1 / 23

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

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. The Ampere (A).

gen
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

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

  2. 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

  3. 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 1018 e-/s is unwieldy • 1A = 1 Coulomb / second • Note: 1 Coulomb = 6.24 x 1018 e-

  4. 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

  5. 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?

  6. 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

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

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

  9. 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

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

  11. 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

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

  13. Resistors: Identifying Values 15KΩ 276Ω

  14. Resistors: Identification Example Note: 10^0 = 1 • The value of the resistor shown above is 339Ω ±1% 3 x 1 3 9 x 10^0 ±1%

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

  16. 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

  17. 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)

  18. 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

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

  20. 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

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

  22. 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

  23. Questions?

More Related