1 / 28

Experiment No. 3 Electric Current, Resistance and Ohm’s Law

Experiment No. 3 Electric Current, Resistance and Ohm’s Law. Electromotive Force emf and terminal voltage Batteries in series and parallel Circuit diagrams and Symbols Resistors and Ohm’s Law Resistivity Electric power Calculate resistance, current voltage and electric power

emccallister
Download Presentation

Experiment No. 3 Electric 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. Experiment No. 3 Electric Current, Resistance and Ohm’s Law • Electromotive Force emf and terminal voltage • Batteries in series and parallel • Circuit diagrams and Symbols • Resistors and Ohm’s Law • Resistivity • Electric power • Calculate resistance, current voltage and electric power • Conduct experiments using resistors • Measure resistance, current and voltage in each circuit arrangement • Test Data Sheet • Learn how to analyze test results • Quiz No. 3 • Homework • New Terminology

  2. Battery Electromotive Force emf and terminal voltage The potential difference across the terminals of a battery when it is not connected to a circuit is called the battery’s electromotive force (emf). It is simply a potential difference, or voltage.

  3. Batteries in series and parallel • Batteries in series have voltages added. Their sum voltage is applied across the resistance R. • Each battery in parallel has the same voltage applled across the resistance R.

  4. Circuit Diagrams and Symbols • Engineers draw circuit diagrams to help them design the actual circuits. These diagrams depict the arrangement of components. R This is the resistor symbol. This is the battery symbol. This is the switch symbol. This is the Ammeter symbol. This is the Voltmeter symbol.

  5. Open, Closed, and Short Circuits • What is an OPEN or CLOSED circuit? • OPEN is a term used by engineers to describe the circuit condition • where current is not flowing(e.g.,a switch is in the open position or a light bulb has a broken filament). A circuit must be CLOSED for current to flow allowing the circuit to perform its intended function (e.g., a light switch is in the closed position allowing the light to function). • What is a SHORT circuit? • SHORT is a term used by engineers to describe the effect of • inadvertent connection of two power supply outlets (plus and minus). A short circuit means that there is a short path between battery terminals, or almost no resistance between them. This causes unintended heating and sometimes “arcs and sparks” that can damage equipment.

  6. Electric Current, I = q/t [C/s = As/s = A] Electric current is defined as the time rate of flow of an electric charge. R

  7. What's a Resistor? A resistor is any material that electric current cannot travel through easily. When electricity is forced through a resistor, often the energy in the electricity is changed into another form of energy, such as light or heat. The reason a light bulb glows is that electricity is forced through tungsten, which is a resistor. The energy is released as light and heat. An Ohm is a unit used to measure the resistance value of a resistor. It is named for scientist Georg Ohm who did research on materials with resistive characteristics. A conductor is the opposite of a resistor. Electricity travels easily and efficiently through a conductor.

  8. or V = R x I or Voltage = Resistance x Current R is the resistance, measured in Ohms. Its symbol is V is the voltage measured in Volts. I is the current measured in Amperes. 1 Ohm = 1 Volt/Ampere Ohm’s Law • Ohm’s Law establishes the relationship between voltage V, resistance R • and electric current I in an electric circuit. • When a voltage, V is applied in the same circuit an electric current, I • flows and it is limited only by the circuit resistance, R. Ohm’s Law states that in a circuit with a given resistance the electric current is directlyproportionally to the applied voltage.

  9. Resistivity (r) and Resistance (R) Electrical resistance is a property of a conductor that opposes the flow of electric current through it. An object of uniform cross section has a resistance proportional to its resistivity and length and is inversely proportional to its cross-sectional area. L Resistance, R = ρx ------ Units: Resistivity, ρ [ m] Length, L [m] Area, A [m2] A Example: If a 1 m × 1 m × 1 m solid cube of material has sheet contacts on two opposite faces, and the resistance between these contacts is 1 Ω, then the resistivity of the material is 1 Ω⋅m.

  10. Table 1 Material Resistivity, ρ (Ω·m) at 20 °C Carbon (graphene) 1×10−8 Silver 1.59×10−8 Copper 1.68×10−8 Annealed copper 1.72×10−8 Gold 2.44×10−8 Aluminium 2.82×10−8 Calcium 3.36×10−8 Tungsten 5.60×10−8 Zinc 5.90×10−8 Nickel 6.99×10−8 Lithium 9.28×10−8 Iron 1.0×10−7 Platinum 1.06×10−7 Tin 1.09×10−7 Carbon steel (1010) 1.43x10−7 Lead 2.2×10−7 Titanium 4.20×10−7 Sea water 2×10−1 Drinking water2×101 to 2×103 Glass 10×1010 to 10×1014

  11. Electric Power Electrons are given energy by the battery. As electric charges pass through the circuit they collide with resistor’s atoms and lose energy. The energy transferred through the collisions is transformed into thermal energy. Travelling around a complete circuit any electric charge loses all its potential energy by the time it returns to the battery. Work: W = qV The rate of energy delivery, Power: P = W/t = qV/t = IV Example No. 1: If electric power, P is equal to V x I and V = I x R what is R equal to _____? No. 2: If the voltage V = 12 V and the power, P = 5 Watts what is the resistance R equal to? _____ No. 3: A refrigerator runs 15% of time; Power used by the refrigerator is 500W; Edison charges $ 0.11 per kilowatt-hour; How much does it cost to operate it monthly (30 days)? Answer: t= 3.6 hr/day; W= Pxt = 500x1.8 = 1.8 kWhr; ; Cost is 1.8 x 0.11x 30 = $5.94/month R

  12. Calculate resistance, current, voltage and power Solve the four problems listed below as applied to the electric circuit diagram below. R Problem No. 1: I = 6 Amps; V = 6 Volts; R = _______ No. 2: V = 6 Volts; R = 10 I =______ No. 3: If electric power, P is equal to V x I and V = I x R what is R equal to _____? No. 4: If the voltage V = 12 V and the power, P = 5 Watts, what is the resistance R equal to? ____________

  13. Resistors In Series • SERIES CIRCUITS • The current that flows in a series circuit flows through every component in the circuit. • All the components in a series connection carry the same current. Rtot = R1 + R2 + R3 +………. Rn

  14. Resistors in Series Derive equation for calculating the total circuit resistance and voltage drop across each resistor I We must use Ohm’s Law to calculate the total resistance Rtot. Vtot = V1 + V2 + V3 = R1 x I + R2 x I+ R3 x I = (R1+ R2+ R3) x I ; Vtot=Rtot x I and we can deduce that Rtot Rtot= R1+ R2+ R3 R1 V1 V R2 V2 R3 V3 Example: R1 = 1500 Ohm; R2 = 1500 Ohm; R3 = 1500 Ohm; V = 6 Volt; Calculate the total circuit resistance R tot, I and voltages V1, V2, V3 Rtot = 1500 + 1500 +1500 = 4500 Ohm I = V/Rtot = 6/4500 = 1.33 mA V1 = 1500 x 1.33 = 1995 mV; V2 = 1500 x 1.33 = 1995 mV; V3 = 1500 x 1.33 = 1995 mV

  15. Resistors in Parallel Derive equation for calculating the total circuit resistance and the current in each resistor Itot We must use Ohm’s Law to calculate the total resistance Rtot. V R2 R3 R1 R1 I1 I2 I3 1. Itot = V / Rtot ; I1 =V /R1 ; I2 = V /R2 ; I3 = V /R3 Itot = V x (1/R1 + 1/R2 + 1/R3) therefore 1/Rtot = 1/R1 + 1/R2 + 1/R3 We’ll first calculate the resistance of R1 and R2in parallel 2. Itot = V / R1,2 and 1/R1,2 = 1/R1 + 1/R2 = R1 /(R2 x R1) + R2 /(R2 x R1) = (R1 + R2)/(R2 x R1) and the reverse of 1/R1,2 R1,2 = (R1 x R2) /(R1 + R2) Itot V R2 I1 I2 We can use the same equation to calculate the total circuit resistance 3. R1,2,3= (R1,2 x R3) /(R1,2 + R3)

  16. 1.5 x 0.47 _1.5 x 1.5 = 0.36 Kohm = 360 Ohm = 0.75 Kohm = 750 Ohm 1.97 3 R1,2 =1,970 Ohm =1.97 Kohm R1,2 = Calculation of Series and Parallel Resistances R1 and R2 in SeriesR1 and R2 in Parallel Example no. 1: R1 = 1500 Ohm = 1.5 KOhm R2 = 1500 Ohm = 1.5 Kohm R1,2= R1+ R2R1,2 = (R1 x R2) /(R1 + R2) R1,2 = 3,000 Ohm = 3 Kohm R1,2 = Example no. 2: R1 = 1500 Ohm = 1.5 KOhm R2 = 470 Ohm = 0.47 KOhm Resistors in SERIES increase total circuit resistance. Resistors in PARALLEL reduce total circuit resistance.

  17. a1 to a5 a10 to a15 b15 to b20 Test No. 1 Resistors in Series • Four resistors are connected in • SERIES with each other • Connect the four 1500 Ohm resistors • R1, R2, R3 and R4 as shown. • Measure the resistance using the 20K resistance scale on the meter. • Expected Actual • R1 = _______;_______ • R1 + R2 = _______;_______ • R1 + R2 + R3 = _______;_______ • R1 + R2 + R3 + R4 = _______;_______ R1 R2 R3 b5 to b10 R4 Note: Enter your Test No. 1 four resistors data on the Data Sheet Pg. 23

  18. Test No. 2 Resistors in Parallel • Two Resistors are connected in PARALLEL with each other. • Each of the two pairs is in SERIES with the other pairs. • Connect the four 1500 Ohm resistors R1, R2, R3 and R4 as shown. Measure the resistance using the 20K resistance scale. • Expected Actual • (R1 & R2) = ______ ______ • (R1 & R2) + (R3 & R4)=______ ______ R1 R2 b1 to b5 R3 a1 to a5 R4 Note: Enter your Test 2 data on the Data Sheet (pg. 23)

  19. Test No. 3 Measure the Current a1 R1 R2 B R1,2 a5 • Set the Am-meter dial at 20m on the DCA scale • Connect the Am-meter BLACK probe alligator with R1 at (a1) • Touch the Am-meter RED probe with the Power Supply (Battery +) • Measure the current through two resistors: I = mA Note: Enter your Test No. 4 data on the Data Sheet (pg. 23) 20

  20. V = ? R1 V V R1 R Calculate the Current a1 R1 I B V a5 Power supply voltage: 5.8 to 6.4 V (Why the variation?) Resistor R1: 1425 to 1575 Ohm (Why the variation?) Current, I = _6_ = 0.004 A = 4.0 mA (nominal) I = = 1500 Note: Enter your calculated current value under Test 3 on the Data Sheet (pgs. 23)

  21. Test No. 4 Measure the Current a1 R1 B R a5 • Set the Am-meter dial at 20m on the DCA scale • Connect the Am-meter BLACK probe alligator with R1 at (a1) • Touch the Am-meter RED probe with the Power Supply (Battery +) • Measure the current: I = mA • Compare with the calculated value of the current: I = mA Note: Enter your Test 4 data on the Data Sheet (pg. 23)

  22. Student Name: ______________________ Experiment No. 3 --- Test Data Sheet Data Sheet with Estimated and Actual Test Results Expected Actual Test No. 1 Four resistors in SERIES ____ Ohm _______Ohm Test No. 2 Two resistors in PARALLEL with each other ____ Ohm _______Ohm Test No. 3 Measure the current with two resistors in parallel ____ mA _______mA Test No. 4 Measure the current in the circuit with one resistor ____ mA _______mA 1. Explain why is there a difference in voltage between No. 1 and 2 tests? ___________ ____________________________________________________________________ ____________________________________________________________________ 2. Explain why is there a difference between the expected and actual current values in Test No. 4? __________________________________________________________ ____________________________________________________________________ 3. Explain why is there a difference between No. 3 and No. 4 tests?____________________________________________________________________ ____________________________________________________________________

  23. New Terminology Resistor--------------A material that resists the flow of electrical current. Resistivity-----------A property of a conductor that opposes electric current flow. Conductor-----------A material through which electricity flows easily. Insulator-------------A material that has exceptionally high resistance, such that it essentially stops the flow of electricity. Ohm’s Law----------The relationship between voltage, current, and resistance. In equation form: I =V/R or V=IR. Circuit Diagram----A drawing depicting an arrangement of components. Open Circuit--------No current flows. The electrical circuit is incomplete. Closed Circuit------ Current allowed to flow. The circuit is complete. Short Circuit--------An unintended closed circuit. SI Units:--------------A system of physical units based on the meter, kilogram, second, ampere, kelvin Electric potential-- The capacity of an electric field to do work on an electric charge, to move it between two specified points. The difference between two electric potentials is Voltage, [Volt or V] The reference point is the Earth or Ground. Electric current-----A flow of electrically charged particles. I = q/t, [A] Electric field---------An electric charge produced without movement. (See above the electric field surrounding a positive and negative charge q. An electric field E, is a space radiated by electric single point charge q, and acted upon by an electric force F where E = F/q = kq/r2. Electric Force--------Coulomb’s Law expresses the magnitude of the force between two point charges: F = kq1xq2/r2 Electric Power-------The rate at which electric energy is transferred by an electric circuit. The unit of electric power is the Watt (W = VxA). Electrical Energy--- -A physical quantity that expresses over time the rate at which electric power W is being transmitted Whr. Battery---------------- A device that converts stored chemical energy into useful electrical energy. Once an external connection is made between its positive and negative terminals a chemical reaction is initiated that generates electrons at the anode to supply the current of the battery to the external circuit.

  24. Summary • You learned: • -How to calculate resistances in series and parallel circuits • -Ohm’s law (I = V/R or its equivalent forms (V = IxR and R = V/I) • ~If we know two among three variables (voltage, current and • resistance), Ohm’s Law allows us to calculate the unknown. • -Electric units of measurement: • Ohm (resistance), Ampere (current), Volt (voltage), Watt (power) • NEW CONCEPT: Power is the rate of doing work. The power dissipated in a resistor is the product of voltage and current. P = VI = V2/R = I2R

  25. Resistor Types

  26. Breadboard Solid lines areimpregnated with flat conducting stripped copper wires. Wire inserted in a front view hole makes contact with one of the copper wires in the back. A breadboard is a piece of a plastic board perforated with small holes and conduction channels that allow electronic parts to be mounted and connected into circuits.

  27. Resistors in Parallel • PARALLEL CIRCUITS • The current in each individual resistor is found using Ohm’s Law. Factoring out the voltage gives: • To find the total resistance of all components, add the reciprocals of the resistances (the conductances) and take the reciprocal of the sum. I1 R1 Iout Iin I2 R2 V Rn

More Related