Electric Potential

What is electric potential?

Electric potential is electric potential per unit charge

What equation gives the electric potential due to point charge?

When should you use

Whenever the problem asks for the electric potential due to a single point charge

What words should make you immediately think of

• Electric potential

• Voltage

• Volts

• Potential at a point charge

What does Q represent?

The source charge

What does r represent?

The distance from the charge to the point

Example:

What are the units of electric potential?

Volts (V)

• 1V = 1 J/C

What’s the difference between electric field and electric potential?

Electric Field

• Vector

• Has direction

Electric Potential

• Scalar

• No direction

When do you use:

V= kQ/r

Instead of:

E=KQ/r²

Use V=kQ/r when finding electric potential

Use E=kQ/r² when finding electric field

True or False: Electric potential is a vector

False

B

Explain why electric potential can be negative

Electric potential is measured relative to a chosen zero (usually infinity). Around a negative charge, the potential is negative because V=kQrV=\frac{kQ}{r}V=rkQ​ and QQQ is negative. A negative value simply means the point is below the chosen reference, just like being below sea level.

How should I think about electric potential?

Think of it like height on a hill

Higher or lower depends on the charge

What is the biggest takeaway from this slide?

Electric potential is a scalar and is calculated using V=kQ/r

When should I NOT use

Don't use it if the problem asks for:

• Electric field

• Force

• Work

• Potential energy

A problem gives you one point charge and asks for the voltage at a point 3 m away. What equation should immediately come to mind?

V=kQ/r

A point charge of +6 uC is located at the origin

Find the Electric potential at a point 3.0m away from the charge

V= 1.80 × 10⁴ V

What kinds of quantities will be found at the midpoint?

• Electric field (E)

• Electric Potential (V)

Before solving a midpoint problem, what are the first two questions you should ask yourself?

1. Am I finding electric field (E) or electric potential (V)?

2. Are the charges similar or opposite

What is work?

Work is the energy transferred by the electric force when moving a charge from one point to another

What equation calculates the work done by electric force?

When should you use

Whenever a problem asks for the work done by the electric force when moving a charge from one point to another

What words should immediately make you think of

• Work done

• Move a charge

• Charge moves from A to B

• Work by the electric force

• Find the work required to move a charge

What does W represent?

Work done by the electric force

What does q represent?

The charge being moved

What does V_A represent?

Electric potential at the starting point

What does V_B represent?

Electric potential at the ending point

Why is there a negative sign in the work equation?

Because work done by the electric force and electric potential energy change in opposite directions

If the electric force does positive work, what happens to electric potential energy?

Electric potential energy decreases

If the electric force does negative work, what happens to electric potential energy

Electric potential energy increases

What does positive work mean?

The charge is easy to move because the electric force helps it move, and the electric potential decreases

What does negative work mean?

The charge is harder to move because it moves against the electric force, and electric potential energy increases

What is the difference between positive work and negative work?

Positive work:

• Electric force helps the motion

• Easy to move

• Electric potential energy decreases

Negative work:

• Move against the electric force

• Harder to move

• Electric potential energy increases

Which equation calculates work done by the electric force?

Which equation calculates electric potential energy?

True or False: The equation is used to calculate electric potential

False

True or False: Positive work means electric potential energy increases

False

Which equation should you use if a problem asks for the work done when moving a charge from one point to another?

C

Explain why positive work makes moving the charge easier

The electric force helps move the charge, so less external effort is needed and the electric potential energy decreases

Explain why negative work makes moving the charge harder

The charge moves against the electric force, so external work is required and electric potential energy increases

How should I remember to use

If you see “move a charge from A to B” and the question asks for work immediately think to use it

What is the biggest takeaway?

• Positive work → Easy movement → Potential energy decreases

• Negative work → Hard movement → Potential energy increases

What is electric potential energy?

The energy a charge has because of its position in an electric field

what happens to the work done by the electric force?

It is stored (or removed) as electric potential energy

What equation calculates the change in electric potential energy?

When should you use

When a problem asks for

• Change in electric potential energy

• Electric potential energy gained or lost

• delta PE

What words in a problem should immediately make you think of

• Electric potential energy

• Gain energy

• Lose energy

• Change in PE

• Find delta PE

When should you not use

• Work

• Electric field

• Electric potential

• Coulomb force

What does delta PE represent?

Change in electric potential energy

What does q represent?

The charge being moved

What does delta V represent?

Change in electric potential

How are work and electric potential energy related?

They are always the opposite

If work is positive, what happens to electric potential energy?

Electric potential energy decreases

If work is negative, what happens to electric potential energy?

Electric potential energy increases

What happens if delta PE is positive?

The charge gains electric potential energy

What happens if delta PE is negative?

The charge loses electric potential energy

What is the difference between positive and negative electric potential energy change?

Positive delta PE

• Gain electric potential energy

Negative delta PE

• Lose electric potential energy

Which equation finds work done by the electric force?

True or False: Work done and electric potential energy always change in the same direction.

False

Which equation should you use if the question asks for electric potential energy?

What is electric potential energy between two charges?

The stored energy due to the positions of two electric charges

Where is the work done stored?

As electric potential energy

What equation gives the electric potential energy between two point charges?

When should you use

Whenever a problem:

• Electric potential energy between two charges

• Stored electric energy

  • U

What words in a problem should make you immediately think of

• Two charges

• Electric potential energy

• Stored energy

• Find U

When should you not use

Dont use it if the problem aks for:

• Electric field

• Electric potentiasl

• Coulomb force

  • Change in potential energy

What does U represent?

Electric potential energy

what does k represent

Coulomb’s constant

What does q1 represent?

The first chargeWha

What does q2 represent?

The second charge

what does r represent?

The distance between the two charges

If the distance rrr increases, what happens to the electric potential energy?

It decreases because rrr is in the denominator.

If the charges become larger, what happens to the electric potential energy?

It increases because UUU is proportional to q1q2q_1q_2q1​q2

When is electric potential energy positive?

When the two charges have the same sign (+/+ or -/-)

When is electric potential energy negative?

When the charges have opposite signs (+/-)

Why can electric potential energy be negative?

Because opposite charges attract each other, giving the system lower (negative) potential energy.

Why does pushing two like charges closer together increase electric potential energy?

Because you must do work against the electric repulsion, and that work is stored as electric potential energy.

Why does pulling opposite charges apart increase electric potential energy?

Because you must do work against their attraction, storing energy in the system.

Which equation finds electric force?

Which equation finds electric potential?

Which equation finds electric field?

Which equation finds electric potential energy between two charges?

What type of electric field does this slide describe?

A uniform electric field

What is a uniform electric field?

An electric field that has the same magnitude and direction everywhere

In the diagram, which plate is positive?

The left plate

Which plate is negative?

The right plate

What direction does the electric field point?

From the positive plate to the negative plate

Which way does a positive charge naturally move?

From the positive plate toward the negative plate

What equation related electric field and electric potential difference?

What is the rearranged form of this equation?

When should you use

• Electric field

• Voltage difference

• Distance between two points

And asks you to find the missing quantity

What words in a problem should make you immediately think of

• Uniform electric field

• Parallel plates

• Volatge difference

• Distance between plates

What does E represent?

Electric field

What does delta V represent?

Electric potential difference (voltage)

What does delta x represent?

The distance moved in the direction of the electric field

Why is there a negative sign in

Because electric potential decreases in the direction of the electric field

As you move in the direction of the electric field, what happens to electric potential?

It decreases

What shortcut can be used for parallel plates?

where d is the distance between plates

When can you use

When the electric field is uniform, such as between parallel plates