Transcript Chapter 30 - Mr. Tanaka`s Website

Electrifying!
Ken McAuliffe
Chapter 30
Summary
Ken McAuliffe
• Explains the complexities of wiring a large building for
electricity.
• Describes how electrical blackouts happen and explains how
the Museum of Science is equipped with a battery and a backup generator to provide uninterrupted power to the server
room and the exhibit hall lights in the event of power loss.
• Explains the difference between AC and DC, and that current
from the uninterrupted power source battery must be
converted from DC to AC.
• Explains why many of the lights in the museum are wired in
parallel instead of in series.
• Explores some important safety precautions that anyone
working with electricity must take, involving circuit breakers,
ground wires, and short circuits.
Alternating Current (AC) &
Direct Current (DC)
Alternating current, or AC, Is the type of current produced
by power plants.
• It moves in one direction and then the opposite direction
repeatedly.
• Many electrical appliances that plug into a wall socket
use AC.
Direct current, or DC, only flows in one direction.
• Batteries provide DC.
• Most electronic devices uses DC.
• AC often needs to be converted to DC, and vice versa.
A series and parallel circuits
A series circuit is a circuit in which resistors or loads are
arranged one right after the other in a chain.
• Total resistance (𝑅𝑇 ) for resistors, 𝑅1 , 𝑅2 , and 𝑅3 in series
is given by the formula: 𝑅𝑇 = 𝑅1 + 𝑅2 + 𝑅3
A parallel circuit is a circuit in which the current branches
on the way to each resistor, then comes back together
after it has passed through the resistors.
• Total resistance for resistors 𝑅1 , 𝑅2 , and 𝑅3 in parallel is
1
1
1
1
given by the formula: = + +
Rt
R1
R2
R3
Example 1
• What is the total resistance across the voltage in the circuit
below?
Example
• What is the total resistance across the voltage in the circuit
below?
Example
• What is the total resistance across the voltage in the circuit
below?
Safety precautions
• You should never work with circuits when water
is present – water can create a short circuit.
• Whenever possible, disconnect the circuit from
the power source when working on it.
• Use ground wire whenever possible.
Short circuit
• A short circuit is an
accidental lowerresistance connection
between two points on
a circuit.
• A short circuit connects
the hot wire to the
ground wire, excluding
the load from the
circuit.
• In a short circuit,
resistance drops and
current can become
dangerously high.
Question 1
Question
• In your own words, write a paragraph explaining how the
museum’s electrical system would work if an electrical
blackout happened?
Answer
• When a blackout occurs, an Uninterrupted Power Source
(UPS) system keeps the power up long enough for a generator
to start functioning and provide power to essential systems.
Question 2
Question
• What is a short circuit? Why can short circuits be dangerous?
Answer
• A short circuit is an accidental lower-resistance connection that
bypasses the load, or the resistance in a circuit? This is dangerous
because in a short circuit the current follows the path of least
resistance, such as a metal bar, another conductor, or a person’s
body. This causes resistance to drop and voltage to remain constant,
which makes the current very strong.
• In wall current, a short circuit will occur when the hot wire is
connected to the ground wire. Current and voltage are already high,
so a short circuit can be very dangerous. Although the electrical
energy in a battery is much less, accidentally connecting the positive
and negative sides of a battery with a wire will drain the battery very
quickly, and heat up the wire.
Question 3
Question
• Find the missing voltages, currents, and ohms
Answers!
Answers
• Diagram A: 3 ohms and 2 amps
• Diagram B: 6 volts and 6 ohms
• Diagram C: 4 volts and 2 amps
• Diagram D: 2 volts and 2 volts
Key points
• The voltage drop in the external circuit equals the voltage
boost from the battery.
• Current is everywhere the same.