Mastering Physics: Electric Current

25 July 2022
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question
What exactly is an ampere?
answer
The flow of one coulomb per second
question
Why are electrons, rather than protons, the principal charge carriers in metal wires?
answer
Electrons are free to move through the metal, whereas protons are fixed in place.
question
What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge?
answer
A difference in temperature is needed for heat to flow. A difference in electrical potential is needed for charge to flow.
question
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?
answer
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 charge to flow.
question
A flow of electric charge in a wire normally requires a _________.
answer
potential difference across the ends of the wire
question
Name two kinds of practical "electric pumps."
answer
Batteries and automobile alternators
question
How much energy is supplied to each coulomb of charge that flows through a 12-V battery?
answer
12 joules
question
Does electric charge flow across a circuit or through a circuit? Does voltage flow across a circuit or is it impressed across a circuit?
answer
Charge flows through and voltage is impressed across a circuit.
question
An electric current of 5 amperes in a wire carries _________.
answer
5 joules per coulomb
question
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?
answer
Wide pipe and thick wire
question
Does heating a metal wire increase or decrease its electrical resistance? Why?
answer
It increases resistance because atoms at higher temperatures jostle into the way of moving electrons.
question
What is the unit of electrical resistance?
answer
Ohm
question
A voltage source in a circuit provides electrical _________.
answer
pressure
question
If the voltage impressed across a circuit is held constant while the resistance doubles, what change occurs in the current?
answer
It is cut in half.
question
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?
answer
It is cut in half.
question
How does wetness affect the resistance of your body?
answer
Wet bodies have lower resistance.
question
What is the function of the round third prong in a modern household electric plug?
answer
It grounds the case to zero potential.
question
The electric resistance in a length of wire is doubled when the wire is _________.
answer
twice as long
question
Does a battery produce dc or ac? Does the generator at a commercial power station produce dc or ac?
answer
The battery is dc; the generator is ac.
question
What does it mean to say that a certain current is 60 Hz?
answer
It alternates back and forth 60 times a second.
question
What property of a diode enables it to convert ac to pulsed dc?
answer
It is a one-way valve that allows electrons to in one direction only.
question
A diode converts ac to pulsed dc. What electrical device smoothes the pulsed dc to a smoother dc?
answer
A capacitor
question
When your body undergoes electric shock, the source of moving electrons is _________.
answer
electrons already in your body
question
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?
answer
Electrons move at 0.01 cm/s. The electric field propagates at nearly the speed of light.
question
Why does a wire that carries electric current become hot?
answer
Moving electrons collide with atoms, which transfers their kinetic energy to atomic motion.
question
What is meant by drift velocity?
answer
The net flow of electrons along a wire.
question
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?
answer
This is the way sound travels but not how electric current travels.
question
What is the error in saying the source of electrons in a circuit is the battery or generator?
answer
Wires are full of mobile electrons.
question
When you make your household electric payment at the end of the month, are you billed for voltage, current, power, or energy?
answer
Energy
question
Where do the electrons come from that produce an electric shock when you touch a charged conductor?
answer
Inside you
question
Electrons in the filament of a lamp that carries ac have a net speed that is _________.
answer
zero
question
What is the relationship among electric power, current, and voltage?
answer
Power is current times voltage.
question
Between a watt, a kilowatt, and a kilowatt-hour, which is a unit of power and which is a unit of energy?
answer
A watt is power, a kilowatt is power, and a kilowatt-hour is energy.
question
The entity that travels about the speed of light in an electric circuit is _________.
answer
electric field
question
How does the heat emitted by lamps affect their efficiency?
answer
Heat emitted reduces the efficiency.
question
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.
answer
1 amp. The same current flows through lamps in series.
question
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.
answer
4 volts. The sum of the voltages across each lamp must add up to the total voltage across both lamps.
question
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?
answer
6 volts
question
How do the currents through the branches of a simple parallel circuit compare with the current in the voltage source?
answer
The sum of the currents in the branches equals the current through the source.
question
What is the function of fuse or circuit breaker in a circuit?
answer
To prevent over loading the circuit with current
question
When the filament breaks in one lamp in a series circuit, other lamps in the circuit normally _________.
answer
go out
question
When the filament breaks in one lamp in a parallel circuit, lamps in other branches of the circuit normally _________.
answer
continue glowing as brightly