Transcript DO NOW: QUIZ

PHYSICS
Ohm’s Law
MR. BALDWIN
31 March 2014
AIM: Upon what factors does resistance
depend?
DO NOW: QUIZ
Take out a sheet of paper and answer
the questions that follow
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
2
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 (–)
PHYSICS
Ohm’s Law
MR. BALDWIN
April 1, 2014
AIM: Upon what factors does the resistance
of a conductor depend?
DO NOW: QUIZ
Take out a sheet of paper and answer
the questions that follow
Recall: 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.
Recall: Charges
The charge (q) is the number of positive charges :
q  ne

What is the magnitude of the charge of an electron?

e  1.6  10
19
C
What is the typical UNIT of a charged object?
microCoulomb  C  10 C
6
TEST YOURSELF
• Calculate the current
where 10 coulombs of
charge pass a point in
5 seconds.
q  10 C;
• Calculate the current
in a lightning bolt that
delivers a charge of 35
coulombs to the
ground in 1ms.
q  35 C ;
t  5s
q 10 C
I 
 2.0 A
t
5s
3
t  1ms  10 s
q 35 C
I   3  35 kA
t 10 s
TEST YOURSELF
A current of 5.0 A passes through the bulb in 20
µs. If the charge of an electron is 1.6 x 10-19 C,
how many electrons flow through the bulb?
Given
Unknown
Substitute
Solve
I  5.0 A
q?
q  I t
q  5.0 A 2.0 10 5 s
t  20s  2.0 10 5 s


q  1.0 10 4 C
n?
q
n 
e
BALDWIN
1.0 10  4 C
14
n

6
.
25

10
1.6 10 19 C
11
Resistance & Resistivity
The electrical resistance of an electrical conductor is the
opposition to the passage of an electric current through that
conductor. The resistance of a wire is directly proportional to
its length and inversely proportional to its cross-sectional area:
The constant ρ is called the resistivity and is a
characteristic of the material. It’s unit is the m
The conducting ability of a material is often indicated by
its resistivity. The lower the resistivity, the better the
conductivity.
Variation of resistance with length and area: the
analogy to water flow
How do you think the flow rate will be affected if
you increase the cross-sectional area or length of the
pipe along which the water has to flow?
resistance increases with length
resistance decreases with diameter or cross-sectional
area
Resistivity
Observation
• Looking at the table of Resistivity, which
metal material is the most conductive?
• Which metal is the least conductive?
• Which substance is the least conductive and
why?
Practice
Determine the resistance of a 1-mile length of 12-gauge
copper wire. (Hint:1 mile = 1609 m; diameter of 12
gauge copper wire = 0.2117 cm)
• L = 1609 m,
• A = πR2
• ρ= 1.7 x 10-8 Ωm.
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 Ωm) • (1609 m) / (3.519 x 10-6 m2)
R = 7.8  (7.7709 ohm)
PHYSICS
Ohm’s Law
MR. BALDWIN
April 2, 2014
AIM: How does the temperature, area and
length of a conductor affects the resistance?
DO NOW: QUIZ
Determine the resistance of a 1-m length of 12gauge gold wire. (Hint: diameter of 12 gauge
wire = 0.2117 cm)
Mathematical Reasoning
• Two wires - A and B - with circular crosssections have identical lengths and are made of
the same material. Yet, wire A has four times
the resistance of wire B. How many times
greater is the diameter of wire B than wire
A?
Resistivity & Temperature
For any given material, the resistivity increases with
temperature:
where T is temperature in 0C and  is the thermal
coefficient with unit 0C-1 or /0C.
Resistivity
RECALL: Resistance & Resistivity
The constant ρ is called the resistivity and is a
characteristic of the material. It’s unit is the m
June 2011
A circuit contains a 12.0-volt battery, an ammeter, a
variable resistor, and connecting wires of negligible
resistance, as shown below. A variable resistor made of
nichrome wire is maintained at 20.°C. The length of the
nichrome wire may be varied from 10.0 centimeters to
90.0 centimeters. The ammeter reads 2.00 amperes
when the length of the wire is 10.0 centimeters.
Determine the resistance of the 10.0-centimeter length
of nichrome wire. [1]
Calculate the cross-sectional area of the nichrome
wire. [Show all work, including the equation and
substitution with units.] [2]
June 2010
A 3.50-meter length of wire with a cross-sectional
area of 3.14 × 10–6 m2 is at 20° Celsius. The current in
the wire is 24.0 amperes when connected to a 1.50-volt
source of potential difference.
Determine the resistance of the wire. [1]
Calculate the resistivity of the wire. [Show all
work, including the equation and substitution
with units.] [2]
PHYSICS
Ohm’s Law
MR. BALDWIN
3 April, 2014
AIM: What is the relationship between the current
and resistance in a circuit? (What is Ohm’s Law?)
DO NOW: QUIZ
1. What is electrical resistance?
2. What variables(factors) do you think might
affect electrical resistance in an electric
conductor?
HOMEWORK: Worksheet 13.3 Ohm’s Law
• opposition to the flow/passage of an electric
current through a conductor.
• factors affecting resistance:
–
–
–
–
Length of conductor
Cross-sectional area
Substance
Temperature
Simple Circuit
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)
Simple Circuit Schematic
A complete circuit is one where current can flow all the
way around. By convention, current is defined as the flow
of positive charges from + to – (high potential to low
potential). Electrons actually flow in the opposite direction.
Simple Circuit must contain?
Battery (power source)
Conductor (wire)
Device
Switch
Ohm’s Law
Experimentally, it is found that the
current in a wire is proportional to the
potential difference between its ends:
Ohm’s Law: Graphically
Materials that do not
follow Ohm’s law are
called nonohmic.
Ohm’s Law: Mathematically
The ratio of the voltage to the current in
a circuit is called the resistance.
Unit of resistance: the ohm, Ω.
1 Ω = 1 V/A.
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
Electric Hazards
Even very small currents: 10 to 100 mA
can be dangerous, disrupting the nervous
system.
Larger currents may also cause burns.
Household voltage can be lethal if you are
wet and in good contact with the ground.
SO…Be careful boys & girls!
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.
Summarize the following terms
•
•
•
•
•
•
•
Ion
Anion
Cation
Anode
Cathode
Current
Circuit
Resistors
Standard resistors are
manufactured for use in
electric circuits; they are
color-coded to indicate
their value and precision.
Resistors
http://www.physicsl
essons.com/exp22
b.htm
Electric Power
Power, as in kinematics, is the energy transformed
by a device per unit time.
In electricity, recall
energy transformed QV Q
P=
=
  V=IV
time
t
t
Electric Power
The unit of power is the Watt, W.
For Ohmic devices, 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.
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.
PHYSICS
Prof.BALDWIN
ELECTRICITY
10 April 2014
AIM: How do we determine the equivalent
resistance in series circuits?
DO NOW:
Draw a simple series circuit and describe its
characteristics in terms of the current and
voltages (potential differences).
BALDWIN
44
DC Circuits
https://phet.colorado.edu/
en/simulation/circuitconstruction-kit-dcvirtual-lab
Resistors in Series
A series connection has a single path from
the battery, through each circuit element in
turn, then back to the battery.
Diagramming Circuits
An ammeter measures the current through a
circuit component.
The same current going through the component
must go through the ammeter, so there can be
only one current path.
A connection with
only one current
path is called a
series connection.
Resistors in Series
The current through each resistor is the same;
the voltage depends on the resistance. The
sum of the voltage drops across the resistors
equals the battery voltage.
Resistors in Series
From this we get the equivalent resistance (that
single resistance that gives the same current in
the circuit).
Resistors in Series
Determine the equivalent resistance
PRACTICE: Determine
a. the equivalent resistance in the circuit
b. The total current in the circuit
c. The current in each component
d. the voltage drop across each resistor
BALDWIN
51
REGENTS QUESTION
A 3.0-ohm resistor and a 6.0-ohm resistor are connected in
series in an operating electric circuit. If the current through
the 3.0-ohm resistor is 4.0 amperes, what is the potential
difference across the 6.0-ohm resistor?
(1) 8.0 V
(3) 12 V
(2) 2.0 V
(4) 24 V
PHYSICS
Prof.BALDWIN
ELECTRICITY
10 April 2014
AIM: How do we determine the equivalent
resistance in parallel circuits?
DO NOW: QUIZ
( you will have 10 minutes to complete the
following problem.)
BALDWIN
53
Consider the following diagram. Calculate:
a. the equivalent resistance of the resistor combination
b. the current in the circuit
c. the potential drop across each resistor
d. the power dissipated across each resistor (extra credit)
Diagramming Circuits
Any time the current has two or more
paths to follow, the circuit is considered a
parallel circuit.
https://phet.colorado.edu/en/simulation/circu
it-construction-kit-dc-virtual-lab
Resistors in Parallel
A parallel connection splits the current, the current
in each component is different; the voltage across
each resistor is the same:
Question?
• What do you think will happen when one of
the bulbs go out?
• More current flows in the other paths making
the other bulbs brighter
Resistors in Parallel
The total current is the sum of the currents across
each resistor:
Resistors in Parallel
This gives the reciprocal of the equivalent
resistance:
NOTE!!! Req< any of the resistors
Solving problems involving complex circuits:
Calculate the equivalent resistance of the circuit?
BALDWIN
60
Consider the following diagram. Calculate:
a. the equivalent resistance of the resistor combination
b. the current in the circuit
c. the current across each resistor
d. the power dissipated across each resistor
Draw the table in your notebook, perform the
calculations and Fill in the spaces
R1
R2
R3
TOTAL
Resistance
10 k
2k
1k
625
Current
0.9 m
4.5 m
9.0 m
14.4 m
Voltage
9
9
9
9
Ohms
Amps
Volts
Power
8.1 m
40.5 m
81 m
129.6 m
Watts
Ammeters, Voltmeters & Ohmmeters
• An ammeter measures current and must be
connected in series to measure the current;
• A voltmeter measures voltage and must be
connected in parallel to measure the voltage.
• An ohmmeter measures resistance and requires a
battery to provide a current. It too must be
connected in parallel to measure the resistance.