January 2012

Electrical Potential

Electrical Potential EnergyJust as masses can have gravitational potential energy, charges can have electrical potential energy

EElectrical potential energy

A mass gains PEg if work is done against gravity (i.e., if the mass is moved against the gravitational field)

A charge gains PEe if work is done against electric force (i.e., if the field is moved against the electric field)

Electrical Potential EnergyJust as masses can have gravitational potential energy, charges can have electrical potential energy

EElectrical potential energy

In both cases, PE is determined by the position of an object within a field.

In both cases, ΔPE = W = Fdcosθ

Electrical Potential Energy & Charge

Does the amount of work done by the monkey change if the charge changes? What does that imply about Pee?

Yes! Stronger charge = more work = greater increase in Pee

Electrical Potential Energy & Electrical Potential

Introducing … Electrical Potential (V) … a measure that doesn’t depend on charge.

Two different things that sound alike!

electric potential energy

electric potential charge

1J1V

1CThe SI unit of electric

potential is the volt.

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+++ ++

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++++++++

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+++++

Electrical Potential Energy vs Electrical Potential

0.5 m

0.5 m

A

B

Two positive charges, A and B, are placed at equal distances from a positively charged sphere.

A has a larger charge than B.

Which has greater potential energy (PE)?1) A2) B3) They have the same PE

Which has greater electrical potential (V)?1) A2) B3) They have the same V

Electrical Potential Energy vs Electrical PotentialThe quantity electric potential is defined as the amount of _____.

1. electric potential energy 2. force acting upon a charge3. potential energy per charge4. force per charge

Electrical Potential Energy vs Electrical Potential

What happens if a positive charge is moved from point A to point B?

In the first case no applied force is needed to move positive charge along electric field.

In the second one, there has to be applied force to move the charge from A to B.

The change in electric potential energy between two points equals the work done per unit of positive charge in order to move it from one point to the other.

Potential Difference (ΔV)

Charge (the positive one) will naturally move from areas of high potential to areas of low potential.

Work / an input of energy is needed to move charge up from low potential to high

VB  − VA  = ∆V= ∆𝑃𝐸q

=Wq 

What is the role of a battery in a circuit?

Potential Difference (ΔV) & Circuits

Batteries supply energy to maintain a potential difference across the circuit.

1) A 12V battery means that the + terminal has an electric potential that is 12V higher than the – terminal.

2) Charges flow through the external circuit (the wire) from high to low potential.

3) As the charges flow through the circuit, they ‘lose’ energy to circuit elements such as lights and motors

4) By the time the charges get to the end terminal, they have used up 12 V of potential

5) The battery supplies the energy necessary to ‘push’ the charges back to high potential

Potential Difference (ΔV) & Circuits

As charges ‘lose’ energy to different circuit elements, the electric potential decreases. This is known as voltage drop.

All of the electric potential difference– i.e. all of the voltage -- is used up by the end of the circuit.

1. The variable we use for potential, potential difference, and the unit for potential difference (volts) is V.

2. Don't let that confuse you when you see V = 1.5V

3. Electric potential energy is not the same as electrical potential.

4. Electrical potential can also be described by the terms, potential difference, voltage, potential drop, potential rise, electromotive force, and EMF.  These terms may differ slightly in meaning depending on the situation.

Terminology Fun

Check your understanding

Compare an electric circuit to a roller coaster ride.

What is the difference in height from the top to the bottom of a rollercoaster analogous to in a circuit?

potential difference

What is the motor that pulls the rollercoaster up the hill analogous to in circuit?

battery

If a battery provides a high voltage, it can ____.

1. do a lot of work over the course of its lifetime 2. do a lot of work on each charge it encounters3. push a lot of charge through a circuit4. last a long time

Check your understanding

Check your understanding

Compared to point D, point A has …

1. 12 V higher potential energy2. 12 V lower potential energy3. Exactly the same potential energy

The electrical potential energy is 0 at …4. A5. B6. C7. D

Check your understanding

The energy required to move +2C of charge from D to A is1. 0.167 J2. 2 J3. 6 J4. 24 J