Transcript Lecture30_BatteriesBulbs

Luigi Galvani A charge applied to the spinal cord
of a frog could generate muscular
(1737-1798)
spasms throughout its body.
While cutting a frog leg, Galvani's
steel scalpel touched a brass hook
that was holding the leg in place.
The leg twitched.
Mary Shelley (1797-1851)
wrote “Frankenstein”(1816)
Alessandro Giuseppe
Antonio Anastasio Volta
(1745-1827)
In 1799 Volta made his 'Voltaic Cell'
by alternating disks of copper and zinc
separated by salt-water moistened pads.
Zn  Zn
2
MORE NEGATIVE
MORE POSITIVE
Electromotive series
Carbon
Gold
Silver
Copper
Tin
Lead
Iron
Zinc
 2e

The zinc electrode
collects a positive
charge as it dissolves.
Both tend to dissolve in even a salt solution!
though often dilute sulphuric acid was used…
As deposits collect on the copper electrode
it develops a positive charge.
Cardboard
Carbon core
++
+++
bitumen
insulating
sleeve
Zinc can
washer
Manganese
dioxide
--Deteriorating zinc canister builds
negative charge at base of this dry cell.
You probably already know,
from replacing batteries in
common devices:
1.5
volts
3.0
volts
1.5
volts
Batteries chained
end-to-end
add their
potential
differences.
The word “battery” actually means a
set of “cells” lined up in sequence:
An automotive
accumulator is
a battery of 6
2-volt leadacid cells.
A plate coated in brown lead dioxide deposits (built
while “recharging”) builds a positive charge when
dissolved in dilute sulphuric acid. Grey lead plates
grow negative as lead sulphate coating develops.
-
Every e that leaves
the zinc electrode and
enters the “circuit”
enables another ion to
dissolve into the acid
A constant voltage
is maintained
(as long as the
Zn and Cu lasts).
So unlike a
single discharge,
this source is
continuous!
+++ ---
- --
“Conventional current”
or just “current”
+
I
e-
The amount of charge
passing any given point
per second is the current:
1 Coul
sec
Q
I
t
= 1 Ampere
Georg Simon Ohm
1789 - 1854
VI
V = IR
1 ohm () allows a current of
1 ampere to pass when a
potential difference of 1 volt
is applied across it.
Ohm’s Law: V = IR
An electric toaster element
(Nichrome) is 20.
How great is the current when
it is connected to a 120v line?
(1) 1/6 A
(2) 1 A
(3) 5 A
(4) 6 A
(5) 120 A
(6) 240 A
0.8A runs through a lamp when
attached to a 32v source.
What is its resistance?
(1) 0.025 
(2) 25.6 
(3) 32 
(4) 40 
Work is continuously being performed
within a battery (or generator)
to maintain steady current flow!
Through friction /atomic collisions
with the crystal lattice of the resistor
the supplied energy is converted to heat.
(if hot enough may even produce light!)
How rapidly is work being done
in an electrical circuit?
POWER =
WORK
TIME
What electrical unit is most closely
associated with work (energy)?
What electrical unit involves time?
What is the unit of POWER?
If an electric iron
draws 5 Amperes
from a 120v line,
at what rate is
it using power?
(1) 6 Watts
(3) 120 Watts
(5) 600 Watts
(2) 12 Watts
(4) 240 Watts
(6) 720 Watts
Which has a greater resistance, a 1
Watt lightbulb or a 25 Watt lightbulb
1) the 25 W bulb
2) the 100 W bulb
3) the same for both
Which carries the greater current?
1) the 25 W bulb
2) the 100 W bulb
3) the same for both
A 1440 W microwave oven is
designed to operate using a
120 V power source. What is
the resistance of this appliance?
1) 0.1 
2) 10 
3) 72 
4) need to know the current
5) none of these
Current
Effect
0.001 Ampere
0.005 Ampere
mild shock
painful shock
<0.010 Ampere
0.010 Ampere
0.020 Ampere
0.030 Ampere
0.100 Ampere
0.200 Ampere
>0.200 Ampere
1.000 Ampere
Fatal?
no
no
tingles
no
paralysis of motor muscles
no
threshold of pain (cannot let go!)
no
breathing becomes labored
no
death due to fibrillation
seconds
no fibrillation, but severe seconds
burns and breathing stops
heart stops
seconds
serious burns
instantly
Galvani was correct in so far as our
central nervous system is in fact
a complex electrical circuit:
Neurons fire electrical pulses along synapses.
Muscle fibers contract in response to these signals.
QUESTION 1
QUESTION 2
(4) 6 A
(4) 40 
32volts / 0.8 Amp
QUESTION 3
(5) 600 Watts
QUESTION 4
Since P = V2/R, P  1/R.
P = IV
Greater resistance: (1) the 25 W bulb
QUESTION 5
The greater current: (2) 100 W bulb
The bulb with the smallest resistance must
draw the greater current!
Also: since P = IV, P  I.
QUESTION 6
(2) 10 
(120V )2 / 1440W  10