Study Guide: Single and Parallel Circuits
Basis for the Electric Motor
If we run a current through a wire loop in a magnetic field, the interaction of these magnetic fields will exert a twisting force, or torque, on the loop causing it to rotate. However, it will only rotate until the magnetic fields are aligned. If we want the loop to continue rotating, we have to reverse the direction of the current, which will reverse the direction of the magnetic field from the loop. The loop will then rotate 180 degrees until its field is aligned in the other direction. This is the basis for the electric motor.
Demo 1: Electric Generator
- Form a wire into a loop and connect the ends to a sensitive current meter, or galvanometer.
- Rotate the wire loop within a magnetic field, and observe that an electric current is induced in the wire.
- Observe that the direction of the current reverses every half turn, producing an alternating current.
Conclusion: This is the basis for the electric generator. Note that it is not the motion of the wire that induces the current, but rather the opening and closing of the loop with respect to the direction of the field. When the loop is face-on to the field, the maximum amount of flux (force) passes through the loop. However, when the loop is turned edge-on to the field, no flux passes through the loop. It is this change in the amount of flux passing through the loop that creates the electric current.
Demo 2: Electric Current
- Form a wire into a loop and connect the ends to a sensitive current meter, or galvanometer.
- Push a bar magnet through the loop, and observe the needle in the galvanometer move, indicating an electric current.
- Stop the motion of the magnet, and observe how the current returns to zero.
Conclusion: The field from the magnet will only induce a current when it is increasing or decreasing. If we pull the magnet back out, it will again induce a current in the wire, but this time it will be in the opposite direction.
Demo 3: Converting Electricity into Light
- Add a light bulb to an electric circuit, and observe how it dissipates electrical energy in the form of light and heat.
- Add a wire loop to the circuit, move a bar magnet in and out of the loop, and feel the resistance to the motion of the magnet.
Conclusion: Although energy is never lost or gained, it can be converted into different forms. In order to move the magnet, we have to do work that is equivalent to the energy used by the light bulb.
Video: Series & Parallel Circuits (5:01)
What is Electricity?
Electric charge is a fundamental property of matter. Although we do not know exactly what it is, we are familiar with how it behaves. The electric field from a localized point charge is relatively simple. It radiates out equally in all directions, like light from a light bulb, and decreases in strength as the inverse square of the distance ($ \frac{1}{r^2} $), in accordance with Coulomb’s Law.
When we move twice as far away, the field strength decreases to one-fourth the original strength.
$$ \frac{1}{r^2} = \frac{1}{2^2} = \frac{1}{4} $$
When we move three times farther away, it decreases to one-ninth the original strength.
$$ \frac{1}{r^2} = \frac{1}{3^2} = \frac{1}{9} $$
Protons have positive charge, while electrons have negative charge. However, protons are mostly immobilized inside atomic nuclei, so the job of carrying charge from one place to another is handled by electrons. Electrons in a conducting material such as a metal are largely free to move from one atom to another along their conduction bands, which are the highest electron orbits.
A sufficient electromotive force (EMF), or voltage, produces a charge imbalance that can cause electrons to move through a conductor from a region of more negative charge to a region of more positive charge. This movement is what we recognize as an electric current.