Download Electrical Potential

January 23-26 2015
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
(e.g., if the positive charge is
moved against the electric
field)
Electrical Potential Energy
How can a charge (or mass) gain potential energy?
By changing location.
Gravitational
potential energy
Work done by
monkey
Work done by
electricity
Low PE
High PE
Electrical potential
energy
Electrical Potential Energy & Charge
Does the amount of work done by the monkey change if the
charge changes? How do we know?
Yes! W = Fd and electrical force is proportional to charge.
F = k𝑞𝑟𝑞
1 2
2
(F between 2 charges)
F = Eq
(F due to a charge in electric field)
What does this imply about potential energy?
Potential energy depends on both location and charge.
Electrical Potential Energy & Electrical
Potential Two different things that sound alike!
Introducing … Electrical Potential (V) … a measure that
depends only on location, not on charge.
electric potential 
The SI unit of electric
potential is the volt.
electric potential energy
charge
Volt =
𝐽
𝐶
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
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
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
Note important difference between
energy and potential:
• A point has potential, charge placed
there has electric potential energy
A
+++++
+
+
++++
+
0.5 m
++
+
+ +
+++
B
++ +
++
0.5 m
Two points that are at the same
distance from the charged object
have the same potential.
So, when two charged objects are
placed there, they are at the same
potential, but the one with more
charge on it has higher electric
potential energy. It is harder to
push it there.
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
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?
Electrical Potential Energy vs
Electrical Potential
What happens if a positive charge is moved from point A to
point B?
Potential Difference (ΔV)
The potential difference between two points is
exactly what the name implies – the difference in
electrical potential between two points.
VB − VA = ∆V
The change in electric potential energy between two
points also equals the work done per unit of positive
charge in order to move it from one point to the
other.
∆𝑃𝐸 W
∆V=
=
q
q
Charge 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.
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.
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.
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
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
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. The electron volt is not a smaller unit of the volt, it's a smaller unit of
the Joule.
5. Electrical potential can also be described by the terms, potential
difference, voltage, voltage drop, 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?
What is the motor that pulls the rollercoaster up the hill
analogous to in circuit?
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
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
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
Energy is required to move a charge
from…
1. Point A to point B
2. Point B to point C
3. Point C to point D
4. Point D to point A
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
Check your understanding
Energy is required to move a charge
from…
1. Point A to point B
2. Point B to point C
3. Point C to point D
4. Point D to point A
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