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January 30, 2017
Potential difference (V)
Recall Work: W = F d cos(q)
In order to bring two like charges together work must be done.
In order to separate two opposite charges, work must be done.
The greater the charge the monkey pushes,
the more work he has to do.
The closer he brings the charge to the other
charge, the harder for him it is.
Poor monkey
Potential difference (V)
Recall Work: W = F d cos(q)
In order to bring two like charges together work must be done.
In order to separate two opposite charges, work must be done.
Electrical potential difference (V or ΔV) is the amount of work required,
per unit charge, to move a charge from point A to point B.
𝑉=
𝑊
𝑞
The SI unit of electric
potential is the volt.
Volt =
𝐽
𝐶
Potential difference (V)
Point A:
Low potential spot
Point B:
High potential spot
Charges will naturally move from high potential to low potential
(the electric force does the work!).
The monkey has to do work / input energy to move charge from
low potential to high potential.
Potential Difference (V) & 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 difference and the unit for potential
difference (volts) are both V.
2. Don't let that confuse you when you see V = 1.5V
3. Electrical potential difference can also be described by the terms,
electrical potential, 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 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
Electric Current
symbol: I
Current = flow of charges
unit: Amperes (A)
(The rate at which charge flows by
a given cross section)
1C
1A =
1s
I=
𝑄
𝑡
Electric Current
symbol: I
Current = flow of charges
unit: Amperes (A)
(The rate at which charge flows by
a given cross section)
1C
1A =
1s
I=
𝑄
𝑡
To have an electric current, you need two things:
• A closed circuit / path for the charges
• A power supply maintain the potential difference
Electric Current
symbol: I
Current = flow of charges
unit: Amperes (A)
(The rate at which charge flows by
a given cross section)
1C
1A =
1s
I=
𝑄
𝑡
To have an electric current, you need two things:
• A closed circuit / path for the charges
• A power supply maintain the potential difference
Remember: Current will flow from high potential to low
potential, but charges need to be pushed (by an energy
source) back from low potential to high potential.
Closed vs. Open Circuits
No, the
switch is
open, so
the path
is not
complete
Yes,
charge
will flow,
BUT the
light will
not light
No, the
circuit
needs to
go from +
to -
Maybe! If
one
battery
has
higher V
than the
other
Direction of Current
Current is defined as the direction positive charges
would flow
• From high potential to low potential
• From + side of battery to – side of battery
Fun Fact
Benjamin Franklin defined
current in this manner long
before we knew much
about charges.
Now, we know that positive
charges stay put and
negative charges flow. So,
electrons actually flow
opposite current.
Calculating Current
Current is the rate of charge flow per unit time
𝑄
I=
𝑡
So, if you have 6 C of charges passing through a
section of wire every 2 seconds, then your current is:
I = 6C / 2 s = 3 A
An important note about current:
Although potential decreases across
a circuit, current is the same
everywhere in a circuit!
Circuit Analogy – CFU
Circuit Analogy - CFU
E
F
B
C
A