric field lines behave in a dipole?

Point away from the positive charge and toward the negative charge.

Can a third charge placed near a dipole follow the field lines?

Yes, a negative charge moves opposite the direction of the field lines.

Can large objects with many charges be simplified using symmetry?

Yes, symmetry can simplify calculations for simple geometric shapes.