1-6 Induction and Discharging

We've Seen Induction Before

Remember induced charge separation? That happened when charge was pushed around on an object, but overall the object stayed neutral. We've seen that a couple of times:

Here, the object on the right has some of its negative charges pulled to the left end. They are attracted by a nearby positive object. They are induced to move left, but overall the object is still neutral.

Here, the negative charges in the wall are induced to move back a little bit, making the surface of the wall positive, allowing the balloon to be attracted to the wall. The balloon easily sticks to the wall, even though the wall is still neutral overall.

If an object's charges are induced to move, we can use that fact to charge this neutral object.

Charging by Induction

Let's revisit an earlier series of diagrams, but this time let's connect the neutral object to ground and see what happens.

The object starts off neutral. It's connected to ground; the Earth is also neutral. So, at this point, nothing happens. But let's bring a positively charged object towards the neutral object.

As we've seen before, the electrons in the neutral object are attracted to the left: there is an induced charge separation. But now let's think about the right side of the neutral object. It's positively charged... and it can now pull some electrons from the wire that it's connected to.

Since the wire is connected to the ground, electrons can easily flow from the ground, through the wire, into the previously-neutral object. But since this object is taking on extra electrons, it is now negatively charged. All without being touched by the positive object!

These extra electrons will spread themselves out a bit. Since these objects are insulators, we know the charges can't move much, so this is probably a bit of an exaggeration. But it still gives you the main idea. Now let's take the positive object away, and remove the grouding wire.

The originally-neutral object has now been charged by induction: you are using induction to charge an object, with some help from the Earth. With induction, you can charge objects without any contact or friction. And, if you wanted to charge other neutral objects by induction, you can use the same original positively-charged object.

Discharging

If you want to get rid of extra charge, you can discharge an object. This means you can make a charged object neutral again by allowing extra electrons to flow to (or from) that object. This can be done with the ground, or another object.

Let's take our now-charged object and see if we can discharge it:

If we connect a ground wire to it, we can allow some of the electrons to "escape" down the wire and back to the ground.

The ground wire provides a path for the extra electrons to escape. And, because electrons repel each other, that's exactly what they do.

Enough electrons run away so that the object becomes neutral again. After we take away the ground wire, we can count up the positive and negative charges to confirm that it's neutral.

And, after a while, the electrons will evenly space themselves out again:

We now can say that this charged object has been "discharged" by connecting it to ground. But you don't always have to discharge to ground: you can sometimes discharge to another conductor.

Let's say you shuffle across a carpeted floor: your body will be charged by friction between your feet and the carpet. But then let's say you touch a metal door knob: you will feel a shock and you might see a spark jump between your finger and the door knob.

If we draw in the charges, we can see what's happening:

The door knob is still neutral, but since it's a metal the electrons it can move easily: there's an induced charge separation. As your negatively charged finger gets close to the door knob, the electrons in the door knob move backwards. The part of the door knob closest to your finger becomes positive, which attracts some electrons off your finger.

If the attraction is big enough, electrons can jump a short distance through the air. We call this a spark: a discharge of electric charge through air. Since you have lots of nerve endings in your fingertips, this might hurt a little!

Lightning

Maybe the most famous type of discharge is lightning: a spark between a cloud and the ground, through the air. Since lightning is so powerful, exactly how it works is still a bit of a mystery for scientists. But, this is the current model, based on observations and simulations:

Somehow -- probably through friction within the cloud, but scientists aren't quite sure about this -- extra negative charge builds up within a cloud. This can gather in the bottom part of the cloud, which repels the electrons in the ground a bit, inducing a charge separation in the ground.

Eventually, enough charge builds up in the cloud, and there's enough charge separation in the ground, that the extra electrons in the cloud jump down to the ground through the air. We call this "lightning."

Lightning usually takes a jagged path to the ground, rather than being a straight line. If a certain part of the air has some more raindrops in it, since water is a better conductor than air is, the lightning bolt will choose that path. Again, scientists at this point aren't quite sure how it "knows" what path to choose, but there are some high-speed video experiments that point the way towards some ideas.

As this video suggests, a lot of lightning takes place within a cloud itself, or between one cloud to another: this is sometimes called "cloud-to-cloud lightning." But the most spectacular type is "cloud-to-ground lightning," which is what we're most familiar with.

Here's a video showing lightning hitting the top of the CN Tower in downtown Toronto several times. (Note: there's a pretty loud yell at the beginning of this clip. You might want to turn the volume down.)

The CN Tower is designed to take lightning strikes: it is connected by a very thick wire down to the ground. This lets the extra energy go safely to the Earth, so it doesn't go through surrounding buildings (or people). This makes the CN Tower a lightning rod, a device that safely carries the energy in lightning to the ground.

Farmers often put lightning rods on roof of a barn, so lightning will go safely to the ground and not catch the wooden structure on fire:

Here is the next video in the series from TVO, which talks about charging and discharging.

The next video examines induction, and here the explanations are really clear, especially when it looks at grounding.

Practice Questions

The Basics

1. Sketch a series of diagrams showing how an object can be charged by induction.

2. Why is grounding such an important part of charging by induction?

Extensions

1. Explain why, after getting out of a car, touching the metal door can give you a shock.

2. Try to locate the lightning rods on the top of the brick chimney near the Caretaking entrance of our school (on the west side, closest to Livingston). Can you see the thick wires that go all the way down to the ground?