Physics 21: 6-15 Applications of Induction and LRC Circuits
Text Reading: Giancoli, Physics - Principles with Applications, Chapter 21: 6-15
- 21.6 Lenz's law predicts that induced EMF will acti to oppose motion, so when a motor turns, it creates a counter emf or "back emf", which depends on the speed of the motor. This prevents motors from having infinite capacity.
- 21.7 Since the voltage difference generated in a wire wrapped around a current-carrying coil depends on the number of coils and the magnetic flux through the coil, coil-and-loop systems can be used to step-up (increase) or step-down (decrease) voltage. The relationship of primary (incoming) to secondary (output) voltage depends only on the number of coils in each system: VS/VP = NS/NP.
- 21.8 Data can be stored using magnetised materials and semiconductors which are charged/polarized in one direction (ON or "1") or uncharged and unpolarized (OFF or "0"). Sensors read the material as data or instructions to carry out computer processes.
- 21.9 Induced current varied by sensitive and rapidly moving magnets are used in microphones, magnetic strip readers, and seismographs. Solenoids are also used as fast circuit breakers.
- 21.10 Mutual inductance occurs when two current-carrying wires are near each other. Changing current in a coil will also induce a back emf in the coil itself (self-inductance).
- 21.11 As with gravity and electrical fields, energy can be stored in a magnetic field: U = 1/2 * LI2.
- 21.12 A circuit with both inductance and resistance is an LR circuit, which follows exponential laws in its behavior.
- 21.13 Alternating current driving an LRC circuit follows specific rules as well:
- Resistance varies as voltage (it will be in phase with voltage changes).
- Current will lag voltage by 90° when an inductor is in the circuit.
- Current will lead voltage by 90° when a capacitor is in the circuit.
- 21.14 Analysis of LRC circuits often involves phasor diagrams, in which voltage drops across each element (Vcapacitor, Vresistor, Vinductor) are drawn as independent vectors that can be added using vector math.
- 21.15 The frequency of the voltage source in an AC circuit will create resonance as a function of the inductance and capacitance in the circuit. Resonance allows TV and radio systems to "tune in" particular frequencies.
- The Generator Equation:
- Transformer Voltages:
- Inductance (emf in coil #2 from changing current in coil #1)
- Energy stored in a magnetic field:
- LR Circuit current
- Inductive reactance
- Capacitive reactance
- Average power in LRC circuit:
- Resonance in AC circuits
Read the following weblecture before chat: Generators and Transformers
Use the electric motor simulator
==>HERE<== to explore how electrical current can be transformed to mechanical rotational motion.
- Draw a diagram of the circuit (you may have to scroll to see all of it).
- Complete the connections to the motor.
- Switch on the curcuit and increase the voltage. How does voltage and current change through the resistor?
- As the motor rotates, what happens to the direction of the magnetic field? What happens to the direction of the current?
- What happens if you revers the connections to the motor?
- How are the directions of magnetic force (Laplace force), current, and magnetic field related?
Chat Preparation Activities
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- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
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