Ohms Law and Electrical Power

Ohms LawandElectrical PowerElectricity in the homepart 3 -- 2010Measuring potential difference and voltageFHIPlan an investigation to measure and report on the relationship between the voltage across a resistor and the current through it.Identify any equipment needed.Tabulate your results and present the data graphically.Conduct this experiment twice, using a lamp and a resistor.

plan, choose equipment for and perform a first-hand investigation to gather data and use available evidence to show the relationship between voltage across and current in a DC circuitMeasuring potential difference and voltageFHITypical results using a 20 ohm resistorTwo ways of recording this data are as atablegraph

Analysis of dataBegin by making qualitative observationswhen the voltage is zero, the current is zero as the voltage increases, the current increasesplan, choose equipment for and perform a first-hand investigation to gather data and use available evidence to show the relationship between voltage across and current in a DC circuitResults

As the current is increased, the applied voltage increases at constant rate. The voltage is proportional to the current.Results

Numerical Analysisvoltage is proportional to the currentvoltage/current = constant (the gradient of the V-I graph)gradient = 190/0.010 = 1900 [dont forget to convert milliamperes to amperes]The constant ratio of the voltage:current is called the resistance, R = 1900ohmscurrent (mA)voltage (V)define resistance as the ratio of voltage to current for a particular conductor: R = V/IResistanceResistance is defined as the ratio of the voltage to the current

Resistance is measured in ohms () if the voltage is in volts and the current is in amperesR = V/IR = 10/0.01R = 1000

define resistance as the ratio of voltage to current for a particular conductor: R = V/IResistanceGenerally, materials that conduct electricity resist the flow of current through the material to some degree.This property is called resistance.

Electronic components designed to control the flow of electric current in a specific manner due to their resistance are called resistors.A colour code is used to identify the value of the resistor. Resistance*Check out this site!

*ResistanceQuiz:Calculate the current through this 1000 ohm resistor if the applied voltage was 20 volts.SolutionR = V/ I1000 = 20/II = 20/1000I = 0.02 A or 20 milliamperes

Solve problems and analyse information applying R = V/IMeasuring potential difference and voltageFHI plan, choose equipment for and perform a first-hand investigation to gather data and use available evidence to show the relationship between voltage across and current in a DC circuit containing an incandescent lampTypical results using a 5 W lamp

plan, choose equipment for and perform a first-hand investigation to gather data and use available evidence to show the relationship between voltage across and current in a DC circuit

Results

Results

ObservationAs the current is increased, the applied voltage increases at an increasing rateThe resistance of the light globe increases as the current increases due to the increasing temperature of the filament.An incandescent lamp is a non-ohmic conductor. Its resistance is not fixed.plan, choose equipment for and perform a first-hand investigation to gather data and use available evidence to show the relationship between voltage across and current in a DC circuit

The filament of a light globeThe filament of a light globe is a coiled tungsten wire.The resistance of the light globe is determined by the intrinsic properties of the filament and the final temperature of the filament

When a potential difference is applied across a conductor, the electric current through the conductor is affected bythe length of the conductorthe cross-sectional area of the conductorthe temperaturethe type of materialDiscuss qualitatively

InvestigationCarefully examine different wattage light globes and describe any differences between them.Examining Light Globes FHIdescribe qualitatively how each of the following affects the movement of electricity through a conductor; length, cross sectional area, temperature, materialThe filament in a light globe is a coil made of tungsten

THINKFilaments in Light Globes

Why is the filament coiled?16Tungsten (symbol W) is a metallic element with a melting point of 3410 C. Other metallic elements, molybdenum (m.pt. 2610C) and hafnium (m.pt. 2227C) are also used to make light bulb filaments.

Electric currentWhen a potential difference is applied across a conductor, the electric current through the conductor is affected by the conductorslengthcross-sectional areatemperaturethe type of materialThe longer the conductor, the more resistance it has to the flow of electrons through it.This is an important factor to be accounted for when electricity is being distributed over long distances.Longer wiresLess brightdescribe qualitatively how each of the following affects the movement of electricity through a conductor; length, cross sectional area, temperature, materialdescribe qualitatively how each of the following affects the movement of electricity through a conductor; length, cross sectional area, temperature, material

Electric currentWhen a potential difference is applied across a conductor, the electric current through the conductor is affected by the conductorslengthcross-sectional areatemperaturethe type of materialThe larger the conductors diameter, the less resistance it has to the flow of electrons through it.This is an important factor to be accounted for when electricity is being distributed over long distances.Thinner wiresLess bright More bright Thicker wiresdescribe qualitatively how each of the following affects the movement of electricity through a conductor; length, cross sectional area, temperature, material

Electric currentWhen a potential difference is applied across a conductor, the electric current through the conductor is affected by the conductorslengthcross-sectional areatemperaturethe type of materialThe higher the conductors temperature, the more resistance it has to the flow of current through it.Experimental distribution of electricity using superconductors at very low temperatures has begun.More bright Cooler wires

Hotter wiresLess bright

Electric currentWhen a potential difference is applied across a conductor, the electric current through the conductor is affected by the conductorslengthcross-sectional areatemperaturethe type of materialCopper and aluminium are both good conductors of electricity.Although copper is a better conductor than aluminium, aluminium is the metal used in most power lines. WHY?More bright copper wires

aluminium wiresLess bright describe qualitatively how each of the following affects the movement of electricity through a conductor; length, cross sectional area, temperature, material

Oct 2002EnergyEnergy is defined as the ability to do work.Symbols for work are W and EWork always involves the conversion of energy from one form to another, or the transfer of energy from one place to another.Electrical energy is transformed into many other useful forms by household appliances, e.g.soundlightheatmovement

PowerPower is defined as the rate at which energy is transformed from one form to another.Electrical power is thus the amount of work that an electric current can accomplish in a given amount of time.

The unit for power is the watt (W)The symbol for power is P

From previous work : V = E/qAnd I = q/tUse the definition of power P = E/tAnd eliminate q and t from these equationsHence P = E/t = qV/t = VIP = VIPower and energy transformationexplain that power is the rate at which energy is transformed from one form to anotherPower and energy transformationThinkWhat happens to the rest of the energy consumed by the 100 W filament lamp if it is less bright than a 17 W fluorescent lamp?The higher the power of an appliance, the greater the amount of electrical energy it consumes in a given time.The brightness of incandescent light bulbs increases with the power rating of the bulb.A 17 watt fluorescent lamp however may produce more light than a 100 watt incandescent light bulbs.The future of lighting - LED

explain that power is the rate at which energy is transformed from one form to another

Power and energy transformation

Most of the energy consumed by an incandescent bulb is converted to heat energyA 100 W incandescent bulb gets very hot while it is operatingA 100 W bulb must never be placed in a lamp fitting rated for less powerful bulbs

hot!!!!explain that power is the rate at which energy is transformed from one form to anotherexplain that power is the rate at which energy is transformed from one form to anotherIncandescent lamps are very sensitive to changes in the supply voltage. These characteristics are of great practical and economic importance. For a supply voltage VLight output is approximately proportional to V3.4Power consumption is approximately proportional to V1.6Lifetime is approximately inversely proportional to V16

Voltage, light output, and lifetime

An image of a 60 W (240 V) light bulb filament. To increase the filament length while keeping its physical size small, the filament takes the form of a coiled coil. By comparison, low voltage lamp filaments usually take the form of a single coil.Power, potential difference and currentPowerThe power produced by the conversion of electrical energy is given by the expressionpower = voltage x currentP = VIEnergy = power x timeE = VIt

identify the relationship between power, potential difference and currentsolve problems and analyse information using P=VI and E = VItPower of Domestic AppliancesTypical values

FirstPower, potential difference and current

FHIPlan a first hand investigation to gather information and use available evidence to demonstrate the relationship between current, voltage and power for a model 12V heating coil.perform a first-hand investigation, gather information and use available evidence to demonstrate the relationship between current, voltage and power for a model 6V to 12V electric heating coil

FirstPower, potential difference and current

ConsiderThe energy, measured in joules (J), drawn from the power supplyE = VItThe energy gained by the waterE = cmTV= voltage (V)I= current (A)c= specific heat of waterm= mass of waterT= temperature changeperform a first-hand investigation, gather information and use available evidence to demonstrate the relationship between current, voltage and power for a model 6V to 12V electric heating coilPower, potential difference and currentQuizA 240V electric room heater has a compliance plate stating that the appliance draws a current of 5 A.What is the power of this appliance?How much energy does it use if operated for 6 hours?

P = VIP = 240 x 5 = 1200 wattsE = VIt = 240 x 5 x (6 x 3600)E = 2.592 x 107 JOr 25.92 megajoules (MJ)identify the relationship between power, potential difference and currentsolve problems and analyse information using P=VI and E = VItPower, potential difference and currentQuizA 240V electric toaster is rated at 980 watts.What current does this appliance draw?

Compare the energy it uses in 10 minutes with that used by a 240V 60watt light globe operated continuously for 12 hours. P= VI980= 240 x II= 4.08 AE= VIt = 980 x 10 x 60 = 5.88 x 105 joulesFor the light globeE= VIt = 60 x 12 x 3600 = 2.59 x 106 joulesHence the globe uses 4.4 times more energy than the toaster.

Power, potential difference and currentQuizHow many 240 V 40 watt lamps can be put in parallel before exceeding the typical 8 ampere current limit for a household lighting circuit? P= VIThe current drawn by one lamp is given byI = P/V = 40/240 = 0.167 ALamps in parallel each draw the same current as if each one was connected independently to the power supply.Hence the number of lights is8/0.167 = 47[Why is it not valid to round the answer, 47.9 upward?]

Energy, potential difference and currentQuizDetermine the time for which each of the appliances* in the following table would have to be run in order to consume the same amount of energy as a frying pan used for 2 hours.

Energy used by frying pan Pt = 1.196 kW x 2 h = 2.392 kWhCarving knife2.392 = 0.092 tt = 26 hours etc

***identify the relationship between power, potential difference and currentsolve problems and analyse information using P=VI and E = Vitidentify that the total amount of energy used depends on the length of time the current is flowing and can be calculatedCompareCompare an electricity bill with a gas bill, describing in particular, the units by which the energy is sold.Report this information back to the class.Typical values QuantityCost ($)____________________________________ConversionA kilowatt.hour is 1000 J/s x 3600 s= 3 600 000 joulesA kilowatt hour is 3.6 MJCost of Energyidentify the relationship between power, potential difference and currentsolve problems and analyse information using P=VI and E = VItElectricity Bill

explain why the kilowatt-hour is used to measure electrical energy consumption rather than the jouleElectricity Bill

explain why the kilowatt-hour is used to measure electrical energy consumption rather than the jouleThe kilowatt hour is used to measure electrical energy consumption because these units are of a convenient size and they do in fact measure energySince energy = power x timeAnd the units for power are watts (or kilowatts)The units for energy, above are kWh (where t is the time measured in hours and power is measured in kilowatts)Electricity Transmission

Gas Bill

explain why the kilowatt-hour is used to measure electrical energy consumption rather than the jouleGas Bill

explain why the kilowatt-hour is used to measure electrical energy consumption rather than the jouleHaving the electricity energy consumption in kilowatt.hours and the gas energy consumption in megajoules makes it almost impossible for the average person to readily compare the two.It could be argued that this is a distinct advantage for sellers of both utilities.Think!explain why the kilowatt-hour is used to measure electrical energy consumption rather than the jouleWhats wrong with this?

Energy, potential difference and currentDiscussionElectrical energy is sold by the kilowatt hour (kWh), whereas energy produced by gas is sold by the megajoule (MJ).Suggest reasons as to why the energy units are different. ConversionA kilowatt.hour is 1000 J/s x 3600 s = 3 600 000 joulesA kilowatt hour is 3.6 MJEnergy, potential difference and current

QuizWhat current is used by a 240 volt coffee maker?Compare the energy used to run the coffee maker for 10 minutes with the energy required by an active human adult male needing 10 MJ per day.

I = P/V = 1200/240 = 5 AEnergy = Pt = 1200 x 10 x 60Energy = 720 000 joulesThe human needs about 10 MJ or 10/0.72 times more 14 times.Power of Domestic AppliancesInvestigationMost electrical appliances are labelled with information about the power rating of the appliance.Examine the compliance plates on a variety of household appliances and tabulate the information found on the appliance compliance plates.

Investigation of the labelling electrical goodsApplianceVoltageCurrentPowerPhone chargerFluorescent lightTVElectric kettleToasterIronCalculatorInvestigation of the labelling electrical goodsApplianceVoltage (V)Current (A)Power (W)Phone charger7.50.43Fluorescent lamp24010TV240120Electric kettle2402400Toaster2404.17Iron2401400CalculatorInvestigation of the labelling electrical goodsApplianceVoltage (V)Current (A)Power (W)Calculator30.00020.0006TelevisionRefrigeratorElectric kettle2402200Keyboard50.5Phone120.2Printer50.42Monitor2401.3312Water heater240153600Labelling Electrical AppliancesPower ratingsMost electrical appliances are labelled with information about the power rating of the appliance.Typical valuesCalculator0.00006 WTelevision115 W (Sony 68 cm CRT)RefrigeratorMicrowave1500 W (input)750 W (output)Electric kettle2200 W...Labelling Electrical Goods with Power RatingsAssessing the practice of labelling appliancesMake a judgment For make customers aware of how much energy appliances use allows comparison between like appliances allow consumers to determine if theyre overloading a circuitAgainst costs money to manufacturers to put on the labels people do not read the labels - they are often inaccessible people do not know how to interpret information on the labelsJudgmentEnergy rating of domestic appliancesAustralian Energy Efficiency Rating (EER)Five major classes of appliance must carry the star ratingrefrigerators freezers dishwashers washing machines and clothes dryersInformation about an appliances energy consumption is found on the label attached to all new appliances in these categories.49How are star ratings calculated?Star ratings have been developed to provide consumers with an easy way of comparing the energy efficiency of different models. In this context, energy efficiency is defined as the "energy service per unit of energy consumption". It is a simple and fair way of comparing the energy consumption of products that do a similar job.If you think of an air conditioner, for example, a model's efficiency is the amount of cooling capacity per unit of energy it consumes. A similar way of measuring the "energy efficiency" has been developed for all labelled appliances. Looking at the energy alone is not all that helpful, as a big appliance will nearly always use more energy than a small one, because it is doing more work. A measure of energy efficiency means that you can directly compare a great range of products.The star rating system has a minimum of 1 star and a maximum of 6, shown in half star increments. Various "algorithms" or equations have been developed to rate the least efficient products at around 1 star.Energy rating of domestic appliancesAustralian Energy Efficiency Rating (EER)

The EER label shows quantitative data produced by a standard testing procedure.The energy efficiency is also represented using a system of stars.50If appliance energy efficiency in the market never changed, the stars could be adjusted so that the best products on the market now rated 6 stars. But we know from experience that manufacturers work hard to improve their products and over time, star ratings gradually improve. So when the stars were re-graded in 2000, it was necessary to leave some room for improvement. Hence under the new system, the most efficient products are generally only 3 stars (although there are some products that rate 3.5 and 4 stars already for some appliance types).

Energy rating of domestic appliancesAustralian Energy Efficiency Rating (EER)This EER label shows quantitative data for a typical small refrigerator.Assess the efficiency of this appliance, based on this product label.Propose some strategies for minimising the amount of energy used by a fridge.

51If appliance energy efficiency in the market never changed, the stars could be adjusted so that the best products on the market now rated 6 stars. But we know from experience that manufacturers work hard to improve their products and over time, star ratings gradually improve. So when the stars were re-graded in 2000, it was necessary to leave some room for improvement. Hence under the new system, the most efficient products are generally only 3 stars (although there are some products that rate 3.5 and 4 stars already for some appliance types).

Energy rating of domestic appliancesAustralian Energy Efficiency Rating (EER) for Refrigerators

Purchase decisions based on factual information about appliances energy consumptions can save significant amounts of money52If appliance energy efficiency in the market never changed, the stars could be adjusted so that the best products on the market now rated 6 stars. But we know from experience that manufacturers work hard to improve their products and over time, star ratings gradually improve. So when the stars were re-graded in 2000, it was necessary to leave some room for improvement. Hence under the new system, the most efficient products are generally only 3 stars (although there are some products that rate 3.5 and 4 stars already for some appliance types).

Reducing Energy Consumption

Electrical energy is sold by the kilowatt hour. A kWh is 3.6 MJWhat is the reading on the electricity meter above?The temperatures in this chart are in Fahrenheit degrees53

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Reducing Energy Consumption

USA domestic consumption dataPropose how a similar graph for a Sydney home might be different

Reducing Energy ConsumptionProblem solvingPropose strategies that can assist in reducing heating and lighting bills in your home.