DC stands for ___
direct current
An electric circuit needs a battery or generator to produce ___
current
a battery/generator that produce currents are called sources of ___ force
electromotive
electromotive force is notated by:
emf or ε
Emf is a ___ btw the terminals where no ___ flows to an external circuit
voltage//current
T/F electromotive force is a force
False, it's a voltage (badly named)
If you want to find the magnitude of a battery, you must first...
disconnect it from the external circuit
the terminal voltage of a real battery ___ (increases/decreases) as we draw more current
decreases
If the current is increased, what happens to the terminal voltage?
it decreasesVab = emf - Ir
When no current is drawn from a battery, the terminal voltage of the battery is... PLa. equal to its emfb. greater than its emfc. less than its emf
a. equal to its emf( Vab = ε - Ir)
Resistors are in series when they are connected end to end along ___ (multiple/single) path
single
T/F the current that goes through each resistor is the same
True
T/F the amount of charge that passes through R1 is the same that passes through R2
True
When a new resistor is added to a circuit and is connected in series w/ the existing resistors, the current thru the battery will ___ (inc/dec/stay the same) PL
decrease
T/F resistors are in parallel when the current from the source splits into separate branches
True
T/F the voltage across each resistor in a parallel resistor is the same
True
T/F the total current is the sum of the current in each resistor
True
1. Resistors in parallel, Req __ Ri2. resistors in serises, Req __ Ri
1. Req < Ri2. Req > Ri
If they ask for the intensity, what are we looking at to see if it increased/decreased/stays the same?
Power
A 10 ohm resistor is placed in parallel with a 1 ohm resistor. The current through the 10 ohm resistor is 1A. What is the voltage across teh 1ohm resistor?a. 10Vb. 1Vc. 100V1. what is known?2. What equation?3. answer
1. R1 = 10ohm R2 = 1 ohm I1 = 1A I2 = ? V = ?2. I = V/R3. a. 10V (V is the same across all resistors in parallel)
Resistors in series:1. the current that goes through each resistor is ___ (different/the same)2. The voltage across the resistors is ___ (different/the same)resistors in parallel:3. the current that goes thru each resistor is ___ (different/the same)4. the voltage across resistors is ___ (different/the same)
1. the same2. different1. different2. the same
resistors in series:1. the equivalent resistance is ___ (larger/smaller) than the resistance of any individual resistorresistors in parallel:2.the equivalent resistance is ___ (larger/smaller) than the resistance of any individual resistor
1. larger2. smaller
T/F complex circuits can still be broken down into series and parallel pieces
False (will use Kirchhoff's rules)
kirchhoff's rules:
junction ruleloop rule
junction rule states that at any junction point, the sum of all currents entering the junction, must ___ the sum of all currents ___ the junction
equal//leaving
the junction rule is based on conservation of ___
electric charge
If the current entering a junction is 1A, what is the total amount of charge leaving the junction in 1 minute? PL1. what is known?2. equation3. answer
1. I = 1A ; t = 60s2. I = Q/t -> Q = It3. 60C
Loop rule: the sum of the changes in potential around any closed path of a circuit must be ___. This is based on the ___
zero//conservation of energy
From points a to b, there is no resistance present. What is the voltage?
0 (V = IR ; R = 0 -> V = I(0))
For a resistor use ___
Ohm's
For a battery use ___
kirchhoff's rules (??? maybe))
for a resistor:1. Change in potential is ___ (+/-) if the loop is in the same direction as the current direction thru that resistor2. The change in potential is ___ if the chosen loop direction is opposite to the chosen current direction
negative//positive
For a battery:1. the change in potential is ___ if teh loop direction is from the negative to positive terminal2. the change in potential is ___ if the loop direction is from the positive to negative terminal
1. positive2. negative
What is true of the loop rule? PLa. the choice of a loop direction is arbitrary, ie. you can choose whatever directionb. the loop direction must be the same as the direction of the currentc. the loop direction must be pointing from the positive terminal of a battery to its negative terminal
a.
What is the first step in applying Kirchhoffs rules?
Label the circuit!!! (resistors and batteries)
For Kirchhoff's rules, is it easier to apply the junction or loop rule first?
junction rule
1. A negative I sign indicated that the actual current direction is the ___ (same/opposite) to that initially assumed2. A positive I sign indicates that the actual current direction is the ___ (same/opposite) as that initially assumed
1. opposite2. same
capacitors connected in a series all carry ___ (different/the same) charge
the same
capacitors in a series, Ceq is ___ (larger/smaller) than any of the individual capacitance
smaller
Capacitors in parallel, have the same ___ across each capacitor
voltage
Capacitors in parallel, Ceq is ___ (smaller/larger) than Ci
larger
which statement is true:a. when capacitors are connected in series, the equivalent capacitance is less than the smallest capacitanceb. when capacitors are connected in series, the equ C is less than the largest Cap.c. When cap. are connected in parallel, the equ. cap. is less than the greatest Cap.
a.
examples of RC circuits in everyday life:
heart pacemakerscamera flashescontrolling the timing of traffic lights and windshield wipers
T/F In a circuit containing resistors, charge appears almost instantaneously
False
T/F In a circuit containing only batteries and capacitors, charge appears almost instantaneously
True
Tau is the ___ of the circuit
time constant
Tau tells how quickly the capacitors can be ___
charged
SI unit of Tau
sec
greater Tau means ___ (longer/shorter) time
longer
T/F after a capacitor is charged, it can be discharged
True (thru a resistor)
A capacitor is being charged in an RC circuit, the current flowing thru a resistor is:a. decreasing with timeb. increasing with timec. constant
a. decreasing with time
When a switch is closed, the current immediately begins to flow thru the circuit. Charge will accumulate on the ___, and the current is ___ until eventually the voltage across the capacitor equals the ___ of the battery. Then the voltage across the resistor is ___, and no further current flows.
capacitor//reduced//emf//zero
A current of about ___ or more can be felt and may cause pain
1mA
Currents above ___ can cause severe contraction of the muscles
10mA
What is the difference btw a 2 prong plug and a 3 prong plug?
A 3 prong plug, has a prong that connects to the ground (ground wire)
If someone touches a metal box that has a wire touching the inside of the box, and it has a 2 prong plug, will it shock them? What about a 3 prong plug?
2 prong: yes3 prong: no
A ___ is an instrument that measures current
Ammeter
T/F an ammeter must be inserted directly into the circuit so that the current passes directly thru it
True
Should an ammeter have a high or low resistance?
low, to have the least disturbance possible
T/F an ammeter has no resistance
False, it has an internal resistance (just want Ra to be minimal)
An ammeter is in ___ (series/parallel) with the circuit
series
A ___ measures the potential drop between two points
Voltmeter
A voltmeter is connected in ___ (series/parallel)
parallel
A voltmeter should have as ___ (small/large) a resistance as possible
large (Rv)
Why must the voltmeter have a large resistance?
it's in parallel, and therefore it must be large to prevent the current from passing thru the voltmeter
Why is an appliance cord w/ a 3 prong plug safer than one with two prongs? PLa. the third prong grounds the case, so the case cannot reach a high voltageb. three prongs fasten more securely to the wall outletc. the third prong controls the capacitance of the appliance, so it can't build up a high voltage
a.
The change in potential is ___ (+,-) if your chosen loop direction is the same as teh chosen current direction thru that resistor.The change in potential is ___ (+.-) if your chosen loop direction is opposite to the chosen current direction.
-//+
In order to have a circuit in an electric current, you need what?
a battery or electric generator (that transforms energy into electric energy)
A device that transforms energy from chemical/mechanical/light energy into electric energy is called a ___
electromotive force or emf
unit of emf
Volt; V
the ____ between the terminals of a source, when no current flows to an external circuit, is called the ___ of the source
potential difference//emf
T/F a battery usually has internal resistance
True; designated r
When no current is drawn from the battery, Vba =
emf
A 65ohm resistor is connected to the terminals of a battery whose emf is 12V and whose internal resistance is 5ohm. Calculate the current in the circuit. BPa. What is known?b. What equation should be used?c. answer
a. R = 65ohm emf = 12V r = 5ohmb. Vab = emf - Ir Vab = IRc. 0.18A
T/F the same current passes through multiple resistors
True
What would it mean if current was not constant across resistors?
that charge wasn't conserved or that charge was accumulating somewhere
When you add more resistance to the circuit, the current through the entire circuit will ___ (decrease/increase/not change)
decrease
why must the current I flowing into a junction equal the current flowing out of the junction?
charge is conserved
Req for resistors in series will be ___ (more/less) than any single resistor
more
Req for resistors in parallel will be ___ (more/less) than any single resistor
less
placing resistors in series effectively increases the ___, and therefore the resistance. placing resistors in parallel increases the ___, and therefore decreases resistance.What equation does this relate to?
length//areaR = pl/A
the ___ is related to the light produced
power
the greater current will produce ___ (more/less) light
moreP = IV
Will two lights in series or parallel produce more light? Why?
parallelRp(parallel) < Rs (series)I = V/RIp > IsP = IVPp > Ps
Are headlights of a car made in parallel or series? Why?
parallelif one went out, both would go out if they were in series
When adding resistors, do you include the internal resistor r?
Yes
kirchoff's first rule is the ___ ruleKirchoff's second rule is the ___ rule
junction//loop
loop rule states that the sum of the changes in ___ around any closed loop of a circuit must be zero
potential
T/F potential can be negative
True
When crossing a 20V battery from + to -, the net change is:
-20V
When crossing a 20V battery - to +, then a second 12V batter + to -, what is the net change?
20V - 12V = 8V
There are two capacitors in parallel, that are then in series with a third capacitor. All three capacitors equal 3F. The battery voltage V = 4V. Determine the charge on each capacitor and the voltage across each. BPa. What is known?b. What must be found first?c. What must be found second?d. What is the Q across C3?e. What is the Q across C1 and C2 (in parallel)?f. What is the voltage for each C?
a. C = 3F V = 4Vb. Ceq 1/Ceq = 1/(C1 + C2) + 1/C3 = 3/2 -> 2/3 Cc. the Q that leaves the battery: Q = CV Q = (2/3)(3F)(4V) = 8Cd. 8Ce. 4C eachf. V1 = Q/C1 = 8C/3F = 2.7V V2 = V3 = Q/C = 4C/3F = 1.3V
RC circuits contain:
resistors and capacitors
What are RC circuits used in?
wind shield wiperstraffic lightscamera flashesheart pacemakers
As current flows, electrons accumulate on the upper plate of the capacitor and electrons flow into the positive terminal of the battery. Does the capacitor have a negative or positive charge on the upper plate? lower plate?
upper plate: negativelower plate: positive
The voltage across a capacitor ___ (inc/dec) as current flows
increases (charge accumulates on plate and therefore Q inc. V = Q/C)
voltage across capacitor as a function of time graph:
0
What is the equation of the time constant of an RC circuit?
T = RC
In a circuit, R = 200kohm and C = 3uF. What is the time constant?
T = RCT = (200000ohm)(3x10^-6F) = 0.60s
What is the Time Constant of an RC circuit?
The time it takes for a fully discharged capacitor to be charged to 63.2% of its maximum value of V and Q
charge on a capacitor ___ (inc/dec) as current flows
Increases
T/F the charge graph is similar to the voltage graph across a charging capacitor
True
If resistance is smaller, the T is ___ (larger/smaller), and therefore the capacitor becomes charged more ___ (slowly/quickly)
smaller//quickly
T/F a capacitor can be charged instantaneously when connected to a battery
False, it can never charge instantaneously due to resistance in wires
current across a resistor ___ (inc/dec) with time
decreases
When is the current largest in a RC circuit?a. at the very "end"b. at the very beginningc. When the S switch is open
b. There is no charge on the capacitor to impede it
As charge builds on a capacitor, current ___ in time
decreases
the max charge on the capacitor is when...
no current flows
What equation can be used to find the max charge on a capacitor?
Q₀ = CεQ₀ is the final charge on a charging capacitor
The capacitance in a circuit is C = 0.3uF, R = 20,000ohms, and the battery emf = 12V. Determine the time it takes for the charge to reach 99% of the max charge. BP (19-12)a. What is known?b. What must be found first?c. What equations can be used?d. answer
a. C = 3x10^-7F R = 20,000ohms ε = 12Vb. Max chargec. Q₀ = Cε = 3.6x10^-6C Q = Q₀(1 - e^-t/T) T = RC = 0.006sd. Q/Q₀ = 0.99 0.99 = 1 - e^-t/T t = 0.028s
At what time is the max current?
t = 0( Imax = ε/R)
Qo is the final charge on a ___ (charging/discharging) capacitor
charging
Qo is the initial charge on a ___ (charging/discharging) capacitor
discharging
For a discharging capacitor, V ___ with time
decreases
The current in a discharging capacitor has its greatest magnitude at ___
t = 0
In a circuit connected to a lightbulb, the capacitor is originally uncharged. What happens when the instant switch S is closed after a long time?
-at first, the bulb will be very bright due to a large I-after some time, I decreases due to an increase in charge which increases V, and therefore the lightbulb dims -after a long time, eventually the capacitor equals the emf and the lightbulb will go out
An ___ measures current and a ___ measure potential difference (voltage)
ammeter//voltmeter
Changes in Req changes the ___ (voltage/current) directly
current
If the current across a resistor increases, then the voltage ___
increases
when Req decreases, does this impact the power output?
Yes, P increasesI = 1(V)/ReqI -> increasesP = IRP -> increases
T/F: Capacitors in series have the same charge
True
T/F capacitors in parallel have the same charge
False
Four 1.50-V cells are connected in series to a 11.0-Ω lightbulb. If the resulting current is 0.31 A, what is the internal resistance of each cell, assuming they are identical and neglecting the resistance of the wires?a. What is known?b. What is the emf eq?c. What can the internal resistance eq be equal to?d. What equation can be used?e. Answer
a. emf = 1.5V Rb = 11ohm I = 0.31Ab. emf eq = 4(1.5V) = 6c. req = 4fd. Req = Rb + req Req = V/I V/I = Rb + reqe. 2.1ohm