Download Electrical Potential

January 2012
Electrical Potential Energy
Just as masses can have gravitational potential energy,
charges can have electrical potential energy
Electrical
potential
energy
A mass gains PEg if work is
done against gravity (i.e., if
the mass is moved against
the gravitational field)
E
A charge gains PEe if work is
done against electric force
(i.e., if the field is moved
against the electric field)
Electrical Potential Energy
Just as masses can have gravitational potential energy,
charges can have electrical potential energy
Electrical
potential
energy
In both cases, PE is determined by the
position of an object within a field.
In both cases, ΔPE = W = Fdcosθ
E
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 Two different things that sound alike!
Introducing … Electrical Potential (V) … a measure that
doesn’t depend on charge.
electric potential 
The SI unit of electric
potential is the volt.
electric potential energy
charge
1J
1V 
1C
Electrical Potential Energy vs
Electrical Potential
A
Two positive charges, A and B, are placed
at equal distances from a positively
charged sphere.
+++++
+
+
++++
+
A has a larger charge than B.
0.5 m
++
+
+ +
+++
B
++ +
++
0.5 m
Which has greater potential energy (PE)?
1) A
2) B
3) They have the same PE
Which has greater electrical potential (V)?
1) A
2) B
3) They have the same V
Electrical Potential Energy vs
Electrical Potential
The quantity electric potential is defined as the amount of
_____.
1. electric potential energy
2. force acting upon a charge
3. potential energy per charge
4. 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.
Potential Difference (ΔV)
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.
∆𝑃𝐸 W
VB − VA = ∆V=
=
q
q
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
Potential Difference (ΔV) & Circuits
What is the role of a battery in
a circuit?
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.
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
Check your understanding
If a battery provides a high voltage, it can ____.
1.
2.
3.
4.
do a lot of work over the course of its lifetime
do a lot of work on each charge it encounters
push a lot of charge through a circuit
last a long time
Check your understanding
Compared to point D, point A has …
1. 12 V higher potential energy
2. 12 V lower potential energy
3. Exactly the same potential energy
The electrical potential energy is 0 at …
1. A
2. B
3. C
4. D
Check your understanding
The energy required to move +2C of
charge from D to A is
1. 0.167 J
2. 2 J
3. 6 J
4. 24 J