Current Electricity - Red Hook Central School Dst

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Transcript Current Electricity - Red Hook Central School Dst

Current Electricity
Continuous motion of charged particles
through a potential /voltage difference.
Potential Dif/Voltage induces charges to
move. Amount of work per charge.
Two models for current.
• Conventional current – positive
charges in motion (IB).
• Real/electron current – electrons in
motion.
Voltage gives free e- a push charges gain lose PE and gain
KE:
DPE = KE
2
qV =1/2 mv .
Units of Current - Fundamental
Amperes (A) measures rate of charge
flow in q/t.
1 A = 1 C/s passing a point or
cross section of wire.
Ex 1. How many e- must pass a point
in a wire every second to carry a
current of 2 A?
• 2 C/s x 1.6 x 10-19 C/e- = 1.25 x 1019 e-.
To get continuous flow of charge:
1. Voltage (EMF) source = to do work on q which
gains E. Batteries, generators, solar cells. The
source raises the PE of charges.
2. Closed Circuit – continuous pathway for
charges to flow –metal wire, ionic solutions.
4. Resistors/Load – device to convert/dissipate
energy (so that source is not immediately
discharged). Resistors lower the PE of charges.
How it happens
P. D. Causes electric field to spread through
wire at near light speed.
All e- in wire respond by moving in field &
colliding with neighboring e- starting to flow.
Drift velocity is net speed in one direction. It’s
slow for e- (mm/s).
Electric field in wire
caused by voltage source induces
e- to move through.
Resistors, Appliances, Loads
Bulbs, toasters, etc. convert Eelc to other forms– heat,
light etc.
These cause the e- to lose PE. Devices are called resistors
or loads. They slow down the e- through collisions so
they resist current flow.
Direct Current (DC) circuits have current flow of
e- in 1 direction fr
Envisioned as traveling from the neg to + terminal
of battery.
Conventional current (pos charge) fr pos to neg..
Resistance
- Caused by internal collisions/interactions.
- Ratio of Voltage to Current
- R = V/I
- Units ohms W.
- V/A
Resistance
Occurs in wires as well as appliances.
Certain factors affect how much resistance
a wire will offer to current flow.
Factors affecting wire resistance.
•
•
•
•
1. Length
2. Area
3. Temperature
4. Type of material
Length – longer wire offers more
resistance. More chances for friction
in wire.
Less resistance
More resistance
Cross Sectional Area
Thick wires offer less resistance.
Temperature
Hot offers more R
Cold offers less R
At a given temperature,
R = resistance
r = constant of resistivity
l = length
A = cross sectional area
See table
Ohm’s Law
Resistance, Current, Voltage
Potential Drop
If a current flows in a resistor or appliance, there
must be a pd across the ends of the resistor.
The voltage pushes the charge.
The resistor “drops” or lowers the PE of the charge.
So is sometimes called potential drop.
-
R
+
e- current
Ohm’s Law
When temperature across a metallic
resistor is constant, the current is directly
proportional to pd across it.
V = IR
V/I = R = constant.
V = volts J/C
I = current A, C/s
R = total resistance
ohm’s W.
V = RI yield direct linear relationship.
V on Y axis. I on x axis. R is slope of
straight line. R = constant.
Switch
axis 1/R is
slope.
Light bulb
heats up as current goes through.
Watch axis - R is 1/slope here.
I
V
Other resistors vary with temperature.
See Kerr pg 133.
When a graph shows non-ohmic behavior,
you can simply find R = V/I at a point –
don’t use slope.
0.2 V/1.5 x 10-3 A
R = 133.3 W.
Film Clips
Resistance 10 mim
http://www.youtube.com/watch?v=YGvu9iqjJq4
• Film Clip Voltage 8.5 min
• http://www.youtube.com/watch?v=F1p3fgb
DnkY&feature=relmfu
Power in Resistors
Resistors/loads convert EElc to other forms.
P is rate E used/converted/dissipated or supplied
J/s or Watts.
Power rating of 500 W means Eelc converted to
other kinds at rate 500 J/s.
P=
W. =
t
Vq.
t
= VI
P = VI
The power is rate thermal E
dissipated & work done in
resistor.
Other Power Equations
For devices that obey Ohm’s Law we use R
= V/I to derive other equations for power.
P = RI2 =
V2.
R
Graphs P=VI
Power
Slope =
Voltage
Current
Power Ratings for Appliances
Devices are rated by the power they use. A bulb
rated 60 W 220 V means:
the bulb will dissipate 60 W when
attached to a 220 p.d.
If a different p.d. is used, then it won’t dissipate
60 W.
Fuses
As current flows, wires heat up.
Fuses designed to break circuit if current
becomes to high for the wires.
Fuse should be rated just above the ideal
operating current for a circuit.
Kilowatt hours kWh.
• Power is a rate of energy use.
• Electric sold in kWh which is Pt = Energy.
• 1 kWh is energy delivered to home in
1hour.
• 1 kWh (1000 W/kW)(60 min/h)(60s/min) =
3.6 x 106 Ws = 3.6 x 106 J
Film Clip Resistance
• http://www.youtube.com/watch?v=YGvu9i
qjJq4&NR=1
IB Packet and Kerr Pg 135 #3, 4, 17, 21.