Winter wk 3 – Thus.20.Jan.05

• Ch.24: Voltage and electric field

• Ch.26: Current and resistance

• Solar applications

• Ch.27: Circuits

Energy Systems, EJZ

Equipotential surfaces and E fields

Equipotential = constant voltage

Conductors are equipotentials, in electrostatics

Potential difference Electric field

dV/dx = -E or, equivalently,

Practice: Ch.24 Q5,8 (p.646), P#3, 4, 6, 35

V E dr ����������������������������

Ch.24 #4

Ch.24 #6

Ch.24 #35

Electrostatics (d/dt=0): charges fields forces, energy

• Charges make E fields and forces

• charges make scalar potential differences dV

• E can be found from V• Electric forces move

charges• Electric fields store

energy (capacitance)

E.dA = q/0=, E = F/q

ldEdr

rV

')'(

4

1)(

VE

F = q E = m a

W = qV, C = q/V

Ch.26: Currents and Resistance

Current = rate of flow of charge I = dq/dt

Units: amps = coulombs/secCurrent density: J = current/area = n e vCh.26 Q1, 2, P.1, 8

Water flow:Electricity flow:pressure voltage V

volume/time current I

Ch.26: Q1, 2, P.1, 8

Resistance

Resistance = resistivity * area/length

R = A/L

Which conductor has the greatest resistivity?

Ch.26: Q3

Ohm’s law

In many substances, for a given resistance R, the stronger the driving voltage, the greater the current that flows:

Voltage = current * resistance

V = I * R

Ch.26 Q5, P.17

Power in electric circuits

Power = rate of energy xfr = voltage*currentP = V I

units: Watts = volts * ampsRecall that work = qV. Units: J = CVSolve for V(J,C) = Then [volts]*[amps] = ____*C/s = ______

If V=IR, find P(I,R) = P(R,V) = Ch.26 #35, 64

Ch.27: Circuits:

Battery pumps electricity current flows

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/watcir.html#c1

Voltage = emf

Voltage = potential difference

Electromotive force = V = dW/dq =work done per unit charge

d/dx = -E = electric field

Emf and electric field E

d/dx = -E = electric field

Using the fundamental theorem of calculus, we can derive another of Maxwell’s eqns:

d dx

d

dx

E

dx E dx

E dx

Ch.27: Practice with simple circuits

Q2

#5, 14

Solar applications

Storms from the Sun:p.13: If a CME travels at 1 million miles per hour, how

long does it take to reach Earth?

p.16: The 2 May 1994 event dumped 4600 GW-hr of electricity into Earth’s upper atmosphere. How much energy is that in Joules?

p.16: If the Earth’s mean magnetic field is B0=0.5 Gauss, and one Tesla=104 Gauss, by what percent does 2000 nanoTesla change Earth’s field?

p.54: For the CME of 1 Sept 1859: calculate its speed v, if it took 18 hours to reach Earth.

more Solar applications

Storms from the Sun:p.77: If Rsun = 100 REarth, then find the ratio of their

volumes, Vsun/VEarth

p.77: If m=5 millions tons of mass is converted to energy (E=mc2) each second, calculate the power (P) produced by the Sun.

p.82: If the Sun’s mass is M=2x1030 kg, and it keeps losing dm/dt = 5 million tons per second, how long (T) can the Sun last?

p.83: If the solar wind pours I=1 million amps into Earths magnetosphere, how much charge (Q) is that per day?

Extra solar applications

p.13: Calculate vthermal from Tsolar wind. Compare to vflow.

p.16: Derive the altitude for a geosynchronous orbit

p.77: If the Sun’s core temperature is about T=107K, calculate the thermal speed vth of protons in the core.