Download Current Electricity How is current produced?

Current Electricity
How is current produced?

When a high potential is connected
by a conductive material to a low
potential.
– Flow of electrons is called electric
current.
– Units: C/s = Amperes (Amps)
Low
Potential
Equal
Potential
q
I
t
High
Equal
Potential
Potential
Low
Potential
High
Potential
Electron
Pump

When the potentials are equal, the
current stops flowing
– To increase the potential of the electrons
an electron pump must convert (do work)
another form of energy into electrical
potential energy.

Electron pump examples:
–
–
–
–
Voltaic or galvanic cell (dry cell)
Several cells connected (Battery)
Photovoltaic cell (solar cell)
Generator
Two types of current

Direct Current – (D.C.) Flow of electrons in
only one direction
–

Battery
Alternating Current – (A.C.) Flow of
electrons at first in one direction and then
the other direction.
–
–
–
Generator
In your House
60 Hertz
What affects current flow?


Potential Difference – Voltage
Resistance - slowing or reduction of
the flow of current
– Resistance depends on the material,
length of material, and the cross
sectional area of the material through
which the current is moving.
L
R
A
R is resistance,
L is length,
 is resistivity,
A is the cross sectional area
Ohm’s Law

Ohm’s Law stated – The ratio of V/I
is a constant, where V is the voltage
applied across a piece of material
(such as wire) and I is the current
through the material. R is the
resistance of the piece of material.
– Units: Ohms (W) = Volt/amp
Voltage
Resistance 
Current
V
R
I
Electric Power

When there is current (I) in a circuit as
a result of a voltage (V), the electric
power (P) delivered to the circuit is:
– Power  current  voltage
P  I V
– Units: watts = amp x volt = J/s
Electric Power

Many electrical devices become hot when provided
with sufficient electric power. Toasters, irons, space
heaters, heating elements in electric stoves, and
incandescent light bulbs. In such cases it is
possible to obtain two equivalent power equations

V2
P
R
PI R
When electric energy is transferred into thermal
energy in a resistor, the increase in thermal energy
is:
2
Q  I Rt
2
– units: Kilowatt hour or Joules
Current in electric Circuits


An electric Circuit is a
closed loop.
Basic Circuits consist of
three things:
– Electron pump (Battery)
– Device that reduces
potential.
(User)
– Conducting connections
(Wires)
Direct Current Circuits:


Circuit – closed loop in
which current can flow
Basic simple circuit
– Voltage source
– Voltage uses
– conducting connections
(wire)

Diagrams for electric
circuits
– Describe in words
– use photographs/drawings
– Use symbols (schematics)
Schematic Symbols
Drawing Schematic
Circuits







Draw symbol for battery and indicate “+” and “-“
terminals
Draw wire from “+” to the voltage user and draw
its symbol.
If a point where there are two current paths,
draw connection symbol and follow one path to
the user. Return to the second path and draw to
its user and continue until paths join.
Follow path to the “-“ terminal
Check work
Connect ammeter in series
Connect voltmeter in parallel
Objectives






Describe the configuration of a working
circuit
Interpret circuit diagrams
Describe the characteristics of series
connections
Determine equivalent resistance of circuits
having two or more resistors.
Explain the cause of short circuits
Homework:
– Read Pages 531-541
– Pg 534 #1-3, Pg 537 # 6-9
Series Circuits


Only one path for current to
follow
User followed by another user
(ie. Resistor followed by a light)
Vequ  V1  V2  V3  ...
…
Series Circuit Rules


Current is the same for each user
The effective resistance is the sum of
all resistors in the series
–

Requ  R1  R2  R3  ...
The sum of the voltage drop is equal
to the total voltage drop
–
Vequ  V1  V2  V3  ...
Parallel Circuits

Two or more paths for current to
follow
Parallel Circuit Rules


Total current in the circuit is the sum
of the current in all its paths
(branches)
–
Itot  I 1 I 2 I3  ...
The equivalent resistance decreases
with more parallel resistors
–

1
1
1
1



 ...
Requ
R1
R2
R3
Voltage is the same in each path.
+
-
Objectives







Describe the configuration of a working circuit
Distinguish between series and parallel circuits
Describe the characteristics of series connections
and of parallel connections.
Interpret circuit diagrams
Determine equivalent resistance of circuits having
two or more resistors.
Explain the cause and prevention of overloading
household circuits.
Homework:
– Read Pages 531-541
– Pg 534 #1-3, Pg 537 # 6-9