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

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

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

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

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

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

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

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

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

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

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

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

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

Resistors: Identifying Values 15KΩ 13 276Ω

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

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

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

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

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.

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

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

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

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

Questions? 23