Download DO NOW

PHYSICS
ELECTRICITY
AIM: What
Mr. BALDWIN
Saturday, May 6, 2017
cause charges to flow?
DO NOW: Answer the following questions.
1. What is the name given to a charged atom or molecule (one
that gains or loses electrons)?
2. When an atom loses an electron, what happens to its net
charge?
3. What is the name given to such atom that loses electrons?
4. When an atom gains an electron, what happens to its net
charge?
5. What is the name given to such atom that gains electrons?
BALDWIN
1
CHARGED PARTICLES
1. ION: When an atom or molecule gains or
loses an electron, it becomes charged.
2. ANION: negatively charged ion
 When an atom gains electrons, its net charge
is negative
3. CATION: positively charged ion
 When an atom loses electrons, its net charge
is positive
The Electric Battery
Volta discovered that
electricity could be created
if dissimilar metals
(electrodes) were connected
by a conductive solution
called an electrolyte.
This is a simple electric cell.
A battery transforms
chemical energy into
electrical energy.
The Electric Battery
Electrode that attracts anions is called an ANODE (+)
Electrode that attracts cations is called a CATHODE (–)
Electric Current
Electric current (I) is the rate of flow of positive
charges through a conductor:
q
I 
t
where
q is the charge and t is the time
Unit of electric current: the ampere, A.
1 A = 1 C/s.
TEST YOURSELF
• Calculate the current
where 10 coulombs of
charge pass a point in
5 seconds.
q  10 C;
t  5s
q 10 C
I 
 2.0 A
t
5s
• Calculate the current
in a lightning bolt that
delivers a charge of 35
coulombs to the
ground in 1ms.
q  35 C ;
3
t  1ms  10 s
q 35 C
I   3  35 kA
t 10 s
PHYSICS
ELECTRICITY
Mr. BALDWIN
Saturday, May 6, 2017
AIM: How do we construct a simple circuit?
DO NOW:
• A current of 5.0 A passes through the bulb in
20 µs. What is the quantity of charge flowing
through the bulb?
• How long does it take a current of 5.0 mA to
deliver 15 C of charge?
• HOME WORK – Prepare for Test on Friday
http://phet.colorado.edu/en/simulation/circuitconstruction-kit-dc
BALDWIN
7
Simple Circuit
A complete circuit is one where current can flow all
the way around.
Note that the schematic drawing doesn’t look much
like the physical circuit!
Electric
Circuit
Components
&
Devices
Electric Current
In order for current to flow, there must be a path
from one battery terminal, through the circuit, to
the other battery terminal AND a potential
difference across the terminal
Which one of these circuits will work?
THIS
ONE
(c)
Electric Current
By convention, current is defined as flowing from + to – .
(High potential to low potential) Electrons actually flow in
the opposite direction, but not all currents consist of
electrons.
Did You Understand?
1. A 60 W bulb draws a current of 500mA.
How much charge passes through the
bulb in 20 s?
2. What is the average current produced by
a lightning strike if a charge of 50 C is
transferred in 2.0 ms?
Summarize the following terms
•
•
•
•
•
•
•
Ion
Anion
Cation
Anode
Cathode
Current
Circuit
PHYSICS
ELECTRICITY
Mr. BALDWIN
Saturday, May 6, 2017
AIM: How are the current, voltage and
resistance related in a simple circuit?
DO NOW:
• A current of 5.0 A passes through the bulb in
20 µs. If the charge of an electron is 1.6 1019 C
How many electrons flow through the bulb?
q
I 
t
HOME WORK – Text pg 546 # 29 -33.
BALDWIN
14
Ohm’s Law: Resistance and Resistors
Experimentally, it is found that the
current in a wire is proportional to the
potential difference between its ends:
Ohm’s Law
The ratio of voltage to current in a
circuit is called the resistance.
Resistance is the hindrance to the flow
of charge.
Ohm’s Law: Resistance and Resistors
In many conductors, the
resistance is independent of the
voltage; this relationship is
called Ohm’s law.
(b) Materials that do not follow
Ohm’s law are called nonohmic.
Unit of resistance: the ohm, Ω.
1 Ω = 1 V/A.
Resistors
Standard resistors are
manufactured for use in
electric circuits; they are
color-coded to indicate
their value and precision.
Resistors
Example1:
You are given a resistor whose stripes are colored from left to right as
brown, black, orange, gold. Find the resistance value.
• Step One: The gold stripe is on the right so go to Step Two.
• Step Two: The first stripe is brown which has a value of 1. The
second stripe is black which has a value of 0. Therefore the
first two digits of the resistance value are 10.
• Step Three: The third stripe is orange which means x 1,000.
• Step Four: The value of the resistance is found as 10 x 1000 =
10,000 ohms (10 kilohms = 10 kohms).
• The gold stripe means the actual value of the resistor mar vary
by 5% meaning the actual value will be somewhere between
9,500 ohms and 10,500 ohms. (Since 5% of 10,000 = 0.05 x
10,000 = 500)
Example 2:
You are given a resistor whose stripes are colored from left to
right as orange, orange, brown, silver. Find the resistance value.
• Step One: The silver stripe is on the right so go to Step Two.
• Step Two: The first stripe is orange which has a value of 3.
The second stripe is orange which has a value of 3. Therefore
the first two digits of the resistance value are 33.
• Step Three: The third stripe is brown which means x 10.
• Step Four: The value of the resistance is found as 33 x 10 =
330 ohms.
• The silver stripe means the actual value of the resistor mar
vary by 10% meaning the actual value will be between 297
ohms and 363 ohms. (Since 10% of 330 = 0.10 x 330 = 33)
Example3:
You are given a resistor whose stripes are colored from left to
right as blue, gray, red, gold. Find the resistance value.
• Step One: The gold stripe is on the right so go to Step Two.
• Step Two: The first stripe is blue which has a value of 6. The
second stripe is gray which has a value of 8. Therefore the first
two digits of the resistance value are 68.
• Step Three: The third stripe is red which means x 100.
• Step Four: The value of the resistance is found as 68 x 100 =
6800 ohms (6.8 kilohms = 6.8 kohms).
• The gold stripe means the actual value of the resistor mar vary
by 5% meaning the actual value will be somewhere between
6,460 ohms and 7,140 ohms. (Since 5% of 6,800 = 0.05 x
6,800 = 340)
Some clarifications:
• Batteries maintain a (nearly) constant potential
difference; the current varies.
• Resistance is a property of a material or device.
• Current is not a vector but it does have a
direction.
• Current and charge do not get used up. Whatever
charge goes in one end of a circuit comes out the
other end.
PHYSICS
Mr. BALDWIN
ELECTRICITY
5/6/2017
AIM: How do we determine the resistance of
a conducting material?
DO NOW:
Go over exam.
HOME WORK – Resitivity hand out
BALDWIN
24
Electric Hazards
Even very small currents: 10 to 100 mA
can be dangerous, disrupting the nervous
system.
Larger currents may also cause burns.
SO…How do we control the amount of
current?
http://hyperphysics.phyastr.gsu.edu/hbase/electri
c/shock.html
Resistivity
The resistance of a wire is directly proportional to
its length and inversely proportional to its crosssectional area:
Where ρ, the resistivity, L is the length & A is the
cross-sectional area of the material.
Resistivity & Temperature
For any given material, the resistivity increases with
temperature:
Practice
Determine the resistance of a 1-mile length of 12gauge copper wire.
1 mile = 1609 m;
diameter of 12 gauge copper wire = 0.2117 cm
•
•
•
•
L = 1609 m, A = πR2 ρ= 1.7 x 10-8 Ωm.
First find the cross-sectional area:
A = π•r2 = (3.14) • [ (0.002117 m) / 2)]2 = 3.519 x 10-6 m2
Now substitute into the above equation to determine the
resistance.
• R = (1.7 x 10-8 ohm •m) • (1609 m) / (3.519 x 10-6 m2)
• R = 7.8 (7.7709 ohm)
Battery
Voltage
(V)
Total
Resistance
(Ω)
Current
(Amps)
1.5 V
3
0.50 Amp
3.0 V
3
1 Amp
4.5 V
3
1.5 Amp
http://www.physicsl
essons.com/exp22
b.htm
PHYSICS
Mr. BALDWIN
ELECTRICITY
Saturday, May 6, 2017
AIM: How do we measure the power in our
homes?
DO NOW: (Turn in)
• Determine the resistance of a 1-cm length of
12-gauge gold wire whose ρ= 2.44 x 10-8 Ω·m
and has a diameter d= 0.2117 cm
HOME WORK – Handout – Power: #s 1-5
BALDWIN
33
Electric Power
Power, as you may recall, is the energy transformed
by a device per unit time.(P=W/t)
In electricity,
energy transformed QV Q
P=
=
 V=IV
time
t
t
Electric Power
The unit of power is the Watt, W.
For Ohmic devices (V=IR), we can make the
substitutions:
Electric Power
What you pay for on your electric bill is not power,
but energy – the power consumption multiplied by
the time.
We have been measuring energy in joules, but the
electric company measures it in kilowatt-hours,
kWh.
TRY IT
• The average amount of electrical energy used
in 30 days is 270kW·h. The cost price for
electricity is 19.2240¢/kW·h. How much will
the bill be in a year?
Power in Household Circuits
Fuses are one-use items – if they blow, the fuse is
destroyed and must be replaced.
Alternating Current
Current from a battery
flows steadily in one
direction (direct current,
DC). Current from a power
plant varies sinusoidally
(alternating current, AC).
Superconductivity
In general, resistivity decreases as temperature
decreases. Some materials, however, have
resistivity that falls abruptly to zero at a very low
temperature, called the critical temperature, TC.
Superconductivity
Experiments have shown that currents, once
started, can flow through these materials for
years without decreasing even without a
potential difference.
Critical temperatures are low; for many years no
material was found to be superconducting above
23 K.
More recently, novel materials have been found
to be superconducting below 90 K, and work on
higher temperature superconductors is
continuing.
Electrical Conduction in the Human
Nervous System
The human nervous system depends on the
flow of electric charge.
The basic elements of the nervous system are
cells called neurons.
Neurons have a main cell body, small
attachments called dendrites, and a long tail
called the axon.
Electrical Conduction in the
Human Nervous System
Signals are received by
the dendrites,
propagated along the
axon, and transmitted
through a connection
called a synapse.