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Chapter 3.4-3.8: Current, Resistance and Ohm’s Law

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Current: Going with the flow What is current? – At its simplest, Electric current is the rate of charge flow past a given point in an electric circuit, measured in Coulombs/second – more commonly known as Amperes 2

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The Ampere (A) Current is measured as the number of e - which flow past a particular point per unit time (generally 1 second) Saying that a device “draws” 6.24 x 10 18 e - /s is unwieldy 1A = 1 Coulomb / second – Note: 1 Coulomb = 6.24 x 10 18 e - 3

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50:50 Chance … but they got it wrong! Early electronics pioneers assumed that current flowed from (+)ve to (-)ve – This is known as “conventional current” – Comes up multiple times in E.E. Turned out to be exactly opposite We will only consider the correct assertion that electromotive force is generated by the flow of electrons: – (-)ve battery terminal to (+)ve – Electrons flow anode → cathode ACID: anode current into device 4

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Anodes.. ACID: Anode current into device – This applies to batteries which are discharging! In electronics, the anode is generally the (+)ve terminal of a component such as a diode – Consider how the electrons flow for a moment.. – See how this is maddening? 5

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Conductors & Insulators Conductor: – Any medium which allows the flow of electrical charge (ie. Electrons) Insulator: – Any medium which (ideally) does not allow the flow of electrical charge – Air breaks down at ~3.3 x 10 6 V/m or 3.3kV/mm 6

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Controlling Current Two methods to control the current in a circuit: 1.Change the voltage applied to the circuit 2.Provide resistance to the flow of electrons 7

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Controlling Current: Voltage By stacking cells of a battery in series, you increase the voltage potential! 8

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Controlling Current: Resistance To influence the flow of electrons (current), you can increase or decrease the ease at which they flow Hallway analogy – Long, narrow hallway limits the number of people which can walk by a point in any given unit of time – Resistors work much the same way 9

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Resistance: Ohms Resistance is defined as the ratio between Voltage (E) and Current (I): R = E I 10

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Conductance: mohs ( ℧ ) The ability of a material to conduct electricity is measured in Siemens (G) – Conductance is seldom used Conductance is effectively the inverse of resistance: – where G = I / E 11

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Resistors: Common Formats There are many resistor packages, depending on design needs Resistance value often identified by resistor colour code 12

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Resistors: Identifying Values 15KΩ 13 276Ω

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Resistors: Identification Example The value of the resistor shown above is 339Ω ±1% 14

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Ohm’s Law E = E.M.F. = Voltage (Volts) I = Current (Amps) R = Resistance (Ohms) E = I x R 15

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Example: Calculate Current If a circuit has a 12V battery and a “load” which has a resistance of 10Ω Ohms, what is the current observed in the circuit? Recall: E = I * R I = E / R I = 12V / 10Ω I = 1.2A 16

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Energy And Work Mechanical forms of energy: – Potential – Kinetic Electrical energy parallels mechanical – Voltage is often also referred to as potential – Current can be thought of some quantity of electrons in motion (kinetic) 17

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Series Resistor Circuit 18 R1 R2 R3 When drawing this schematic, I should have (by convention) labeled the Resistors R1 through R3 as the electrons (EMF) flow. I inadvertently labeled them in the direction of conventional current. This is more stylistic than anything else, though it is worth mentioning.

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Series Resistor Circuit What do we need to know in order to calculate how much current flows in this circuit? 19

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Kirchhoff’s Laws Loop Rule: – The sum of voltages across all resistors in a series circuit is equal to the applied EMF – Put another way, the total voltage drop equals the supply voltage Point Rule: – At any node (junction) in a circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node – Restated, the current in a loop is the same at every component 20

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How much current flows in the following circuit? To find the total resistance in a series circuit, simply add the resistances! Worked Example: Current E = I / R Rearrange the equation to: I = E / R I = 40V / (5Ω + 25Ω + 10Ω) I = 40V / 40Ω I = 1A 21

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Worked Example: Voltage Drop What is the voltage drop experienced by each component in the following circuit? Recall I = 1A E 1 = I x R 1 E 1 = 1A x 5Ω E 1 = 5V E 2 = I x R 2 E 2 = 1A x 25Ω E 2 = 25V E 3 = I x R 3 E 3 = 1A x 10Ω E 3 = 10V + + = 40V 22

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Questions? 23

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Current and Resistance El Paso Independent School District.

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