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# Category Archives: skeptic

## If we assume global warming is a hoax, what should we expect to see

### This analysis is by Phil Plait, Mar 9, 2017

I will ask you to indulge me for a moment in a thought experiment. It’s not hard, and it leads to a startlingly simple yet powerful conclusion, one I think you may find both important and terribly useful.

Still, it starts with a big ask, so forgive me. And that is: Let’s make an assumption, one you’ve heard many times before. Let’s say that global warming is a hoax.

I know, I know. But go with this, here. So, yes, let’s say that climate change deniers —people like House Science, Space, and Technology Committee chairman Lamar Smith, Senator James Inhofe, and even Donald Trump himself— are right. Whatever the reasons (Chinese hoax, climatologist cabal clamoring colossal cash, carbon dioxide isn’t a powerful greenhouse gas, or just a liberal conspiracy), let’s say that the Earth is *not* warming up.

In that case, the temperatures we see today *on average* should be much like the ones we saw, say, 20 years ago. Or 50. Sure, you’d see fluctuations. In a given spot on a given day the temperature in 1968 might have been a degree warmer than it was in 1974, or three degrees cooler than in 2010. But what you’d expect is that *over time*, a graph showing the temperature would be pretty much flat, with lots of short-term spikes up and down.

Now, statistically speaking, you expect some records to be broken every now and again. Over time, every few years for a given day you’d get a record high, and every few years a record low. The details will change from place to place and time to time, but again, *if the average temperature trend is flat*, unchanging, then you would expect to see just as many record cold days as record warm days. There *might* be small deviations, like, say, a handful of more cool than warm days, but the difference would be very small depending on how many days you look at.

It’s like flipping a coin. On average, you should get a 50/50 split between heads and tails. But if you flip it 10 times, say, you wouldn’t be shocked to see seven heads and three tails. But if you flip it a thousand times, you’d really expect to see a very even split. Seeing 700 heads and 300 tails would be truly extraordinary.

So, if we remind ourselves of our basic assumption —global warming isn’t real— then we expect there to be as many record high days as there are record lows. Simple statistics.

So, what *do* we see?

Guy Walton, a meteorologist in Georgia, took a look at the data from the NOAA’s National Centers for Environmental Information. Whenever a weather station in the US breaks a record, high or low, it’s catalogued (Walton has more info on this at the link above). He found something astonishing: For February 2017, the number of record highs across the US recorded was **6,201**.

The number of record lows? **128**.

That’s a ratio of over 48:1. In just one month.

Again, if temperatures were flat over time, and record highs and lows were random fluctuations, you’d expect a ratio much closer to 1:1. In other words, out of 6329 records set in total, you’d expect there to be about 3165 record highs, and 3165 record lows.

For fans of statistics, with a total of 6329 records broken, one standard deviation is the square root of that, or about 80. So, sure, something like 3265 highs and 3064 lows wouldn’t be *too* unusual. If you start to see more of an imbalance than that, it would be weird.

Seeing *6201* record highs to *128* lows is very, very, very weird. Like, zero chance of that happening by accident.

Now, Phil, I can hear you thinking, that’s just for the US (2% of the planet) over one month. And you’ve told us before that weather isn’t climate; weather is what you expect now, climate is what you expect over long periods of time. So, maybe this is a fluke?

Walton notes that, if you look at records in the US going back to the 1920s, the six highest ratios of record highs to lows all occur since the 1990s. Huh.

And making this more global, a pair of Australian scientists looked at their country’s data, and found that their ratios *were* about even…until the 1960s. After that, highs always outnumber lows. From 2000-2014, *record highs outnumbered lows there by 12:1*.

The University Corporation for Atmospheric Research collated data from 1800 stations across the US and binned the data by decade — by *decade*, which is a huge sample; any deviation from a 1:1 ratio would be extraordinary over that timescale.

Source of the above image: RECORD HIGH TEMPERATURES FAR OUTPACE RECORD LOWS ACROSS U.S. The National Center for Atmospheric Research/UCAR, Nov 12, 2009

We are seeing far more record high temperatures than record lows in the US… and in other countries, too. Credit: UCAR

Huh. Not only are there more record highs than lows, the ratio between the two is getting higher with time.

So, looking back at our initial assumption — the Earth isn’t warming, and temperatures are flat— there’s a conclusion these data are screaming at us: *That assumption is completely and utterly wrong.*

And of course, all the evidence backs this up. *All* of it. Earth’s temperature *is* increasing. That’s because of the 40 billion tons of extra carbon dioxide humans put into the atmosphere every year (the amount we will see this year, expected to top 410 parts per million, has never been seen before in history as long as humans have walked the Earth). This CO2 allows sunlight to warm the Earth, but prevents all of it from escaping so that a little bit of extra heat remains behind, and that’s warming our planet.

Over time, we’re getting hotter. 2014 was a record hot year, beaten by 2015, itself beaten by 2016. In fact, 15 of the 16 hottest years ever recorded have been from 2001 – 2016. That’s exactly what you’d expect if we were getting warmer, and that means our initial assumption of hoaxery was dead wrong.

The science on this is so basic, the evidence of this so overwhelming, that “not a single national science academy disputes or denies the scientific consensus around human-caused climate change”, and also the overwhelming majority of scientists who study climate do, too.

Maybe you should listen to *them*, and not politicians who seem ideologically opposed to the science.

Or, you could flip a coin. But if it comes up science dozens of times more often than anti-science, well —and forgive me if I sound like a broken record— the conclusion is obvious.

___________________________

Fair use: This website is educational. Materials within it are being used in accord with the Fair Use doctrine, as defined by United States law.

§107. Limitations on Exclusive Rights: Fair Use

Notwithstanding the provisions of section 106, the fair use of a copyrighted work, including such use by reproduction in copies or phone records or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use, the factors to be considered shall include: the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes; the nature of the copyrighted work; the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and the effect of the use upon the potential market for or value of the copyrighted work. (added pub. l 94-553, Title I, 101, Oct 19, 1976, 90 Stat 2546)

## You’re Not Going to Believe What I’m Going To Tell You

### From “You’re Not Going to Believe What I’m Going To Tell You”, from The Oatmeal/ Matthew Boyd Inman.

### You’re Not Going to Believe What I’m Going To Tell You.

I’m going to tell you some things.

You’re not going to believe these things that I tell you.

And that’s Ok. You have good reason not to.

But I need you to keep listening, regardless of what you believe.

I don’t care if you’re liberal, conservative, or somewhere in between.

I don’t care if you’re a cat person, a dog person, or a tarantula person.

Morning person or night owl. iPhone or Android. Coke or Pepsi.

I don’t care. All I care about is that you read this to the end.

Sound good? Then let’s begin.

### “You’re Not Going to Believe What I’m Going To Tell You”, from The Oatmeal

## Ampère’s circuital law

I’m caching a copy of www.maxwells-equations.com/ampere/amperes-law.php

This isn’t to negate the copyright of the original website, which I direct people to! I create backups like this on occasion, because even favorite teaching websites sometimes disappear (maybe the owner didn’t pay to renew the domain name.) And I wouldn’t want something so valuable to disappear.

___________________

On this page, we’ll explain the meaning of the last of Maxwell’s Equations, **Ampere’s Law**, which is given in Equation [1]:

Ampere was a scientist experimenting with forces on wires carrying electric current. He was doing these experiments back in the 1820s, about the same time that Farday was working on Faraday’s Law. Ampere and Farday didn’t know that there work would be unified by Maxwell himself, about 4 decades later.

Forces on wires aren’t particularly interesting to me, as I’ve never had occassion to use the very complicated equations in the course of my work (which includes a Ph.D., some stints at a national lab, along with employment in the both defense and the consumer electronics industries). So, I’m going to start by presenting Ampere’s Law, which relates a electric current flowing and a magnetic field wrapping around it:

Equation [2] can be explained: Suppose you have a conductor (wire) carrying a current, *I*. Then this current produces a Magnetic Field which circles the wire.

The left side of Equation [2] means: If you take any imaginary path that encircles the wire, and you add up the Magnetic Field at each point along that path, then it will numerically equal the amount of current that is encircled by this path (which is why we write for encircled or enclosed current).

Let’s do an example for fun. Suppose we have a long wire carrying a constant electric current, *I*[Amps]. What is the magnetic field around the wire, for any distance *r* [meters] from the wire?

Let’s look at the diagram in Figure 1. We have a long wire carrying a current of *I* Amps. We want to know what the Magnetic Field is at a distance *r* from the wire. So we draw an imaginary path around the wire, which is the dotted blue line on the right in Figure 1:

Figure 1. Calculating the Magnetic Field Due to the Current Via Ampere’s Law.

Ampere’s Law [Equation 2] states that if we add up (integrate) the Magnetic Field along this blue path, then numerically this should be equal to the enclosed current *I*.

Now, due to symmetry, the magnetic field will be uniform (not varying) at a distance *r* from the wire. The path length of the blue path in Figure 1 is equal to the circumference of a circle of radius *r*: 2 x Pi x r.

If we are adding up a constant value for the magnetic field (we’ll call it *H*), then the left side of Equation [2] becomes simple:

Hence, we have figured out what the magnitude of the **H** field is. And since *r* was arbitrary, we know what the H-field is everywhere. Equation [3] states that the Magnetic Field decreases in magnitude as you move farther from the wire (due to the 1/r term).

So we’ve used Ampere’s Law (Equation [2]) to find the magnitude of the Magnetic Field around a wire. However, the **H** field is a Vector Field, which means at every location is has both a magnitude and a direction. The direction of the H-field is everywhere tangential to the imaginary loops, as shown in Figure 2. The right hand rule determines the sense of direction of the magnetic field:

Figure 2. The Magnitude and Direction of the Magnetic Field Around a Wire.

### Manipulating the Math for Ampere’s Law

We are going to do the same trick with Stoke’s Theorem that we did when looking at Faraday’s Law. We can rewrite Ampere’s Law in Equation [2]:

On the right side equality in Equation [4], we have used Stokes’ Theorem to change a line integral around a closed loop into the curl of the same field through the surface enclosed by the loop (*S*).

We can also rewrite the total current (I enclosed, I enc) as the surface integral of the Current Density (**J**):

So now we have the original Ampere’s Law (Equation [2]) rewritten in terms of surface integrals (Equations [4] and [5]). Hence, we can substitute them together and get a new form for Ampere’s Law:

Now, we have a new form of Ampere’s Law: the curl of the magnetic field is equal to the Electric Current Density. If you are an astute learner, you may notice that Equation [6] is not the final form, which is written in Equation [1]. There is a problem with Equation [6], but it wasn’t until the 1860s that James Clerk Maxwell figured out the problem, and unified electromagnetics with Maxwell’s Equations.

### Displacement Current Density

Ampere’s Law was written as in Equation [6] up until Maxwell. So let’s look at what is wrong with it. First, I have to throw out another vector identity – the divergence of the curl of any vector field is always zero:

So let’s take the divergence of Ampere’s Law as written in Equation [6]:

[Equation 8] |
---|

So Equation [8] follows from Equations [6] and [7]. But it says that the divergence of the current density **J** is always zero. Is this true?

If the divergence of **J** is always zero, this means that the electric current flowing into any region is always equal to the electric current flowing out of the region (no divergence). This seems somewhat reasonable, as electric current in circuits flows in a loop. But let’s look what happens if we put a capacitor in the circuit:

Figure 3. A Voltage Applied to A Capacitor.

Now, we know from electric circuit theory that if the voltage is not constant (for example, any periodic wave, such as the 60 Hz voltage that comes out of your power outlets) then current will flow through the capacitor. That is, we have **I** not equal to zero in Figure 3.

However, a capacitor is basically two parallel conductive plates separated by air. Hence, there is no conductive path for the current to flow through. This means that no electric current can flow through the air of the capacitor. This is a problem if we think about Equation [8]. To show it more clearly, let’s take a volume that goes through the capacitor, and see if the divergence of **J** is zero:

Figure 4. The Divergence of **J** is not Zero.

In Figure 4, we have drawn an imaginary volume in red, and we want to check if the divergence of the current density is zero. The volume we’ve chosen, has one end (labeled side 1) where the current enters the volume via the black wire. The other end of our volume (labeled side 2) splits the capacitor in half.

We know that the current flows in the loop. So current enters through Side 1 of our red volume. However, there is no electric current that exits side 2. No current flows within the air of the capacitor. This means that current enters the volume, but nothing leaves it – so the divergence of **J** is not zero. We have just violated our Equation [8], which means the theory does not hold. And this was the state of things, until our friend Maxwell came along.

Maxwell knew that the Electric Field (and Electric Flux Density (**D**) was changing within the capacitor. And he knew that a time-varying magnetic field gave rise to a solenoidal Electric Field (i.e. this is Farday’s Law – the curl of E equals the time derivative of **B**). So, why is not that a time varying **D** field would give rise to a solenoidal **H** field (i.e. gives rise to the curl of **H**). The universe loves symmetry, so why not introduce this term? And so Maxwell did, and he called this term the *displacement current density*:

[Equation 9] |
---|

This term would “fix” the circuit problem we have in Figure 4, and would make Farday’s Law and Ampere’s Law more symmetric. This was Maxwell’s great contribution. And you might think it is a weak contribution. But the existance of this term unified the equations and led to understanding the propagation of electromagnetic waves, and the proof that all waves travel at the same speed (the speed of light)! And it was this unification of the equations that Maxwell presented, that led the collective set to be known as Maxwell’s Equations. So, if we add the displacement current to Ampere’s Law as written in Equation [6], then we have the final form of Ampere’s Law:

[Equation 10] |
---|

And that is how Ampere’s Law came into existance!

### Intrepretation of Ampere’s Law

So what does Equation [10] mean? The following are consequences of this law:

- A flowing electric current (
**J**) gives rise to a Magnetic Field that circles the current

- A time-changing Electric Flux Density (
**D**) gives rise to a Magnetic Field that circles the**D**fieldAmpere’s Law with the contribution of Maxwell nailed down the basis for Electromagnetics as we currently understand it. And so we know that a time varying

**D**gives rise to an**H**field, but from Farday’s Law we know that a varying**H**field gives rise to an**E**field…. and so on and so forth and the electromagnetic waves propagate – and that’s cool.

### This website is educational. Materials within it are being used in accord with the Fair Use doctrine, as defined by United States law.

§107. Limitations on Exclusive Rights: Fair Use

Notwithstanding the provisions of section 106, the fair use of a copyrighted work, including such use by reproduction in copies or phone records or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use, the factors to be considered shall include:

the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;

the nature of the copyrighted work;

the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and

the effect of the use upon the potential market for or value of the copyrighted work. (added pub. l 94-553, Title I, 101, Oct 19, 1976, 90 Stat 2546)

## How science works – examples

### Science is a process used to approach claims. We approach claims skeptically: That doesn’t mean that that we don’t believe anything. Rather, it means we don’t accept a claim unless we are given compelling evidence. Skepticism is a provisional approach to claims.

### In 1976 during the Viking missions, NASA scientists found a pattern of chemical reactions that indicated some form of bacterial life may be living in the martian soil.

### In the late 1990s, studies of a Martian meteorite provided evidence that microscopic, bacteria-like life on Mars may have existed. Did simple forms of life once lived on Mars? Does bacterial life live in the Martian soil today?

### If this interests you, look up Viking lander biological experiments, and the meteorite Allan Hills 84001 (ALH84001)

### Many people in Scotland reported a creature swimming in Loch Ness (a large freshwater lake in the Scottish Highlands.) A few blurry photographs have been taken of an object in the water. Newspapers named this supposed creature “the Loch Ness Monster”. Are there unknown, large sea monsters living in this lake?

### If this interests you look up Loch Ness “monster”

### In the 1970’s doctors created an oral pill, Loniten, to control high blood pressure. It works by dilating the blood vessels, so blood can flow better. One of the side effects that patients reported was excess body hair growth. Could this be the first drug to regrow more hair? If this interests you look up the discovery of Minoxidil.

### Charles Darwin (1809 –1882) was an English naturalist. He discovered evidence that today’s animals are modified versions of animals that lived in the past; he discovered that many forms of life have descended over time from common ancestors. Has life on Earth evolved from earlier forms of life? If this interests you look up the discovery of evolution by natural selection.

## How can we tell which claims are true?

### Use the scientific method to investigate such claims.

**Learning Objectives**

**2016 Massachusetts Science and Technology/Engineering Standards
**Students will be able to:

* plan and conduct an investigation, including deciding on the types, amount, and accuracy of data needed to produce reliable measurements, and consider limitations on the precision of the data

* apply scientific reasoning, theory, and/or models to link evidence to the claims and assess the extent to which the reasoning and data support the explanation or conclusion;

* respectfully provide and/or receive critiques on scientific arguments by probing reasoning and evidence and challenging ideas and conclusions, and determining what additional information is required to solve contradictions

* evaluate the validity and reliability of and/or synthesize multiple claims, methods, and/or designs that appear in scientific and technical texts or media, verifying the data when possible.

**A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)**

Implementation: Curriculum, Instruction, Teacher Development, and Assessment

“Through discussion and reflection, students can come to realize that scientific inquiry embodies a set of values. These values include respect for the importance of logical thinking, precision, open-mindedness, objectivity, skepticism, and a requirement for transparent research procedures and honest reporting of findings.”

**Next Generation Science Standards: Science & Engineering Practices**

● Ask questions that arise from careful observation of phenomena, or unexpected results, to clarify and/or seek additional information.

● Ask questions that arise from examining models or a theory, to clarify and/or seek additional information and relationships.

● Ask questions to determine relationships, including quantitative relationships, between independent and dependent variables.

● Ask questions to clarify and refine a model, an explanation, or an engineering problem.

● Evaluate a question to determine if it is testable and relevant.

● Ask questions that can be investigated within the scope of the school laboratory, research facilities, or field (e.g., outdoor environment) with available resources and, when appropriate, frame a hypothesis based on a model or theory.

● Ask and/or evaluate questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of the design

**MA 2016 Science and technology**

Appendix I Science and Engineering Practices Progression Matrix

Science and engineering practices include the skills necessary to engage in scientific inquiry and engineering design. It is necessary to teach these so students develop an understanding and facility with the practices in appropriate contexts. The Framework for K-12 Science Education (NRC, 2012) identifies eight essential science and engineering practices:

1. Asking questions (for science) and defining problems (for engineering).

2. Developing and using models.

3. Planning and carrying out investigations.

4. Analyzing and interpreting data.

5. Using mathematics and computational thinking.

6. Constructing explanations (for science) and designing solutions (for engineering).

7. Engaging in argument from evidence.

8. Obtaining, evaluating, and communicating information.

Scientific inquiry and engineering design are dynamic and complex processes. Each requires engaging in a range of science and engineering practices to analyze and understand the natural and designed world. They are not defined by a linear, step-by-step approach. While students may learn and engage in distinct practices through their education, they should have periodic opportunities at each grade level to experience the holistic and dynamic processes represented below and described in the subsequent two pages… http://www.doe.mass.edu/frameworks/scitech/2016-04.pdf

## Soundly Proving the Curvature of the Earth at Lake Pontchartrain

Excerpted from an article by Mick West

A classic experiment to demonstrate the curvature of a body of water is to place markers (like flags) a fixed distance above the water in a straight line, and then view them along that line in a telescope. If the water surface is flat then the markers will appear also in a straight line. If the surface of the water is curved (as it is here on Earth) then the markers in the middle will appear higher than the markers at the ends.

Here’s a highly exaggerated diagram of the effect by Alfred Russel Wallace in 1870, superimposed over an actual photograph.

This is a difficult experiment to do as you need a few miles for the curvature to be apparent. You also need the markers to be quite high above the surface of the water, as temperature differences between the water and the air tend to create significant refraction effects close to the water.

However Youtuber Soundly has found a spot where there’s a very long line of markers permanently fixed at constant heights above the water line, clearly demonstrating the curve. It’s a line of power transmission towers at Lake Pontchartrain, near New Orleans, Louisiana.

The line of power lines is straight, and they are all the same size, and the same height above the water. They are also very tall, and form a straight line nearly 16 miles long. Far better than any experiment one could set up on a canal or a lake. You just need to get into a position where you can see along the line of towers, and then use a powerful zoom lense to look along the line to make any curve apparent

One can see quite clearly in the video and photos that there’s a curve. Soundly has gone to great lengths to provide multiple videos and photos of the curve from multiple perspectives. They all show the same thing: a curve.

One objection you might make is that the towers could be curving to the right. However the same curve is apparent from both sides, so it can only be curving *over* the horizon.

c

People have asked why the curve is so apparent in one direction, but not in the other. The answer is *compressed perspective*. Here’s a physical example:

c

That’s my car, the roof of which is slightly curved both front to back and left to right. I’ve put some equal sized chess pawns on it in two straight lines. If we step back a bit and zoom in we get:

Notice a very distinct curve from the white pieces, but the “horizon” seems to barely curve at all.

Similarly in the front-back direction, where there’s an even greater curve:

There’s a lot more discussion with photos here Soundly Proving the Curvature of the Earth at Lake Pontchartrain

## Fuses

Fuses

### You can use adapters to turn one outlet into two… two outlets into four, and so on. What happens if you turn on all the devices connected to all these cords at once? They draw a lot of current through the wires to that outlet – and those wires can overheat, and start an electrical fire.

### Electrical fire

### This is why we need something in the house which can detect abnormally high electrical currents – and cut them off.

### Circuit breakers and fuse boxes.

### Here we see what could be a potentially fatal accident – a wet electrical appliance could conduct enough electricity to kill a person. How can we avoid this?

### “A ground fault circuit interrupter (GFCI) or Residual Current Device (RCD) is a device that shuts off an electric power circuit when it detects that current is flowing along an unintended path, such as through water or a person.”- Simple Wikipedia

### A GFCI on a hair dryer.

Lab Measuring Voltage Current DC circuits

- Learn how to build a simple circuit, measure voltage, and current
- Build a DC series circuit and DC parallel circuit

==========================

## The fuse

The fuse breaks the circuit if a fault in an appliance causes too much current flow. This protects the wiring and the appliance if something goes wrong. The fuse contains a piece of wire that melts easily. If the current going through the fuse is too great, the wire heats up until it melts and breaks the circuit.

Fuses in plugs are made in standard ratings. The most common are 3A, 5A and 13A. The fuse should be rated at a slightly higher current than the device needs:

- if the device works at 3A, use a 5A fuse
- if the device works at 10A, use a 13A fuse

Cars also have fuses. An electrical fault in a car could start a fire, so all the circuits have to be protected by fuses.

## The circuit breaker

The circuit breaker does the same job as the fuse, but it works in a different way. A spring-loaded push switch is held in the closed position by a spring-loaded soft iron bolt. An electromagnet is arranged so that it can pull the bolt away from the switch. If the current increases beyond a set limit, the electromagnet pulls the bolt towards itself, which releases the push switch into the open position.

from http://www.bbc.co.uk/schools/gcsebitesize/science/edexcel_pre_2011/electricityworld/mainselectricityrev3.shtml

======================

## Additional resources

### How does a Residual Current Circuit Breaker Work?

### External resources

http://www.electronicsteacher.com/direct-current/physics-conductors-insulators/fuses.php

https://www.allaboutcircuits.com/textbook/direct-current/chpt-12/fuses/

https://en.wikipedia.org/wiki/Thermal_management_(electronics)

https://www.howequipmentworks.com/electrical_safety/

https://www.howequipmentworks.com/electricity_basics/

### ===========================================

## Learning Standards

**Massachusetts 2016 Science and Technology/Engineering (STE) Standards
**

HS-PS2-9(MA). Evaluate simple series and parallel circuits to predict changes to voltage, current, or resistance when simple changes are made to a circuit

HS-PS3-1. Use algebraic expressions and the principle of energy conservation to calculate the change in energy of one component of a system… Identify any transformations from one form of energy to another, including thermal, kinetic, gravitational, magnetic, or electrical energy. {voltage drops shown as an analogy to water pressure drops.}

HS-PS3-2. Develop and use a model to illustrate that energy at the macroscopic scale can be accounted for as either motions of particles and objects or energy stored in fields [e.g. electric fields.]

HS-PS3-3. Design and evaluate a device that works within given constraints to convert one form of energy into another form of energy.{e.g. chemical energy in battery used to create KE of electrons flowing in a circuit, used to create light and heat from a bulb, or charging a capacitor.}

## Power (electrical)

If you look carefully at a stereo, hair dryer, or other household appliance, you find that most devices list a “power rating” that tells how many watts the appliance uses. In this section you will learn what these power ratings mean, and how to figure out the electricity costs of using various appliances.

The three electrical quantities

We have now learned three important electrical quantities:

Paying for electricity

Electric bills sent out by utility companies don’t charge by the volt, the amp, or the ohm. You may have noticed that electrical appliances in your home usually include another unit – the watt. Most appliances have a label that lists the number of watts or kilowatts. You may have purchased 60-watt light bulbs, or a 900-watt hair dryer, or a 1500-watt toaster oven. Electric companies charge for the energy you use, which depends on how many watts each appliance consumes in a given month.

A watt is a unit of power

The watt is a unit of power. Power, in the scientific sense, has a precise meaning. Power is the rate at which energy is flowing. Energy is measured in joules. Power is measured in joules per second. One joule per second is equal to one watt. A 100-watt light bulb uses 100 joules of energy every second. Where does the electrical power go?

Electrical power can be easily transformed into many different forms. An electric

motor takes electrical power and makes mechanical power. A light bulb turns electrical power into light and a toaster oven turns the power into heat. The same unit (watts) applies to all forms of energy flow, including light, motion, electrical, thermal, or many others.

Power in a circuit can be measured using the tools we already have. Remember

that one watt equals an energy flow of one joule per second.

Amps = flow of 1 coulomb of charge per second

Volts = an energy of 1 joule of energy / coulomb of charge

If these two quantities are multiplied together, you will find that the units of

coulombs cancel out, leaving the equation we want for power.

Watts equal joules/second, so we can calculate electrical power in a circuit by

multiplying voltage times current.

# P = VI

power measured in watts; voltage in volts; current in amps

A larger unit of power is sometimes needed.

A 1500-watt toaster oven may be labeled 1.5 kW.

kilowatt (kW) is equal to 1000 watts, or 1000 joules per second.

Horsepower – another common unit of power often seen on electric motors

1 horsepower = 746 watts.

Electric motors you find around the house range in

size from 1/25th of a horsepower (30 watts) for a small electric fan to 2 horsepower (1492 watts) for an electric saw.