Physics Chapter 34 Chapter Assessment
(skip 34.8)
...
How many joules per coulomb are given to charges that flow in a 120-volt circuit?
120 joules
The resistance of a certain wire is 10 ohms. What would the resistance of the same wire e if it were twice as long? If it were twice as thick?
20 Ω; 2.5 Ω, because twice the diamter gives four times the cross sectional area and one-fourth the resistance
Calculate the yearly cost of running a 5-W electric clock continuously in a location where electricity costs 10 cents per kW*h.
5 W * $0.10/kWh * (1 kW)/(1000 W) * (24 h)/(1 day) * (365 day)/(1 yr) = $4.38/yr
Does voltage flow through a circuit, or is voltage established across a circuit?
Across
What is Ohm's Law?
Current = voltage/resistance, or I =V/R
What is the effect on current if both the voltage and the resistance are doubled? If both are halved?
Current remains the same in both cases.
Distinguish between DC and AC. Which is produced by a battery and which is usually produced by a generator?
DC--current flows in one direction; AC--direction alternates; DC is produced by a battery, AC by a generator.
What is a typical "drift" speed of electrons that make up a current in a typical DC circuit? In a typical AC circuit?
DC--less than 1 cm/s; AC--zero
Do an ampere and a volt measure that same thing, or different things? What are those things, and which is a flow and which is the cause of the flow?
Different quantities; the potential difference (or voltage) causes a flow of charge, which is the current.
If the resistance of a circuit remains constant while the voltage across the circuit decreases to half its former value, what change occurs in the current?
Drops to half
If the voltage impressed across a circuit is constant but the resistance doubles, what change occurs in the current?
Drops to half
What is voltage?
Electric "pressure," Potential Energy/charge, that produces electric current
What is an ampere?
Flow of 1 charge/1 second
What is electric current?
Flow of charge
Use the relationship power = current * voltage to find out hoe much current is drawn by a 1200-watt hair dryer when it operates on 120 volts. Then use Ohm's law to find the resistance of the hair dryer.
From P = I * V, I = P/V = (1200 W)/(120 V) = 10 A; R = V/I = (120 V)/(10 A) = 12 Ω
The wattage marked on a lightbulb is not an inherent property of the bulb but depends on the amount of voltage to which it is connected, usually 110 V or 120 V. Calculate the current through a 40-W bulb connected to 120 V.
From P = I * V, I = P/V = (40 W)/(120 V) = 1/3 A
Calculate the current in a 140-W electric blanket connected to a 120-V outlet.
From P = I * V, I = P/V= (140 W)/(120 V) = 1.17 A
How many amperes flow through a 60-watt bulb when 120 volts are impressed across it?
From P = IV, 60 W = I * 120 V; I = 0.5 A
From where do the electrons originate that flow in a typical electric circuit?
From the conductors themselves
Is electric resistance greater in a short fat wire or a long thin wire?
Greater in a long, thin wire
Calculate the current of a lightning bolt that delivers a charge of 35 coulombs to the ground in a time of 1/1000 second.
I = (35 C)(0.001 s) = 35,000 A
Calculate the current in a toaster that has a heating element of 14 ohms when connected to a 120-V outlet.
I = V/R = (120 V)/(14 Ω) = 8.6 A
Calculate the current in the coiled heating element of a 240-V stove. The resistance of the element is 60 ohms at its operating temperature.
I = V/R = (240 V)/(60 Ω) = 4 A
How much current moves through your fingers (resistance: 1200 ohms) if you touch them to the terminals of a 6-volt battery?
I = V/R = (6 V)/(1200 Ω) = 0.005 A
Electric socks, popular in cold weather, have a 90 ohm heating element that is powered by a 9-volt battery. How much current warms your feet?
I = V/R = (9 V)/(90 Ω) = 0.1 A
Calculate the current where 10 coulombs of charge pass a point in 5 seconds.
I = q/t = (10 C)/(5 s) = 2 A
How much current, in amperes, is in a lightning stroke that lasts 0.05 second and transfers 100 coulombs?
I = q/t = (100 C)/(0.05 s) = 2000 A
How does wetness affect the resistance of your body?
Lowers skin resistance
Two lightbulbs designed for 120-V use are rated at 40 W and 60 W. Which lightbulb has the greater filament resistance? Why?
More current flows in the 60-W bulb, which means the resistance of the filament if less. [From P = IV = V²/R, R = V²/P = (120 V)²/(60 W) = 240 Ω; for the 40-W lamp, R = (120 V)²/(40 W) = 360 Ω.]
Will the current in a lightbulb connected to 220 V be more or less than when the same bulb is connected to 110 V? How much?
More; for the same resistance, current willdouble when the voltage is doubled.
Why is it that a bird can perch without harm on a high-voltage wire?
Negligible potential difference across the body
Is this label on a household produce cause for concern? "Caution: This produced contains tiny electrically charged particles moving at speeds in excess of 10 000 000 kilometers per hour."
No; it simply states a typical speed of electrons within the material.
Calculate the power dissipated in a toaster that has a resistance of 14 ohms plugged into a 120-V outlet.
P = 1 * V = (V/R) * V = V²/R = (120 V)²/(14 Ω) = 1030 W
What is power?
P = energy/time, rate of doing work
What is meant by the term potential? What is meant by potential difference?
Potential energy/charge; Change in Potential Energy/charge between points
Which of these is a unit of power and which is a unit of electrical energy; a watt, a kilowatt, and a kilowatt-hour?
Power--watt, kilowatt; energy--kilowatt-hour
What condition is necessary for the sustained flow of water in a pipe? What analogous condition is necessary for the sustained flow of charge in a wire?
Pressure difference (voltage); potential difference
Calculate the resistance of the filament in a lightbulb that carries 0.4 A when 3.0 V is impressed across it.
R = V/I = (3.0 V)/(0.4 A) = 7.5 Ω
What is the function of the third prong in a household electric plug?
Serves as a ground connection
What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge?
Temperature difference; voltage difference
What is electric resistance?
That which resists flow of charge (measured in ohms)
Why is it important that the resistance of an extension cord be small when it is used to power an electric hea
The amount of energy converted to thermal energy depends on the resistance of the wire and the amount of current flowing in it. An electric heater draws a lot of current, so if the connecting wire has a high resistance, much energy will be wasted in heating the wire.
Would you expect to find DC or AC in the dome lamp in an automobile? In a lamp in your home?
The electrical system in a car is powered by a battery so it is DC; electricity in a home is powered by an alternating current generator so it is AC.
why will an electric drill operating on a very long extension cord not rotate as fast as one operated on a short cord?
The longer an extension cord, the greater its resistance, and the more energy is dissipated in it, with less available for the electric drill.
Why are thick wires rather than thin wires used to carry large currents?
Thick wires have less electrical resistance and will carry greater amounts of current without overheating.
Does charge flow through a circuit or into a circuit?
Through
In 60-Hz alternating current, how many times per seconds does an electron change its direction? (Don't say 60!)
Twice per cycle so 120 times per second
A battery does 18 joules of work on 3 coulombs of charge. What voltage does it supply?
V = E/q = (18 J)/(3 C) = 6 V
How much voltage is required to make 2 amperes flow through a resistance of 8 ohms?
V = I * R = (2 A)(8 Ω) = 16 V
How much charge flows in a pocket calculator each minute when the current is 0.0001 ampere?
q = I * t = 0.00001 A * 60 s = 0.006 C
