Physics 19: 1-4 EMF, Resistors, and Circuits
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 19: Sections 1 to 4
- 19.1 EMF or electromotive force is not really a force; it is the potential difference between two terminals in a battery. Since there is always a little resistance r inside the battery, the practical actual potential difference delivered by a battery and available to do work is a little less than the emf: we calculate V = emf - Ir.
- 19.2 Resistance in series is calculated by adding up the resistances. This is because all the electrons have to go through each resistor. Resistance in parallel is calculated by adding up the inverses of the resistors: 1/Rtot = 1/R1 + 1/R2 + .... 1/Rn. This is because the electrons will try to take the path of least resistance -- but some of them will go down each path.
- 19.3 We can summarize the behavior of current in circuits using Kirchhoff's rules.
- Kirchhoff's rule #1 says the sum of all currents entering the junction must equal the sum of all currents leaving the junction. This just means that the electrons that go into the junction must come out; junctions cannot create or destroy electrons.
- Kirchhoff's rule #2 says that the sum of changes in potential around a loop must be zero. Since the loop starts and ends at the same potential, any potential change in one part of the circuit must be countered in another part of the circuit.
- We can apply the above two rules to complex circuits and create "equivalent circuits" that summarize resistance (and capacitance) in a given section of the circuit.
- 19.4 Charging a battery (a source of EMF) requires that we drive the normal flow of current in reverse, using another battery or source of current.
- Resistance in a series circuit:
- Resistance in a parallel circuit:
Read the following weblecture before chat: DC Circuits
Use the lab simulation at Glencoe's Voltage, Current, and Resistance Lab to create circuits and vary current.
- Follow the instructions to place a battery and light bulb (resistance) on the circuit and calculate the resulting current. Repeat your circuit construction and vary the batteries and light bulbs.
- How can you maximize the current in your series circuit?
- How can you minimize the current?
I'm sending you back to the Learn Physics Today! site. Please study Chapter 14 on circuits.
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