Physics Exam 2
What would be the effect on B inside a long solenoid if (a) the diameter of all the loops was doubled, (b) the spacing between loops was doubled, or (c) the solenoid's length was doubled along with a doubling in the total number of loops.
(a) There would be no effect on the magnetic field since it only depends on the current and the number of turns per unit length. (b) There would be half as many turns in the solenoid so the magnetic field would decrease by a factor of two. (c) There would be the same number of turns per unit length so the magnetic field would be the same.
A long straight wire carries a current to the right as shown in the diagram. what is the direction of the magnetic field in the plane of the screen?
------> so its out of the screen above the wire and out of the screen below the wire.
Describe electric and/or magnetic fields that surround a moving electric charge.
A moving electric charge has a radial electric field around it, similar to a stationary electric charge. The magnitude of the electric field is inversely proportional to the square of the distance from the charge and the direction is away from the charge if it is positive and towards the charge if it is negative. A moving electric charge is an electric current so it has a magnetic field around it, similar to the field of a long straight wire if the charge is moving in a straight line. The field lines are circles centered on the line along which the charge moves and the direction is given by the same right hand rule as for a wire carrying current in the direction the charge is moving if the charge is positive. If the charge is negative the direction is opposite the one given by the right hand rule (or use your left hand for a negative charge). The magnitude of the field is inversely proportional to the distance from the line along which the charge is moving.
Can you set a resting electron into motion with a magnetic field? With an electric field? Explain
A resting electron cannot be set into motion with a magnetic field since there a magnetic force on the electron only if it is moving. It can be set into motion with an electric field since there is an electric force on the electron whether it is moving or not.
For what use are batteries connected in series? For what use are they connected in parallel? Does it matter if the batteries are nearly identical or not in either case?
Batteries are connected in series to increase the voltage. They are connected in parallel to increase the maximum current. In series it doesn't matter if the batteries are nearly identical or not, but it does in parallel, because if they aren't when the batteries are in parallel then the battery with the larger voltage will charge the one with the smaller voltage.
You have a single 60 W bulb on in your room. How does the overall resistance of your room's electric circuit change when you turn on an additional 100 W bulb?
Buildings are wired in parallel so when you turn on the 100 W bulb the overall resistance decreases.
A beam of electrons is directed toward a horizontal wire carrying a current from left to right. In what direction is the beam deflected?
By the right-hand rule for the magnetic field of a current carrying long, straight wire, the direction of the magnetic field below the wire is into the page. By the right-hand rule for the force on a moving charged particle the force on a positive charge would be to the left. But since electrons are negatively charged the force is in the opposite direction and the beam is deflected to the right.
In what direction are the magnetic field lines surrounding a straight wire carrying a current that is moving directly away from you
Clockwise (Point your right thumb in the direction of the current; your fingers curl in the direction of the magnetic field lines.)
Explain in detail haw you could measure the internal resistance of a battery
First measure the emf of the battery by connecting a high resistance voltmeter to it. Then, measure the maximum current when the battery terminals are shorted out using a low resistance ammeter. The internal resistance will be the emf divided by the maximum current. An alternative method is to first measure the emf as before. Then connect the battery to a known resistance and measure the terminal voltage. The internal resistance is thenr = R⎛E −VT ⎞ . ⎜⎝ VT ⎟⎠
ohms law
I=V/R or V=IR
A negatively charged particle is moving in a uniform magnetic field which is perpendicular to the particle's velocity. What happens to the particle's kinetic energy?
It remains the same since the magnetic force does no work on the particle.
Two iron bars attract each other no matter which ends are placed close together. Are both magnets? Explain.
One of the iron bars is a magnet and the other is not. If both were magnets then the ends that were like poles would repel, not attract.
which resistors in the circuit if any are in series and which if any are in parallel if there is a bridge?
R2 and R3 are in parallel, and they combined are in series with R1
Two ideal light bulbs with resistances 5 Ω and 10 Ω are connected in series or in parallel. Which of the following statements is correct? Hint: The brightness of a bulb is related to the rate at which it changes electrical energy to light, i.e. its power. P = IV = I2R = V2/R
The 10 Ω bulb is brighter in series and the 5 Ω bulb is brighter in parallel.
If two identical resistors are connected in series to a battery, does the battery have to supply more power or less power than when only one of the resistors is connected? Explain.
The battery has to supply less power since with two resistors the current through the battery is less and the power supplied is the voltage across the battery times the current through it. If the battery is ideal the current is half and the power supplied is half with two resistors compared to one. For a real battery the power supplied is less, but not half.
If all you have is a 120 V line, would it be possible to light several 6 V lamps without burning them out? How?
The easiest way would be to use 20 lamps in series so that each lamp has (120 V)/20 = 6 V across it. You could also use fewer lamps in series with a resistor so that the voltage across the lamps and the resistor added up to 120 V. In order to calculate the necessary resistance, you would first have to measure the resistance of each lamp.
An unmagnetized nail will not attract an unmagnetized paper clip. However, if one end of the nail is in contact with a magnet, the other end will attract a paper clip. Explain.
The magnet magnetizes the nail so the nail becomes a magnet. The nail then attracts the paper clip by induction.
With two identical lightbulbs and two identical batteries, how would you arrange the bulbs and batteries in a circuit to get the maximum possible total power out? (Assume the batteries have negligible internal resistance.)
The power dissipated by the lightbulbs is proportional to the square of the voltage and inversely proportional to the resistance, so to get the maximum power you would want to connect the batteries in series to get the maximum voltage and the resistors in parallel to get the minimum resistance.
When applying Kirchhoff's loop rule (such as in the figure), does the sign (or direction) of a battery's emf depend on the direction of current through the battery? What about the terminal voltage?
The sign of the emf does not depend on the direction of the current through the battery. Instead it depends on the direction you go around the loop. If you go from the negative terminal to the positive terminal the voltage increases and the sign is positive; going from positive to negative the voltage decreases and the sign is negative. The terminal voltage does depend on the direction of the current through the battery. If the battery is discharging (the current in the battery going from the negative terminal to the positive terminal) then there is a voltage drop across the internal resistance and the terminal voltage is less than the emf. If the battery is charging (the current in the battery going from the positive terminal to the negative terminal) then there is a voltage gain across the internal resistance and the terminal voltage is greater than the emf.
Household outlets are often double outlets. Are these connected in series or parallel? How do you know.
They are connected in parallel. If they were in series the current would have to be the same through each one and if one outlet was not connected to an appliance or if the appliance was turned off, the current through it would be zero so the current through the other outlet would also be zero. Since you can use just a single outlet, they can't be connected in series.
Two lightbulbs of resistance R1 and R2 (R2 > R1) are connected in series. Which is brighter? What if they are connected in parallel? Explain.
When the light bulbs are connected in series the current through them is the same. The power dissipated by the light bulbs is I 2R so the light bulb with the larger resistance is brighter. When the light bulbs are connected in parallel the potential difference (voltage) across them is the same. The power dissipated by the light bulbs is V 2/R so the light bulb with the smaller resistance is brighter.
Can the terminal voltage of a battery ever exceed its emf? Explain.
Yes, if the battery is being charged then the terminal voltage equals the emf plus the current times the internal resistance.
what device was a graph of I vs. V a straight line?
a resistor
which of the following objects are magnetic: a steel paper clip, a sheet of paper, and iron nail, a rubber eraser, the earth?
all of them. all objects are magnetic to some extent. only the paper clip and the nail are ferromagnetic and therefore strongly magnetic. the earth has electric currents in its outer core that produce its magnetic field.
which resistors in the circuit if any are in series and which if any are in parallel.
all three resistors are in series. if its just around in one square.
if the resistance of a small immersion heater (two heat water for tea or soup) was increased, would it speed up or slow down the heating process?
assume voltage is constant, it would slow down the heating process since for constant voltage the power is inversely proportional to the resistance.
when electric lights are operated on low-frequency ac (say 5 hz) they flicker noticeably.. why?
at 5 Hz th lights turn on and off 10 times per second, twice each cycle. the metal filament has time enough to cool down and get dim during the low current parts of the cycle and the eye can detect this. at 50 or 60 hz, the filament never cools enough to dim significantly.
when a flashlight is operated, what is being used up?
battery energy
when a flashlight is operated, what is being used up?
battery energy is being used up. a battery changes chemical energy into electrical energy. when a flashlight is operated the light bulb changes electrical energy into visible light and thermal energy. when a battery goes dead it is because of all its chemical energy has been changed into electrical energy.
what is the main difference between alternating current and direct current?
charges only move in one direction in direct current. their direction of motion changes in alternating current.
real batteries
circuits a real battery can be considered to be an ideal batter in series with a resistor r.
electric circuit.
consists of one or more sources of electrical energy one or more circuit devices that store electrical energy or change it to some other form, and wires to connect them. in order to have a flow it has to leave one terminal of battery and return to the other terminal. its closed if there is a continuous conducting path b/t terminals of at least one source. open if no current. Resistivity of metals increases with temp.
for which of the following would the strength of the magnetic field inside a long solenoid double?
doubling the current and keeping everything else the same along with doubling the number of turns in the solenoid and keeping everything else the same.
what is the direction of the magnetic force on a wire carrying a current toward the left in a uniform magnetic field directed inwards
down
what does a resister use up?
electric energy
no series or parallel
if there are two batteries interrupting.
shorted out
if there is a wire and the current can travel an alternate rought
What is the direction of the magnetic force on an electron moving upwards in a magnetic field directed toward the left?
in
resistor
is an ohmic device designed to have a specific value of resistance. the current through a resistor is the potential difference across it divided by resistance. impedes the charge flow. like a bumper.
direct current
is one in which the direction of the current doesn't change. usually the magnitude of the current is constant
Which draws more current, a 100 W lightbulb or a 75 W bulb? Which has the higher resistance?
is voltage is the same, then I= PV and the 100 W bulb draws more current than the 75W bulb. the 75W bulb has the higher resistance since R=V^2/P
two iron bars attract each other no matter which ends are placed close together. are both magnets?
no because if they were both magnets two pairs of ends would repel.
Is current used up in a resistor?
no. current is flow of electric charge and charge is conserved. the current in the circuit is the same on either side of a resistor. what is used up by a resistor is electrical energy which is changed to thermal energy by the resistor.
resistance and resistivity
resistance of the object is the ration of the applied voltage to the current. R=V/I. Expressed as Ω. the resistance of a wire depends on the material it is made of R=p (L/A) where p is the resistivity.
The heating element in a toaster is made of nichrome wire. Immediately after the toaster is turned on, is the current in the wire increasing, decreasing or staying constant?
resistance of the wire increases as the wire heats up, so the current decreases.
which of the following arrangements of two identical ideal lightbulbs and two identical ideal batteries would give the maximum possible total power out.
the batteries in series and the lightbulbs in parallel b/c V is maximum for the batteries in series and Req is minimum for the lightbulbs in parallel.
alternating current
the direction periodically reverses.
coaxial cable consists of a central metal wire which carries a current in one direction and a metallic shield which carries an equal current in the opposite direction. why is the magnetic field outside the cable zero?
the net current carried by the cable is zero, and the magnetic field due o the central wire is cancelled by the magnetic field due to the shield.
the temperature coefficient of resistivity of a semiconductor is negative (ie. the resistance decreases with increasing temperature.) how could this be explained by the microscopic model of resistivity.
the number of charge carriers in a semiconductor increases with increasing temperature.
electric current
the rate of flow of electric charge. I= (delta) Q/(delta)t. the SI unit of of current is ampere. 1A=1 C/s. by convention, the direction of current is taken to be the direction of the flow of positive charges (from + to -) electrons actually flow in the opposite direction.
electric power
the rate that electrical energy is transformed to some other form of energy or the rate at which some other form of energy is transformed into other energy. P = ΔPE/t = qV/t = IV. for an ohmic device, P=IV=I2R=V2/R
The equation P = V ^2/R indicates that the power dissipated in a resistor decreases if the resistance is increased whereas the equation P = (I^ 2)R implies the opposite. Is there a contradiction here?
there is no contradiction in the two equations since the current, potential, and resistance are related through Ohm's law (V=IR). P = V ^2/R is used if the voltage is constant in which case the current also decreases when resistance increases. P= (I^ 2)R should be used if the current is constant in which case the voltage also increases when the resistance increases
the resistivity of copper is less than the resistivity of aluminum. can a copper wire and the aluminum wire of the same length have the same resistance?
yes if the aluminum wire is thicker.
can a copper wire and an aluminum wire of the same length have the same resistance
yes. the resistivity of copper is less than that of aluminum, but the copper wire can have the same resistivity as an aluminum wire of the same length if the copper wire has a smaller cross sectional area. (ie. if the copper wire is thinner than the aluminum wire)
