Physics
ys cs 132:
3 Lecture
ectu e 15
5
Elements of Physics II
A
Agenda
d for
f T
Today
d


Resistance
 Resistors
 Series
 Parallel
 Ohm’s law
Electric Circuits
 Current
 Kirchhoff
Kirchhoff’s
s laws
Physics 201: Lecture 1, Pg 1
Electromotive force




Electromotive force (emf) is the maximum
potential difference a battery or power source
can give a circuit.
A car b
battery h
has an emff off 12 V
Sometimes shown as a script (E)
Symbol for emf
-
+
Physics 201: Lecture 1, Pg 2
Electrical Current
When we connect the two terminals of an emf to
a circuit ( a continuous conducting path), charge
will move continuously through the circuit.


Since there is a potential difference electrons in circuit will feel
a force

emf
Electrons will flow in
direction of positive
terminal
 Battery “pushes”
electrons
Physics 201: Lecture 1, Pg 3
Electrical Current

This flow is called electric current

Net amount of charge through a point in the
circuit per unit time.

Units Ampere = Coulomb/second
q
I
t
Physics 201: Lecture 1, Pg 4
Example
The electron drift speed in a wire is exceedingly
slow-typically only a fraction of a millimeter per
second. Yet when you turn on a flashlight switch,
the light comes on almost instantly. Resolve this
apparent paradox.
Physics 201: Lecture 1, Pg 5
Example
The electron drift speed
p
in a wire is exceedingly
g y slow-typically
yp
y
only a fraction of a millimeter per second. Yet when you turn
on a flashlight switch, the light comes on almost instantly.
pp
p
paradox.
Resolve this apparent
Correct:
It'ss like water analogy on page 877. as water (electrons) start flowing into
It
one end of the pipe(wire), water(electron) from the other end of pipe (wire)
start turning paddle wheel immediately. therefore light can come instantly as
you turn on the switch.
y
There is a line of electrons in the wire. That means the electrons are right
next to each other, so as soon as you turn the flashlight on, electrons will
start to move, but there is an electron already there behind the part that
produces the light so as electrons are added from the electrical current, the
light is produced immediately.
Physics 201: Lecture 1, Pg 6
Clicker Question 1:
When a light
g bulb is emitting
g light
g in a circuit as
below the number of electrons entering the light
bulb is:
(a) larger than the number leaving the light bulb
(b) equal to the number leaving the light bulb
(c) smaller than the number leaving the light bulb
Physics 201: Lecture 1, Pg 7
Plinko




Disks = electrons
Disk bounces through
atoms on it’s way down the
incline
Angle
g of incline = emf
(potential difference)
Y t b demo
Youtube
d
Physics 201: Lecture 1, Pg 8
Resistance

Resistance: Traveling through a conductor, electrons bump into
things which slows them down
down.
 : Resistivity: Density of bumps
 L: Length of conductor
 A: Cross sectional area of conductor
R ρ

L
A
A
All electrical devices will resist the flow
of electrical current
L
Physics 201: Lecture 1, Pg 9
R ρ
L
A
Plinko
Physics 201: Lecture 1, Pg 10
Clicker Question 2:
Below we have a skinny wire and a thick wire
connected to identical batteries. Which case
will have the larger current?
Iskinny

( ) Thick Wire
(a)
(b) Skinny Wire
(c) They will have the same current
Ithick

Physics 201: Lecture 1, Pg 11
Clicker Question 3:
Below we have circuit with a resistor and a wire. Which part will have
the larger current?
?
(a) Wire
(b) Resistor
(c) They will have the same current
Iwire

Iresistor
Physics 201: Lecture 1, Pg 12
Conservation of Current
Physics 201: Lecture 1, Pg 13
Charge Distribution

The nonuniform distribution of surface charges along a
wire creates a net electric field inside the wire that points
from the more p
positive end toward the more negative
g
end
of the wire.

This is the internal electric field that pushes the electron
current through the wire
wire.
Physics 201: Lecture 1, Pg 14
Clicker Question 4:
Is this a possible surface charge distribution for a current carrying
wire?
?
(a) yes
(b) no
Physics 201: Lecture 1, Pg 15
Electric Current

The direction of current flow – from the positive terminal to
the negative one – was decided before it was realized that
electrons are negatively charged
charged. Therefore
Therefore, current flows
around a circuit in the direction a positive charge would
move; electrons move the other way. However, this does
not matter in most circuits.
circuits
Physics 201: Lecture 1, Pg 16
Kirchhoff’s Junction Law
For a junction, the law of
conservation of current
requires that
where the  symbol
y
means summation.
This basic conservation
statement is called
Kirchhoff’s junction
law
law.
Physics 201: Lecture 1, Pg 17
Kirchhoff s Loop Law
Kirchhoff’s
Voltage drops must add to zero

We can think of the voltage difference as a height
difference
I
Down
Volts

R
Up
Volts
Electron must end up at same place it started!!
Physics 201: Lecture 1, Pg 18
Ohm’s law

With conductors the more resistance the less
current can flow

•
Ohm’s law
V  IR
Hi h resistance
Higher
i t
llower currentt

Units are Ohms ()
V
I 
R
Physics 201: Lecture 1, Pg 19
Resistor

A device that resists current

Used to
o co
control
o cu
current
e flow
o in a ccircuit
cu

Resistor in a circuit:

Generally in a circuit the resistance of the
connecting wires are very small and we can
neglect them.

Wires will resist current, but we will assume they have zero
resistance!!
Physics 201: Lecture 1, Pg 20
Resistor

R ρ
L
A
Generally in a circuit the resistance of the connecting wires
are very small and we can neglect them.
Student: How come the resistance of a wire is very very small, considering
wires are usually longer than they are wide, so the Length is greater than
the Area – shouldn’t the resistance be large?
Student: The textbook says about an ideal wire, but I don't understand how it
i possble
is
bl to
t create
t a currentt if voltage
lt
= 0.
0 Thanks
Th k
Physics 201: Lecture 1, Pg 21
Kirchhoff s Loop Law
Kirchhoff’s
for the Simplest Circuit in the World
I

R
• Example
– Calculate I when emf =24 Volts and R = 8 
– Ohm’s Law: V=IR
– I = 24/8 = 3 Amps
Physics 201: Lecture 1, Pg 22
Clicker Question 5:
Below we have two circuits, with identical batteries and three identical
cylindrical conductors. Compare
C
the current in the two circuits:
(a) IA > IB
(b) IA = IB
(c) IA < IB


Circuit A
Circuit B
Physics 201: Lecture 1, Pg 23
Clicker Question 6:

A light bulb is connected to a battery in case A.
In case B an identical light bulb is added in
series. What happens to the brightness of the
original bulb?
(a) it stays the same
(b) it decreases
(c) it increases
Case A
Case B
Physics 201: Lecture 1, Pg 24
Series wiring


When two objects are connected end to end in a circuit, they are
said
id tto b
be iin series.
i
Here R1 and R2 are in series
R1
R2

Current must be same in each resistor!!
Voltage drop across R1 plus Voltage drop across R2
should equal the emf of the battery
Physics 201: Lecture 1, Pg 25
Resistors in series

One wire:
 If charge goes through one resistor, it must go
through other.
 I1 = I2 = Ieq
 V1 + V2 = Veq

Add eq
1
2
 Adding Voltage IR1 + IR2=I(R1 + R2)
 Adding length Req=(L1+L2)/A
R
=R +R
R1
Req
R2

The equivalent resistance of resistors in series is just
the addition of the resistors
Physics 201: Lecture 1, Pg 26
Clicker Question 7:
In the circuit below, what is the voltage across R1?
a) 12 V
b) zero
R1 = 4 
R2 = 2 
c) 6 V
d) 8 V
e) 4 V
12 V
Physics 201: Lecture 1, Pg 27
Clicker Question 7:
In the circuit below, what is the voltage across R1?
R1 = 4 
R2 = 2 
12 V
Physics 201: Lecture 1, Pg 28
Clicker Question 8:
Below we have two circuits, with identical batteries and three identical
cylindrical conductors. Compare
C
the current drawn ffrom the
batteries in the two circuits:
(a) IA > IB
(b) IA = IB
(c) IA < IB


Circuit A
Circuit B
Physics 201: Lecture 1, Pg 29