Lecture series for Conceptual Physics, 8th Ed.

1
Lecture series for Conceptual Physics, 8th Ed.
2
Flow of Charge p398
Recall that heat flows from the hot end to the
cold end.
Current flows from the high potential (V) end to
the low V end of a conductor.
Current flows as long as there is a potential
difference, ?V.
Potential (V) is like pressure (kPa)
A pump does the same job that a battery does.
A difference in pressure causes water to flow.
3
Electric Current p399
Water current is made up of water molecules.
Electric current is the flow of electric charges.
In a wire, conduction electrons flow while the
protons stay put.
A slight problem current (I) flows from
positive to negative. The electrons are going
the other way.
The electric field also goes from the positive
end of the wire to the negative end of the wire.
current

-

e-
Current (I) is measured in Amperes (A).
4
Voltage Sources p400
Batteries (DC) and Generators (AC)
Oh! And
The work done by batt.s and gen.s to separate
charges creates the potential difference, ?V,
between their terminals.
The and terminals have a difference in
potential energy per charge or voltage.
Become familiar with these symbols.
5
Electrical Resistance p401
A hose resists the flow of water.
Longer, skinnier hoses have more resistance than
short, fat hoses.
This wire has more resistance than
this wire.
Most of the time, hot wires have higher
resistance than cold wires. Something to do with
the greater jostling of atoms at higher temp.s.
Electrical resistance is measured in ohms,
6
Ohms Law p401
V I R
V
I
Try this
R
What is the current in a wire with a potential
difference of 10 V across its ends and a
resistance of 2 ohms?
10 V
5 A
I
2 ohms
Now, what if you
doubled the voltage?
doubled the resistance?
Inside electronic devices, current is regulated
by resistors.
7
12V
V
0.2 A
I

60 ohms
R
120 V
V
10 ohms

R
I
12 A
Why isnt the bird shocked?
His feet have the same potential.
There is no potential difference, V.
But, if it were to touch something else
8
Ohms Law and Electric Shock p402
See Table 22.1 p403 Small currents can be
dangerous.
12 V
V
I

0.00012 A
R
100,000 ohms
To ground.
Protects you from any wire touching the inside of
the appliance.
9
Direct Current and Alternating Current p404
Electrons go from to terminals.
Batteries produce dc.
AC travels better.
Generators produce ac.
Electrons just move back and forth in the 1/60 Hz
change in electric field polarization.
Converting ac to dc p405
A gadget called a diode takes the ac going one
way only and passes it along in pulses.
A capacitor is used to smooth out the pulses.
A capacitor works in a manner similar to the
water system at left.
10
Speed and Source of Electrons in a Circuit
p405
When you flip the switch on, an electric field is
established immediately in the circuit.
But, the electrons dont travel very far or very
fast.
In fact, they vibrate back and forth in the 1/60
Hz ac current.
Fig 22-13 shows how a wire channels the elec.
field.
Fig 22-14 shows how the elec.s travel through a
wire.
Elec.s are jostled by atoms as they try to
accelerate and end up starting over.
Basically, es just vibrate.
11
Electrons jostle back and forth in the filament
to produce heat and light with their friction.
Do you get the elec.s from the power company?
No! They come with the appliance.
In a dc current, the elec.s would drift along at
33cm/hour.
12
Electric Power p407
Some of the power is in the form of light and a
bit more than half goes as heat.
At 120V this bulb would produce 100 W.
Thats 100 joules per second.
P I V
P I V (15 A)(120 V) 1800 W
Yes. With 600 W to spare.
1 kW
10 cents
1200W x
12 cents
x 1 hr x
1000 W
kWh
13
Electric Circuits p408
e-s flow along a circuit.
The current stops when the switch is open,
1.5 V
or a light burns out.
open
The current is the same everywhere in a series
circuit.
Resistors in a series act like one resistor
according to this formula
Rtotal R1 R2 R3 ..
If each resistor is 5 ohms, all of them are 15
ohms. AND
1.5 V
V
0.1 A
I

The current in the circuit is
R
15 ohms
14
What current would be indicated on the ammeter?
Rtot R1 R2 R3 2 3 1 6 ohms
I V / R 6 V / 6 ohms 1 A
Watt is the power output of the total circuit
P I V (1 A)(6 V) 6 W
What is the voltage drop (IR) of each resistor?
I R2 (1 A)(2 ohms) 2 V
I R3 (1 A)(3 ohms) 3 V
I R1 (1 A)(1 ohm) 1 V
15
Again
What is the power output of each resistor?
P2 I V2 (1A)(2V) 2 W
P3 I V3 (1A)(3V) 3 W
6 W total power
P1 I V1 (1A)(1V) 1 W
16
Parallel Circuits p410
There are 3 pathways for the electrons to follow.
If a light fails in one, the others will continue.
1.5V
The voltage through each light equals the voltage
of the battery.
The resistors in parallel act like one resistor
according to this formula
1/Rtot 1/R1 1/R2 1/R3 ..
If each resistor is 5 ohms, what is the total
resistance?
1/5 1/5 1/5 3/5
5/3 1.7 ohms
What is the total current through the circuit?
I V / R 1.5V / 1.7 ohms 0.88 A
17
What is the current through the ammeter?
First, find the total resistance.
1/2 1/3 1/1 3/6 2/6 6/6 11/6
0.545 ohms
I V/R 1.50V / 0.545A 2.75 A
What current is going through each resistor?
1.5V
I2 V/R2 1.50V/2 ohms 0.750 A
I3 V/R3 1.50/3 0.500 A
I1 V/R 1.50/1 1.50 A
What is the total power?
What power is going through each resistor?
P2 I2V (0.750)(1.50) 1.13 W
PIV(2.75)(1.50)
P3 I3V (0.500)(1.50) 0.750 W
P1 I1V (1.50)(1.50) 2.25 W
P 4.13 W
18
Parallel circuits and Overloading P411
Adding more appliances decreases the resistance
AND increases the current through the parallel
circuit.
Safety Fuses p412
Now, circuit breakers have replaced this type of
fuse.
The end