Why does the light turn on? – Ben, aged three, UK.

The electric light was invented more than 200 years ago, and has been in common use for over a century. It works by converting electricity into light (and a bit of heat).

The two most common electric lights in use today are incandescent lights (which are the oldest) and light emitting diodes (LEDs). The word “incandescent” refers to something that’s so hot it glows white.

Incandescent lights have an outer shell made of glass, which has all the air sucked out of it.

Inside this vacuum, there’s a thin coiled wire called a “filament”, made of a metal called tungsten.

The glass shell has to have all the air removed so that the tungsten doesn’t rust away or “oxidise” when it gets hot.

Curious Kids is a series by The Conversation, which gives children the chance to have their questions about the world answered by experts. If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.com. We won’t be able to answer every question, but we’ll do our very best.

Tungsten is used in light bulbs because it has a really high melting point (over 3,000°C), which is much higher than the temperature needed to give a nearly white light (2,000°C).

Heating up

These types of light bulbs are now mostly used in vehicles and low cost pocket torches because they are cheap and reliable. But they also make a lot of heat – in fact, it’s only by getting very hot that they can light up at all – and this is seen as being wasteful of energy.

When the light bulb is connected to a source of electricity, the electricity can easily travel along the thick wires to the light bulb. But when it reaches the filament, with its very thin wire, the electricity has to force its way through, using up a lot of energy which makes the filament very hot, and very bright.

The more energy used by the incandescent light, the brighter it will be. The amount is usually written on the side of the light bulb; for example 20 watts, 40 watts and so on.

Lighting the way

In modern houses, schools and workplaces, the incandescent light has mostly been replaced with LED lights. This is because they use much less energy – about one sixth – to make the same amount of light, since they don’t really get hot. LEDs can also last a very long time, compared to other lights.

The way that LED lights work is not actually very different to the way incandescent lights work.

But instead of heating up a wire to produce the white light, an LED light has a special material inside it called a “semiconductor” to produce the light.

The particular type of semiconductor in an LED reacts when an electric current is passed through it by a process called electrolumincescence and this gives out lots of light, but not very much heat.

LEDs can be made to produce light in almost any colour, by changing the chemicals in the semiconductor.

Children can have their own questions answered by experts – just send them in to Curious Kids, along with the child’s first name, age and town or city. You can:

• email curiouskids@theconversation.com
• tweet us @ConversationUK with #curiouskids
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Here are some more Curious Kids articles, written by academic experts:

• Why do I have boogies and why does my nose keep replicating them? – Duncan, aged seven, Sydney, Australia.

• Can we live on Kepler 452-b? – Year Five, Globe Primary School, London, UK.

• When fish get thirsty do they drink sea water? – Torben, aged nine, Sussex, UK.

Roger Clarke, Honorary Visiting Senior Research Fellow, University of Bradford

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Curious Kids is a series by The Conversation, which gives children of all ages the chance to have their questions about the world answered by experts. All questions are welcome: find out how to enter at the bottom of this article.

What causes the northern lights? – Ffion, age 6.75, Pembrokeshire, UK.

I first saw the northern lights three years ago, while driving home one night. They were so beautiful, I had to stop the car and get out to have a proper look, even though it was cold. Although the northern lights might look like magic, they can actually be explained by science – with a bit of help from the Sun, birds and fizzy drinks.

The energy for making the northern lights comes from the Sun. The Sun creates something called the “solar wind”. This is different to the light that we get from the Sun, which keeps us warm and helps us to see during the day.

This solar wind drifts away from the Sun through space, carrying tiny particles called protons and electrons. Protons and electrons are some of the tiny building blocks that make up most of the stuff in the universe, like plants and chocolate and me and you.

Think of the smallest Lego bricks you have in your toy box, which can be stuck together to make bigger things - these are what protons and electrons (and neutrons too) are to the universe. These particles carry lots of energy from the Sun, on their journey through space.

The solar wind

Sometimes the solar wind is strong, and sometimes it’s weak. We can only see the northern lights at times when the solar wind is strong enough.

When the solar wind reaches planet Earth, something very interesting happens: it runs into the Earth’s magnetic field. The magnetic field forces the solar wind away, and makes it travel around the Earth instead.

The magnetic field is what makes the needle on a compass point north, and is how birds know where to go when they migrate – it’s also why we have the north and south poles at all.

The magnetic field interacts with the solar wind and guides the protons and electrons down towards Earth along the magnetic field, away from the middle of the planet and toward the north and south poles.

Because of this, we get both northern and southern lights – also known as the aurora borealis and the aurora australis.

Shake it up

When the solar wind gets past the magnetic field and travels towards the Earth, it runs into the atmosphere. The atmosphere is like a big blanket of gas surrounding our planet, which contains lots of different particles that make up the air that we breathe and help to protect us from harmful radiation from the Sun.

As the protons and electrons from the solar wind hit the particles in the Earth’s atmosphere, they release energy – and this is what causes the northern lights.

Here’s how it happens: imagine you have a bottle of fizzy drink, and you give it a good shake. This puts lots of energy into the bottle, and when you open it, this energy will be released in a big stream of fizzy bubbles.

In the same way, the protons and electrons from the Sun “shake up” the particles in the atmosphere. Then, the particles let out all that energy in the form of light (instead of bubbles).

Different types of particles in the atmosphere make different colours after they’re shaken up – oxygen makes red and green light, and nitrogen makes blue light. Our eyes see green best out of all the colours, so we see green the brightest when we look at the northern lights.

It is easiest to see the northern lights in winter when is it very dark at night, and also outside of cities and away from street lights. You are more likely to see them the further north you are too. Check out this great website Aurora Watch from Lancaster University – it might just help you find them!

This article has been corrected: the Earth’s magnetic field is not weaker at the poles, as the article originally stated.

Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question – along with your name, age and town or city where you live – to curiouskids@theconversation.com. Send as many questions as you want! We won’t be able to answer every question, but we’ll do our best.

More Curious Kids articles, written by academic experts:

• Why do spiders have hairy legs? - Audrey, age five, Melbourne, Australia

• Why do we have different seasons at specific times of the year? – Shrey, age nine, Mumbai, India

• How is water made? – Clara, age eight, Canberra, Australia

Paul O'Mahoney, Post-Doctoral Research Assistant in Photobiology, University of Dundee

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to CuriousKidsUS@theconversation.com.

How will the universe end? – Iez M., age 9, Rochester, New York

Whether the universe will “end” at all is not certain, but all evidence suggests it will continue being humanity’s cosmic home for a very, very long time.

The universe – all of space and time, and all matter and energy – began about 14 billion years ago in a rapid expansion called the Big Bang, but since then it has been in a state of continuous change. First, it was full of a diffuse gas of the particles that now make up atoms: protons, neutrons and electrons. Then, that gas collapsed into stars and galaxies.

Our understanding of the future of the universe is informed by the objects and processes we observe today. As an astrophysicist, I observe objects like distant galaxies, which lets me study how stars and galaxies change over time. By doing so, I develop theories that predict how the universe will change in the future.

Predicting the future by studying the past?

Predicting the future of the universe by extending what we see today is extrapolation. It’s risky, because something unexpected could happen.

Interpolation – connecting the dots within a dataset – is much safer. Imagine you have a picture of yourself when you were 5 years old, and then another when you were 7 years old. Someone could probably guess what you looked like when you were 6. That’s interpolation.

Maybe they could extrapolate from the two pictures to what you’d look like when you are 8 or 9 years old, but no one can accurately predict too far into the future. Maybe in a few years you get glasses or suddenly get really tall.

Scientists can predict what the universe will probably look like a few billion years into the future by extrapolating how stars and galaxies change over time, but eventually things could get weird. The universe and the stuff within might once again change, like it has in the past.

How will stars change in the future?

Good news: The Sun, our medium-sized yellow star, is going to continue shining for billions of years. It’s about halfway through its 10 billion-year lifetime. The lifetime of a star depends on its size. Big, hot, blue stars live shorter lives, while tiny, cool, red stars live for much longer.

Today, some galaxies are still producing new stars, but others have depleted their star-forming gas. When a galaxy stops forming stars, the blue stars quickly go “supernova” and disappear, exploding after only a few million years. Then, billions of years later, the yellow stars like the Sun eject their outer layers into a nebula, leaving only the red stars puttering along. Eventually, all galaxies throughout the universe will stop producing new stars, and the starlight filling the universe will gradually redden and dim.

In trillions of years – hundreds of times longer than the universe’s current age – these red stars will also fade away into darkness. But until then, there will be lots of stars providing light and warmth.

How will galaxies change in the future?

Think of building a sand castle on the beach. Each bucket of sand makes the castle bigger and bigger. Galaxies grow over time in a similar way by eating up smaller galaxies. These galactic mergers will continue into the future.

In galaxy clusters, hundreds of galaxies fall inward toward their shared center, often resulting in messy collisions. In these mergers, spiral galaxies, which are orderly disks, combine in chaotic ways into disordered blob-shaped clouds of stars. Think of how easy it is to turn a well-constructed sand castle into a big mess by kicking it over.

For this reason, the universe over time will have fewer spiral galaxies and more elliptical galaxies because the spiral galaxies combine into elliptical galaxies.

The Milky Way galaxy and the neighboring Andromeda galaxy might combine in this way in a few billion years. Don’t worry: The stars in each galaxy would whiz past each other totally unharmed, and future stargazers would get a fantastic view of the two galaxies merging.

How will the universe itself change in the future?

The Big Bang kick-started an expansion that probably will continue in the future. The gravity of all the stuff in the universe – stars, galaxies, gas, dark matter – pulls inward and slows down the expansion, and some theories suggest that the universe’s expansion will coast along or slow to a halt.

However, some evidence suggests that some unknown force is starting to exert a repulsive force, causing expansion to speed up. Scientists call this outward force dark energy, but very little is known about it. Like raisins in a baking cookie, galaxies will zoom away from each other faster and faster. If this continues into the future, other galaxies might be too far apart to observe from the Milky Way.

To summarize the best current prediction of the future: Star formation will shut down, so galaxies will be full of old, red, dim stars gradually cooling into darkness. Each group or cluster of galaxies will merge into a single, massive, elliptical galaxy. The accelerated expansion of the universe will make it impossible to observe other galaxies beyond the local group.

This scenario eventually winds down into a dark eternity, lasting trillions of years. New data might come to light that changes this story, and the next stage in the universe’s history might be something totally different and unexpectedly beautiful. Depending on how you look at it, the universe might not have an “end,” after all. Even if what exists is very different from how the universe is now, it’s hard to envision a distant future where the universe is entirely gone.

How does this scenario make you feel? It sometimes makes me feel wistful, which is a type of sadness, but then I remember we live at a very exciting time in the story of the universe: right at the start, in an era full of exciting stars and galaxies to observe! The cosmos can support human society and curiosity for billions of years into the future, so there’s lots of time to keep exploring and searching for answers.

Hello, curious kids! Do you have a question you’d like an expert to answer? Ask an adult to send your question to CuriousKidsUS@theconversation.com. Please tell us your name, age and the city where you live.

And since curiosity has no age limit – adults, let us know what you’re wondering, too. We won’t be able to answer every question, but we will do our best.

Stephen DiKerby, Postdoctoral Researcher in Physics and Astronomy, Michigan State University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Curious Kids is a series for children of all ages, where The Conversation asks experts to answer questions from kids. All questions are welcome: find out how to enter at the bottom of this article.

What is a species? – Finlay, age four, London, UK

Thanks for the question, Finlay. In the past, it seemed like a sensible and simple idea to put living creatures – including animals, plants, fungi, bacteria and so on – into different categories called “species”.

Scientists mostly told different species apart from the way they looked, or where they could be found. But sometimes that proved very tricky indeed.

For example, it’s clear that giraffes and mice are very different groups of creatures, and that you can easily tell them apart; in other words, they are two different species. But what about those two little brown birds, which look so similar?

It was also easy to say that emus and ostriches were probably different species, even though they look a bit similar, because they live on different continents, so they must be different groups of birds.

As time passed and scientists got to study more and more creatures, they realised that some creatures could be quite different, but still be part of the same group.

Have you seen a peacock and a peahen next to each other? In the animal kingdom, mums and dads can look quite different, but they should definitely be part of the same species.

So, by the end of the 1800s, the scientists realised that you can’t always decide if creatures belong to the same species, just by how they look.

Around this time, a man called Charles Darwin started to convince other scientists with his idea of evolution: he showed how creatures change over time, to become better at living in their environment.

One example is how the peppered moths in the UK changed from light to dark colour, after pollution from factories darkened the tree trunks where the moths like to hide. Light coloured moths became easy to spot and got eaten by birds, while the dark coloured moths survived and had more dark coloured babies.

By the beginning of the 1900s, scientists also started to understand that a baby looks like their mum and dad because some kind of information is passed between parents and their children, inside their bodies. Nowadays, we know that this information is called DNA.

With this knowledge, scientists decided that it’s better to define a species as a group of living things that can exchange DNA, by creating “viable offspring”. “Viable offspring” means a baby that can survive, and make babies of its own later on.

This is important, because some species can make babies together, like a zebra and a horse. But this baby – called a zorse – is sterile; it cannot have babies of its own.

This “viable offspring” definition of species is useful, and it’s the one that scientists rely on most often today.

But if you want to have some fun and see a scientist get very hot under the collar, you could mention that sometimes it’s possible to bring together, which would never meet without the help of humans, and that they can produce a viable offspring.

This is the case with tigers and lions. They do not exist in the same location, but humans have bred them together to produce the “liger”. Now what? Are they the same or different species?

As you can see, defining species can get tricky… But I think most scientists will agree that if these two groups wouldn’t meet and have babies without humans getting involved, they are probably two different species.

So how can we define a species? Well, in most cases the “viable offspring” test will work just fine.

You just need to remember that groups of creatures are constantly evolving, so sometimes the differences between species might become a bit blurry.

Hello, curious kids! Have you got a question you’d like an expert to answer? Ask an adult to send your question to us. You can:

* Email your question to curiouskids@theconversation.com
* Tell us on Twitter by tagging @ConversationUK with the hashtag #curiouskids, or
* Message us on Facebook.

Please tell us your name, age and which town or city you live in. You can send an audio recording of your question too, if you want. Send as many questions as you like! We won’t be able to answer every question, but we will do our best.

More Curious Kids articles, written by academic experts:

• How do moths eat our clothes? – Albie, age five, Australia

• Why do leaves change colour? – Isaac, age eight, Guildford, UK

• Why do we need food? – Milo, age five, Cowes, Australia

Paula Kover, Reader in Biology and Biochemistry at The Milner Centre for Evolution, University of Bath

This article is republished from The Conversation under a Creative Commons license. Read the original article.

When I was playing “splash rocks”, I noticed that when I threw the rock into the river it made a circle shape, which got bigger. How does it make the ripple? Why do the circles spread out further and further? Why do they stop? – Rowan, aged six, UK.

Hi Rowan, these are good questions, and a fun experiment to do.

When you throw a rock into a river, it pushes water out of the way, making a ripple that moves away from where it landed. As the rock falls deeper into the river, the water near the surface rushes back to fill in the space it left behind.

The water usually rushes back too enthusiastically, causing a splash – and the bigger the rock, the bigger the splash. The splash then creates even more ripples that tend to move away from where the rock went into the water.

When water is in its calmest, lowest energy state, it has a flat surface. By throwing the rock into the river, you have given the water some energy. That causes the water to move around, trying to spread out the energy so it can go back to having a still, flat surface.

This follows a powerful principle of physics, which is that everything seeks to find a state where its energy is as small as possible.

Curious Kids is a series by The Conversation, which gives children the chance to have their questions about the world answered by experts. If you have a question you’d like an expert to answer, send it to curiouskids@theconversation.com. We won’t be able to answer every question, but we’ll do our very best.

One way energy can move around is by forming waves. For example, the waves you see at the beach are formed by energy from the wind. Light and sound also move in waves, though we can’t see that directly. And the ripples that you see in the river are small waves carrying away the energy from where you threw the rock.

Up and down

You might already know that everything you can touch is made up of lots of tiny molecules, which are themselves made up of even smaller parts called atoms.

Water is also made of molecules. But during a ripple, the water molecules don’t move away from the rock, as you might expect. They actually move up and down. When they move up, they drag the other molecules next to them up – then they move down, dragging the molecules next to them down too.

That’s what creates the peaks and troughs you see on the surface of the water. And that’s how the ripple travels away from your rock – a bit like a human wave around a stadium.

Dragging neighbouring water molecules up and down is hard work, and slowly uses up energy, so the ripples get smaller as they get further away. Eventually, the ripples use up all the energy from the rock and the splash, and shrink until we can no longer see them.

Rippling out

Ripples often spread out in circles, but this isn’t the only possibility. If you throw a stick into the water it will create straight ripples on the sides, and round ripples near the ends. So your rock probably made circular ripples because the rock itself was quite round.

But something else is happening too: different waves move at different speeds. Waves with a lot of energy move more quickly. For example, really big tidal waves, or tsunamis, race across the ocean as fast as a plane flies (up to 800 kilometres per hour).

When you throw a stick into the water, the ripples from the middle of the stick eventually catch up with the ripples from the ends, because of the different ways they spread out. So far away from the stick, the ripples are round … just like they were for your rock.

Children can have their own questions answered by experts – just send them in to Curious Kids, along with the child’s first name, age and town or city. You can:

• email curiouskids@theconversation.com
• tweet us @ConversationUK with #curiouskids
• DM us on Instagram @theconversationdotcom

Here are some more Curious Kids articles, written by academic experts:

• How does our brain send signals to our body? – Aarav, aged nine, Mumbai, India.

• How can we see what we are imagining but still see what’s in front of us? – Malala Yousafzai class, Globe Primary School, London, UK.

• Why is the sea salty? – Torben, aged nine, Sussex, UK.

Simon Cox, Professor of Mathematics, Aberystwyth University

This article is republished from The Conversation under a Creative Commons license. Read the original article.