Download Chp. 22 Outline Induction - Redlands High School

Chp. 22 Induction and AC
Lecture Questions:
1) When will a current cause a magnetic field? Under what conditions will a magnetic field cause a
current? What variables affect the amount of current that is induced in a loop of wire? Write
an equation which reflects these relationships (Faraday’s Law).
2) What is the purpose of a generator? How is a generator similar to a motor? What is the
direction of the loop vs. the magnetic field that produces the maximum voltage. How about
the orientation which produces the minimum voltage?
3) Distinguish the generator from the motor effect.
4) State Lenz law. What conservation law is this an extension? What does the magnitude of the
back voltage and current depend upon?
5) A bar magnet sits to the left of a coil of wire that is hooked up to an ammeter. The north pole of
the magnet is closest to the coil. State which direction the induced current at the top of the coil
when the following changes occur: a) magnet is moved to the right b) magnet is moved to the
left c) coil is moved away from the magnet d) coil is moved towards the magnet e) there is no
motion between the magnet and coil.
S
N
A
6) Two circuits a separate from one another. The first has a DC source connected in series to a
switch and a coil of wire. The second has a coil of wire connected to an ammeter. Will a
current be induced in the second circuit when the following actions occur: a) the switch is first
closed b) the switch is kept closed c) the switch is opened d) The switch is kept open?
7) What does a transformer do? What quantities does a transformer change? What quantity does
a transformer not change? Distinguish between a step-up and step-down transformer in terms
of the number of turns on the primary and secondary and whether voltage or current are
stepped up or down. How is Ohm’s law used for a transformer?
8) Why was AC picked over DC to transfer electrical energy? Why are high voltage and small
currents picked over low voltage and high currents to deliver power?
9) Define rms voltage and current. How are these values related to the maximum values for
current and voltage in an AC circuit?
10) What causes a changing magnetic field? What causes a changing electric field? At what speed
will electric and magnetic fields regenerate one another? How is this connected to light?
Practice Problems:
1) A single circular loop with a radius of 22 cm is placed in a uniform external magnetic field with a
strength of 0.50 T so that the plane of the coil is perpendicular to the field. The coil is pulled
steadily out of the field in 0.25s. Find the average induced EMF during this interval.
2) A circular wire loop with a radius of 0.33 m is located in an external magnetic field of strength
0.35 T. The field strength changes to -0.25 T in 1.5 s. What is the average EMF induced in the
coil during this time?
3) In a model generator a 510 turn rectangular coil 0.082 m by 0.25 m rotates with an angular
frequency of 12.8 rad/s in a uniform magnetic field of 0.65 T. What is the maximum EMF
induced in the coil?
4) A square coil with an area of 0.045 m2 consists of 120 turns of wire. The coil rotates about a
vertical axis at 157 rad/s. The horizontal component of the Earth’s magnetic field is 2.0x10-5T.
Calculate the maximum emf induced in the coil.
5) What is the rms current in a light bulb that has a resistance of 25 ohms and an rms potential
difference of 120 V? What are the maximum values for current and potential difference?
6) A toaster is plugged into a source of alternating potential difference with an rms value of 110 V.
The heating element is designed to convey a current with a maximum value of 10.5 A. a) What
is the rms current in the heating element? B) What is the resistance of the heating element?
7) An AC generator has a maximum potential difference of 155 V. a) What is the rms voltage? b)
Find the rms current in the circuit when the generator is connected to a 53 ohm resistor.
8) A step down transformer providing electricity for a residential neighborhood has exactly 2680
turns on its primary and 55 turns on its secondary. If the potential difference across the primary
is 5850 V, what is the potential difference across the secondary?
9) A potential difference of 0.750 V is needed to provide a large current for arc welding. If the
potential difference across the primary of a step-down transformer is 117 V, what is the ratio of
turns on the primary to secondary required?
10) A television picture tube requires high potential difference, which in older models is provided by
a step-up transformer. The transformer has 12 turns in its primary and 2550 turns in its
secondary. If 120 V is placed across the primary, what is the output voltage?
Schedule:
Monday
Lecture: EM Induction
None
Tuesday
Lecture: Transformers
None
Wednesday
Practice Problems
None
Thursday
NTQ, HW Review, WS
Chp. 22 1-9, 10, 12,14,16-25 (omit 19
and 20), 26, 28, 30,32-37, 38, 40
Friday
Quiz
Notebook