Electricity for All

Honors Physics spent time working with Ohm’s Law in the laboratory. The rate of charge motion depends on the degree of potential difference to which it is exposed and the amount of opposition it faces moving through materials (resistance). For some materials, there is a constant, proportional relationship between voltage and current and these substances are said to be “ohmic.” Your resistors are constructed of ohmic materials and you produced clearly linear relationships for the entire range of tested voltages. The slopes of the lines were close matches to the stated resistances. For the light bulb, however, the ratio of voltage to current changed over the range of tested voltages. Light bulb filaments are “non-ohmic,” although they do follow Ohm’s Law quite well at higher voltages. Your LED’s are examples of diodes, which only allow current to flow in one direction. So, the LED lit when inserted one way in the circuit and didn’t when the polarity was reversed. Your motors allowed charge to move through regardless of how it was inserted into the circuit, but the direction the shaft turned changed when the motor was reversed. Tomorrow, we’ll talk more about the lab (including taking care of lab equipment/supplies) and continue on discussing potential difference and electric potential energy.

Physical Science B went over their Ohm’s Law lab and found that their resistors definitely followed Ohm’s Law. Very nice, straight lines were produced when voltage versus current was plotted by the computer, and the slopes of these lines were quite close to the values of the resistors you tested. The light bulb did not have so clear a linear relationship. The first few points on the graph had a markedly different slope than later points. Your bulb filament was non-ohmic., but Ohm’s Law could be used to predict the electrical properties at higher voltages. Then, students had the opportunity to practice the calculations associated with this chapter. We’ll go over the worksheet tomorrow and then turn attention to constructing and analyzing electrical circuits.

Physical Science E discussed electric power and electric safety. Electric power is not conceptually different than mechanical power – it is the rate at which the charges’ energy is used to do work. The unit for electric power is still the watt (W) and a device with more electrical power than another still doesn’t do more work than another; it simply does the work in a shorter amount of time. We then discussed the use of fuses, circuit breakers and ground-fault interrupters for electric safety. Tomorrow, math practice with circuits, Ohm’s law and electric power.

Physics F and G discussed the ability of charges to generate electric fields and forces. The use of Coulomb’s law to calculate the magnitude of electric force between two charges was examined and the use of field lines to represent electric fields was described. Tomorrow, G Block will conduct an investigation that will propel you towards our study of electric circuits by laying out one of the top gun relationships in electricity – Ohm’s Law.

Homework

Honors Physics – Complete lab write up
Physical Science B: Complete electricity worksheet
Physical Science E: None
Physics F: Complete Chapter 17 Chapter Review items #11-17, 19
Physics G: Read lab protocol sheet and complete Chapter Review #11-17, 19 for Monday