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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 ewhich flow past a particular point per unit time (generally 1 second) • Saying that a device “draws” 6.24 x 1018 e-/s is unwieldy • 1A = 1 Coulomb / second – Note: 1 Coulomb = 6.24 x 1018 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 106 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 276Ω 13 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=IxR 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 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. R2 R1 18 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 Worked Example: Current • How much current flows in the following circuit? E=I/R Rearrange the equation to: I=E/R I = 40V / (5Ω + 25Ω + 10Ω) I = 40V / 40Ω I = 1A • To find the total resistance in a series circuit, simply add the resistances! 21 Worked Example: Voltage Drop • What is the voltage drop experienced by each component in the following circuit? • Recall I = 1A E1 = I x R1 E1 = 1A x 5Ω E1 = 5V E2 = I x R2 E2 = 1A x 25Ω E2 = 25V E3 = I x R3 E3 = 1A x 10Ω E3 = 10V + + = 40V 22 Questions? 23