Down to Earth

It’s currently the summer holidays, and we must follow in the steps of the ‘cool’ kids and go outside and ‘socialise’ (pfft what is that?). That means, of course, that we see a screening of Finding Dory.

As we wait for the tram back home, Harvey wanders alarmingly close to the rails, evoking a memory of See Track Think Train in which a girl is on the verge of being crushed because she is preoccupied in playing I Spy. Tom casually glances up at the overhead lines.

“If you touch those cables while standing on the ground, you still won’t get shocked, right?”

Multiple pairs of eyes turn to glare at Tom, because this guy is doing A-level physics (although he did only get a B at GCSE so who knows?).

For any point in a circuit, you can assign an electrical potential, which determines how much potential energy an electric charge would have at that location. Charge always likes to move from an area of high potential to an area of low potential, and this flow generates a current.

However, the concept of potentials is pointless if you don’t set a reference level against which all other potentials can be compared. This reference level is defined to be the Earth, that is, the literal dirt or rock that you may or may not be standing on right now, and is hence named ground, or earth. The Earth has a very low potential so charge will flow to ground if possible.

The majority of electrical devices and appliances employ a grounding (or earthing) system, a safety measure that prevents users from receiving electrical shocks. Take a fridge for example, which is in essence a metal box with rubber feet. If electricity were to somehow come into direct contact with the metal box, its path to ground would be obstructed since rubber is an excellent insulator. Charge therefore lingers around the box, waiting patiently for a hungry person to touch the box and open up a path to ground; people are made of squishy stuff and have low electrical resistances. I can’t vouch for other people but I prefer not to have high currents flowing through me.

Electric modes of transportation, such as electric trains or trams, receive electrical power from overhead lines through a current collector, typically a pantograph.

Electric Pantograph.jpg
Courtesy of Wikipedia.

The original pantograph refers to a mechanism that allows you to create a scaled copy of a drawing, as there existed no Microsoft Word to copy and paste and resize pictures.

Mechanical Pantograph
Courtesy of bidorbuy.

Apparently the current collector looks like a pantograph (I kind of see it to be honest).

For a train or tram, the current typically arrives from one overhead line, and returns through steel rails on the ground. For a trolleybus, rubber tyres prevent current from flowing to ground, so a second overhead line is required for the return.

Let’s revisit the original situation. There is one overhead tram line, and you want to test out your high jumping skills. If you hang from it, you’re pretty safe, because air is another excellent insulator and current can’t flow to ground. On the other hand, if you are able to touch the ground while still clutching the wire, you’re roasted, because you provide an easy path to ground.

This is the reason why birds can nonchalantly perch on overhead wires without becoming cooked meat – because they are not in contact with the ground.

Perhaps your journey home also involves a trolleybus. If you hang from one of the wires, you’re totally fine. Touch the other though, even if you’re not on the ground… roasted, because (you guessed it) you provide an easy path from a high potential to a low potential.

Later, there roars a thunderstorm outside yet you develop a sudden urge to fly a kite. When you fly the kite you’re essentially holding up an enormous sign inviting charge to flow through you, and as expected, you’re roasted (boy, you’re being roasted quite a few times today). But… if you happen to be suspended in the air somehow, you’re probably safe because, once again, it’s all about not touching the ground.

I have to say this, because there will always exist these kinds of people: do NOT test this out (even if it’s for scientific curiosity). Seriously. It’s not worth it.



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