Wireless power

Massachusetts Institute of Technology: 7-Jun-2007

Imagine cell phones, household robots, MP3 players and laptop computers that can recharge their batteries without being plugged in. Some might not even need bulky batteries at all.

A team from MIT has just taken a step toward this vision of wireless power. The work will be reported in the June 7 issue of Science Express. This is the advance online version of the journal Science.

The story starts late one night a few years ago. Professor Marin Soljacic (pronounced Soul-ya-cheech) was standing in his pyjamas, staring at his cell phone on the kitchen counter. "It was probably the sixth time that month that I was wakened by my cell phone beeping to let me know I had forgotten to charge it.

"It occurred to me that it would be so great if the thing took care of its own charging."

Soljacic started thinking about physics and engineering that could make this happen. Several ways to send power without using wires have been known for centuries. The best known is electromagnetic radiation, such as radio waves.

These are great for sending information. But they are no use for sending power. Electromagnetic radiation spreads out in all directions, so most of the power would be wasted. It is possible to beam electromagnetic radiation in one direction. Lasers do it.

But this needs an uninterrupted line of sight between the source and the device. It also needs some kind of tracking device if the receiver is moving around. And it could be dangerous.

So the MIT team came up with a novel idea. They call it WiTricity. Using this they have now lit up a 60 watt light bulb from a power source over 2 metres away. There was no physical connection between source and appliance.

WiTricity is based on something called coupled resonance. Two objects that have the same resonant frequency exchange energy very efficiently. But very little energy will pass to other nearby objects.

Frequency is number of vibrations per second. Everything has a natural frequency, the frequency it vibrates at naturally. A child on a swing is a good example.

If given one big push this will swing back and forth at the natural frequency. A short swing will move faster than a long swing, so it will have a greater natural frequency.

Now suppose Mum pushes the swing any old time she feels like it. Sometimes she will speed the swing up. Sometimes she will slow it down. Overall very little energy will be given to the swing.

But suppose she times her pushes just right. The swings will get higher and higher. Energy is being transferred from the parent to the child on the swing. The reason is that Mum times it so she pushes at the natural frequency of the swing. This is resonance.

There are lots of examples of resonance. A wine glass vibrates at a certain frequency if it is tapped. Its natural frequency will be different depending on the amount of wine in the glass. So imagine a room with 100 wine glasses, each filled with wine to a different level.

If an opera singer produces a single note inside the room, the glass that vibrates naturally at the same frequency as the note gains energy from the sound waves. It might even gain enough to burst. The other glasses would not be affected because their resonant frequencies are all different from the frequency of the note.

These ideas apply to all kinds of resonances (for example acoustic, mechanical, electromagnetic). But the MIT team concentrated on one particular type, namely resonators that are coupled magnetically. The team explored a system of two electromagnetic resonators coupled mostly through their magnetic fields.

They found that the source and receiver were strongly coupled, even when the distance between them was several times larger than they were. This means power was being transferred efficiently.

Magnetic coupling is particularly good for everyday applications. This is because most common materials interact very little with magnetic fields. So power is not transferred to other objects in the room. "The fact that magnetic fields interact so weakly with biological organisms is also important for safety," Andre Kurs, a graduate physics student points out.

The design the team studied has two copper coils. Each is a resonant system. One of the coils, attached to the power source, is the sending unit. Instead of irradiating the environment with electromagnetic waves, this fills the space around it with a magnetic field. This oscillates at megahertz (millions of times a second) frequencies but does not send out any energy at first.

But when the receiving unit, which has the same resonant frequency, is brought into this field, energy is transferred. The coupled resonance means that the sending unit and receiving unit are strongly connected. But everything else around is not. Moffatt, an MIT undergraduate in physics, explains:

"The crucial advantage of using the non-radiative field lies in the fact that most of the power not picked up by the receiving coil remains bound to the vicinity of the sending unit, instead of being radiated into the environment and lost."

With such a design, power transfer has a limited range. The range is shorter for smaller-size receivers.

Even so, for a laptop-sized coil, power levels more than enough to run a laptop can be transferred over a distance the size of a room. This works well in pretty much all directions. It even works when there are objects between the source and the receiver.

Professor Peter Fisher says: "As long as the laptop is in a room equipped with a source of such wireless power, it would charge automatically, without having to be plugged in. In fact, it would not even need a battery to operate inside of such a room."

In the long run, this could reduce our need for batteries, which are heavy, expensive and harmful to the environment.

WiTricity is based on very well known laws of physics. So why has no one thought of it before? "In the past, there was no great demand for such a system, so people did not have a strong motivation to look into it," says Professor John Joannopoulos.

In recent years portable electronic devices, such as laptops, cell phones, iPods and even household robots have become much more common, he says. "All of these require batteries that need to be recharged often."

As for the future, Soljacic says, "Once, when my son was about three years old, we visited his grandparents' house. They had a 20-year-old phone and my son picked up the handset, asking, 'Dad, why is this phone attached with a cord to the wall?'

"That is the mindset of a child growing up in a wireless world. My best response was, 'It is strange and awkward, isn't it? Hopefully, we will be getting rid of some more wires and also batteries soon.'"

More help with words

cycles

device

electromagnetic

electron

energy

field

frequency

natural frequency

radiation

resonance

response

spirals

vibrate

vibration

What's it all about?

1. This story is about being able to recharge everyday things without having to plug them in to an electricity supply. This is called -------- power.
2. Why was Prof Soljacic in his pyjamas when the idea for this research first came to him?
3. How many times had he been wakened that month by his mobile phone beeping?
4. Why was it beeping?
5. One way to send energy without using wires is by electromagnetic radiation. Give one reason this is not likely to work well.
6. Why might it be dangerous?
7. What does MIT call their new method?
8. What electrical item did they use in their demonstration?
9. In your own words what does "no physical connection" mean?
10. What is a natural frequency?
11. Resonance is when energy gets transferred very effectively because it is supplied at the same -------- as the natural frequency of an object.
12. The writer gives a couple of examples of resonance to help explain what the scientists have been doing. Pick one of these and describe very briefly what happens at resonance.
13. Resonance can take place with all sorts of things, such as sounds or moving objects. What did the MIT scientists concentrate on in their work?
14. Resonance is all about the frequency of the energy supplied being the same as the ------- frequency of the thing receiving the energy.
15. Frequency is fairly easily understood when you can see something moving back and forth like a child on a swing. The frequency of a child on a swing is the ------ of times she swings back and forth in a certain time.
16. But frequency is a useful idea for anything that is moving or changing, even when it's too small or moving too fast to see. The frequency of a sound for instance is the number of pressure waves in the --- that hit your ear in a second.
17. The frequency of a magnetic field is the number of times it ------- direction in a second.
18. The frequency of the magnetic field the scientists used was several megahertz, the writer says. This means it changes direction -------- of times a second.
19. At first there is a coil in the room that is producing a rapidly changing magnetic field. But it is not sending out any ------.
20. Then when a second coil is brought fairly close to the first, energy passes. But only if the natural frequency of the second coil is the ---- as the natural frequency of the first.
21. Moffatt refers to a "non-radiative field". What does "non-radiative" mean?
22. A radiative field produces electromagnetic radiation. In question 5 you gave one reason for not using electromagnetic radiation for wireless energy. Now find as many reasons in the story as you can for not using electromagnetic radiation for wireless energy.
23. State one disadvantage of the method these scientists have developed.
24. If you were these scientists what would you like to do next?
25. Why?

What kind of story is this?
Learning to do science is about learning to think. Experiments, direct teaching, group activities and discussions all have a part to play. So do science news stories.

Like other non-fiction texts, science stories contain different kinds of statements. To get at the science behind the words - and to make reading them an active experience - students should pull a text apart and explore the kinds of statement it contains.

We've met some of these in the later questions of the previous activity. Science news stories usually include the aims of the research or reasons for doing it. They often contain a hypothesis. Sometimes evidence for a hypothesis is given, or a hypothesis is used to make a prediction. Towards the end of a story the direction of future research the scientists are planning is often discussed, as well as outstanding questions the research will be designed to answer.

All these types of statement occur in some science stories. Virtually all science stories, however, will contain statements of the following four types:

• new findings or developments;
• the technology and methods the scientists used;
• previous or accepted knowledge, which may or may not be supported by the new findings;
• issues, implications and applications of the research.

So the next activity is designed to engage students with the latest science news by exploring the meaning and structure of a story as revealed by the content and balance of these four statement types:

Pulling it apart
In groups students should read through the story looking for new findings or developments. Once they have reached agreement, or at least consensus, and have underlined all the statements about what the scientists have just discovered or achieved, they can compare and discuss.

In groups they should go through the story again looking for the technology and methods the scientists used in their research. Once they have reached agreement or consensus, and have underlined the statements that talk about the methods and equipment the scientists used, they can compare and discuss.

They should repeat the activity for existing knowledge.

Any areas of disagreement in these activities - whether among the students or between teacher and students - should be regarded as opportunities for discussion rather than errors to be corrected.

Having fully engaged with the latest science news through the above activities, students will be far better able to talk and think about the science and its implications than someone who has simply read about it in a newspaper or watched a brief item on television.

Now it's time for them to get to grips with the issues raised by the research.

Young people have opinions. But school science traditionally allowed little scope for forming and expressing these - which is why it turned many of them off the subject for life.

Putting it together again

In groups, students should read through the latest story looking for issues, implications and applications. Once they have reached agreement, or at least consensus, and have underlined all the relevant statements in the story, they can compare and discuss.

Having done all this the students are well armed to explore the issues raised by the story. A suggested discussion topic specific to this new story is provided below.

Topic for discussion, research or pupil presentations

This is a tough story for schoolkids. Resonance is university level science and engineering, and electromagnetic induction is upper high school. But the philosophy of this website, and the whole science in society movement, is that you don't need a PhD in nuclear physics to engage with the key ideas of modern science.

The key ideas in this story are resonance, natural frequency and the difference between WiTricity and electromagnetic radiation.

A) In groups students should research as many examples of resonance as they can find. They should then prepare a short presentation to explain how their favourites among these work, and the principle that is common to them all.

They may find the following websites useful starting points:

Earthquakes, bridges, seashells, all from Discovery Education

Vibrating strings from Nova

Musical instruments from the Physics Classroom

Guitars and rockets from NASA (podcast and transcript)

Tacoma Narrows collapse from Nova

B) In groups they should compile a list of similarities between WiTricity and electromagnetic radiation. They should then compile a list of differences. They should brainstorm these tasks at first, writing down every suggestion from the group, no matter how trivial or even frivolous they seem. Only when this is complete should they examine each item, retaining any that has a grain of truth.

Tips for science class discussions and groupwork

No 51

As part of their report they must submit 5 new words/terms they learned, summarize the article, give me a complete citation as for a bibliography (MLA format), 5 questions that they would like to ask the author, the subject, or anyone about the article. They must tell me how the illustrations helped or detracted from their understanding of the article; and give me 2 questions that refer to the illustrations.

"You might try magazine articles - I've done this with my older kids using 5 magazine articles from an 'approved magazine' typically, Smithsonian or National Geographic.

This has been great for pushing them to look at periodicals, and using another source for reading material. For my purposes, I don't restrict the topics, so it's fun to see the things they are interested in beyond what we cover in class. I take a day and try to answer their questions if I can, or point them to where they can find out more for themselves.

National Science Teachers Association (NSTA) forum entry by teacher Kathleen Gorski (May 2007)