Transcript Where Does Current Come From?

Where Does Current Come From?
• The free electrons in a conductor need to be
replaced when they move through the
conducting medium (the wire).
• These electrons are replaced by electrons
from a source of energy such as a battery.
EGR 101
1
Circuit Components
• Batteries
+
+
EGR 101
2
Source of Voltage in a Battery
• A chemical reaction within the battery
separates positive and negative ions
(charged atoms)
• When the battery is connected to a circuit, the
negative charges move toward the positive
terminal and a current is produced
• The amount of current is constrained by the
resistance of the circuit
EGR 101
3
Example of A Battery
The source of energy E in (Joules) releases charges Q in
(Coulombs)
Voltage (V) – the difference in potential that uses E = one Joule
(J) of energy to move a charge Q = one coulomb (C).
EGR 101
4
Current Direction
We will adopt “Conventional Flow” for all of our work!
EGR 101
5
Mathematical Relationship
• E: Energy required to move charges in Joules
(J).
• Q: Charge value in Coulombs (C).
• V: Voltage or potential difference across the
battery's posts in volts (V).
•
Thus, V = E / Q.
• In units: 1 Volt = 1 Joule/Coulomb
EGR 101
6
Team In-class Activity (15 minutes)
• A 10 V source uses 2 Joules of energy per
second. Calculate the current being used by
the source.
• Hint, turn the word problem into equations;
write down what you know.
EGR 101
7
When batteries are
connected with their
polarities in the same
direction, they are in a
series-aiding
arrangement.
EGR 101
8
When batteries are
connected with their
polarities in the
opposite direction, they
are in a
seriesopposing arrangement.
EGR 101
9
In Class Activity
• What is the total voltage between terminals A
and B in the figure below?
A
B
9V
4V
15V
Which terminal is at a higher potential?
EGR 101
10
Introduction to Ohm’s Law
• A 1 Ohm resistance limits the current to 1
Amp when 1 Volt is applied.
V = I.R
EGR 101
11
Ohm’s Law
V  IR
• Three forms:
V
I 
R
V
R 
I
• Use the form that best
describes the quantity
that you are trying to
determine
EGR 101
12
Determine the Current
I
+
_
V
I 
R
EGR 101
13
Determine the Voltage
+
_
V  IR 
EGR 101
14
Determine the Resistance
+
_
V
R 
I
EGR 101
15