egr 1011 where does current come from? the free electrons in a conductor need to be replaced when...
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EGR 101 1
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 2
Circuit Components• Batteries
+
+
EGR 101 3
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 4
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 5
Current Direction
We will adopt “Conventional Flow” for all of our work!
EGR 101 6
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 7
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 8
When batteries areconnected with their polarities in the same direction, they are in a series-aiding arrangement.
EGR 101 9
When batteries areconnected with their polarities in theopposite direction, they are in a series-opposing arrangement.
EGR 101 10
In Class Activity
• What is the total voltage between terminals A and B in the figure below?
Which terminal is at a higher potential?
9V 4V 15V
A B
EGR 101 11
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 12
Ohm’s Law
• Three forms:
• Use the form that best describes the quantity that you are trying to determine
V IR
VI
RV
RI
EGR 101 13
Determine the Current
R
VI
+
_
I
EGR 101 14
Determine the Voltage
IRV
+
_
EGR 101 15
Determine the Resistance
I
VR
+
_
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