Chapter 18 Flashcards
When an electric cell is connected to a circuit electrons flow away from the negative terminal in teh circuit. But within the cell, electrons flow to the negative terminal why
In the circuit the electrons flow from the negative high potential to positive low potential. Inside the cell chemical reaction does work and electrons move from the positive to negative. Conservation of electric charge and energy apply.
When a flashlight is operated what is being dues up, battery current voltage energy power or resistance
Battery energy is being used up. As charges leave the terminal they have high potential energy. Then as the charges move though the flashlight bulb, they lose potential energy. The batter uses a chemical reaction to replace potential energy of the charges by lowering the battery chemical potential energy. When used up the battery cannot give potential energy to the charges since the change in voltage will be zero.
What quantity is measured by a battery rating oven in ampere hours why
Ampere hour is a measure of charge=3600 C.
Can a copper wire and an aluminum wire of the same length have the same resistance
Yes. If one has a greater rho (Resistivity) than the cross sectional area of the other must be larger by the same proportion then they will have the same R. It is true with any metal.
One terminal of a car battery is said to be connected to ground. Since it is not really connected to the ground what is meant by this expression
The terminal of the battery (negative one) is connected to the metal chassis, frame and engine block of the car. This means all voltage used for electric devices in the car are measured with respect to the cars frame. Also since the frame is a large mass of metal, it can supply charges for current without changing it electric potential (voltage)
The equation p=v squared divided by R indicated that the power dissipated in a resister decreases if the resistance is increased whereas the equation p=I squared times R implies the opposite . Is there a contradiction here
No to say that. P=v^2/r Indicated a decrease in P as R increases implies that voltage is constant. To say that. P=I^2R Indicates an increase in power as R increases implies that current is constant. ONLY ONE OF THOSE CAN BE TRUE FOR ANY GIVEN situation in which R is changing. If R changed and V is constant the I must change. If R changes and I is constant then V must also change.
What happens when a lightbulb burns out
The filament breaks so no current can flow. When the light is off the filament I’d cold and the lower temperature allows more I to pass through it. With the excess current, it heats up and vaporizes.
If a rectangular solid made of carbon has sides of lengths a, 2a, 3a to which faces would you connect from a battery so as to obtain. The least resistance, the greatest resistances
For minimum R- small length and large cross section area. For maximum R long length and small cross section area
Explain why light bulbs almost always burn out just as they are turned on and not after they have been on for some time
This answer describes incandescent light bulbs. When the bulbs is first turned on, it will be cool and the filament has a lower R than when hot. The lower R means more I through the bulb than when cool. This momentary high I will make the filament hot. If it is weak it can snap. The current I will no longer flow.
A device that produces electricity to by transforming chemical Energy into electrical energy is called a
Battery
A car battery
has an emf of 12 V consisting of six 2-V cells connected in series
The total amount of charge that passes through a wires full cross section at any point per unit of time is referred as
Current
The direction of convention current is taken to be the direction that
Positive charges would flow
A coulumbs per second is the same as
An ampere
Car batteries are rated in amp-hours this is a measure of their
Charge
The resistance of a wire is defines as
Voltage over current
What is 1 Q equivalent to
1 V/A
The resistance of a wire is
Proportional to its length and inversely proportional to its cross sectional area
The resistive try of a wire depends on
the material out of whihc it is composed
Which conducting material has the lowest resistivity value
Silver
Consider two copper wires one has twice the length if the other how doe the resistivities of these two wires compare
Both wires have the same resistivity bc both copper wires not resistance but resistivity
Consider two copper wires one has twice the length of the other how do teh resistance of these two wires compare
The longer wire has twice the resistance of the shorter wire BC REISTANCE not resistivity
Consider two copper wires one has twice the cross sectional area of the other how to the resistances of these two wires compare
The thicker wire has half the resistance of teh shorter wire
Consider two copper wires one has twice the length and twice the cross sectional area of the other. How do teh resistance of these two wires compare
Both wires has the same resistance
If teh resistance in a Constant voltage circuit is doubled the power dissipated by that circuit will
Decrease to 1/2 it’s original value bc p=V^2/R
A superconducting wires chief characteristic is which of the following
No resistance
Tripling the current in a circuit with constant resistance has teh effect of changing the power byt what factor
9 bc p=v^2/R
The unit of electric current the ampere is dimensionally equivalent to which of the following
Volt/ohm bc ohms law
The unit of electric resistance the ohm is dimensionally equivalent to whcih of the following
Volt/amp
Teh quantity electromotive force is dimensionally equivalent ti whcih of the following
Joule/coulumb bc EMF if voltage and 1j/c=1V
When an electric current exists within a conducting wire whihc of the following statements describes the condition of any accompanying electric field
Must be parallel to current flow bc charges carrying electric field with them
If a certain resistor obeys ohms law its resistance will change
No bc resistance is a cosntant for a given resistor, 5here is some tolerance but not a lot
10 C of charge start from negative terminal of a battery flow through the battery =then leave the positive terminal through a wire flow through a resister then return to the starting point on this closed circuit in this complete process the 10 C
Has no net change in potential energy
Which is not a force
EMF bc it’s a misconception
Two conducting spheres of differences radii each hae a charge of -Q whihc of the following occurs when teh two spheres are connected with a conducting wire
Negative charge flows flows from teh smaller sphere to the larger sphere until the electric potential of each sphere is the same, smaller one has a higher charge density so current wil flow until it is even and if they were the same size then no charge would flo w
Which fo teh following will cause th electrical resistance of certain material known as superconductors to suddenly decrease to essentially zero
Cooling teh material below a certain threshold temperature
A metal wire has a resisitance R when it is at a temperature T the wire is melted and all of teh metal is used to reform it into a new wire 4 times as long wht is teh resistance of the new wire at temperature T
16 R bc increase length make cross sectional area taller 4 from top and 4 from bottom
Wire made of brass and a wire made of silver hae the same length but the diameter of the brass wire is 4 times the diameter of the silver wire. The resistivity of brass is 5 times greater than teh resistivity of silver if R’b denotes teh resisitance of the brass wire and R’s denotes the resisitance o the silver wire which of the following is true
R’b=5/16Rs bc R’b=PL/A= 5Ps L/(4)^2A of silver = 5/16 of silver
For an. Ohm conductor double the voltage without changing the resisitance will cause the current to
Increase by a factor of 2 bc I=V/Rand it has to be a metal
if a 60 watt light bulb operates at a voltage of 120 V what is the resistance of the bulbs
R=V^2/P=120^2/60=240 0hms
How much energy is dissipated as heat in 20 s by a 100 ohm resistor that carries a current of .5 A
Dissipated as heat means it is wasting energy so P=I^2 X R= .5^2 X 100= 25 W … E =PT= 25 Wx 20s= 500 J
The power dissipated in a wire carrying a constant electric current I may be written as a function of I the length L of the wire the diameter d of teh wire and the resistivity p of the material in the wire. In this expression the power dissipated is directly proportional to which of the following
L and p only
Two conduction cylindrical wires are made out of the same matierla wire X has the twice the length and twice the diameter of wire Y what is the ration of X/y of their resistances
4
A hair dryer is rated 1200 W 120 V it’s effective internal resistance is
12 ohms p=v^2/r r=v^2/p
A particle of charge Q and mass m is accelerated from rest though a potential difference V attaining a kinetic energy K what is the kinetic energy of. A particle of charge 2 Q and mass 1/2 that is accelerated from rest through the same potential difference
K=1/2mv^2 k=Qv =2Qv —-2K
