## How much charge can a metal sphere hold?

#### Suppose you have a metal sphere in vacuum and you begin putting electric charge on that sphere. Neglecting possible discharges, how much charge can the sphere store? An unlimited amount? — BC

By asking me to “neglect possible discharges,” you’re asking me to neglect what actually happens. There will be a discharge, specifically a phenomenon known as “field emission”. Neglect that discharge, then yes, the sphere can in principle store an unlimited amount of charge. But on route to infinity, I will have had to ignore several other exotic discharges and then the formation of a black hole.

What will really happen is a field emission discharge. The repulsion between like charges will eventually become so strong that those charges will push one another out of the metal and into the vacuum, so that charges will begin to stream outward from the metal sphere.

Another way to describe that growing repulsion between like charges involves fields. An electric charge is surrounded by a structure in space known as an electric field. An electric field exerts forces on electric charges, so one electric charge pushes on other electric charges by way of its electric field.

As more and more like charges accumulate on the sphere, their electric fields overlap and add so that the overall electric field around the sphere becomes stronger and stronger. The charges on the sphere feel that electric field, but they are bound to the metal sphere by chemical forces and it takes energy to pluck one of them away from the metal.

Eventually, the electric field becomes so strong that it can provide the energy needed to detach a charge from the metal surface. The work done by the field as it pushes the charge away from sphere supplies the necessary energy and the charge leaves the sphere and heads out into the vacuum. The actually detachment process involves a quantum physics phenomenon known as tunneling, but that’s another story.

The amount of charge the sphere can store before field emission begins depends on the radius of the sphere and on whether the charge is positive or negative. The smaller that radius, the faster the electric field increases and the sooner field emission starts. It’s also easier to field-emit negative charges (as electrons) than it is to field-emit positive charges (as ions), so a given sphere will be able to hold more positive charge than negative charge.

## How do electronic touch pads and switches work?

#### The new soft drink dispenser at a nearby store has touch pads that release soda as long as you are pressing on them. I noticed that if I press a pad with something other than my fingers (like a straw or car key) nothing happens, no matter how hard I press. Yet with my fingers, I sometimes don’t even have to make actual contact — just very close proximity. What is happening here? — RLB

Those touch pads are sensing your presence electronically, not mechanically. More specifically, electric charge on the pad pushes or pulls on electric charge on your finger and the pad’s electronics can tell that you are there by how charge on the pad reacts to charge on your finger.

Because your finger and your body conduct electricity, the pad’s electric charge is actually interacting with the electric charge on your entire body. In contrast, a straw is insulating, so the pad can only interact with charge at its tip, and while your car keys are conducting, they are too small to have the effect that your body has on that pad.

The second way for the pad’s electronics to sense your presence is to let your body act as an antenna for electromagnetic influences in the environment. We are awash in electric and magnetic fields of all sorts and the electric charge on your body is in ceaseless motion as a result. You’ve probably noticed that touching certain input wires of a stereo amplifier produces lots of noise in the speakers; that’s partly a result of the electromagnetic noise in our environment showing up as moving charge on your body. The little pad on the soda dispenser picks up a little of this electromagnetic noise all by itself. When you approach or touch the pad, however, you dramatically increase the amount of electromagnetic noise in the pad. The pad’s electronics easily detect that new noise.

In short, soda dispenser pads are really detecting large electrically conducting objects. Their ability to sense your finger even before it makes contact is important because they need to work when people are wearing gloves. I first encountered electrical touch sensors in elevators when I was a child and I loved to experiment with them. Conveniently, they’d light up when they detected something and there was no need to clean up spilled soda. We’d try triggering them with elbows and noses, and a whole variety of inanimate objects. They were already pretty good, but modern electronics has made touch pads even better. The touch switches used by some lamps and other appliances function in essentially the same way.

## If a bird lands on a high-voltage wire, will it be injured?

#### A bird lands on an uninsulated 10,000 volt power line. Will it become extra crispy? — RKS, Texas

No. Birds do this all the time. What protects the bird is the fact that it doesn’t complete a circuit. It touches only one wire and nothing else. Although there is a substantial charge on the power line and some of that charge flows onto the bird when it lands, the charge movement is self-limiting. Once the bird has enough charge on it to have the same voltage as the power line, charge stops flowing. And even though the power line’s voltage rises and falls 60 times a second (or 50 times a second in some parts of the world), the overall charge movement at 10,000 volts just isn’t enough to bother the bird much. At 100,000 volts or more, the charge movement is uncomfortable enough to keep birds away, so you don’t see them landing on the extremely high-voltage transmission lines that travel across vast stretches of countryside.

The story wouldn’t be the same if the bird made the mistake of spanning the gap from one wire to another. In that case, current could flow through the bird from one wire to the other and the bird would run the serious risk of becoming a flashbulb. Squirrels occasionally do this trick when they accidentally bridge a pair of wires. Some of the unexpected power flickers that occur in places where the power lines run overhead are caused by squirrels and occasionally birds vaporizing when they let current flow between power lines.

## Why do I sometimes shock myself when I kiss Uncle Al?

#### Why do I sometimes shock myself when I kiss Uncle Al? — BS

If both of you were electrically neutral before the kiss, nothing would happen. Evidently, one of you has developed a net charge and that charge is suddenly spreading itself out onto the other person during the kiss. That charge flow is an electric current and you feel currents flowing through your body as a shock.

Most likely, one of you has been in contact with a insulating surface that has exchanged charge with you. For example, if you walked across wool carpeting in rubber-soled shoes, that carpeting has probably transferred some of its electrons to your shoes and your shoes have then spread those electrons out onto you. Rubber binds electrons more tightly than wool and so your shoes tend to steal a few of electrons from wool whenever it gets a chance. If you walk around a bit or scuff your feet, you’ll typically end up with quite a large number of stolen electrons on your body. When you then go and kiss Uncle Al, about half of those electrons spread suddenly onto him and that current flow is shocking!