# Where Does Current Come From? - UMass D

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 Ohms Law A 1 Ohm resistance limits the current to 1 Amp when 1 Volt is applied. V = I. R EGR 101

11 Ohms 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

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