Polarity

Myth:

Positive voltage is the opposite of negative voltage.

Two types of electricity?

Perhaps the most common misconception in electronics is that positive voltage is the opposite of negative voltage. Before Benjamin Franklin, it was thought that there were two types of electricity with opposite characteristics. Franklin’s most important contribution to the science of electricity was the discovery that there was only one type of electricity. Different pressures actually caused the apparent opposite characteristics. Let’s take a look at an example of this by simply taking a breath.

Negative pressure is not the opposite of positive pressure

When you inhale your diaphragm expands your lungs. There is now more space inside your lungs, so the air expands to fill that space. Now the air molecules are farther apart and thus exerting less repulsive force on each other. This causes a drop in pressure. The air outside your lungs, having the higher pressure naturally follows your windpipe to the lower pressure inside your lungs. Now exhale. Your diaphragm now squeezes your lungs creating a greater pressure inside the lungs, so the higher pressure air inside your lungs follows the windpipe to the lower pressure outside.

When you inhale you create a negative pressure inside your lungs. This just means that the pressure in your lungs is lower than the outside pressure. When you exhale you create a positive pressure inside your lungs. This means that the pressure in your lungs is higher than the outside pressure. Do you create a different type of air in your lungs when you inhale and exhale? No. All you are doing is changing the pressure.

Negative voltage is not the opposite of positive voltage

Positive and negative electricity work the same way. If you have a voltage that is higher than some other reference voltage we call that a positive voltage. If you have a voltage that is lower than some other reference voltage we call that a negative voltage. Electricity will try to flow from a positive voltage to a negative voltage.¹  If you look at a battery, you will see one side labeled positive (+) and the other negative (-). All that is telling you is that the higher voltage is at the positive end and the lower voltage is at the negative end. What do you think will happen if you connect a wire from the positive end to the negative end of the battery? Electricity will flow out of the positive end through the wire to the negative end (actually, don’t do that. Shorting a battery in such a way can cause some unpleasant things to happen).² Positive and negative electricity are not opposites, just higher and lower voltages.

Voltage and altitude are both potential energy

Voltage is potential energy and is often called electrical potential. Height or altitude is also potential energy. Here's an example. Hold a one-kilogram weight one meter above the floor. You could drop the weight and it would fall. When you do, it will hit the floor with 9.8 Joules of kinetic energy (kinetic energy is the energy of motion). However, when you are holding it, it only has the potential to fall and gain kinetic energy. Its height and mass constitute potential energy. Voltage has the potential to move electrons, but the electrons will move only if there is a circuit through which the electrons can flow.

Negative altitude is not the opposite of positive altitude

Since height (or altitude) and voltage are both potential energy, they can be expressed in similar ways. Altitude, like voltage, can have both positive and negative values. For example, you can go to the beach in San Diego, California, USA. At the beach, you are at an altitude of zero feet. Could you go lower? Sure, if you have a submarine. From the beach, you can drive for about 45 minutes to the east to Mt. Laguna. That is about 6,000 feet above sea level or an altitude of +6,000 feet. From Mt. Laguna you can see the Salton Sea. On the way there you will reach a point where you are again at sea level or zero feet in altitude. Drive eastward and you will go still lower. When you reach the Salton Sea, your GPS receiver will display an altitude of about -230 feet. Are you upside down? If you throw a rock will it fall up? No. Negative altitude is not the opposite of positive altitude. It's just lower than the altitude that the world has agreed to call zero, which is sea level.