Transcript Electric circuits

11.2 Electric Circuits:
Analogies and Characteristics
(Pages 446-453)
Water circuit and electrical circuit analogy.
Learning Goals
• I can describe how electric current moves
through circuits.
• I can explain what current, voltage and
resistance are, and how these quantities
are measured.
An electric circuit is a closed path along
which electrons powered by energy source
can flow.
In a circuit that is connected to a cell,
Electrons move very slowly from the
negative terminal to the positive terminal
in the external circuit.
The electric field in an operating circuit
transmits energy at almost the speed
of light.
Electric current is the rate of flow of electric charge in a circuit,
and it is measured in Amperes (A) using an ammeter.
Electrical resistance is a property of a substance that hinders
electric current and converts electrical energy to other forms of
energy.
Potential difference (voltage) is the difference between the
electric potential energy per unit of charge at two points in a
circuit, and it is measured in Volts (V) using a voltmeter.
Electric circuits:
What is an electric circuit?
An electric circuit is a closed path along which electrons that are
powered by an energy source can flow.
What are the 2 types of terminals?
There is a positive terminal & a negative terminal.
Is a switch essential for a circuit to operate?
No, a switch is not essential for a circuit to operate, but it is
included for convenience.
What are the most popular connecting wires being used to make an electrical circuit?
The most popular wire is copper metal covered with an insulator
made of plastic or rubber.
Distinguish between a closed and open circuits?
An open circuit is one in which there is a gap or break in the
circuit. A closed circuit has no gaps and allows electrons to flow
from and return to an energy source.
Movement of Electrons:
Describe the movement of electrons in a circuit.
The electric field produced by the cell travels at almost the
speed of light and forces the free electrons to move, overall, in
one direction, although there may be collisions among the
electrons. The movement of the electrons is from the negative
terminal to positive terminal
Electric Circuits
• Terminal: location on a cell that must be
connected to other components to form a
circuit
• Switch: a control device that can complete
or break the circuit to which it is connected
• Open circuit: a circuit that contains a gap
or break
Electric Circuits
Electrons Flow in One Direction
in a Circuit
As each electron moves uniformly through a conductor, it pushes on
the one ahead of it, such that all the electrons move together as a
group. The starting and stopping of electron flow through the length of
a conductive path is virtually instantaneous from one end of a
conductor to the other, even though the motion of each electron may
be very slow. An approximate analogy is that of a tube filled end-toend with marbles:
If a single marble is suddenly inserted into this full tube on the lefthand side, another marble will immediately try to exit the tube on the
right. Even though each marble only traveled a short distance, the
transfer of motion through the tube is virtually instantaneous from the
left end to the right end, no matter how long the tube is. With
electricity, the overall effect from one end of a conductor to the other
happens at the speed of light: a swift 186,000 miles per second!!!
Each individual electron, though, travels through the conductor at a
much slower pace. Actually, about only 0.1 mm/s.
Electrons Flow in One Direction in a Circuit:
Describe the flow of electrons when a
conductor is not connected and when a circuit is
connected.
If a conductor is not connected, the electrons
move in random directions, frequently colliding
with other electrons or with ions.
The instant a circuit is completed, the electric
field causes the electrons to move in one
direction through the wire.
Electric Current
• Electric current: the rate of movement of
electric charge
• Unit of charge  Coulomb (C) the quantity
of charge that is equal to the charge of
6.25 × 1018 electrons
• Unit of current  Ampere (A) the unit of
electric current, equivalent to one coulomb
per second
• Measured using an ammeter
Electric Fields, Circuits and
Curents
• Separation of charges
between two electrodes
in a cell gives rise to an
electric field
• This field transmits an
electric force (and
current) through a
circuit at almost the
speed of light
• Electrons flow slowly
Conventional Current vs.
Electron Flow
• Electrical engineers
often describe the
direction of electric
current as the
direction in which a
positive charge would
move
• Either method is
acceptable, as long
as it is used
consistently
Electrical Resistance
• Electrical resistance: the property of a
substance that hinders electric current and
converts electrical energy to other forms of
energy
• Resistor: a device used in an electric
circuit to decrease the current through a
component by a specific amount
• Measured using an ohmmeter
Electrical Resistance
A load is a resistor or any other device that transforms electrical
energy into heat, sound, motion or light. Examples of loads are
resistors, motors, speakers & light bulbs.
Potential Difference:
Compare and contrast a water tap in your home and an electric switch.
A tap acts to allow or prevent water
from flowing. It acts much like an
electric switch, which allows or
prevents electricity from flowing.
A tap can also change the amount or
flow of water flowing. Most electric switches
cannot do this, except for dimmer switches.
Water circuit and electrical circuit analogy.
Potential Difference and the Volt
• Potential difference (voltage): the
difference between the electric potential
energy per unit of charge at two points in a
circuit
• Volt: the unit for potential difference,
equivalent to one joule (J) per coulomb (C)
• Measured using a voltmeter
Potential Difference and the Volt
Electrical Cells and Voltage
Practice
• Page 454 # 1-8