Transcript Lecture #25 - UCF Physics

Lecture #25
What’s in the wall??
Last Time
We discussed how charge can be studied.
 There are two kinds of charge

– NEGATIVE
 electrons, light, mobile and easy to push around
– POSITIVE
 protons, located in the nucleus of atome.
 “heavy”, tough and not to be messed with.

Charges interact with each other.
Results of our Study
Charges can be created by mechanical
rubbing or separation of bound surfaces.
 Charge cannot be created or destroyed

– It is there before we rub
– All we mortals can do is move it around
Like charges repel each other
 Un-Like charges attract each other.

So…
insulator
what about this balloon
trick?
insulator
A Van de Graaff generator
What about our Volunteer??
This thing also has the capacity to
store charge
+
+
+
+
+
+
+
-
This is called
a capacitor
Charging a Capacitor takes WORK
The work per unit charge is called the Potential Difference
+
+
+
+
+
+
+
-
+
+
+
+
+
+
+
-
Capacitor
separated charge
POTENTIAL DIFFERENCE
Units are VOLTS
 It takes one Joule of energy (work) to
move one coulomb through a potential
difference of one volt.
 We usually refer to potential differences as
“the voltage”

Another Definition
If a capacitor has a potential difference V
from one terminal (plate) to the other
 And if the capacitor has a charge Q on the
plate,
 Then

Q
C
V

where C is measured in Farads.
Example

A 0.001 F capacitor has 5 volts across it. How
much charge is on the capacitor?
Q
C
V
so
Q  CV  0.001F  5volts  0.005Coul.
Big Cap Demo
What’s Happening?
+
+
+
+
+
+
+
-
Huh?
The electrons were “pushed” by the
difference in potential (voltage).
 They came around to the (+) plate and
neutralized the positive plate.
 The two plates and the wires are said to
form an electrical circuit.
 The movement of charge is called
electrical current.

Symbols & Definitions
“Stand” at one point in a circuit where electrical
current is flowing.
 Count the amount of charge in coulombs that
pass where you are standing in one second.
 This amount of charge, divided by the time of
passage is defined as the CURRENT.
 The current is measured in AMPERES and we
use the symbol I.
 One ampere is a current of one coulomb per
second.

The Battery
The battery behaves in a way like a
capacitor.
 As charge is withdrawn from one side of
the battery the battery replaces it via a
chemical reaction in the battery cell.
 The battery therefore can maintain a
constant potential difference or voltage
between its terminals.

Electricity made a
giant leap in 1800
when Volta
invented the
electric cell and
the battery (of
cells).
For the first time
investigators had a
source of steady
electric current.
Alessandro Volta, (1745-1827)
Pretty
Complicated
Lots of sources on
the internet to find
out how these things
really work.
The Commercial Battery
Something that we ALL Know!
The Circuit – A Thunker
I1
A
I2
Current must be the same!
V
A
I
Define Resistance R
V
R
(Unit  Ohm  )
I
V  IR
Ohm Sweet Ohm
The more resistance, the less the current.
 Like stepping on a garden hose.
 If you stepped in two spots on a garden
hose, the flow would even be less.
 The “resistance” would double.


So let’s look at a few bulbs.
Example
6 volts
10 ohm
V  IR
V 6
I    0.6 A
R 10
Add a Bulb
B
6V
C
10
10
Total = 20
I=V/R=6/20=0.3 amperes … ½ the previous current
Parallel Circuit