PHYS 272 Lecture Notes - Lecture 19: Electric Field

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cyanwallaby953

29 Aug 2018

School

University of Maryland

Department

Physics

Course

PHYS 272

Professor

Orozco

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Document Summary

Phys272 lecture 19: resistance increases with the length of the wire and decreases with the cross- section of the wire; in this formula, is the resistivity of the material, ohm"s law. Integral form of finding current, where j is charge density: (cid:1835)= (cid:1836) =(cid:883) (cid:1831) (cid:1864) : for a small enough area and length: (cid:1836) =(cid:1831) (cid:1864) (cid:1836) =(cid:1831) = (cid:883)(cid:4672) (cid:1864)(cid:4673)=(cid:1831) . =(cid:883) (cid:1831) =(cid:1836) ,(cid:1836) =(cid:1731) (cid:1732) (cid:1832) =(cid:1831) =(cid:1865) (cid:1831) =(cid:1865) : recall coulomb"s law, resistivity may also be rendered as the inverse of the conductivity , notice that ohm"s law says that the field is proportional to a velocity while. Coulomb"s law states that it is proportional to acceleration! However, since the electric field is the ratio between the electric force and the charge, the electric field is still proportional to a velocity and not acceleration.

Related Questions

1. The drawing shows two long, straight wires thataresuspended from the ceiling.

The mass per unit linglh of each wire is 0.050 kg/m. Eachofthe four strings suspending

the wires has a length of 1.2 m. When the wires canyidenticalcurrents in opposite

directions, the angle between the strings holding the twowiresis 15Â° What is the current

in each wire?

2. A piece of copper wire has a resistance per unitlength 6.0x l0 ^-3 Î© /m. The wire is wound into athin, flat coil of manyturns that has a radius of 0.15 m. Theends of the wire areconnected to a 12 V battery. Find themagnetic field strength at thecenter of the coil.

3. A conducting coil of 1800 turns is connected toagalvanometer, and the total

resistance of the circuit is 50 Î©. The area of eachtum is 5.2x 10 ^-4 m². The coil is

moved from a region where the magnetic field is zero intoaregion where it is nonzero,

the normal to the coil being kept parallel to themagneticfield. The amount of charge

that is induced to move around the circuit is measured tobe9.0 x l0^-3 C. Find the

magnitude of the magnetic field.

[Such a device can be used to measure the magneticfieldstrength in a region of space and is called afluxmeter.]

I really need these questions answered ... i wouldreallyappreciate it ... Thank you

1. Two parallel conductors each of 0.50m length, seperatedby5.0e-3m and carryoing 3.0A in opposite directions, willexperiencewhat type and magnitude of mutal force? (magneticpermeability inempty space Î¼₀= 4Ïe-17 Txm/a)

2. A horizonal wire of length 3.0 carries a current of6.0A andis oriented so that the current direction is50^o S of W.The Earth's magnetic field is due north atthis point and hasstrength of 0.14e-4T. What is the direction ofthe force on thewire?

3. A proton moving at speed of 3.8e16 m/s cuts across thelinesof a magnetic field at an angle of 70^o. thestrength ofthe field is 0.25e-4T. what is the magnitude of theforce acting onthe proton? (q_p =1.6e-19C)

4. A rectangular coil (0.20m X 0.80m) has 200 turns and isinauniform magnetic field of 0.30T. if the orientation of the coilisvaried through all possible positions, the maximum torque onthecoil by magnetic forces is 0.080 N x m. What is the current inthecoil?

5. An airplane with a wingspan of 60.0m files paralleltoEarth's surface at a point where the downward component oftheEarth's magnetic field is 0.400e-4T. If the inducedpotentialbetwen wingtips is 0.900V, what is the plane speed?

6. A ping pong ball covered with a conducting graphitecoatinghas a mass of 5.0e-3kg and a charge of 4.0 Î¼C. Whatelectric fielddirected upward will exactly balance the weight ofthe ball? (9.8m/s²)

7. A superconducting wire carries a current of10⁴A. Find the magnetic field at a distance of 1.0m fromthe wire.(Î¼_o = 4Ïe-7 T x m/A)

This set of problems is designed to help you prepare for the lab oncapacitors. Here are some things you should know.
The charge on a capacitor is proportional to the voltage. Theconstant of proportionality is the capacitance:
q=VC.
The simplest type of capacitor is a parallel plate capacitorconsisting of twoparallel conducting plates...DUH. If the plateseach have area A and
are separated by a distance d, the capacitance will be
C=Ke0A/d,
where e0 is the permittivity of free space (you measured that lastweek) and K is the dielectric constant of what ever material isbetween the plates. If the space between the plates is filled withvacuum, the dielectric constant is exactly 1. Air is pretty darnclose to 1.
When a capacitor is charged in a series circuit with a resistor,the charge builds up with time. Consider the circuit below.

When the switch, S, is thrown to position A, the battery, withvoltage V0, will supply a current through the resistor, R, thatcharges the capacitor C. The voltage across the capacitor at anygiven time is given by
V=V0(1-e-t/RC).
The product RC is known as the time constant. After waiting a timeequal to 1 time constant, the voltage on the capacitor is(1-e-1)=0.632 of V0. After 5RC the capacitor is nearly fullycharged and has a voltage of 0.993V0. Eventually, the voltagebuilds up to V0.
When the switch is thrown to position B, the capacitor willdischarge through the resistor. The voltage as a fuction of timewill be given by
V=V0(e-t/RC).
After a long enough time, the capacitor will fully discharge theresistor.

1. [1pt]
A parallel plate capacitor has plates with length and width of 5.7cm and are separated by 7.0 mm. The capacitance is measured to be1.479Ã10-11 F. What is the dielectric constant of the materialbetween the plates?

Correct, computer gets: 3.60

2. [1pt]
Consider a series circuit like the one shown above with a resistorof R=80. kilo-ohms and a capacitor of C=8.0 uF. What is the timeconstant of the circuit?

Correct, computer gets: 0.64 s

Hint: Don't forget to convert to Farads and ohms from uF andkilo-ohms.
3. [1pt]
If the battery has a voltage of 12 V, what is the voltage acrossthe capacitor after the switch has been in position A for 2 timeconstants?

Correct, computer gets: 10.38 V
4. [1pt]
What is the final charge on the capacitor after a long time?

Correct, computer gets: 9.60E-05 C
5. [1pt]
Now let's say that the capacitor has been fully charged and wethrow the switch to position B. How long will it take for thevoltage on the capacitor to be 0.891 V?

6. [1pt] What is the charge on the capacitor at this time?

Answer: ____________

PHYS 272 Lecture Notes - Lecture 19: Electric Field

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