Chapter 34 Flashcards
Heat flows through a conductor when…
a difference in temperature exists between its ends
Heat flows from…
the end of higher temperature to the end of lower temperature.
When both ends reach the same temperature…
the flow of heat ceases
When the ends of an electric conductor are at different electric potentials…
charge flows from one end to the other
When does charge flow?
when there is a potential difference, or difference in porential (voltage), between the ends of a conductor
Until when will the flow of charge end?
when both ends reach a common potential
When there is no potential difference…
there is no longer a flow of charge through the conductor
To attain a sustained flow of charge in a conductor…
some arrangement must be provided to keep one end at a higher potential then the other. The situation is analogous to the flow of water from a higher reservoir to a lower one. Water will flow in a pipe that connects the reservoirs only as a difference in water level exists.
The flow of water in the pipe, like the flow of charge in the wire that connects the Van ge Graaff generator to the ground, will cease when…
the “pressures” at the two ends are equal
In order that the flow be sustained…
there must be a suitable pump of some sort to maintain a difference in water levels. Then there will be a continual difference in water pressures and a continual flow of water. The same is true of electric current
What is electric current?
The flow of electric charge
In solid conductors the electrons…
carry the charge through the circuit because they are free to move throughout the atomic network.
What are electrons in solid conductors called?
conduction electrons
What is electric currents charged in?
Amperes
Whats is an ampere?
The flow of 1 coulomb of charge per second
A current-carrying wire has a net electric charge of…
zero
While the current is flowing…
negative electrons swarm through the atomic network that is composed of positively charged atomic nuclei
When electrons flow in a wire…
the number entering one end is the same as the number leaving the other. So we see that the net charge of the wire is normally at every moment
Charges do not flow unless…
there is a potential difference
A sustained current requires a…
suitable “electric pump” to provide a sustained potential difference.
What is a voltage source?
Something that provides a potential difference
What happens if you charge a metal sphere positively and another negatively? Why is this not a good voltage source?
You can develop a large voltage between them. This is not a good voltage source b/c when spheres are connected by a conductorm the potentials equalizr in a single brief surge of moving charges
Voltage sources such as batteries and generators supply…
energy that allows charges to move steadily
In a battery a chemical reaction occuring inside releases…
electrical energy
Generators convert mechanical energy to…
electrical energy
The electrical potential energy produced by whatever means is available…
at the terminals of the battery or generator
The potential energy per coulomb of charge available to the electrons moving between terminals is the…
voltage
What does the voltage provide?
the 120 volts delivered to home outlets
Water will flow through the pipe if…
there is a difference in pressure across sthe pipe or between its ends
Does pressure flow?
No
What does voltage cause?
Current
What does the amount of charge that flows in a circuit depend on?
The voltage provided by the voltage source
What does current also depend on?
electric resistance
What is electric resistance?
The resistance that the conductor offers to the flow of charge
The resistance of a wire depends on…
the conductivity of the material used in the wire (that is, how well it conducts) and also on the thickness and length of the wire.
Thick wires have ________resistance than thin wires
less
Longer wires have _______ resistance than short wires
more
What does electric resistance depend on?
temperature
The greater the jostinling about ofatoms within the conductor…
the greater resistance the conductor offers to the flow of charge
For most conductors increased temperature means…
increased resistance
When does the resistance of some materials become zero?
At very low temperatures,
Superconductivity
The resistance of some materials becomes zero at very low temperatures, a phenomenon
Do certain metals acquire superconductivity?
Yes
Once electric current is stablished in a superconductor…
the electrons flow indefinitely
Ohms
electric resistance is measured in units
What is Ohm’s law?
The relationship among voltage, current, and resistance
Ohm’s law states that the ______ in a circuit is ________proportional to the _______ impressed across the circuitm and is __________ proportional to the resistance of the circuit.
current; directly; voltage; inversely
What is the formula for current?
current = voltage/resistance
What is the formula for ampere?
1 ampere = 1 volt/ohm
What is the source of electrons in a circuit?
the conducting circuit material itself
Do electrons flow from the power utility through the power lines and into the wall outlets of their homes.
No
The outlets in homes are…
AC
Do electrons travel appreciable distances through a wire in an AC circuit.
No, they vibrate to and fro about relatively fixed positions
When you plug a lamp into an AC outlet, what happend?
energy flows flows from teh outlet into the lamp, not electrons
Energy is carried by…
the electric field and causes a vibratory motion of the electrons that already exist in the lamp filament
If 120 volts AC are impressed on a lamp, then…
an average of 120 Jolues of wnergy are dissipated by each coulomb of charge that is made to vibrate.
Most of this electrical energy appears as heat, while
some of it takes the form of light
Do power utilies sell electrons?
No. they sell energy. you supply the electrons
When you are jolted by an AC electric shock…
the electrons making up the current in your body originate from your body
What comes out of the wire and through your body into the ground?
energy
What does energy cause?
Free electrons in your body to vibrate in unison.
Small vibrations __________; large vibrations can be _________
tingle; fatal
Unless it is a superconductor…
a charge moving in a circuit expends energy. This may result in heating the circuit or in turning a motor
What is electric power?
the rate at which electrical energy is converted into another form such as mechanical energy, heat, or light
What is electric power equal to?
the product of current and voltage
Formula for electric power
electric power = current x voltage
If the voltage is expressed in volts and the current in amperes, then the power is expressed in watts. What’s the formula?
1 watt = (1 ampere) x (1 volt)
If a lamp at 120 watts operates on a 120-volt line, you can see that it will draw a current of 1 amp. What’s the formula?
120 watts = 1 ampere x 120 volts
60 watts = 0.5 ampere x 120 volts
