Physics 112 Practice midterm

A man stands in front of a mirror and his eyes are 1.95 meters above the floor and the top of his head is .15 meters above his eyes. A. What is the height above the floor for the bottom edge of the smallest mirror required to see both the top of head and his feet? B.What is the total height needed (distance of floor to top of mirror)?

1. For height from floor to bottom of mirror, just add the height of the entire body including eyes to head, and divide it in half for answer. 1 meter. 2. The answer is the height of feet to eyes + the height of eyes to head divided by 2. so 1.95+(.15/2)= 2.025

Kirchoff's current rule example: If 4.2 amps is going into a 4 way junction (the going in part takes up one of the 4 ways in the junction leaving 3 remaining) and 1 route leaves the junction with 2.3 Amps, the other routes leaves with 3.5 Amps, what does the last route leave with?

1.6, because you put them together like this 4.2+(-2.3)+(-3.5)=x and x is 1.6. Kirchoff's voltage loop rule does make sense and would simply be 6.8v = 3.2v+x+2.1v --> and x would be 1.5

If a light goes through a medium that is .35m thick and has a N index of 2.2, at 59 degrees, and is refracted off course, but then exits the medium and follows its original angle right next to the original projected course, how far off track did that medium make it go?

1st find angle inside medium. sin 59 (1) = sinx (2.2). You should get .389 which you inverse sin to get 22.93. 2nd. You plug it into formula thickness*sin(angle 1- angle 2)/(cos angle 2) to get answer. ==.223

For a certain circuit, Resistor 1 is 37.5, Resistor 2 is 31.8 and Resistor 3 is 19.8. Resistors 1 and 2 are in parallel and resistor 3 is in series right after it. If the circuit is connected to a 12.5 V battery, then what is the current in the circuit? (at the end maybe???)

1st the parallel. 1/32.5+1/31.8 =.0581. --> 1/.0581 = 17.20 --> 17.20 +19.8(R3) equals a total resistance of 37. V=RI or I=V/R -->12.5/37 = a current of ~ .3377A

A 25 microAmp current passes through a wire for 63 seconds. How much charge passed through it?

25 microamps * 63 seconds =1572 microcharge?

What is the speed of light in water when water has a N index of 1.333?

3*10^8 (1) = new speed (1.333). divide 3*10^8 by 1.333 to get 225056264

What is the constant of earths magnetic field?

5*10^-5. Its not on your sheet and its often the magnetic field if one is not given in the problem.

750 C of charge passes through a flashlight in a .25 of an hour. What is the average current in mA?

750/(.25H)(3600) <--3600 being the amount of seconds in a hour. Answer is .833 Amps, but 833.33 mA

A laser hits to mirrors like the one in this picture, and the angle of the rays when they overlap is 138 degrees. What is the angle of the two mirrors attached together?

ADD IMAGE Its angle/2 so 138/2 = 69

What is the formula for the scenario of an electron being sped up as it goes through parallel plates?

Acceleration = Eparr.*(Q)/mass

Assume two mirrors are perpendicular forming a right angle, and a ray of light reflects onto mirror 1 with an incident angle 74 degrees, then bounces on to mirror 2. Whats its angle as it comes off mirror 2 and what is the relationship of the ray coming off mirror 2 to the starting ray?

Angle of reflection is equal to angle of incident, so it came on to mirror 1 at 74 degrees and will come off mirror one at 74 degrees. You then use the mirrors right angle and 74 deg to find the last angle. 180-90+74= 16 left. Since it inters at 16 it leaves at 16. This leaving ray is parallel with the entering ray

There are certain formulas that have area as part of the formula, but they require you to do different things to find the area. What formulas are these and what special Bullshit do you have to do?

Area of Loop Formula: A = (pi(radius is meters^2)) Area for Bwire formula: A = 2piR Area for resistance of long wire: A = (pi/4)(Diameter in meters)^2

A circular wire loop with a diamete of 10cm is in a magnetic field of .50 T strength. (B) what if the loop was rotated so the normal to the plane of the loop was 32 degrees

BAcos(0) --> .50(Pi(.05)^2)*cos(32) = 3.4mWb

Find the magnetic field 5 cm from a straight wire with a current of 1.5 Amps

Bwire = UoI/2pi(r) so 1.5*(4pi*10^-7)/2pi(.05) should get you .000006

When it asks for the magnetic field which formulas should you probably use?

Bwire, Bloop, Bsolenoid

What are the formulas for capacitance

C=Q/V and C=KEoA/D

What are three ways to induce emf?

Change area of loop a, change angle theta, change magnetic field B. Youll notice these are all things that make up the faraday equation

A circular wire loop with a diamete of 10cm is in a magnetic field of .50 T strength. (A) Find flux through loop of if magnetic field is perpendicular to plane of loop

Circle strikethrough= BAcos(0) --> .50(Pi(.05)^2)*cos(actual 0) = 3.9mWb When you're given the diameter or radius and you're trying to find the area of the circle, remember the formula is (pi(radius in meters)squared)

What does a transformer do?

Converts AC into a higher or lower voltage because its more efficient to transport it at high voltage.

What are kirchoffs laws?

Current rule- total current entering junction is equal to total current leaving junction (Not the way its sounds) Voltage rule- total potential drop in a loop is equal to the total potential increase.

A radar detects an aircraft 5.5*10^-5 seconds after it was transmitted. How far away is the plane?

D= V*T 5.5*10^-5 times 3*10^8 = 16500. But 16500 is the distance to the plane and back, we only want to know how far the plane is so 16500/2 is 8250

What is formula for potential energy?

Delta PE= q Delta V.

Whats the formula for finding how much energy is stored in a capacitor?

E= (1/2)*C*V^2

What are the two versions of faradays law and when would you use them?

E= -N Delta (circle strike through or BAcos(0))/ Delta Time. For when you are trying to find/induce EMF or Flux. 2. E=BLV for when you are trying to find motional emf (or like the bar moving through a field)

What formula is for the parallel plate?

E=4(Pi)kQ/A

What is the generic formula for parallel plates that can't be found on the formula sheet?

E=4(pi)KQ/Area

Motional EMF problem: Assume a bar has a length of 45 cm and moves to the right 15 cm in 1.2 seconds. The magnetic field is going into the page, and has a strength of 1.5 T. What is the induced EMF?

E=BLV --> (1.5T)(.45m)(d/t=v -->.15m/1.2s) = .084

What formulas do you use for electric fields?

E=F/Q and Eparallel = V/d

There are parallel plates that are both 5 by 5 cm that are 3.6 mm apart. They have a potential difference of 12 volts. What is electric field charge on each plate and their capacitance?

Eparallel =V/d --> 12/.0036m = 3333. CHARGE is E=4piKQ/A or Q= EA/4piK --> q= 3333*.0025/4pi(k CONSTANT)= 7.36*10^-11 now that you have charge, capacitance = Q/V or 7.36*10^-11/12volts = 6.13*10^-12

A DC powerline carries 850A at an angle of 32 degrees to the earths magnetic field of 5*10^-5. And the length is 95 meters. What is the magnetic field force of that wire

F=BILSin(0) (5*10^-5)(95)(850)Sin(32) = 2.14

How do you know what formula to use when it asks for magnetic force?

F=qvb is for magnetic field on a charge while F=BIL is for magnetic field on a charge in a wire (or maybe just force on wire)

When using a defibrillation device, the resistance of the pathway is 475(omega sign) and 12mA is needed. What voltage should be applied in Amps?What voltage should be applied in amps?

First convert the 12mA to Amps --> .12Amps. as the formula requires all units to be consistent with their prefixes. Then do I*R =V. so .12 *475 = 5.7 volts. i dont know why you had to change from milliamps to amps the question just said so

If a problem asks for magnetic field force on a charge or on a wire, what should you know?

Formulas F=qvb or F= Bil. also first set of right hand rules, where your hand is in the shape of an L with middle finger out. Thumb = direction of electrons, Index finger is direction of established magnetic field, and middle finger is magnetic force on a positive charge

When light goes into a new medium, what does not change?

Frequency. Everything else can change.

generator vs. electric motor

Generator: converts mechanical energy into electrical energy, while an electromotor converts electric energy into mechanical energy.

A lightening bolt with 17500A of current moves 39 C of charge. What was the duration of this moving in milliseconds?

I=Delta Q/Delta T or T= Q/I. The charge is 39 and the current is 17500 so 39/17500 =.00222 then times 10^-3??? so .0000022?? I think so

An electron is going 79 m/s around a circle with a radius of 1.2 m. What is the value of I for the time it takes to go around the circuit once.

I=ev/2Pi(r) E= Elementary charge constant v is velocity. You should get 1.67*10^-18

What is lens law of Electromagnetic Inertia?

Induced EMF in a loop gives rise to a current whose magnetic field opposes a change in flux. When doing problems involving this, take into consideration that magnetic field comes out of the north pole and goes into the south pole.

For total internal reflection occur.....

It has to start in the denser material and head into a less dense material.

A wire is placed horizontally in a magnetic field directed into the page (a field of x's) if the length of the wire is 5 cm and the current 2.6A, and the magnetic field is .25 T what is the force on the wire?

Its force on a wire so F=BILsinTheta. (.25)(2.6)(.05)sin90 degrees because the wire perpendicular to the field and intersects at 90 degrees

Whats the formula electron pushed by magnetic field?

Mass*Volume/Radius = Charge(q)* Magnetic field(B)

Motional EMF problem: Assume a bar has a length of 45 cm and moves to the right 15 cm in 1.2 seconds. The magnetic field is going into the page, and has a strength of 1.5 T. Which direction is that EMF going?

Note that it said magnetic field goes into the page. Also note that the bar moved in a way that allowed more magnetic field into it when it stopped then when it started. These are both important because induced EMF/flux always goes the opposite direction of magnetic field (pointing the fingers at face, thumb to the right) making it counter clockwise, but if either of those "notes" changed it would go the opposite way. if the both changed it would go counterclock wise again.

What is the formula for power?

P = Volts^2/current. This is important because on the formula sheet says P=V*I but we are remembering it having a square for voltage.

A defibrillator sends a 6.25 A current, applying 11500 Volts at a time. What is the resistance in K (omega sign)

R=V/I so 11500/6.25 =1840(omega sign) --> 1.840K(omega sign)

Diameter of a copper wire is 8.013mm and has resistivity of 1.72*10^-8 Find resistance on a 1.1km length of wire.

Resistance= resistivity*length/area. Area= (Pi/4)(Distance in meters)^2 Plug it in and you should get .375

Find the magnetic force on a proton moving at 3*10^6 m/s at 30 degrees south of east. What direction is the force?

So first identify its on a charge, and not on a charge in a wire so We use F=QVBsinTHETA. Then note its 30 degrees south of east. Our starting point is always north in these types of problems so that 30 becomes 90+30 =120. If not shown or given in any way, then the magnetic field also points north (and they might be related) Also note theres two constants in this formula, 1 of them is not on the equation sheet. plug it in and you should get 2.07*10^-17. Direction: field points fingers north, proton moves east(ish) so force is out of paper at you.

For a certain circuit, Resistor 1 is 37.5, Resistor 2 is 31.8 and Resistor 3 is 19.8. Resistors 1 and 2 are in parallel and resistor 3 is in series right after it. If the circuit is connected to a 12.5 V battery, What is the potential difference among the resistors? How much power is dissipated over R1 and R2 in watts?

Starting with series resistors, we times the current by the resistance to get the volts dissipated over it. .3377A * 19.8R = 6.68 volts are used on R3. 12.5 (starting volts) -6.68 = 5.81 volts go over R1 and R2, and since they are in parallel, it goes across them equally. flipping the I*R=V formula, we do 5.81v/37.5R = .1549 A is dissipated in R1. to convert it to watts,P=I^2R (squared because parallel means 2?) .1549^2*37.5=.900W. You use 5.81 V for R2 as well.

What is dispersion?

Taking light and refracting it into a spectrum. Red bends the least and travels the fastest while violet bends the most and travels the slowest. Most materials can disperse to some degree

What is the critical angle? What is its formula?

The angle of incidence when the angle of refraction is 90 degrees Sin(0c) =N2/N1 as long as N1>N2 or N=1/Sin(0c) REMEMBER N = INDEX, YOU DO NOT NEED ANY ANGLES TO FIND CRITICAL ANGLE

When power is dissipated over a circuit with how is power dissipated over resistors that are in parallel that have different resistance amounts?

The power that is dissipated over the resistors in parallel is exactly equal, even if the resistance amount is different for each.

What is magnetic flux?

The total number of lines (or magnetic fields) going through a give area

Whats important to remember for capacitors when they are in a circuit?

They behave in an opposite way as resistors for example. Resistors in parallel are added up like 1/x+1/y+1/z = total, while for capacitors in SERIES operate the same way with adding fractions. Capacitors in parallel act like resistors in series

What does it mean when resistance is in parallel and how do you add them up?

They split up and come back. You add them up like 1/r1+1/r2+1/r3.... to get an answer of X. then You do 1/X to get the total resistance.

What does to the normal mean?

To the a line that comes out of the flat angle. So a line that bounces of a mirror 4 degrees to the normal is almost exactly perpendicular as opposed to parallel

Right hand rules 1. You see the electron is heading south and that magnetic field is coming at you, out of the page. What direction is the magnetic field force on the charge?

To the left. Index finger is pointing at you just like magnetic field, thumb goes straight down like electron, leaving middle finger to go left

Right hand rules1 If you see an electron heading away from you into the paper and the magnetic fields are point up, which way is magnetic force?

To the right because index finger is pointing north for magnetic field, thumb is going into paper just like electron, leaving middle finger out going right.

For the movement of a wave through a field, how do youi know which way is the electric field/magnetic field?

Use the right hand rules to figure this out. The direction of wave = thumb (direction of electron), the magnetic field is index finger, and the electric field is force/middle finger

Visible light wavelengths are between 400-700 Nanometers, what is minimum and maximum frequency visible in hertz?

V=F/Lambda --> F =V/Lambda. 3*10^8/700*10^-9 = 4.28*10^14 3*10^8/400*10^-9 = 7.5*10^14

A radio station uses a frequency of 11.04 MHz. What is its wavelength?

V=F/Lambda --> Lambda = V/F. So 3*10^8/11.04*10^6 = 27.17

A battery supplies a current of 9mA to a flashlight that has a resistance of 7.7K(omega sign) What is voltage?

V=IR so(9*10^-3)(7.7*10^3) = 69.3

A step down transformer is used to reduce the voltage from 15kV to 220V. if the primary coil has 4500 turns, how many turns does the secondary coil have?

Vs/Vp = Ns/Np so 220/15,000=4500/x x = 66 number of turns

A circular wire loop with a diamete of 10cm is in a magnetic field of .50 T strength. (C) if the time of rotation of loop was 1.2 seconds what is the average induced emf?

When it talks about the rotation of loop, it means it started out like it was in problem A then became problem B. and remember it states only 1 loop. so -1(3.4mWb-3.9mWb)/1.2s = .42mV (which is the flux)

Consider 2 parallel wires placed 18.5cm apart. the current in the first wire is 18.5 A and the current in the second wire is 10.5 A. The currents are going in opposite directions. What is the magnitude of the combined magnetic fields at the midpoint of the wires. Answer in microtesla.

You would use Bwire = UoI/2pi(r) with r being the distance to the measuring spot. And you do this formula twice for both magnetic fields. Uo is the vacuum permeability constant and given. When you do the curving right hand rule on it this mid point, you realize both magnetic fields push same direction so the are both positive. Do the Bwire 1 + Bwire 2 and you should get .0000622 -->62.2 microtesla

As light goes through a medium and is refracted, how does speed play a role in it?

if the light is slowed down, it goes towards the normal, if it speeds up, it goes away from the normal.

Right hand rules 1. You see the magnetic field is going to the right and the electron is going north, which direction is the magnetic field force?

into the page. because thumb goes up, index finger goes right meaning middle finger goes into the page.

What are the parts of a shadow?

umbra is the dark center and penumbra is semi dark area around umbra