 Physics Honors/AP 1 and 2

#### Course Materials are always under revision! Weblecture content may change anytime prior to two weeks before scheduled chat session for content. Homework

# Physics 21: 6-15 Applications of Induction and LRC Circuits

## Homework

### Reading Preparation

Text Reading: Giancoli, Physics - Principles with Applications, Chapter 21: 6-15

##### Study Points
• 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.

### Key Equations

• 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

### Web Lecture

Read the following weblecture before chat: Generators and Transformers

### Study Activity

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

• Forum question: The Moodle forum for the session will assign a specific study question for you to prepare for chat. You need to read this question and post your answer before chat starts for this session.
• Mastery Exercise: The Moodle Mastery exercise for the chapter will contain sections related to our chat topic. Try to complete these before the chat starts, so that you can ask questions.

### Chapter Quiz

• 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!

### Lab Work

If you want lab credit for this course, you must complete at least 12 labs (honors course) or 18 labs (AP students). One or more lab exercises are posted for each chapter as part of the homework assignment. We will be reviewing lab work at regular intervals, so do not get behind!