Week 8
When a pair of identical lamps are connected in parallel _____________.
(current in each is the same, power dissipated in each is the same, voltage across each is the same) all of the above
No net force acts on a loop of wire in a magnetic field when _____________.
(no magnetic field lines pass through the loop, no current is in the loop) both of these
Although a magnet can change the direction of travel of an electron beam, it cannot change its _____________.
(speed, kinetic energy) both of these
By convention, the direction of current in a circuit is _____________.
(the direction in which positive charge flows, from the positive terminal of a source to the negative terminal) both of these
A) Using the formula power = current x voltage, find the current drawn by a 1500-W toaster connected to 70 V. B) Then, using the equation current = voltage/resistance, find the resistance of the toaster.
A) I = 21 A B) R = 3.3 Ω
Magnetic domains are composed of clustered _____________.
aligned iron atoms
In what way are magnetic poles very different from electric charges?
all magnets have both south and north poles electric charges can exist as singular entities
A circuit breaker often serves the same purpose as a _____________.
fuse
If the voltage impressed across a circuit is held constant while the resistance doubles, what change occurs in the current?
it is cut in half
The fundamental rule for the attraction and repulsion of magnets is that _____________.
like poles repel each other while opposite poles attract
How is the rule for the interaction between magnetic poles similar to the rule for the interaction between electrically charged particles?
like poles repel, unlike poles attract, and the force is proportional to the inverse square of the distance between two poles
Two light bulbs are connected to a battery, one at a time. The bulb that draws more current has the ______________.
lower resistance, and is brightest
Into which stable force field can a proton be placed at rest without being acted upon by a force?
magnetic field
What exactly is an ampere?
the flow of one coulomb per second
A beam of singly-charged ions entering a magnetic field with the same speed are swept into circular paths. The wider paths are those of _____________.
the heavier ions
In Chapter 22, we learned that the direction of the electric field about a point charge is radial to the charge. What is the direction of the magnetic field surrounding a current-carrying wire?
the magnetic field is directed along concentric circles surrounding the wire
Why is the magnetic field strength greater inside a current-carrying loop of wire than about a straight section of wire?
the magnetic field of each segment of wire in the loop, due to electrons moving in the wire, adds together inside the loop, thereby making the field become bunched-up
Why will dropping an iron magnet on a concrete sidewalk make it a weaker magnet?
vibrations provide energy to randomize the magnetic directions of the domains
One joule per coulomb is a unit of _____________.
voltage
The energy dissipated in a light bulb in a circuit is provided by the _____________.
voltage source
How does wetness affect the resistance of your body?
wet bodies have lower resistance
Will water flow more easily through a wide pipe or a narrow pipe? Will current flow more easily through a thick wire or a thin wire?
wide pipe and thick wire
What is the error in saying the source of electrons in a circuit is the battery or generator?
wires are full of mobile electrons
Is it correct to say that an electric motor extends the physics that underlies a galvanometer? Why?
yes, the deflection forces are the same, but the direction of current in the motor is reversed cyclically
Electrons in the filament of a lamp that carries ac have a net speed that is _____________.
zero
Ohm's law tells us that the amount of current produced in a circuit is ______________.
(directly proportional to voltage, inversely proportional to resistance) both of these
A galvanometer can be calibrated to measure _____________.
(electric current, electric voltage) both of these
Current in a conductor can be increased by ______________.
(increasing the voltage across it, reducing its resistance) both of these
In a circuit powered by a battery, charge _____________.
(is energized by the battery, flows through the battery) both of these
Which force field can accelerate an electron?
(magnetic field, electric field) both of these
A current-carrying wire in a magnetic field _____________.
(may be deflected, may experience a force) both of these
An electron beam directed through a magnetic field _____________.
(may be deflected, may experience a force) both of these
In a circuit of two lamps in series, where the current through one lamp is 1 A, what is the current through the other lamp? Defend your answer.
1 amp the same current flows through lamps in series
How much energy is supplied to each coulomb of charge that flows through a 12-V battery?
12 joules
The current in two identical light bulbs connected in series is 0.25 A. The voltage across both bulbs is 110 V. The resistance of a single light bulb is ______________.
220 ohms
When a 12-V battery power a single 6-ohm lamp, _____________.
24 joules flow in the lamp each second
The electric power supplied to a lamp that carries 2 A at 120 V is _____________.
240 watts
If a voltage of 6 V is impressed across the circuit in the preceding question and the voltage across the first lamp is 2 V, what is the voltage across the second lamp? Defend your answer.
4 volts the sum of the voltages across each lamp must add up to the total voltage across both lamps
An electric current of 5 amperes in a wire carries ______________.
5 coulombs per second
The resistance of a filament that carries 2 A when a 10-V potential difference across it is ______________.
5 ohms
In a circuit of two lamps in parallel, where there is a voltage of 6 V across one lamp, what is the voltage across the other lamp?
6 volts
Consider the power dissipated by the two circuits in the video. The ratio of power dissipation in the parallel circuit to that in the series circuit is _____________.
9
A) Let's start with a simple circuit that you will have seen in the Ohm's Law and Power PhET tutorial, if you completed that one. What is the voltage reading across the resistor, as shown in the figure below? You should see the blue electrons flowing through the circuit, whereas (conventional) current is the flow of positive charge. You can select which current to display in the panel in the upper-right corner. B) Construct a circuit containing two 10-Ω resistors in a series, as shown below. Place the crosshairs of the noncontact ammeter just before each resistor to measure the current through the top and bottom resistors. How do these currents compare? C) The current through each resistor in the two-resistor circuit is _____________ the current through the resistor in the one-resistor circuit (the circuit in Part A). The voltage across each resistor in the two-resistor circuit is _____________ the voltage across the resistor in the one-resistor circuit. D) For the series circuit in the previous part, change the resistance of the bottom resistor to 20 Ω. What is the voltage across this 20-Ω resistor? E) What must be the resistance of a single resistor connected to a 9-V battery for the current coming out of the battery to be the same as that of the circuit in Part D? F) How does the current coming out of the battery change when the switch is closed? G) With the switch closed, how does the voltage across the 20-Ω resistor compare to the voltage across the 10-Ω resistor? H) For the parallel circuit in the previous part (with the switch closed), the current through the 20-Ω resistor is _____________ the current through 10-Ω resistor. I) For the circuit in the previous part, what happens to the current flowing through the resistor on the right when the switch is closed (allowing current to flow through the resistor on the left)? J) With the switch open, roughly what must be the resistance of the resistor on the right for the current out of the battery to be the sane as when the switch is closed (and the resistances of the two resistors are 20 Ω and 10 Ω)? K) The brightness of a light bulb having a specific resistance increases if the current through it increases. For the circuit shown in in the figure below, how does the brightness of the light bulb change when the resistance of the 10-Ω resistor is decreased?
A) 9.0 V B) the currents are the same C) half / half D) 6.0 V E) 30 Ω F) the current increases G) the voltage across the 20-Ω resistor is equal to the voltage across the 10-Ω resistor H) less than I) the current flowing through the resistor on the right does not change J) 7 Ω K) the brightness does not change
A) What is the relationship between current, resistance, and voltage difference? B) How does the current change if you increase the resistance, keeping the voltage difference the same? C) What factors does the resistance offered by a piece of conductor depend upon? D) How does the resistance of a thick piece of copper wire compare to the resistance of a thin piece of copper wire? E) What is the current in a bulb if the resistance of its filament is 2 ohms and it is connected across a 6-volt battery?
A) current = voltage difference/resistance B) the current decreases C) both the geometry and the material of the conductor D) the resistance of the thin piece is greater than that of the thick piece E) 3 amps
A) Which of the following would double the amount of current flowing through a piece of metal wire? B) If the resistivity of copper is less than that of gold at room temperature, which of the following statements must be true? C) How do the resistances of two conducting wires compare if they have the same length, but one is twice the radius of the other? D) You are given three pieces of wire that have different shaped (dimensions). You connect each piece of wire separately to a battery. The first piece has a length L and cross-sectional area A. The second is twice as long as the first, but has the same thickness. The third is the same length as the first, but has twice the cross-sectional area. Rank the wires in order of which carries the most current (has the lowest resistance) when connected to batteries with the same voltage difference.
A) double the voltage across it B) electrons in gold are more likely to be scattered than electrons in copper at room temperature when they are accelerated by the same electric field C) the thicker wire has one-fourth the resistance of the thinner wire D) wire of length L and area 2A, wire of length L and area A, wire of length 2L and area A
A) What does the power company provide to our homes? B) Of what physical quantity is Hertz a unit of? C) What type of field causes the electrons to do what they do in the wire? D) What kind of current runs through the electric wiring in a home?
A) it provides energy to move the electrons B) number of times per second that the electrons move back and forth in the wire C) electric field D) alternating current
A) What is the effect on the current in a wire if both the voltage across it and its resistance are doubled? B) If both are halved?
A) the current remains unchanged B) the current remains unchanged
A) Drag a battery into the construction panel, and use the voltmeter to determine which end of the battery is the positive terminal. The positive terminal has a higher potential than the negative terminal (recall the the voltmeter measures the potential difference between the red probe and the black probe). Which end of the battery is the positive terminal? B) In what direction is the (conventional) current flowing through the circuit? Recall that current is the flow of positive charge. C) Use the noncontact ammeter to measure the current flowing through the circuit. What is the current? D) For the circuit in the previous part, the current flowing in the wire between the positive terminal of the battery and the resistor is _____________ the current flowing between the resistor and the negative terminal of the battery. E) Double the resistance of the resistor by changing it from 10 Ω to 10 Ω. What happens to the current flowing through the circuit? F) For the circuit containing one resistor and one battery, what happens to the current if the voltage is doubled? G) For the circuit containing one resistor and one battery, what happens to the current if the voltage is tripled and the resistance is doubled? H) What is the voltage reading across the resistor, as shown in the figure below? I) Hook up a light bulb to a 5-V battery. Right click (or control click) on the light bulb, and change its resistance. How does the brightness of the light bulb depend on its resistance? J) In Part H, you discovered that the luminosity of a light bulb increases if the current increases. The rate at which electric potential energy is converted into heat depends on the current flowing through the bulb and the voltage across the bulb. This energy is supplied by the battery. Mathematically, the luminosity P of the light bulb is given by P=ΔVI, where ΔV is the voltage across the bulb and I is the current.What happens to the luminosity of the light bulb if the voltage of the battery is doubled? (Note that the PhET simulation does not display a numerical value for the luminosity, so you should use the relationship between the luminosity, the voltage across the bulb, and the current.)
A) the orange end B) the current flows from the positive terminal, through the wires and resistor, and into the negative terminal C) 0.9 A D) equal to E) the current decreases by a factor of two F) the current increases by a factor of two G) the current increases H) 9.0 V I) the light bulb gets dimmer as the resistance is increased J) the luminosity increases by a factor of four
A) What happens when Dr. Hewitt places a current-carrying wire between the poles of the magnet for the first time? B) What happens when Dr. Hewitt turns the magnet around and places the current-carrying wire between the poles of the magnet for the second time? C) What kinds of appliances work on the principle of deflection of a current-carrying wire in a magnetic field?
A) the wire jumps down B) the wire jumps up C) electric motor
A) What happens when a switch is closed in a circuit that contains a battery and a wire that is placed between the poles of a magnet? B) What happens to the shape of a wire that is part of a circuit with a battery when the switch is closed and the battery voltage is increased? C) What happens to the shape of a wire that is part of a circuit with a battery when the switch is closed and the direction of the current is reversed in the wire?
A) the wire will either bend up or down between the poles of the magnet due to the force of the magnetic field B) the wire bends more than it did before C) the wire will bend the same amount but in the opposite direction
A) A bar magnet is oriented perpendicular to a uniform magnetic field as shown in Figure 1. Describe the force and/or torque on the magnet. B) Two concentric loops of wire are carrying currents in opposite directions as shown in Figure 2. Describe the net force and the torque on either of the current loops. C) The magnetic dipole moment of a coil depends on the current (I), the area of the coil (A), and the number of loops (N). Rank the three different coils of wire shown in Figure 3 in order by the magnitude of their magnetic dipole moment. coil 1 = single coil with area 10 cm^2 and I=6 A coil 2 = single coil with area 20 cm^2 and I=3 A coil 3 = coil with 6 loops each with an area of 10 cm^2 and I=1 A D) The torque that is exerted on a magnetic dipole moment depends on the orientation of the magnetic dipole moment. Rank the three loops of wire carrying current shown in Figure 4 in order by the magnitude of the torque exerted on them.
A) there is a net torque on the magnet in a clockwise direction B) there is a net force on each loop that causes them to repel each other C) all the same D) normal to loop is perpendicular to magnetic field, normal to loop is at 45 degrees with respect to magnetic field, normal loop is parallel to magnetic field
By whom, and in what setting, was the relationship between electricity and magnetism discovered?
Hans Christian Oersted in a classroom demonstration
Ohm's Law: I = V/R Calculate the current in the 320-Ω filament of a bulb connected to a 150-V line.
I = 0.47 A
Power = I x V Calculate the power of a hair dryer that operates on 170 V and draws a current of 14 A.
P = 2400 W
Rearrange the equation current = voltage/resistance to express resistance in terms of current and voltage. Then solve the following: A certain device in a 140-V circuit has a current rating of 30 A. What is the resistance of the device (how many ohms)?
R = 4.7 Ω
A diode converts ac to pulsed dc. What electrical device smoothes the pulsed dc to a smoother dc?
a capacitor
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?
a continuous pressure difference, often provided by a pump, is needed for water to flow a continuous potential difference, often provided by a battery, is needed for a charge to flow
What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge?
a difference in temperature is needed for heat to flow a difference in electrical potential is needed for charge to flow
A galvanometer indicated _____________.
a flow of electrons
Between a watt, a kilowatt, and a kilowatt-hour, which is a unit of power and which is a unit of energy?
a watt is power, a kilowatt is power, and a kilowatt-hour is energy
When do cosmic rays penetrate your body?
all the time
When current reverses direction in a wire, the surrounding magnetic field _____________.
also reverses direction
What is a galvanometer called when it has been calibrated to read current? When it has been calibrated to read voltage?
ammeter, voltmeter
In units of measurement, power in watts is equal to _____________.
amperes x volts
How does a galvanometer detect electric current?
an electric current flows through a wire in a magnetic field the deflection force on the wire acts against a spring the compression of the spring is proportional to current
A current-carrying coil of wire is _____________.
an electromagnet
The direction of the force exerted on a moving charge in a magnetic field is _____________.
at right angles to the direction of the motion
Which of the following are creatures that are known to harbor tiny magnets within their bodies?
bacteria, pigeons, bees, wasps, monarch butterflies, and sea turtles
Name two kinds of practical "electric pumps".
batteries and automobile alternators
When a current-carrying wire is bent into a loop, its magnetic field inside the loop _____________.
becomes concentrated
Does electric charge flow across a circuit or through a circuit? Does voltage flow across a circuit or is it impressed across a circuit?
charge flows through and voltage is impressed across a circuit
Magnetic field lines about a current-carrying wire _____________.
circle the wire in closed loops
The shape of a magnetic field surrounding a current-carrying conductor is _____________.
circular
The magnetic field about a straight length of current-carrying wire is _____________.
circular in shape
What is a magnetic domain?
clusters of atoms with their magnetic fields aligned
When the filament breaks in one lamp in a parallel circuit, lamps in other branches of the circuit normally _____________.
continue glowing as brightly
Why are the magnetic fields of superconducting magnets often stronger than those of conventional magnets?
conventional magnets based on iron have a maximum magnetization that occurs when all of the domains are aligned this saturation magnetization is weaker than the magnetic field of superconducting coils
Power is defined as the energy expended per unit of time. When translated to electrical terms, power is equal to _____________.
current multiplied by voltage
When an electron passes through the magnetic field of a horseshoe magnet, the electron's _____________.
direction is changed
What produces a magnetic field?
electric charges in motion
What is the source of the magnetic force?
electric charges in motion are the sources of magnetic forces
The entity that travels about the speed of light in an electric circuit is _____________.
electric field
Which force field can increase a moving electron's speed?
electric field
The current produced by voltage in a circuit is impeded by ______________.
electric resistance
The source of a magnetic field is a moving _____________.
electron
When your body undergoes electric shock, the source of moving electrons is ______________.
electrons already in your body
Why are electrons, rather than protons, the principal charge carriers in metal wires?
electrons are free to move through the metal, whereas protons are fixed in place
At what speed do electrons in a battery driven automotive circuit travel along a wire? At what speed does the electric field propagate along a wire?
electrons move at 0.01 cm/s the electric field propagates at nearly the speed of light
One kilowatt-hour is a unit of _____________.
energy
When you make your household electric payment at the end of the month, are you billed for voltage, current, power, or energy?
energy
Electric power in a circuit is the rate at which _____________.
energy dissipates
Compared to the huge force that attracts an iron tack to a strong magnet, the force that the tack exerts on the magnet is _____________.
equally huge
The cosmic rays deflected most by Earth's magnetic field are above the _____________.
equator
Magnet A has twice the magnetic field strength of Magnet B and at certain distance pulls on Magnet B with a force of 100 N. The amount of force that Magnet A exerts on Magnet B is _____________.
exactly 100 N
A current-carrying loop of wire experiences no tendency to rotate in a magnetic field when _____________.
field lines pass directly through the loop
When the filament breaks in one lamp in a series circuit, other lamps in the circuit normally _____________.
go out
How is the magnetic force on a particle moving in a magnetic field different from gravitational and electric forces?
gravitational and electric forces act along the line connecting two interacting objects, whereas magnetic forces are perpendicular to the motion of the particle and the magnetic field
A 100-W lamp glows brighter than a 25-W lamp. The current drawn by the 100-W lamp is _____________.
greater
How does the heat emitted by lamps affect their efficiency?
heat emitted reduces the efficiency
At the micro level, what is the difference between an unmagnetized iron nail and a magnetized iron nail?
in an unmagnetized nail, the magnetic domains have a random orientation so that the net magnetism adds to zero in a magnetized nail, many of the magnetic domains are aligned
Place an iron rod inside a current-carrying coil of wire and you _____________.
increase the strength of the electromagnet
When you turn on a lamp, the initial current in its filament is greater at first, rather than a moment later, which indicates ______________.
increased temperature means increased resistance
Heat a copper wire and its electric resistance ______________.
increases
If you stretch a copper wire so that it is thinner, the resistance between its ends ______________.
increases
Where do the electrons come from that produce an electric shock when you touch a charged conductor?
inside you
What does it mean to say that a certain current is 60 Hz?
it alternates back and forth 60 times a second
What is the function of the round third prong in a modern household electric plug?
it grounds the case to zero potential
Does heating a metal wire increase or decrease its electrical resistance? Why?
it increases resistance because atoms at higher temperatures jostle into the way of moving electrons
What property of a diode enables it to convert ac to pulsed dc?
it is a one-way valve that allows electrons to in one direction only
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?
it is cut in half
What effect does Earth's magnetic field have on the intensity of cosmic rays striking Earth's surface?
it reduces the intensity
How often is current reversed in the loops of a simple dc electric motor?
it reverses twice each cycle
A 100-W lamp glows brighter than a 25-W lamp. The electrical resistance of the 100-W lamp is _____________.
less
Compared to a single lamp connected to a battery, two identical lamps connected in series to the same battery will carry _____________.
less current
If two copper wires of the same length have different thickness, then the thicker wire has ______________.
less resistance
When two lamps are connected in parallel to a battery, the electrical resistance that the battery senses is _____________.
less than the resistance of either lamp
Electric resistance in our skin is lowered when our skin is ______________.
moist
Connect a pair of lamps in series and current is drawn from the battery. Connect the same lamps in parallel and the current drawn is _____________.
more
Compared to a single lamp connected to a battery, two lamps connected in parallel to the same battery will carry _____________.
more current
The voltage across a 10-ohm resistor carrying 5 A is ______________.
more than 20 V
When two lamps are connected in series to a battery, the electrical resistance that the battery senses is _____________.
more than the resistance of either lamp
Why does a wire that carries electric current become hot?
moving electrons collide with atoms, which transfers their kinetic energy to atomic motion
Compared with a simple galvanometer, an electric motor is _____________.
much the same with small modifications
The figure shows the path of a charged particle moving in a magnetic field directed into the screen. What is the particle's charge?
negative
The unit of electrical resistance is the ______________.
ohm
What is the unit of electrical resistance?
ohm
Modern automobile headlights are connected in _____________.
parallel
A beam of electrons can pass through a magnetic field without being deflected if the direction of the beam is _____________.
parallel to the field lines
In what direction relative to a magnetic field does a charged particle move in order to experience maximum deflecting force? To experience minimum deflecting force?
perpendicular for maximum force, parallel for minimum force
The force that acts on a current-carrying wire placed in a magnetic field is _____________.
perpendicular in direction to the magnetic field
The force that acts on an electron traveling in a magnetic field is _____________.
perpendicular in direction to the magnetic field
A beam of electrons passing through a magnetic field experiences maximum deflection if the direction of the beam is _____________.
perpendicular to the field lines
The force a magnetic field exerts on a current-carrying wire is maximum when the wire is oriented _____________.
perpendicular to the magnetic field
The force exerted on an electron moving in a magnetic field is maximum when the electron moves _____________.
perpendicular to the magnetic field
A flow of electric charge in a wire normally requires a ______________.
potential difference across the ends of the wire
One kilowatt is a unit of _____________.
power
What is the relationship among electric power, current, and voltage?
power is current times voltage
A voltage source in a circuit provides electrical ______________.
pressure
If a magnet produces a force on a current-carrying wire, the wire _____________.
produces a force on the magnet
What two kinds of motion do electrons in an atom appear to have?
revolution and spin
Two lamps with different filament thickness, and therefore different resistances, are connected in series. Greater current is in the lamp with the ______________.
same in each
On some early automobiles both headlights failed when one bulb burned out. The headlights were likely connected in _____________.
series
The safety fuse in an electric circuit is connected to the circuit in _____________.
series
The magnetic field about Earth takes a shape _____________.
similar to that of an interior bar magnet
As the number of loops in a current-carrying wire is increased, the _____________.
stronger the enclosed magnetic field
A superconducting magnet uses _____________.
superconducting coils
What is the cause of the aurora borealis (the northern lights)?
the Earth's magnetic field guides trapped charged particles to follow field lines toward the poles, where they collide with the atmosphere, causing a glow
Does a battery produce dc or ac? Does the generator at a commercial power station produce dc or ac?
the battery is dc; the generator is ac
When an iron rod is placed inside a current-carrying coil of wire _____________.
the coil becomes a stronger electromagnet
We repeat the experiment from the video, but this time we connect the wires in parallel rather than in series. Which wire will now dissipate the most heat?
the copper wire (resistance 0.1 Ω)
Why are there probably no permanently aligned magnetic domains in Earth's core?
the core is too hot to allow atoms to maintain a constant magnetic orientation
Why does a piece of iron in a current-carrying loop increase the magnetic field strength?
the electromagnetic field of the current-carrying loop is amplified because it aligns the domains in the iron
How does magnetic field strength relate to the closeness of magnetic field lines about a bar magnet?
the field strength is stronger where the field lines are closer
What happens to the direction of the magnetic field about an electric current when the direction of the current is reversed?
the magnetic field reverses direction at every point a clockwise pattern of concentric circles becomes a counterclockwise pattern of concentric circles and vice versa
Why is iron magnetic and wood not?
the magnetic fields of individual iron atoms are strong enough to align the magnetic fields of neighbor atoms the atoms in wood have much weaker magnetic fields
What is meant by drift velocity?
the net flow of electrons along a wire
What are magnetic pole reversals?
the poles of the Earth's magnetic field undergo reversals
The force on an electron moving in a magnetic field will be least when its direction is _____________.
the same as the magnetic field direction
How do the currents through the branches of a simple parallel circuit compare with the current in the voltage source?
the sum of the currents in the branches equals the current through the source
What relative direction between a magnetic field and a current-carrying wire results in the greatest force?
the wire should be perpendicular to the field
The force between electrically charged particles depends on the magnitude of each charge, their separation distance, and what else?
their motion
A tipped domino sends a pulse along a row of standing dominoes. Is this a good analogy for the way electric current, sound, or both travel?
this is the way sound travels but not how electric current travels
What is the function of fuse or circuit breaker in a circuit?
to prevent over loading the circuit with current
Sometimes the current in lamps is too feeble to produce glowing of the lamp filaments, which occurs when _____________.
too many lamps are connected in series
The electric resistance in a length of wire is doubled when the wire is ______________.
twice as long
If an electric toaster rated at 110 V is accidentally plugged into a 220-V outlet, the current drawn by the toaster will be ______________.
twice its normal value
