Wednesday, January 19, 2011

Hello everybody! Today's class consisted of answering the three worksheets we received the previous day, so I'll go ahead and post up the answers!


Transparency 25-2 Worksheet
" AC Generator"

1. As a wire moves through position 1, how is it moved with respect to the magnetic field?
- The wire is moving perpendicular to the field.

2. How much current is carried through the wires as it moves through position 1?
- Max amount of current, Imax.

3. As the wire reaches position 2, how is it moving with respect to the magnetic field?
- The wire is moving parallel to the field.

4. How is the change is the orientation of the wire in the magnetic field related to the amount of current induced in the wire?
- Only the component of motion that is perpendicular to the field induces a current.
Position 1 & 3 - Max I (current)
Position 2 & 4 - zilch

5. How much current is carried through the wire as it moves through position 3?
- Imax.


Transparency 25-3 Worksheet
"Lenz's Law"

1. What is the direction of the current induced in the wire as a result of moving through the magnetic field? What rule is used to determine this direction?
- Induced current ia downwards, using the 3rd right hand rule.

2. How does the direction of the force exerted by the magnetic field on the wire related to the original motion of the wire?
- Force exerted by B opposes original motion of wire.

3. What is Lenz's law?
- It states that the direction of the induced current is such that the magnetic field resulting from the induced current opposes the change in the field that caused the induced current.

4. If the generator produces a small current, will the armature be easy or hard to turn? Why?
- Easy to turn, the opposing force on the armature is small.

5. Why does placing a mechanical load on a motor cause more current to be carried through the motor?
- The rotation of the motor slows. The slowing decreases back EMF and allows more current to be carried through the motor.


Transparency 25-4 Worksheet
"Step-Up and Step-Down Transformers"

1. In the step-up transformer, the primary coil has two turns, and the secondary coil has four turns. By what factor is the voltage in the primary circuit stepped up by the transformer?
- Vp/Vs = Np/Ns = 2/4 = 1/2
2Vp/Vs = 1/2
Doubles Vp

2. If the voltage across the primary circuit of the step-up transformer were 120 V, what would be the voltage across the secondary circuit?
- Vs = Vp.Ns/Np = (120 V)(4 turns)/(2 turns
Vs = 240 V

3. If the voltage across the primary circuit of the step-down transformer were 350 V, what would be the voltage across the secondary circuit?
- By not knowing the number of turns on the primary or secondary, you cannot calculate Vs.

4. If the current in the primary circuit of the step-down transformer were 40 A, what would be the current in the secondary circuit?
- Increase I (current) on the secondary circuit, but can't calculate accurately due to not knowing both voltages.

5. If the voltage in a transformer is stepped up, the current has been stepped down. Ohm's law indicates that increase voltage typically produces increased current. Explain this seeming contradiction.
- Ohm's law holds true in a single circuit. In this instances, the current being compared are actually in two different circuits.


Chapter 37: Electromagnetic Induction
"Transformers"

1. What will be the voltage output of the secondary?
- 1200 V

Vs = Vp.Ns/Np

2. What current flows in the secondary circuit?
- 1.2 A

Is = Vs/R

3. Now that you know the voltage and the current, what is the power in the secondary coil?
- 1440 W

Ps = Is.Vs

4. Neglecting small heating losses, and knowing that energy is conserved, what is the power in the primary coil?
- 1440 w

5. Now that you know the power and the voltage across the primary coil, what is the current drawn by the primary coil?
- 12 A

Ip = Pp/Vp

6. (up)(down)
7. (more)(less)
8. (voltage)(energy and power)
9. (step-down)(20/1)
10. (ac)(continually change)

Finally, filling out the last of our chapter 25 Study Guide!

Self-Inductance

Current generates a(n) magnet field, creating new field lines, which cut through the wires. As a result, and EMF is generated and it opposes the current change. The faster you try to change the current, the larger the opposing EMF, and the slower the current change. Because of this self-inductance, work has to be done to increase the current that flows, and the energy is stored in the magnetic field.

Transformers

A transformer is used to increase or decrease AC voltage without a loss of energy. A transformer contains two coils that are insulated electrically from each other, but that share the same core. When the primary coil is connected to a source of AC voltage, the changing current creates a varying magnet field. A varying current is induced in the secondary coil, by the process of mutual inductance. The ratio of primary and secondary voltages depends on the ratio of the number of turns in the two coils. I a step-up transformer, the primary voltage is less than the secondary voltage, and the current in the primary circuit is greater than the current in the secondary. In a step-down transformer, the primary voltage is the secondary voltage and the current in the primary circuit isgreater than less than the current in the secondary circuit.

Now after all that we received 3 more pages!

Chapter 26 Review
Transparency 22-4 "Electric Power Transmission"

And last but not least THE EXAM REVIEW ANSWER KEY!

Don't forget to pick those up and FAST! Especially the Answer Key. I'll upload that as soon as I can. (Too lazy to change computers.)

Less than, but about, 2 more weeks until exams! Studystudystudy, and an expected test (our last!) on Tuesday. Good luck everyone!