Ohms Law and Power ELPT 1311 Basic Electrical
Ohm’s Law and Power ELPT 1311 Basic Electrical Theory Chapter 04_Ohm’s Law Introduction to Electricity by Paynter & Boydell and Pearson Illustrations
Objectives • Describe the relationship among voltage, current and • • • resistance Predict how either a change in voltage or resistance will affect the circuit current Use Ohm’s Law to calculate any one of the following basic electrical values, given the other two: current, voltage, resistance Describe how Ohm’s Law is used in troubleshooting Define “power” and its relationship to current and voltage Calculate power from any two of current, voltage resistance • • Describe the relationship between power and heat • Define and calculate efficiency and the kilowatt hour (k. Wh) unit of energy • Calculate any two of the following given the other two: current, voltage, resistance, power Determine minimum acceptable power rating for a resistor in a circuit
The Relationship Between Current and Voltage • Current is directly proportional to voltage. • Current is inversely proportional to resistance. • Voltage is the force or potential difference that causes current to flow
• Double the voltage, double the current
• Double the resistance, current decreases by 1/2 60 120
The Relationship Between Current and Voltage • • Conclusion § Increases in resistance causes decreases in current § Decreases in resistance causes increases in current § Increases in voltage causes increases in current § Decreases in voltage causes decreases in current Thus the amount of current flow through a circuit is dependent upon the amount of voltage and resistance
Using Ohm’s Law to Calculate Current • The mathematical relationship between voltage, current and resistance is derived from Ohm’s Law: where I = the circuit current E = the applied voltage R = the circuit resistance • Calculating current:
Using Ohm’s Law to Calculate Current • Question: will the fuse below blow? 220 V
Using Ohm’s Law to Calculate Current • Question: will the fuse below blow? Yes, 6. 7 A exceeds 5 A 220 V
Using Ohm’s Law to Calculate Voltage • Calculating voltage: 15 V
Using Ohm’s Law to Calculate Voltage • Question: what is the maximum voltage that can be applied before the fuse blows?
Using Ohm’s Law to Calculate Circuit Values • Question: what is the maximum voltage that can be applied before the fuse blows?
Using Ohm’s Law to Calculate Resistance • Calculating resistance:
Using Ohm’s Law to Calculate Resistance • Question: If a speaker has 10 V applied with 1. 25 A flowing through the speaker, what is the resistance of the speaker?
Using Ohm’s Law to Calculate Resistance • Question: If a speaker has 10 V applied with 1. 25 A flowing through the speaker, what is the resistance of the speaker?
Using Ohm’s Law to Calculate Resistance • Question: What is the minimum allowable setting for the rheostat below to prevent the fuse from blowing?
Using Ohm’s Law to Calculate Resistance • Question: What is the minimum allowable setting for the rheostat below to prevent the fuse from blowing?
Current Relationships • Question: What does current change to if voltage doubles?
Current Relationships • Question: What does current change to if voltage doubles?
Current Relationships • Question: What does current change to if resistance doubles?
Current Relationships • Question: What does current change to if resistance doubles?
Ohm's Law Wheel - Ohm's Law Triangle
Troubleshooting Using Ohm’s Law • • Problem: What could be wrong if the circuit shows the current to be 4 A, but the drawing shows that it should be 1 A? Question: Either _____ is too high, or ____ is too low.
Troubleshooting Using Ohm’s Law • • Problem: What could be wrong if the circuit shows the current to be 4 A, but the drawing shows that it should be 1 A? voltage is too high, or ____ resistance is Question: Either _____ too low.
Troubleshooting Using Ohm’s Law • • Question: what would the voltage need to be to produce 4 A at the resistance of 10 Ω? Question: what would the resistance need to be to produce 4 A at 10 V?
Troubleshooting Using Ohm’s Law • • Question: what would the voltage need to be to produce 4 A at the resistance of 10 Ω? Question: what would the resistance need to be to produce 4 A at 10 V?
Power • Power is the rate at which a component, circuit or system uses energy • The more power a circuit consumes, the more heat it produces; example is a resistor dissipating heat • Technically, power is the amount of energy consumed per unit of time • • Electrical power is measured in watts Some important relationships - 1 volt = 1 joule per coulomb (the joule is a measure of energy) - 1 amp = 1 coulomb per second (6. 25 x 1018 electrons per second) - 1 watt = 1 joule per second
Power • Calculating power when current and voltage is known: 1 W
Power • Question: How much power is being provided by the voltage source?
Power • Question: How much power is being provided by the voltage source?
Power • Question: How much power is being provided by the voltage source?
Power • Question: How much power is being provided by the voltage source?
The Power Wheel
Power • Calculating power when current and resistance is known:
Power • Question: How much power is being consumed in the circuit?
Power • Question: How much power is being consumed in the circuit?
Power • Question: How much power is being consumed in the circuit? 6 kΩ I = 2. 25 A
Power • Question: How much power is being consumed in the circuit? 6 kΩ I = 2. 25 A
Power • Calculating power when voltage and resistance is known:
Power • Question: How much power is being consumed in the circuit? 50 V 125 Ω
Power • Question: How much power is being consumed in the circuit? 50 V 125 Ω
Power • Question: How much power is being consumed in the circuit? 480 V 2. 75 kΩ
Power • Question: How much power is being consumed in the circuit? 480 V 2. 75 kΩ
The Complete DC Ohm's Wheel
Power and Heat • • • Energy cannot be created or destroyed Energy can be transferred from one part of a circuit to another - a toaster converts electrical energy to heat - a light bulb converts electrical energy to heat and light - a motor converts electrical energy to heat and motion - a speaker converts electrical energy to mechanical energy (sound) A component such as a resistor converts a given amount of energy in a unit of time.
Resistor Power Ratings • • The power rating of a resistor is a measure of its ability to dissipate heat measured in watts (W) When choosing a resister, the power rating of the resistor should be greater than the amount of power expected to be dissipated or the resistor is likely to fail . 25 W. 5 W 1 W 2 W 25 W ALUMINUM COOLING FINS
Power • Question: What is the minimum power rating that the resistor should have?
Power • Question: What is the minimum power rating that the resistor should have?
Efficiency • Every circuit contains a variety of components that use some amount of power • Therefore the output power from a circuit is always less than its input power • The ratio of the a circuit’s (or component’s) output power to its input power is its efficiency where η = efficiency Pout = the circuit output power Pin = the circuit input power
Efficiency • A DC power supply has the following ratings: Pin = 1600 W Pout = 1250 W
Efficiency • Question: what is the efficiency of the DC to AC inverter with the following ratings: Pin = 1. 76 k. W Pout = 950 W
Efficiency • Question: what is the efficiency of the DC to AC inverter with the following ratings: Pin = 1. 76 k. W Pout = 950 W
Energy Measurement
Energy Measurement • How much energy is required to run twenty 60 W incandescent light bulbs for six hours? - total wattage is the sum of the individual wattages • If you pay 13. 5 cents per k. Wh, what is your cost?
Energy Measurement • Question: you keep a 150 W incandescent light on in your barn 24 hours a day, how many kilowatt hours are you using? • If you pay 13. 5 cents per k. Wh, what is your cost?
Energy Measurement • Question: you keep a 150 W incandescent light on in your barn 24 hours a day, how many kilowatt hours are you using? • If you pay 13. 5 cents per k. Wh, what is your cost?
Energy Measurement • Question: you replace the 150 W incandescent light with a compact florescent (CFL) with the same lumens that is rated at 32 watts. • At the same 13. 5 cents per k. Wh how much do you save?
Energy Measurement • Question: you replace the 150 W incandescent light with a compact florescent (CFL) with the same lumens that is rated at 32 watts. • At the same 13. 5 cents per k. Wh how much do you save?
Energy Measurement • Question: you replace the 150 W incandescent light with a LED lamp with the same lumens that is rated at 23 watts. • At the same 13. 5 cents per k. Wh how much do you save?
Energy Measurement • Question: you replace the 150 W incandescent light with a LED lamp with the same lumens that is rated at 23 watts. • At the same 13. 5 cents per k. Wh how much do you save?
Energy Measurement • Reading a k. Wh meter
Problems: 500 m. A - R = _______ 60 V - + 2. 75 m. A 2. 2 A V = _______ 1. 5 k 100 + + + 2. 2 M 90 V - + 4160 V - V = _______ R = _______ + 12 V 1. 25 A 250
Problems: 500 m. A 24 Ω R = _______ 60 V - + 2. 75 m. A 220 V V = _______ - 1. 5 k 2. 2 A - + + 1. 89 m. A 360 m. A + 2. 2 M 90 V - + 4160 V 100 + 4. 125 V V = _______ 48 Ω R = _______ - 12 V 1. 25 A 250
Problems: 750 m. A - POWER = _______ 35 V POWER = _______ + 480 V - 333 - + 2. 7 k + + POWER = _______ 2. 5 A . 275 m. A 120 - 1. 35 k + + 48 V - 120 V 1. 2 A POWER = _______
Problems: 750 m. A 90 W POWER = _______ 35 V - + 480 V - 333 - + 2. 7 k + + 6. 9 W POWER = _______ W POWER = 85. 3 _______ 2. 5 A . 275 m. A 120 - 1. 35 k + + 48 V 42 W POWER = _______ - 120 V 1. 2 A 750 W POWER = _______ m. W POWER = 10. 2 _______
Problems: 2. 5 A - 60 V What is the minimum resistance that the circuit can have and not blow the fuse? + R = _______ 555 m. A + _____ V What is the maximum voltage that the circuit can have and not blow the fuse? 332
Problems: 2. 5 A - 60 V What is the minimum resistance that the circuit can have and not blow the fuse? 24 Ω R = _______ + 555 m. A + 184. 26 V V _____ What is the maximum voltage that the circuit can have and not blow the fuse? 332
Problems: 180 W 75 W What is the efficiency of the circuit? 40 W How many k. Wh are used to cook a stuffed 30 lb turkey for 5 hours and 30 minutes in a 2, 200 watt oven? How much did it cost to cook the turkey at 12. 72 cents per k. Wh?
Problems: 180 W 75 W What is the efficiency of the circuit? 40 W How many k. Wh are used to cook a stuffed 30 lb turkey for 5 hours and 30 minutes in a 2, 200 watt oven? How much did it cost to cook the turkey at 12. 72 cents per k. Wh?
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