A Physics Digest—Part 5: Electricity and Magnetism

Our world is filled with electrical devices—in fact, all matter is electrical in nature. Every atom contains negatively-charged electrons orbiting a positive nucleus of protons. Positives and negatives attract each other. This is what holds an atom together. So what keeps all those mutually repulsive protons squeezed into an atom’s nucleus? The strong nuclear force—which we’ll get to in a later entry.

Some materials can be charged with static electricity by rubbing them. Shuffle across a rug in winter (when the humidity is low) and touch a door handle—watch the spark. Rub a balloon on your shirt and stick it to a wall: electrical attraction.

Some materials are good conductors of electricity; e.g., metals, which also conduct heat well. Other materials are poor conductors; e.g., rubber, so we coat electrical wires with rubber, in order to be able to handle them without a shock.

When electrical charges move along a wire, we have an electrical current; which can be thought of as analogous to water flowing through a hose. Voltage corresponds to pressure in the hose, and a battery is like the pump that pushes the water.

We can have direct current (DC), in which all the electric charge flows in one direction, or alternating current (AC), in which the charge reverses direction (60 times a second for the electric power coming into our houses). Voltage can do work as it pushes current, such as when it turns an electric motor. We measure the work that is done by the power it produces—in watts.

The discovery of magnetism predated our grasp of electricity by a few millennia. The Greeks played with magnetic stones they found. Material gets magnetized when clusters of atoms line up, military style. That lineup creates directional properties, for which we assign a north and a south pole to the magnet. Where did we get that convention? The Earth’s iron core creates a magnetic field that points towards the north and south poles. A magnetized compass needle aligns itself with this field. Some birds have a built-in magnetic compass to navigate themselves via Earth’s magnetic field.

A wonderful property of matter is that electricity and magnetism are coupled. When electricity flows in a wire, it sets up a magnetic field around it. Conversely, if you move a wire through a magnetic field, it causes an electric current to flow in the wire. Better yet, if you move a wire carrying current through a magnetic field a force on the wire can be felt. This is the essence of electric motors. That force can do work rotating a motor, maybe even moving an electric car.

One more fascinating property of electricity and magnetism is that their interaction creates electromagnetic radiation. If we vibrate a wire that’s carrying an electric current back and forth, we cause the wire’s surrounding magnetic field to similarly vibrate. This interaction sends out an electromagnetic wave, which happens to travel at the speed of light. In fact, these waves are light! They can move through the vacuum of space, like the heat radiation we saw earlier.

How fast we wave the wire back and forth determines the frequency of the electromagnetic wave. At low frequencies we get radio and microwaves (the latter can cook food). At mid frequencies we get infrared, visible light, and ultraviolet radiation. At high frequencies we get X-rays and gamma rays. These are all the same kind of waves, all moving at the speed of light.

On to more light next time.