Transcript Resistivity and Circuits

ISNS 3371 - Phenomena of Nature
Determining Planet Mass and Orbit Animation
ISNS 3371 - Phenomena of Nature
Remember - Doppler shift only tells us radial motion. If plane of orbit
perpendicular to our line of sight - no shift seen. If we view it from edge on,
maximum Doppler shift seen. Orbit generally tilted at some angle - star’s full
speed not measured. So mass derived from Doppler technique is minimum
possible. If varying position in sky measured (as in one case) orbital tilt can be
determined and mass measured accurately.
ISNS 3371 - Phenomena of Nature
Planetary Transit
• The Doppler technique yields only planet masses and orbits.
• Planet must eclipse or transit the star in order to measure its radius.
• Size of the planet is estimated from the amount of starlight it blocks.
• We must view along the plane of the planet’s orbit for a transit to occur.
– transits are relatively rare
• They allow us to calculate the density of the planet.
– extrasolar planets we have detected have Jovian-like densities.
Planetary Transit Animation
ISNS 3371 - Phenomena of Nature
Orbital distances and
approximate masses of
first 77 planets
discovered
ISNS 3371 - Phenomena of Nature
Electricity and Magnetism
ISNS 3371 - Phenomena of Nature
Charge
Charge - basic property of some elementary particles:
The electron has a negative charge (-1.6 X 10-19 C)
The proton has a positive charge of (+1.6 X 10-19 C)
The atom has an equal number of protons and electrons - it has no net
charge.
Like charges repel one another and unlike charges attract one another.
The force between two charged particles is inversely proportional to the
square of the distance between them and is called the Coulomb force:
kq1q2
F 2
d
(Coulomb’s Law)
k is a constant similar to G in Newton’s Law of Gravity (= 9 X 109 N m2/C2)
ISNS 3371 - Phenomena of Nature
Electrostatic Charge
The buildup of electric charge on the surface of an insulated body
Examples: electroscope, balloon rubbed on hair, glass
rod rubbed by fur, on body by rubbing feet on carpet
 Can damage sensitive electronics - why technicians wear
clothing made of special fabrics and wear grounding
straps.
 Can cause discharges on spacecraft - can cause damage
or even failures
 Can cause gasoline explosions - why plane is always
grounded to fuel pump before refueling
 Partially responsible for process of bacteria sticking to
surfaces or other cells
ISNS 3371 - Phenomena of Nature
Electrostatic charging on spacesuits
while ISS travels through the aurora
potentially hazardous situation
Display
and
Control
Module
(DCM)
Metal waist and neck rings and other
metal portions of the suit make contact
with the sweat soaked ventilation
garment providing possible conducting
path for discharge through astronaut’s
thoracic cavity.
 Surface of spacesuit could charge to Safety
Tether
high voltage leading to subsequent
discharge.
 Discharge to the station through safety
tether:
•Tether is a metallic cable - connected to
astronaut via non-conducting (nylon)
Mini Work Station
(MWS)
housing.
• Station maintained at plasma potential
- arc path closed when tether gets
wrapped around astronaut.
Body Restraint
Tether (BRT)
ISNS 3371 - Phenomena of Nature
Voltage
Electric potential - change in electric potential is work done on charge by
an electric (Coulomb) force.
Remember: work = force X distance (over which force is applied)
Analogous to gravitational
potential energy: work done
by the force of gravity in
moving a mass through a
certain distance
Electric Potential
Voltage 
Ch arg e
Units of voltage are Volts (Joules/Coulomb). In a nine volt battery the
potential difference between the positive and negative terminals is precisely
nine volts. 
ISNS 3371 - Phenomena of Nature
Current
Establish a potential difference between two points and release some
charges - charges will be acted on by the electrical force and start to move
- this is a electric current - a flow of electrical charges. Just like the flow of
H2O is a water current.
 A current is the amount of charge that flows by a given point in a
certain time interval. Unit is Ampere (A = Coulombs/second)
In electrical circuits, consist of electrons in the wire moving from lower to
higher electrical potential.
By convention, electrical current flows from positive to negative - electrons
flow in opposite direction. In other words, a positive current flows from
higher to lower electrical potential.
ISNS 3371 - Phenomena of Nature
Resistivity and Circuits
If a wire is connected between two poles of a battery, a current flows.
This is a circuit. The electrons do not move completely freely - there is a
resistance to their flow - has to do with number of collisions electrons
make with other electrons and atoms. Amount of current dependent of
resistivity. Unit of electrical resistance is Ohm ().
Insulator - high resistivity
Conductor - low resitivity
Superconductor - zero resistivity
Dependent on material, temperature, length of wire
Electrical circuit like
hydraulic circuit with
flow of water
analagous to flow of
electrons.
ISNS 3371 - Phenomena of Nature
Power
Power is the rate of energy flow. Unit is Watt ( Watt = Joules/second)
Power = Voltage X Current - 1 W = 1 VA
Total amount of energy used = Watt X Time, This is how the power
company charges us for electricity - by the kilowatt hour (kWh)
The power rating of an appliance tells us the rate at which it uses electrical
energy. For example, a 1500 watt hair dryer uses 1500 joules of electrical
energy per second.
Keep a 60 watt light bulb on for a full day:
uses 60 watts x 24 hours = 1440 watt hours = 1.44 kilowatt hours
of energy. At 9 cents per kilowatt hour this would cost just under
15 cents.
kilowatt hour is a unit of energy - can convert it to joules:
1.44 kWh = 1440 J/s X 3600 s = 5,184,000 J
ISNS 3371 - Phenomena of Nature
Electrical Quantities
QUANTITY SYMBOL UNIT
MEANING
Charge
Q
Coulomb
(C)
A basic property of some
elementary particles. The electron
has a charge of -1.6 X 10-19 C
and the proton has a charge of
+1.6 X 10-19 C
Current
I
Ampere
(A)
(1 A = 1
C/sec)
Rate of flow of charge.
Potential
Difference
(voltage)
V
Volt (V)
(1 V = 1
J/C)
J= Joule
Potential energy difference per
coulomb of charge between two
points; corresponds to pressure in
water flow.
ISNS 3371 - Phenomena of Nature
QUANTITY SYMBOL UNIT
MEANING
Resistance
R
Ohm (Ω)
1Ω =
1 V/A
A measure of the opposition to the
flow of charge in a particular
circuit. For a given voltage, the
higher the resistance, the lower
the current.
Power
P
Watt (W)
(1 W =
1 VxA)
Rate of energy flow.
ISNS 3371 - Phenomena of Nature
Ohm’s Law
Ohm’s Law states that the amount of current in a circuit is proportional
to the voltage across the circuit and inversely proportional to the
resistance in that circuit
I=V/R
or
V = IR
Voltage = Current x Resistance
ISNS 3371 - Phenomena of Nature
Electrical Shock
“It’s not the voltage but the current.”
The current is what actually causes a shock - human body has
resistance of ~500,000  with dry skin - ~100  wet! Requires
conducting path.
Can cause: (1) burning of tissue by heating, (2) muscle contractions, (3)
disruption of cardiac rhythms.
Current (A)
Effect
0.001
Can be felt
0.005
Is painful
0.010
Causes spasms
0.015
Causes loss of muscle control
0.070
Goes through the heart - fatal after more than
1 second
ISNS 3371 - Phenomena of Nature
Circuits in Series
•Resistance (light bulbs) on same path
•Current has one pathway - same in every part of the circuit
•Total resistance is sum of individual resistances along path
•Current in circuit equal to voltage supplied divided by total resistance
•Sum of voltages across each lamp equal to total voltage
•One bulb burns out - circuit broken - other lamps will not light (think of
string of old Christmas lights)
ISNS 3371 - Phenomena of Nature
Water Analogy for Series Circuits
ISNS 3371 - Phenomena of Nature
Parallel Circuits
•Bulbs connected to same two points of electrical
circuit
•Voltage same across each bulb
•Total current divides among the parallel branches equals sum of current in each branch - current in each
branch inversely proportional to resistance of branch
•Overall resistance of circuit lowered with each
additional branch
•Household wiring (and new Christmas light strings)
designed in parallel - too many electrical devices on too much current - trip fuse/breaker
ISNS 3371 - Phenomena of Nature
Water Analogy for Parallel Circuits