Electrons flow due to potential difference If the potential difference is removed then electrons stop flowing As electrons move they bump into other atoms this slows them down and impedes their motion ID: 1046110
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1. Ohm’s LawAnd Power
2. Electrical resistance (symbol R)Electrons flow due to potential difference. If the potential difference is removed, then electrons stop flowing.As electrons move, they bump into other atoms, this slows them down and impedes their motion..atomsfree electron(actually positive ions)pathResistance (R) is a measure of the degree to which an object impedes the flow of current.Resistance is measured in Ohms ()
3. Resistance Across a CircuitHow do resistors affect circuits?The energy of electricity comes from the movement of the electrons. So, by slowing down the electrons, resistance causes the charges to lose energy / electric potential.Most of the loss of potential comes from the circuit elements, such as lights and motors (often just called resistors) which are using the kinetic energy of the electrons to do something (make light, turn a motor, etc.). We call this loss of electrical potential voltage drop (or potential drop).
4. Resistance in WiresSo, in most cases, we can completely ignore the resistance of the wires within our circuits. But, in some cases, the wire is designed to have high resistance (and high ‘loss’ of energy to heat), usually by making the wire very very long …Most of the time, wires are designed to have as little resistance as possible, mainly by making them short, thick, and out of a good conductor, such as copper.
5. What is the relationship between resistance and voltage drop?Introducing …. Ohm’s LawV is potential difference across a resistorI is current R is resistanceV = I R
6. ExamplesIf a 3 volt flashlight bulb has a resistance of 9 ohms, how much current will it draw? I = V / R = 3 V / 9 = 1/3 AmpsIf a light bulb draws 2 A of current when connected to a 120 volt circuit, what is the resistance of the light bulb? R = V / I = 120 V / 2 A = 60
7. Check for UnderstandingWhich of the following would cause the current in a circuit to decrease the most?Increased voltage and increased resistanceIncreased voltage and decreased resistanceDecreased voltage and decreased resistanceDecreased voltage and increased resistance
8. Check for UnderstandingIf the resistance of a circuit were tripled, then the current through the circuit would be ____.one-third as much three times as much3. unchanged4. ... nonsense! There would be no way to make such a prediction.
9. Just for Fun … Effects of electric current on the BODY- electric shockCurrent (A)Effect0.001can be felt0.005painful0.010involuntary muscle contractions (spasms)0.015loss of muscle control0.070if through the heart, serious disruption; probably fatal if current lasts for more than 1 second
10. human body resistance varies: 100 ohms if soaked with salt water; moist skin - 1000 ohms; normal dry skin – 100 000 ohms, extra dry skin – 500 000 ohms. What would be the current in your body if you touch the terminals of a 12-V battery with dry hands? I = V/R = 12 V/100 000 W = 0.000 12 A quite harmlessBut if your hands are moist and you touch 12 V battery, how much current would you draw? I = V/R = 12 V/1000 W = 0.012 A a dangerous amount of current.
11. Electric PowerElectric power is the rate at which electric energy is converted into another form such as mechanical energy, heat, or light. What does a 60W light bulb mean?It converts electrical energy into light/heat energy at a rate of 60 J per second.What is power?Power = = If we play around with the electric potential energy equation, we can derive another form of the power equation that is more useful in circuits: Unit: Js-1 = WP = VI
12. How much current is drawn by a 60 Watt light bulb connected to a 120 V power line?What is the resistance of the bulb? Electric Power – We Do
13. How much current is drawn by a 60 Watt light bulb connected to a 120 V power line? P = 60 W = I V = I x 120 so I = 0.5 AWhat is the resistance of the bulb? I = V/R R = V/I = 120 V/0.5 A R = 240 Electric Power – We Do
14. Calculate the resistance and the current of a 1500-Watt electric hair dryer plugged into a US household outlet (120 V).2. The sticker on a compact disc player says that it draws 288 mA of current when powered by a 9 Volt battery. What is the power (in Watts) of the CD player?Electric Power – You Do
15. Calculate the resistance and the current of a 1500-Watt electric hair dryer plugged into a US household outlet (120 V).I = P / V = (1500 W) / (120 V) I = 12.5 AmpR = V / I= (120 V) / (12.5 Amp) R = 9.6 2. The sticker on a compact disc player says that it draws 288 mA of current when powered by a 9 Volt battery. What is the power (in Watts) of the CD player? P = I • V = (0.288 A) • (9 V) P = 2.59 WElectric Power – You Do
16. Paying for electricityYou pay for electricity by the Kilowatt-hour (kWh). What is a kWh? Simply another unit for energy.
17. Paying for electricityYou pay for electricity by the Kilowatt-hour (kWh). What is a kWh? Simply another unit for energy. Physicists measure energy in joules, but utility companies customarily charge energy in units of kilowatt-hours (kW h), where : Kilowatt-hour (kWh) = 1000 W x 3600 s1 kWh = 3.6 x 106 J1W x 1s = 1J
18. Paying for electricityYou pay for electricity by the Kilowatt-hour (kWh). What is a kWh? Simply another unit for energy. Physicists measure energy in joules, but utility companies customarily charge energy in units of kilowatt-hours (kW h), where : Kilowatt-hour (kWh) = 1000 W x 3600 s1 kWh = 3.6 x 106 J1W x 1s = 1JAt a rate of 14 cents per kWh, how much does it cost to keep a 100 W light bulb on for one day?energy (kWh) = power (kW) x time (h) energy (kWh) = 0.1 kW x 24 h = 2.4 kWh cost / day = 2.4 kWh x 14 cents/kWh = 33.6 ¢ for one month that amounts to $ 10.1.
19. Paying for electricity – You do A 9V battery costs $1.25. If placed in a cd player that draws a 300 mA current, it will last for 32 hours. What is the price per kWhr? = 0 .086 kWhr$1.25/0.086 kWh = $14 /kWhr