## If you connect two direct current motors so that the current flowing through one…

#### If you connect two direct current motors so that the current flowing through one also flows through the other, then turning one motor will cause the other motor to turn as well. If you reverse the direction of rotation, the other motor will also reverse its direction of rotation. Why does this happen?

DC motors turn in a direction that depends on the direction of that current. If you reverse the direction of current flowing through the motor, its direction reverses, too. When you use one DC motors as a generator, it produces DC current! The direction of that current depends on which way you turn the motor. Thus as you turn the first motor clockwise, it generates current in a particular direction through the circuit connecting the two motors and the second motor also turns clockwise. If you then reverse the first motor, the current in the circuit reverses and so does the second motor.

## What happens to the current when it “stops”?

#### What happens to the current when it “stops”?

Current refers to moving charged particles. In most solids, the particles that do the moving are negatively charged electrons that move in the opposite direction from the way we say that current is flowing. These charged particles are the components of atoms and molecules, so they are always there inside a wire or the filament of a light bulb, even if they are not moving. Thus when the current “stops”, these electrically charged particles simply stop moving. You can imagine a pipe full of water. The water can be flowing to the right or left (a current) or it can be standing still (no current). The water itself, like the charged particles, doesn’t disappear when the flow stops.

## When flashbulbs were used with cameras, was there a coil in the camera and a mag…

#### When flashbulbs were used with cameras, was there a coil in the camera and a magnet, or how did they get it to light? Also, how are flashes used on cameras today different than flashbulbs?

Flashbulbs contain a wad of very fine magnesium wire that burns almost instantly in a gas of pure oxygen. The wire is ignited by a small piece of gunpowder-like primer material that is itself ignited by the camera. There are/were three techniques for igniting the primer: impact (a little lever smacked the side of a tube containing the primer and it burst into flame, just like a cap), electric current (a thin filament inside the bulb overheated when current ran through it), and spark (a spark jumped between two wires and ignited the primer). A camera that uses/used the current-ignited bulbs has a battery in it and taking a picture closes a circuit that then sends current through the bulb. A camera that uses/used the spark-ignited bulbs used a piezoelectric spark igniter, like the ones in outdoor gas grills. A camera that uses/used the impact-ignited bulbs just hit the primer itself. Modern cameras uses gas discharges to produce light. Since the flashlamp isn’t burned up during a flash, it can be used many times.

## Why does a moving magnet excite charges?

#### Why does a moving magnet excite charges?

A moving magnet, which carries with it a magnetic field, creates an electric field. That’s just the way our universe works. Changing magnetic fields create electric fields. Since an electric field exerts a force on any electrically charged particle, the charges in a wire are pushed around whenever a magnet moves past them.