Physics Quiz 3
ConceptTest 19.1a: Assume that the voltage of the battery is 9 V and that the three resistors are identical. What is the potential difference across each resistor?
c) 3V because the sum of voltage drops across the resistors equals the battery voltage
Power is always positive. Also, when a question specifies "In the US, standard line voltage is 120 V ac," the 120V = Vrms. Similarly, if they specify a current, its Irms. If they specify a power, its the average power.
know this
Concept Test
the same
Ohm's Law: current always moves from (high/low) potential to (high/low) potential. The potential difference across the wire = the current through the wire * the resistance of the wire. V =
V = IR.
ConceptTest 19.1a: What is the voltage across R₁?
V(across resistor) = IR(of resistor). V (across a battery) = IReq. 12 V = I x 6, I = 2 A. V₁ = 2 A x 4 ohms = D) 8 V
Resistors in parallel: How do you find the equivalent resistance (Req), also called net resistance. How do you find total current from Req?
1/Req = (1/R₁) + (1/R₂) + (1/R₃)...Look at the picture for finding total current from Req
If two 1.5 V AA batteries are said to be connected in series, what is the total voltage?
3.0 V
Concept Test: Two wires, Aand B, are made of the same metal and have equal length, but the resistance of wire Ais four times the resistance of wire B. How do their diameters compare?
4) dA= 1/2 dB
Concept Test: A wire of resistance R is stretched uniformly (keeping its volume constant) until it is twice its original length. What happens to the resistance?
5) increases by a factor of 4. If L increases by 2, A decreases by a factor of 2 because the volume is kept constant. AL = V. (1/2)*2L = AL = V. R₁ = L/A. R₂ = 2L/(1/2)A = 4L/A.
What is electromotive force (EMF), internal resistance, and terminal voltage? Whats the equation? When does terminal voltage = EMF
As you can see in the picture, batteries have their own resistance within the battery (not talking about the circuit the battery supplies, but the literal battery). This resistance is called INTERNAL RESISTANCE (r). As the battery then draws a current, its voltage output to the circuit would decrease because of the internal resistance. The voltage, if there is no current drawn, therefore is the maximum voltage, and it is called Electromotive force (EMF). TERMINAL VOLTAGE (Vab) is the voltage the battery supplies once the current is drawn, and therefore depends on the resistor. If there is no current drawn, terminal voltage = EMF
Battery: positive terminal is connected to the negative terminal of the other. This is said to be connected __ _________. The voltages add up.
Battery: positive terminal is connected to the negative terminal of the other. This is said to be connected IN SERIES. The voltage add up.
Why does the sum of voltage drops across the resistor equal the battery voltage?
Because we know that for a 9V battery, by definition, has a potential difference of 9V from the positive end to the negative end. So, as we get to the negative end, the voltage should be 0.
Concept Test 18.1: Which is the correct way to light a lightbulb with a battery?
C) because, remember, a closed circuit is the only circuit that works, and you need to to go from the positive terminal to the negative terminal
Complete circuit vs open circuit
Complete circuit is where CURRENT CAN FLOW ALL THE WAY AROUND (from the positive terminal all the way around to the negative terminal). An open circuit if (if a wire was cut): NO CURRENT FLOWS.
Note: Current cannot go through a resistor in two directions. Why? This will help with Kirchoff's loop rule.
Current is technically the flow of protons, and protons will repel if they near each other. That's why current cannot flow across a resistor in both directions and this should help with identifying the direction of the current in loop rule.
direct current (DC) vs alternating current (AC)
Direct current--current steadily moves in one direction (therefore current and voltage are constant (e.g battery) Alternating current--current moves sinusoidally with time (from power plant)--current keeps reversing direction multiple times per second
AC: The average current is _____. The average power is ______. How do you calculate average power?
For AC, the average current is zero, but this doesn't mean the average power is zero (electrons are still moving and producing heat). Average power = (I₀²R)/2 = V₀²/2R
For resistors in series, Req is (greater/lesser) than the individual resistances. For resistors in parallel, Req is (greater/lesser) than the individual resistors.
For resistors in series, Req is GREATER than the individual resistances. For resistors in parallel, Req is LESSER than the individual resistors.
Concept Test 18.5b: wo space heaters in your living room are operated at 120 V. Heater 1 has twice the resistance of heater 2. Which one will give off more heat?
Heater 2. P = V²/R, R = V²/P More resistance = less power
Electric current (I) = amount of charge passing through a conductor / the time it takes =
I = ∆Q/t (C/S)
Peak current, peak voltage. Root mean squared current, root mean squared voltage. All used to calculate average power output in AC circuits. Know these equations
If average I² = (1/2)I₀², and average V² = (1/2)V₀², then the square root of these is the rms (root mean square) value of the current or voltage.
You have to know how to use kWh (kilowatt hours) for the quizzes and midterms. 1 kWh = ____ J?
It is a unit of energy. 1 kWh = (1000 W) (3600 s) = 3.60 x 10⁶ J
P = IV is power transmitted. P = V²/R and P = I²R are power lost due to the resistor (power consumed by the resistor. Usually the power transmitted is the (same/different) as the power lost. ACTUALLY I BELIVEVE YOU ONLY USE P = I²R FOR POWER LOST
P = IV is power transmitted. P = V²/R and P = I²R are power lost due to the resistor (power consumed by the resistor. Usually the power transmitted is the SAME AS the power lost. This is not true for when there is a fixed power transmitted (from a company).
Power = energy transformed per unit time = current times voltage = ? What is the unit of power?
P = QV/T = IV. (Q/T) = I, thats why P = IV. Unit of power = [W]
Resistance is directly proportional to the resistor's length and inversely proportional to it cross-sectional area. R=
R = ρL/A, where ρ is the resistivity constant (units = Ωm). cross section IS USUALLY pi *r² cause its a circle
Conceptest: The three lightbulbs in the circuit all have the same resistance of 1 ohm. By how much is the brightness of bulb B greater or smaller than the brightness of bulb A? (brightness proportional to power)
READ ALL OF THIS. The voltage supplied to A is 10 V. The voltage supplied to B is NOT 10 V, because there are two resistors on that line (in series) and therefore they share the voltage, just as they would in series. Vb = Vc = 5V. P = V²/R =5²/1 = 25. P = 10²/1 = 100. The power for bulb B is 1/4, so the answer is D. THEREFORE, FOR PARALLEL RESISTORS, THE VOLTAGE SUPPLIED TO EACH LINE IS THE SAME
Resistors in parallel: why are the currents different but the potentials the same?
READ ALL OF THIS. Think about it, for resistors in series, the SAME current had no choice but to go through each resistor. However, for resistors in parallel, the current reaches a node and "choose" which resistor it will go though. More current will go through the lesser resistor, and less current will go through the greater resistor. But, V = IR, the current times the resistance = voltage will be the SAME.
ConceptTest: The lightbulbs in the circuit below are identical with the same resistance R. Which circuit produces more light? (brightness proportional to power)
Refer to the picture on flash card 42. When resistors are in parallel, the total resistance is lower and the total power output is higher than when in series. So b) circuit 2
Reminder: as you go across a resistor, charge and therefore current (increases/decreases/stays the same) and voltage (increases/decreases/stays the same).
Reminder: as you go across a resistor, charge and therefore current STAYS THE SAME and voltage DECREASES. This is known as voltage drop.
Resistivity (ρ) (increases/decreases) as temperature increases. Therefore resistance (increases/decreases).
Resistivity INCREASES. as temperature increases. Therefore, resistance INCREASES. Resistance increases because electrons acquire more KE, so they touch each other more and repel more, interfering with current.
Resistors are used for? Does every conducting material have resistance? How do you indicate resistance on a circuit diagram?
Resistors are used to control the amount of current going through a wire. Every conducting material has resistance, but good conducting materials have negligible resistance. Resistors are indicated with squiggly lines
Resistors in parallel: the current traveling through each resistor is (different/the same), but the voltage drop from each resistor ONLY IF THERE IS ONE RESISTOR PER PARALLEL LINE is (different/the same). The voltage across each resistor is the voltage of the _______. The sum of currents across each resistor equals _________. If you disconnect one device, the current to the other is NOT interrupted.
Resistors in parallel: the current traveling through each resistor is DIFFERENT, but the voltage drop from each resistor ONLY IF THERE IS ONE RESISTOR PER PARALLEL LINE is THE SAME. The voltage across each resistor is the voltage of the BATTERY. The sum of currents across each resistor equals the TOTAL CURRENT. If you disconnect one device, the current to the other is NOT interrupted.
Resistors in series: the current traveling through each resistor is (different/the same), but the voltage drop from each resistor is (different/the same). The sum of voltage drops across the resistor equals the _______ _______.
Resistors in series: the current traveling through each resistor is THE SAME, but the voltage drop from each resistor is DIFFERENT. The sum of voltage drops across the resistor equals the BATTERY VOLTAGE.
Concept Test 19.2a: In the circuit below, what is the current through R1?
The voltage through resistor 5 = the voltage of the battery = 10 V. V = IR. 10 = 5I, I = 2. d) 2A
There is a continuous flow of ________ once the wire is connected to both terminals. Current flows from the (positive/negative) terminal to the (positive/negative terminal), which is in the (same/opposite) direction as electron flow.
There is a continuous flow of electrons once the wire is connected to both terminals. Current flows from the POSITIVE terminal to the NEGATIVE terminal, which is in the OPPOSITE direction as electron flow. Why is it like this? It's just due to convention, because scientists screwed up lol.
Resistors in series: How do you find the equivalent resistance (Req), also called net resistance. The voltage of the battery =
To find the equivalent resistance (Req) also called net resistance: Req = R₁ + R₂ + R₃... The voltage of the battery = IReq
VERY important clarifications about ohm's law and resistors: 1) Batteries (the supplier of voltage) maintain a nearly constant _________, but the current __________ (due to resistance) 2) Current is NOT a vector, although it does have direction 3) Charge is conserved as charges flow through resistors. Therefore, _______ at point A _______ at point B 4) The potential at point A is (more/less) Than the potential at point B. This is called a:
VERY important clarifications about ohm's law and resistors: 1) Batteries (the supplier of voltage) maintain a nearly constant POTENTIAL DIFFERENCE, but the current VARIES (due to resistance) 2) Current is NOT a vector, although it does have direction 3) Charge is conserved as charges flow through resistors. Therefore, CURRENT at point A CURRENT at point B 4) The potential at point A is MORE Than the potential at point B (BECAUSE POTENTIAL DECREASES IN THE DIRECTION OF THE ELECTRIC FIELD--AS YOU MOVE TOWARD THE NEGATIVE CHARGE. This is called a: POTENTIAL DROP (VOLTAGE DROP)
If an instrument is rated at 250 W and connected across a 120 V power supply: a) what is the current? b) what is the resistance? c) How many kilowatt hours does it use in a day if its left on all the time?
a) 2.1 A b) 57.6 Ω c) 6 kWh 250 W * 24 h = 6000 Wh = 6 kwH
Concept Test: Two lightbulbs operate at 120 V, but one has a power rating of 25 W while the other has a power rating of 100 W. Which one has the greater resistance?
a) The 25 W bulb. P = V²/R, R = V²/P More resistance = less power
ConceptTest: Two lightbulbs A and B are connected in series to a constant voltage source. When a wire is connected across B, bulb A will:
a) glow brighter than before. If the shunt is closed, it will make A and B in parallel. When they are in parallel, there is less resistance. Instead of SHARING the battery's voltage, they both get the batteries voltage.
What do batteries do?
adds energy to a circuit by having a potential difference (once there is a potential difference, charges can flow)
In many conductors, resistance (R) is independent of voltage. Therefore it is __________. What is the unit for resistance?
constant. The unit is ohm Ω = V/A. Resistance is constant because the resistance has to do with the material.
Concept Test 19.6: Which resistor has the greatest current going through it? Assume that all the resistors are equal.
d) R₅ because thats the path of least resistance
I(across resistor) = V/R(of resistor). I (across battery) =V/Req
know this
In series: V(across resistor) = IR(of resistor). V (across a battery) = IReq
know this
