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PHY 1104 | Exam 3
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Gravity
Key Concepts:
Terms in this set (84)
electrolyte
a simple electric cell
battery
transforms chemical energy into electrical energy
electric current
the rate of flow of charge through a conductor: I = Delta Q/ Delta t
unit of electric current
the ampere, A. 1 A = 1 C/s
in order for current to flow...
there must be a path from one battery terminal, through the circuit, and back to the other battery terminal
conventional current
current is defined as flowing from + to -
electron flow
electrons are negative and flow to the positive and so electrons actually flow in the opposite direction of conventional current
resistance
ratio of voltage to current: R = V/I
ohm's law
V = IR
unit of resistance
the ohm, Omega. 1 Omega = 1 V/A
resistance is...
a property of a material or device
current is...
not a vector but it does have a direction
does not get "used up"
whatever charge goes in one end of a circuit comes out the other end
resistivity
resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area. this is a characteristic of the material and is represented by rho. R = rho (L/A)
thermistors
rely on varying resistivity with temperature
heating element
electric energy is transformed into thermal energy in a wire resistance
mass of an electron
9.10938291 × 10^-31 kilograms
heating light bulbs
traditional incandescent & halogen incandescent
light producing light bulbs
compact fluorescent & light-emitting diode
unit of power
watt, W. 1 W = 1 J/s
P = IV
for ohmic resistive devices
P = IV = I (IR) = I^2*R
energy
what you pay on your electric bull rather than power
electric company measures it in kilowatt-hours
kW
h = (kJ / s)
h ...or... [(energy/time) * time]
fuses
one-use items - if they blow, the fuse is destroyed and must be replaced
circuit breakers
are switches that will open if the current is too high; they can then be reset
direct current, DC
current from a battery flows steadily in one direction
alternating current, AC
current from a power plant varies sinusoidally
examples of DIRECT CURRENT (DC)
batteries and PV panels
example of ALTERNATING CURRENT (AC)
wall outlets
voltage varying sinusoidally
V = V0 sin 2pi ft = V0 sin wt
current varying sinusoidally
I = V/R = V0/R sin wt = I0 sin wt
photoelectric effect
solar radiation can release an electron from a semi-conductor
p-n junction in a PV panel...
directs these electrons in a prescribed path so that we can harness the energy of their movement
transformer (sources of emf)
a device that transforms one type of energy (chemical, mechanical, light,...) into electrical energy
emf
electromotive force
emf is...
potential difference between the terminals of such a device, when no current flows to a circuit
cursive E
units are volts
what reduces the actual voltage from the ideal emf
a battery, ab = cursive E - Ir
series connection
a single path from the battery, through each circuit element in turn, then back to the battery
current in series
I1 = I2 = I3 =Itot
voltage in series depends on resistance
the sum of the voltage drops across the resistors equals the battery voltage. V = I*(R1+R2+R3) or... V = V1+V2+V3 = IR1+IR2+IR3
equivalent resistance
Req = R1+R2+R3
parallel connection
splits the current; the voltage across each resistor is the same: V1=V2=V3=Vtot
total current in parallel
the sum of the currents across each resistor:
I= I1+I2+I3
for parallel circuits:
the sum of the currents across each resistor:
I = I1+I2+I3
V/Req = V/R1+V/R2+V/R3
reciprocal of the equivalent
1/Req = 1/R1 + 1/R2+ 1/R3
series circuit
the more devices, the more resistance
the more resistance, the less current
current through each device is the same, voltage is not
parallel circuit
the more devices, the less resistance
current is split between different legs of circuit (total current is the sum of the current through each leg).
voltage across each leg is the same, current is not
Kirchhoff's rules
used for circuits that cannot be broken down into simple series and parallel connections
junction rule
the sum of currents entering a junction equals the sum of the currents leaving it (conservation of electric charge)
loop rule
the sum of the changes in potential around a closed loop is zero (conservation of energy)
capacitor
a device used to store an electric charge, consisting of one or more pairs of conductors separated by an insulator.
Q = CV
capacitors in parallel...
have the same voltage across each one. in this case, the total capacitance is the sum.
Q = CeqV
capacitors in series...
have the same charge.
Q/Ceq = Q/C1 + Q/C2 + Q/C3
resistor / capacitor combination in series
can be used to provide a consistent timed flow of electric current
2 factors are required for electric shock:
1. A potential (voltage) difference (no difference in V across bird's body)
2. A path (note that the current is inversely proportional to resistance)
ammeter
measures current
voltmeter
measures voltage
ammeters must be connected in...
series
voltmeters are connected in
parallel
poles
the ends of a magnet (north and south)
like poles...
repel
unlike poles...
attract
natural magnet
magnetite or lodestone
magnetic field lines
used to visualize a magnetic field. always a closed loop.
geographic north pole
really a south magnetic pole
uniform magnetic field
constant in magnitude and direction
right hand rule
determines the direction of the magnetic field
examples of electro magnets
metal recycle sorting, mag-lev train
moving electrons...
create a magnetic field
constant magnetic field produces...
no current
a CHANGING magnetic field...
can produce an electric current. such a current is called an INDUCED CURRENT.
electricity
changing magnetic fields induces the movement of electrons
2 connections between electricity and magnetism:
1 - moving electrons create a magnet
2 - a moving magnet makes electrons move!
Magnetic Flux (PhiB)
the number of magnetic field lines passing through a surface (such as a loop of wire)
electric motor
transforms electrical energy into mechanical energy
generator
the opposite of a motor - it transforms mechanical energy into electrical energy
examples of electric generators
wind turbine, generator for micro-hydro, hydro-power plant
if the electric current is AC
the magnetic field will be changing polarity with time (north to south pole) rather than a stationary
transformers
convert electricity (AC) to a changing magnetic field and back to electricity (AC) in the 2nd loop
INCREASING the voltage using a transformer...
allows us to DECREASE the current and transmit the same power. (P=IV)
ratio of the emfs (voltage)
equal to the ratio of the number of turns in each coil. Vs/Vp = Ns/Np
Is/Ip = Ns/Np
the ratio of the currents must be the inverse of the ratio of turns
step-up transformer
the induced voltage in the secondary coil is greater than the voltage of the primary coil
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