Crawling and electrical outlets

My daughter has just started moving. We are fortunate that she started moving later than some other babies we know.  We have had a blissful 10 months of not having to chase after her and put up gates in the house. Alas, that is all over now.

She is not crawling exactly, but she scoots forward. She reaches forward onto her hands while sitting and uses one leg to pull herself forward and drags the other leg behind. I have seen many of her cousins doing something similar, at pretty high speeds. This may turn into a standard crawl someday soon, or maybe not. Either way, she is definitely moving around, and faster every day.

Last week, I put her down on the floor in the bedroom so I could pick up some clothes to do some laundry. She usually goes straight to playing with our window shades, which is relatively safe. That day, though, I turned my back for just a second and she went straight across the room to the electrical outlet. Yikes! I had not yet put the safety plugs in upstairs. It is crazy how she always goes straight for the electrical outlets. They are right at eye level and are apparently quite fascinating.

So why are electrical outlets so dangerous for little ones?

First of all, can she even get her little fingers in those tiny little holes? In order for an electrical connection to be made, she needs to touch the metal connections. Even with her very little fingers, I do not think she could get them into the outlets. That being said, I certainly do not want to let her try!

I think the real danger is probably having her put one of her toys into the outlet and making an electrical connection that way. Many of her toys are plastic or rubber right now, which do not conduct electricity effectively, but she does love to play with metal things. Also, she gets into everything now that she is mobile and I let her play with anything that does not seem dangerous, so it is not unlikely that she would find something to put into that outlet.

What happens if she does manage to get something into the electrical outlet (finger, fork, etc.)? The outlet has 120 V of potential – that means that it has energy that is ready to start flowing. If you ‘complete the electrical circuit’, i.e. give that energy somewhere to flow, it will start flowing – through you.

So how much current will flow through your body if you touch an electrical outlet and what is a dangerous level? Ohm’s Law tells us that voltage, V, (that 120 V for a standard household outlet) is related to current, I, and resistance, R, through the following equation:

The amount of current that flows through us depends on our resistance to that current.

So what is our resistance? That turns out to be a complicated question because it depends on things like how dry (or wet) our hands are, and how much contact we make with the metal wires. If we connect with the wires over a large area of our bodies, our resistance is lower – it is easier for current to flow through us.

Dry skin has a larger resistance than wet skin. This is fortunate for me as a teacher. I accidently shocked myself with a 500 V source in the lab once, but my hands were covered in chalk and the contact between my hands and the wire was small. It was definitely not a pleasant shock, and I felt dizzy for a little while after, but there was no serious or permanent damage. On the other hand, I have also shocked myself on a 120 V outlet in a lab (no chalk) and it was quite painful. Who knew the life of a physicist could be so dangerous?

But I have not answered the question. What is our resistance? I had my students measure their resistance in a teaching lab using very small metal probes (small area of contact). They measured from one hand to the other and found that they had a resistance of about a megaohm – that’s one million ohms. However, an article on Electric Shock on Wikipedia cites the International Electrotechnical Commission as showing adult resistances at 100 V of ~2000 ohms. They use larger contact areas, but you never know when you shock yourself how much contact you will have, so it is much safer to assume your resistance will be low.

Okay, let us assume, to be safe, that our resistance is about 2000 ohms and the voltage of the outlet is 120 V. From the equation above (Ohm’s Law), we can calculate that this situation would send 60 milliamps (mA) of current through us. That does not sound like a lot, right? Well, an amp is a LOT of current. The same Wikipedia article cited above states that humans can feel 1 mA of current. Currents as low as 60 mA (and sometimes lower) can cause fibrillation of the heart muscles, which can lead your heart to stop.

Depending on the voltages and contact situation, electrical shocks can also cause serious burns. I had minor burns on both hands when I was shocked by the 500 V lab source. (I should note that I am always very careful around electrical sources and in both cases of being shocked, I was working with wires that had been previously damaged  – a good example of why you should always have broken or old wiring repaired immediately.)

Okay, I am convinced. I should cover up my outlets and keep my daughter safe! So far we have just put in some standard outlet covers, though I have read in several places online that these covers are too easy for toddlers to remove. Fortunately, my daughter is too young to have figured that out yet (and trust me, she has tried!). As she gets older, and stronger, and more coordinated, and smarter, we may have to come up with better ways to keep her safe.

We are definitely entering a new stage of parenting. It is amazing to watch her learn to move and explore her world and I am very much enjoying it, but with every new development comes new challenges.


Airport body scanners

I have been in the middle of a cross-country move the past few weeks, which has been quite challenging with a baby in tow. According to Google Maps, we moved 1423 miles from our old home to our new home. We also moved 4292 ft higher in elevation (according to Wikipedia). No wonder I am exhausted

For work reasons, we needed to move rather quickly. Normally, we would have driven, which takes about 24 hours. With stops for feeding, changing, and soothing a baby, that would have been at least 3 long days, or 4 or 5 if we took our time and stopped to see things on the way – after all, if you are going to drive more than 1400 miles, you might as well see the sights along the way. Unfortunately, we did not have time for that this trip, and so I flew with my daughter to our new home.

I am a relatively frequent flyer and adept at getting through security quickly and efficiently, though it is definitely more of a challenge now that I have a baby with me. I, like all of you who fly I’m sure, have noticed the increasing number of airport body scanners in the security line over the past couple of years. My first experience with one was returning to the US from an international flight in the spring of 2010. Now they seem to be in all the airports that I frequently fly through.

There are a number of issues surrounding these scanners: privacy, effectiveness, cost, and safety. There are huge numbers of articles around the web on each of these issues. I started to think seriously about the last one – safety – last year when I was pregnant. I went on three plane trips while pregnant and was wondering if the scanners were safe for my unborn baby. I never had to walk through one of these scanners during those trips, and so I did not give it much more thought. Then I started traveling with my daughter and wondered once again whether or not these scanners are safe for her. It is interesting that it never occurred to me to worry about this for my own safety, but I guess we are always more concerned about keeping our children safe.

Ignoring all the other issues (which may also be very important to you), let’s take a look at how these scanners work and whether or not they are safe for us to walk through.

There are two types of scanners that are currently in use in the United States: millimeter wave three dimensional scanners, and x-ray backscatter (two dimensional) scanners. They use different wavelengths of light (both invisible to the eye) and different methods of making an image.

A few weeks ago, I had a blog entry on how CT scans work. These body scanners are very similar in a lot of ways, so let’s review a few of those ideas, starting with wavelength. The electromagnetic spectrum (the different wavelengths and frequencies of light) is shown below:

I did not mention the relationship between wavelength () and frequency (f) in my previous blog entry, but here it is:


where c is the speed of light, 3 x 108 meters/second (or 186 miles/second). This is true for all wavelengths of light traveling through air. Who cares, you say? Well, this means that the millimeter wave beams have a frequency of ~ 300 GHz and below. For reference, your cell phone uses waves that have a frequency of ~ 1 GHz. The x-ray beams have frequencies of greater than a million GHz, or about 10,000 times that of the millimeter beam waves.

Again…this is interesting and all (if you are into numbers), but why should I care? Well, the energy contained in the individual balls of light (photons) in these beams is directly proportional to the frequency of light. That means that the x-ray photons hitting your body have more than 10,000x more energy than the millimeter wave photons. These high energy photons can actually ionize atoms – that means they can remove electrons from the atoms in your body, which can change the ways your atoms chemically bond and form all the organic material your body needs to function correctly.

So that is why x-rays are more dangerous than millimeter waves (which have not been shown to have any lasting harm on your body as far as I know). The question of safety seems to lie in the dosage. The backscatter scanners only send a very small amount of x-rays at your body.

According to the Transportation Security Administration (TSA):

“Advanced imaging technology is safe and meets national health and safety standards. Backscatter technology was evaluated by the Food and Drug Administration’s (FDA) Center for Devices and Radiological Health (CDRH), the National Institute for Standards and Technology (NIST), and the Johns Hopkins University Applied Physics Laboratory (APL). For comparison, a single scan using backscatter technology produces exposure equivalent to two minutes of flying on an airplane, and the energy projected by millimeter wave technology is thousands of times less than a cell phone transmission. Millimeter wave imaging technology meets all known national and international health and safety standards. In fact, the energy emitted by millimeter wave technology is 1000 times less than the international limits and guidelines.”

So this sounds like the scanners are safe, right? But the European Union has banned the x-ray backscatter scanners. From a European Commission Press Release:

“In order not to risk jeopardising citizens’ health and safety, only security scanners which do not use X-ray technology are added to the list of authorised methods for passenger screening at EU airports.”

Why? Well, the backscatter machines concentrate the x-rays on our skin. Unlike for CT scans, the x-rays do not travel through our bodies, but are reflected backwards to a detector to form an image. Some scientists, like the ones from the University of California at San Francisco that wrote a Letter of Concern printed in this news article, are concerned that the scanners have not be accurately tested and evaluated for this use. They believe that until these tests are done, the scanners should not be used in airports.

Okay, so we have heard this all before. They are safe. They are not safe. What does this mean? I feel that there are enough intelligent people (scientists, officials in Europe) who do not feel the machines are safe, especially for children. I will not walk through one of the x-ray backscatter machines with my daughter. (Of course, it is unlikely I will ever have to since I currently have to carry my daughter and that would defeat the purpose of the scanners – I could just hold her in front of my body wherever I was carrying something forbidden on airplanes.)

I do believe that the millimeter scanners are safe and have no problem walking through those. My next question was this: How can I tell which is which? It turns out they look completely different. There are number of pictures online showing what they look like. This article on The Science Behind Airport Body Scanners shows the scanners side by side so you can see what they look like.

Now I can make an educated decision about whether or not to walk through the scanner I see in the airport. I have only seen the millimeter wave machines and am not sure what airports have the x-ray backscatter scanners. If anyone has found a list of what airports carry which scanners, please leave a comment – I would be interested in knowing.

Side note: We are still in the midst of settling in and so the blog entries will likely be shorter and/or later than usual for a few weeks until I can find the time to work on them.