Electric Current: Movement of Charges
Electricity is any effect resulting from the presence and/or movement of electrical charges. An electric current is the flow of electric charges. But the reason that an electric current makes a lamp come on instantly when you flip a switch is not because electrons race through the wire to the lamp. Instead, an electrical impulse, passed from electron to electron, moves through the wire to the lamp.
In this project, you will demonstrate the way an electrical impulse moves through a wire. You will determine how voltage affects an electric current. You will also investigate how to test the electrical conductivity or measure of the ability of a material to conduct an electric charge and the effect that the material's resistance has on an electric current passing through it.
Purpose To demonstrate how an electrical impulse moves through a wire.
1/4 × 16-inch (.31 × 40-cm) dowel
four 11/8-inch (2.8-cm) ceramic disk magnets, each with a hole through the center
- Stack the books on a table so that you have two equal-size piles that are each at least 11/2 inches (3.75 cm) high.
- Separate the stacks of books so that they are 12 inches (30 cm) apart. Lay the 12-inch ruler on the table between them.
- Stick the dowel through the hole in one of the magnets.
- Stick the dowel through the hole of a second magnet and push the two magnets on the dowel together. If the magnets cling together, remove the second magnet, turn it around, and put it back on the dowel. (1be magnets are to push away from each other.)
- Repeat step 4 twice, placing the remaining two magnets on the dowel.
- Support the ends of the dowel on the edges of the books so that the magnets hang above the ruler. Place a book over the ends of the dowel to secure the dowel.
- Push magnet D (see Figure 16.1) toward the zero end (the left side) of the ruler, which will force the rest of the magnets to move, until magnet A rests against the books and is above the zero end of the ruler. Slowly move your hand away from magnet D. Record the starting position of each of the magnets in a Magnet Movement Data table like Table 16.1. Magnet A's initial position is 0 inches (cm).
- Push magnet A slowly forward (to the right) 1 inch (2.5 cm).
- While holding magnet A at its final position of 1 inch (2.5 cm), wait until all the magnets have finished moving, then measure and record the position of each of the magnets.
- Determine how far each magnet moved by calculating the difference between the starting and the final position of each.
Magnets B, C, and D appeared to move the instant that magnet A moved. Magnets Band C moved about the same distance as magnet A, which was 1 inch (2.5 cm). Magnet D moved farther than the others.
In this experiment the electrons in some solids, particularly metals, are attracted relatively equally to all nearby atoms and are not tightly bound to a single site. These electrons are relatively free to move through the solid, so they are called free electrons. The motion of free electrons results in the transfer of energy from one electron to the next. This transfer of energy is an electrical impulse caused by the repulsive force between negatively charged electrons. When magnet A was pushed forward, magnets Band C moved about the same distance because they each had about the same force pushing from their front and back. Thus they had the same net force acting in a forward direction. The magnets represent free electrons in a metal wire that is part of an electric circuit (the path that electric charges follow). An electric circuit is made of material called an electrical conductor or conductor (material with a large concentration of free electrons). If an electric circuit forms a loop so that the free electrons move in a continuous unbroken path, it is called a closed circuit. If there is a break in the materials forming the circuit so no current can flow, it is called an open circuit. There is no movement of charged particles in an open circuit. This experiment represents only a section of a closed circuit. Magnet D does not have a magnet in front of (i.e., to the right of) it, so it does not simulate the movement of an electron in a closed electric circuit. It moves farther than the rest of the magnets because it does not run into an opposing force, as do the others. (For more on the distance magnet D travels, see ''Try New Approaches" in this chapter.)
Electricity is any effect resulting from the presence and/or movement of electrical charges. Current electricity is the result of moving electric charges. The flow of electric charges through a conductor is called an electric current or current. Energy associated with electricity is called electrical energy.
The electrical energy that causes an electric current to move can be compared to the stored potential energy of two opposing magnets in this experiment, such as magnets A and B. The closer the magnets are, the harder it is to push them together; thus as they move closer together, their potential energy increases. In like manner, the potential energy of two electrons increases when the two charges move closer together. The electrical energy needed to move a charge from one point to another in an electric circuit is called potential difference (difference in electric potential energy between two points).
In current electricity, the motion of the charges is very slow in comparison to the electrical impulse. Free electrons in a metal wire can wander from atom to atom through the metal. Imagine a single row of electrons in a wire. When electron A moves forward, electron B in front of it is pushed forward by the repulsive electrical force of their like charges. Electron C in front of electron B is then pushed forward by the repulsive electrical force, and so on, like the movement of the magnets in this experiment. While the individual electrons move at a speed of about 0.0004 inch (0.001 cm) per second, which is actually a great distance for such a small particle, the electrical impulse that they pass along moves almost as fast as light—186,000 miles (300,000 km) per second. In this experiment, while each magnet moves only a small distance, the magnet moves forward almost instantaneously, representing each transfer of electrical energy through a row of electrons (represented by magnets) by an electrical impulse.