Physics II Chapters 20/21 Discussion Questions
Could a person on intravenous infusion (an IV) be microshock sensitive?
"Microshock" refers to the occurrence of when some type of shock is close enough to the heart to disrupt its normal activity. Based on what I'm reading on the internet, this is not possible from an IV unless the IV where to be placed directly into the heart. Microshock supposedly only happens during certain medical procedures where things come in direct contact with the heart.
If such a string operates on 120 V and has 39 remaining identical bulbs, what is then the operating voltage of each?
120V/39 = 3.08 V
If two household lightbulbs rated 60 W and 100 W are connected in series to household power, which will be brighter? Explain.
A lower watt bulb has a thinner wire filament, which will give that bulb a higher resistance. The power associated with the bulb is given by several equations including P=I2R. The bulb with the higher resistance (the 60 W) will have the higher power and glow more brightly. (Remember that series resistors have the same current, so the I in our equation will be the same for both lightbulbs.)
A sinusoidal wave on a voltage vs. time graph represents __________________.
AC current
The flow of electric charge that periodically reverses direction.
AC current
Give an example of a use of AC power other than in the household. Similarly, give an example of a use of DC power other than that supplied by batteries.
AC: electric motors (e.g. household appliances) DC:the power supply for electronics; smelting aluminum
Knowing that the severity of a shock depends on the magnitude of the current through your body, would you prefer to be in series or parallel with a resistance, such as the heating element of a toaster, if shocked by it? Explain.
Based on I=V/R, the smaller the resistance, the larger the current. Therefore, we want the largest resistance possible to have the smallest current. Thus, we need our resistors in series.
Explain why R = R0(1 + αΔT) for the temperature variation of the resistance R of an object is not as accurate as ρ = ρ0(1 + αΔT) , which gives the temperature variation of resistivity ρ .
Because the R equation contains the ρ equation. (Anytime we use a calculated value in another equation, it lessens the accuracy of that equation.)
Why isn't a short circuit a shock hazard?
Because the circuit is broken, and a shock can't occur.
Car batteries are rated in ampere-hours ( A ⋅ h ). To what physical quantity do ampere-hours correspond (voltage, charge, . ..), and what relationship do ampere-hours have to energy content?
Charge. Remember that I=Q/time, which has units of C/s defining the ampere. Thus, if we multiply A*hour, we're multiplying I*time to give charge.
The flow of charge in only one direction
DC current
Is every emf a potential difference? Is every potential difference an emf? Explain.
Emf is the value for a source when NO current is running. Thus, any potential difference when a current is running is not the emf.
How is the IR drop in a resistor similar to the pressure drop in a fluid flowing through a pipe?
From our notes: When a water pump (the battery) releases water (electric current), the water flows through the pipes. When the water reaches a larger area within the pipe (the resistor), the pressure drops and the flow decreases. In the case of the resistor, the resistant material is actually converting some of the electrical energy into another form of energy, e.g. thermal.
Two different 12-V automobile batteries on a store shelf are rated at 600 and 850 "cold cranking amps." Which has the smallest internal resistance?
I=E/r (or V/r), which rearranges to r=E (or V)/I. Thus, the battery with the higher amps has the smaller resistance.
The power dissipated in a resistor is given by 𝑃 = 𝑉 ^2/ 𝑅 . , which means power decreases if resistance increases. Yet this power is also given by 𝑃 = 𝐼^2𝑅, which means power increases if resistance increases. Explain why there is no contradiction here.
If we substitute 𝐼 ∝ 𝑉 /𝑅 in for current in the second equation, then we will reduce it to the first equation. See the following... 𝑃 = 𝐼^2𝑅 = (𝑉^2/ 𝑅)𝑅 Thus, really, the resistance is always in the denominator.
Suppose you are doing a physics lab that asks you to put a resistor into a circuit, but all the resistors supplied have a larger resistance than the requested value. How would you connect the available resistances to attempt to get the smaller value asked for?
In parallel because the 1/Rp means that the total resistance of all resistors used will be smaller than any of the resistors.
What are the advantages and disadvantages of connecting batteries in series? In parallel?
In series, their voltages (emfs) add and their internal resistances add. Thus, there is more voltage for the same amount of time of a single battery. In parallel, the voltage stays the same as a single battery, but the current is increased by all the batteries.
What determines the severity of a shock? Can you say that a certain voltage is hazardous without further information?
It's mainly the current. No, you can't say that a certain voltage is hazardous or not without accompanying information about the current.
Can all of the currents going into the junction in Figure 21.46 be positive? Explain
Kirchoff's junction rule tells us that the sum of two of these will equal the third. Thus, we could write any of the following: I2=I1+I3 or I1=I2+I3 or I3=I2+I1 But one or more of these currents will have to be negative because they are not going in the same direction. For example, I1 and I3 could be the same direction (e.g. +) while I2 is negative. Or I1 and I2 could be + while I3 is negative.
states that current is proportional to voltage
Ohm's Law
Some light bulbs have three power settings (not including zero), obtained from multiple filaments that are individually switched and wired in parallel. What is the minimum number of filaments needed for three power settings?
Only two filaments are needed because the lowest speed is the filament with the highest resistance. The medium speed is the filament with the lower resistance. And the third speed is both filaments being used together. (The "zero" speed in most lamps is the off position, which means the circuit is open (not flowing electricity).
If aluminum and copper wires of the same length have the same resistance, which has the larger diameter? Why?
Table 20.1 shows that copper has a lower resistivity. If we take our resistance formula and rearrange it to solve for radius, we see that copper will have a lower resistivity and therefore a lower radius. Thus, aluminum will have the larger diameter wire. 𝑅 = 𝜌 (𝐿 /𝐴) → 𝑅 = 𝜌 (𝐿 /𝜋𝑟 ^2) → 𝑟 = √𝜌 (𝐿 /𝜋R)
Explain which battery is doing the charging and which is being charged in Figure 21.45.
The battery with the higher current (more + number) will be charging the other battery because the higher current determines the direction of flow. Thus, if we use I=E/r, the 12.0 V battery has a 12 A current while the 18.0 V battery has a 36 A current. Thus, the 18.0 V battery is charging the 12.0 V.
In cars, one battery terminal is connected to the metal body. How does this allow a single wire to supply current to electrical devices rather than two wires?
The circuit is always completed whether it's through use of a wire or some other conducting material. As long as there is a continuous pathway of conducting material (wires or metal components), then the current can flow.
If such a string operates on 120 V and has 40 identical bulbs, what is the normal operating voltage of each?
The current is constant for series resistors, but voltage changes. Therefore, Vtot=V1+V2+V3+...=40V1---->V1=120V/40=3V
You are riding in a train, gazing into the distance through its window. As close objects streak by, you notice that the nearby fluorescent lights make dashed streaks. Explain.
The fluorescent lights run on AC power, which actually flickers due to the cycle of AC power. The motion of the train causes the flickers to actually be noticeable in "streaks".
Note that in Figure 20.28, both the concentration gradient and the Coulomb force tend to move Na+ ions into the cell. What prevents this?
The repolarization of the region.
A student in a physics lab mistakenly wired a light bulb, battery, and switch as shown in Figure 21.44. Explain why the bulb is on when the switch is open, and off when the switch is closed. (Do not try this—it is hard on the battery!)
The switch is not in the circuit with the battery and the light bulb. The switch should be between the lamp and the battery in order to work properly. With the switch open as shown, the current flows through the lamp (device). When the switch is closed, the current will flow through the switch and not the device causing the device to have no current flowing through it.
Why do incandescent lightbulbs grow dim late in their lives, particularly just before their filaments break?
The wire (filament) actually starts to disintegrate, which causes a reduction in power. Remember from section 3 that a decrease in the cross-sectional area of the wire will cause R (resistance) to increase. Also, 𝑃 = 𝑉^ 2/ 𝑅 ., so an increase in R causes a decrease in power.
In which of the three semiconducting materials listed in Table 20.1 do impurities supply free charges? (Hint: Examine the range of resistivity for each and determine whether the pure semiconductor has the higher or lower conductivity.)
There are three impure substances listed: carbon, germanium, and silicon. Carbon (pure) 3.5×10^5 Carbon (3.5 − 60)×10^5 Germanium (pure) 600×10^−3 Germanium (1 − 600)×10^−3 Silicon (pure) 2300 Silicon 0.1-2300 The question gives us a hint to look at the range of numbers. Since resistance means that it resists the flow of charge, the high number means higher resistance. Thus, we're looking for the impure substances whose pure substances are on the TOP of the range, e.g. germanium and silicon. For example, when germanium is doped with other substances, it will LOWER its resistance, which means it's a better conductor.
What are the two major hazards of electricity?
Thermal (an increase in heat that causes a short) and shock (when electric current passes through a person).
Why are two conducting paths from a voltage source to an electrical device needed to operate the device?
This question is actually asking why are there two wires coming out of battery or something like that. There needs to be a complete circuit, which almost always means that two wires must be used and must be connected together at both ends unless there's some other conducting material in between the wires.
An electrified needle is used to burn off warts, with the circuit being completed by having the patient sit on a large butt plate. Why is this plate large?
To create a larger grounding surface.
Some devices often used in bathrooms, such as hairdryers, often have safety messages saying "Do not use when the bathtub or basin is full of water." Why is this so?
Water conducts electricity and will complete the circuit between you and the appliance causes a shock.
Before World War II, some radios got power through a "resistance cord" that had a significant resistance. Such a resistance cord reduces the voltage to a desired level for the radio's tubes and the like, and it saves the expense of a transformer. Explain why resistance cords become warm and waste energy when the radio is on.
When the radio is turned on, the circuit is "closed" (all connected), which allows the electricity to run through the circuit. Every substance has resistance. Thus, when electricity is running through the resistance cord, some electrical energy is transformed to thermal energy (heat). This heat dissipation is a waste of the electrical energy.
Does the resistance of an object depend on the path current takes through it? Consider, for example, a rectangular bar—is its resistance the same along its length as across its width? (See Figure 20.37.) Figure 20.37 Does current taking two different paths through the same object encounter different resistance?
Yes. Consider one of the resistance formulas, 𝑅 = 𝜌 𝐿 𝐴 . This formula indicates that R increases with an increasing length but a decreasing area. In our figure above, the length is longer for the diagram on the left, and it will have an increase in resistance. (Of course, resistance is also inversely proportional to the cross-sectional area, which will change, too. So we really need both the actual numbers of length and area to determine the resistance.)
the rate at which charge flows
electric current
defined as current times voltage, but other equations can be generated using Ohm's law.
electrical power
T or F: Shock is only dependent on voltage.
false --> current
T or F: Decreasing the diameter of a wire in a circuit helps reduce voltage drop.
false --> increasing the diameter
T or F: The direction of a current is always towards the positive end of a battery.
false --> negative end of a battery
T or F: A shock hazard occurs when more heat is produced by an electrical appliance than is desired.
false --> thermal hazard
Semitractor trucks use four large 12-V batteries. The starter system requires 24 V, while normal operation of the truck's other electrical components utilizes 12 V. How could the four batteries be connected to produce 24 V? To produce 12 V? Why is 24 V better than 12 V for starting the truck's engine (a very heavy load)?
in series; in parallel; The larger the emf, the more terminal voltage the system should have for starting the truck engine.
a factor that helps slow the flow of charge.
resistance
see notes for #20-#23
see notes for #20-#23
Resistance is dependent on ____________ and _______________ of the wire.
shape; material
T or F: AC current is easier to transport longer distances because less energy is lost due to resistance.
true
Some surgery is performed with high-voltage electricity passing from a metal scalpel through the tissue being cut. Considering the nature of electric fields at the surface of conductors, why would you expect most of the current to flow from the sharp edge of the scalpel? Do you think high- or low-frequency AC is used?
"The sharp edges increase the current density" 1 , which would cause a better, more precise cut.
Why do voltage, current, and power go through zero 120 times per second for 60-Hz AC electricity?
60 Hz means that there are 60 cycles in one second. This cycle is a sine wave, and one cycle crosses the zero axis twice. Thus, 60*2=120 times.
Why is the resistance of wet skin so much smaller than dry, and why do blood and other bodily fluids have low resistances?
Because water has a low resistivity, ρ, which correlates to the number of free electrons in a material. Pure water doesn't conduct well; it's the ions in the water that cause the conductivity. The ions act like free electrons with their movement.
Define depolarization, repolarization, and the action potential.
Depolarization occurs when the membrane becomes permeable to the Na+. Repolarization is when the membrane becomes impermeable to the Na+. Active transport is the series of depolarization and repolarization.
The IR drop across a resistor means that there is a change in potential or voltage across the resistor. Is there any change in current as it passes through a resistor? Explain.
IR drop is defined as V=I*R. R (as we'll learn in section 3) does change with an increase in area or length of the wire. But I doesn't change. Therefore, no, there is no change in current, I, as it passes through the resistor. Just a change in voltage.
Before working on a power transmission line, linemen will touch the line with the back of the hand as a final check that the voltage is zero. Why the back of the hand?
Muscle contractions occur with shock, and the front of the hand would close over the wire causing further shock.
Why isn't a bird sitting on a high-voltage power line electrocuted? Contrast this with the situation in which a large bird hits two wires simultaneously with its wings.
The bird sitting isn't part of the circuit. The bird that hits two wires actually creates a new circuit that flows from one wire through the bird to the other wire.
In view of the small currents that cause shock hazards and the larger currents that circuit breakers and fuses interrupt, how do they play a role in preventing shock hazards?
The main way is that the circuit breaker will "throw" and break the circuit cause the shock to stop. (The shock usually causes a large enough change in the current due to traveling through the human body, and this change "throws" the breaker.)
Using an ohmmeter, a student measures the resistance between various points on his body. He finds that the resistance between two points on the same finger is about the same as the resistance between two points on opposite hands—both are several hundred thousand ohms. Furthermore, the resistance decreases when more skin is brought into contact with the probes of the ohmmeter. Finally, there is a dramatic drop in resistance (to a few thousand ohms) when the skin is wet. Explain these observations and their implications regarding skin and internal resistance of the human body.
The resistance is on average the same across the human body. The resistance equation, 𝑅 = 𝜌 (𝐿 /𝐴) , shows that R is inversely proportional to area. Thus, if more skin is in contact with the probes, then the total R decreases. Water allows for better contact between the skin and the probe causing an increase in A, which causes a decrease in R.
Some strings of holiday lights are wired in series to save wiring costs. An old version utilized bulbs that break the electrical connection, like an open switch, when they burn out. If one such bulb burns out, what happens to the others?
They all go out.
Newer versions use bulbs that short circuit, like a closed switch, when they burn out. If one such bulb burns out, what happens to the others?
They stay lit.
Would your headlights dim when you start your car's engine if the wires in your automobile were superconductors? (Do not neglect the battery's internal resistance.) Explain
This is similar to Exercise 9 where we saw that the fridge lightbulb dimmed when the fridge motor turned on. (The power output by the fridge bulb is decreased when the motor is on because the current is reduced by the motor taking some of the current (essentially from the lightbulb). Remember that the bulb and the motor are wired in parallel, which has a constant voltage over the resistors. But their current is different. If one resistor, the motor, takes part of the current (and also part of the power), then there is less current (and, therefore, less power output) for the other resistor. If the car's headlights and engine are wired in parallel (they are), then the same effect will happen with the car. Now, if the wires are superconductors, they do have less resistance than regular wires. Therefore, the dimming effect shouldn't be as noticeable because we will still have a lot of power. But if we consider the battery's internal resistance, which is a lot higher than the superconductor wires, then there is still enough loss of current that we will notice the dimming.
We are often advised to not flick electric switches with wet hands, dry your hand first. We are also advised to never throw water on an electric fire. Why is this so?
Water conducts electricity and will provide an "extension" to the circuit. Therefore, a spark could jump out of the switch to you, and water on an electrical fire could cause it to spread.
Can a wire carry a current and still be neutral—that is, have a total charge of zero? Explain.
Yes. The net amount of electrons doesn't change. When one electrons enters the wire, it pushes another electron out of the wire.
T or F: Resistance is independent of current and voltage.
true
T or F: Resistivity increases with increasing temperature.
true
