FE Exam Review Electrical Circuits The FE exam consists of 180 multiple-choice questions. During the morning session, all examinees take a general exam common to all disciplines. During the afternoon session, examinees can opt to take a general exam or a discipline-specific (chemical, civil, electrical, environmental, industrial, or mechanical) exam. See exam specifications for more details. •XI. Electricity and Magnetism 9% •A. Charge, energy, current, voltage, power •B. Work done in moving a charge in an electric field (relationship between voltage and work) •C. Force between charges •D. Current and voltage laws (Kirchhoff, Ohm) •E. Equivalent circuits (series, parallel) •F. Capacitance and inductance •G. Reactance and impedance, and admittance •H. AC circuits •I. Basic complex algebra Exam Strategies •Only 4 minutes per problem. –Don’t dwell on a problem. •Do the ones you know. Make an “educated guess” at the ones you don’t know. •Answers are typically in SI unit. Set your calculator to engineering notation. •Pay attention to units (degrees vs. radians) FE supplies equations You can visit their page To get one http://www.ncees.org/exams/study%5Fmaterial s/fe%5Fhandbook/ Electric Field Electric field due to single charge: E = k k=8.89x109Nm2/C2 q r2 Uniform electric field due touniform distribution of surface charge: Eplane 2 0 kq V Electric potential due to single charge r Potential difference in uniform electric field: ΔV = E●d Potential energy : ΔU = qΔV Charge in uniform electric field qΔV= Kf -Ki F =qE qE = ma Capacitance Capacitance C = Q/ΔV, unit: farad [F] C =εoA /d, with dielectric C=koA/d Cseries = (1/C1+1/C2 + … + 1/Cn)-1 Cparallel = C1 + C2 + …. + Cn U = ½CV2 εo =8.85x10-12C2/Nm2 Example 1 Charges Q, -Q = 2 nC are placed at the vertices of an equilateral triangle with side a = 2 cm as shown. Find the magnitude of electric force on charge q = 6 nC placed at point A. Example 2,3 2. An electron with a speed of 5 x106 m/s i enters an uniform electric field E =1000 N/C i. a. How long will it take for the electron to come to stop? qe = 1.6x10-19 C me = 9.11x10-31kg 3. Find the potential difference needed for the electron to obtain a speed of 3x107 m/s. Example 4 Determine the charge on capacitor C1 when C1=10 µF, C2=12 µF, C3= 15 µF, Ceq= 4μF and V0=7 V. (Hint: If capacitors are connected in series, then charge on each capacitor is the same as that on equivalent capacitor a. 0.5x10-5 C b. 2.8x10-5 C c. 5.2x10-5 C d. 7.0x10-5 C e. 1.1x10-4 C 1 2 Example 5 a. b. c. d. e. Determine the charge stored in C2 when C1 = 15 µF, C2 = 10 µF, C3 = 20 µF, and V0 = 18 V. Hint: find the equivalent capacitor first 180μC 120μC 90μC 60μC 30μC Direct Current Current (A) – flow of charge Q in time t I = ΔQ /Δt units: ampere [A] Current density J = (ne)vd e = 1.6x10-19C Ohm’s Law: V = IR, unit: volts [V] Resistance, unit: ohm [Ω] – opposition to flow of charge R = ρL / A {in a conductor of length L and area A} Power: P = I.V = I2R = V2/R, unit: watt [W] Combination of Resistors Rseries = R1 + R2 + …. + Rn Rparallel = (1/R1 + 1/R2 + … + 1/Rn)-1 A wire carries a steady current of 0.1 A over a period of 20 s. What total charge passes through the wire in this time interval? a. 200 C I=Q/t 1A=1C/1s b. 20 C c. 2 C Q = It Q = 0.1A*20s = 2C d. 0.005 C A metallic conductor has a resistivity of 18 106 m. What is the resistance of a piece that is 30 m long and has a uniform cross sectional area of 3.0 mm2? Resistivity a. 0.056 b. 180 R =r * L/A c. 160 Resistance R=18*10-6 Ωm*30m / d. 90 3*10-6m2 R = 180 A 60-W light bulb is in a socket supplied with 120 V. What is the current in the bulb? a. 0.50 A b. 2.0 A P = V*I = V2/R = I2*R c. 60 A d. 7 200 A 60 = 120*I >> I = 60/120 = 0.5 If a lamp has resistance of 120 when it operates at 100 W, what is the applied voltage? a. 110 V b. 120 V P = V*I = V2/R = I2*R c. 125 V d. 220 V 100 = V2 /120 V = sqrt(120*100) = 11*10 = 110 14. If R1 =R2=R3=R4 = 10Ω and R = 20 Ω, what is the equivalent resistor of the circuit? R*0/(R+0) = 0 Example 14 cd • Req =(1/R2+ 1/R3)-1 + R4 + R • R eg =(1/10+1/10)-1 + 10 +20 = 35Ω resistors in parallel I1+I2 = I – Kirkchoff’s second law I2 voltage V across resistors is the same I2 I1 I Current I splits but 6*I1 = 7*I2 The larger the resistor the smaller the current I Rseries = R1 + R2 + …. + Rn Rseries = is always larger than any of the elements if R1 and R2 are the same (R) Rseries is 2R Current through each resistor is the same. Rparallel = (1/R1 + 1/R2 + … + 1/Rn)-1 Rparallel = is always smaller than any of the elements Rparallel = (1/R1 + 1/R2 + … + 1/Rn)-1 if either of R1, R2, and, … Rn is 0 (wire or closed switch) while in parallel Rparallel is 0 0 Rparallel = (1/R1 + 1/R2 + … + 1/Rn)-1 if R1 and R2 are the same (R) in parallel Rparallel is R/2 Example a. b. c. d. e. What is the magnitude of the potential difference across the 20-Ω resistor? 3.2 V 7.8 V 11 V 5.0 V 8.6 V Charging a Capacitor • At the instant the switch is in position a the charge on the capacitor is zero, the capacitor starts to charge. The capacitor continues to charge until it reaches its maximum charge (Q = Cε) • Once the capacitor is fully charged, the current in the circuit is zero. • Once the maximum charge is reached, the current in the circuit is zero – The potential difference across the capacitor matches that supplied by the battery • The charge on the capacitor varies with time – q(t) = C(1 – e-t/RC) = Q(1 – e-t/RC) t is the time constant • t = RC Discharging a Capacitor in an RC Circuit • When a switch is thrown from a to b the charged capacitor C can discharge through resistor R – q(t) = Qe-t/RC • The charge decreases exponentially Force on a Charge Moving in a Magnetic Field Force on a charge moving in a magnetic field is given by equation: FB qv B – FB is the magnetic force q is the charge – v is the velocity of the moving charge – B is the magnetic field The magnitude of the magnetic force on a charged particle is FB = |q| v B sin θ Charged Particle in Magnetic Field • Equating the magnetic and centripetal forces: mv FB qvB r • Solving for r: mv r qB 2 Mass Spectrometer • Example: The magnetic field in the deflection chamber has a magnitude of 0.035 T. Calculate the mass of a single charged ion if the radius r of the its path in the chamber is 0.278 m and its velocity is 7.14x104m/s Inductance, Inductors Inductance, unit: henry [H] = ability to store magnetic energy A circuit element that has a large self-inductance is called an inductor. The circuit symbol is Potential across inductor: vL(t) = L diL(t) / dt L = N2 μA / ℓ UM = ½LI2 Lparallel = (1/L1 + 1/L2 + … + 1/Ln)-1 Lseries = L1 + L2 + …. + Ln Symbols DC current source – keeps constant current flowing out in the direction shown DC voltage source – keeps constant potential between + and – side of battery AC source V(t) = V0sin(wt) or I(t) = I0sin(wt) R=0 R = infinity Complex Numbers rectangular form z=a+jb, z=zcosθ+jzsinθ) phasor form z=c/θ c = (a2+b2)½ θ = tan-1(b/a) z1+z2 = (a1+a2)+j(b1+b2) z1·z2= c1·c2/(θ1+θ2) z1/z2=c1/c2/(θ1-θ2 ) AC circuits: impedance Z=R+jX In series Zeq = (R1+R2)+j(X1+X2) In parallel Zeq=[1/(R1 +jX1)+1/(R2 +jX2)]-1 AC Circuits • The instantaneous voltage would be given by v = Vmax sin ωt • The instantaneous current would be given by i = Imax sin (ωt - φ) – φ is the phase angle, Imax= Vmax /Z Z is called the impedance of the circuit and it plays the role of resistance in the circuit, where Z R X L XC 2 2 Impedance has units of ohms X – reactance of the circuit; X=ωL – 1/ωC XL = ωL XC = 1/ωC AC Circuits Root mean square value of V and I is given by expressions: Vrms = Vmax/√2 , Irms = Imax /√2 Z=V/I θ =tan-1(X/R) V = Vrms sin ωt, I = Irmssin (ωt+ θ) in phasor form V=Vrms∟0 I = Irms∟θ Impedance in rectangle form: Z =R+jX X=XL-Xc Xc = 1/(ωC) XL = ωL AC Circuits R = Zcosθ X = Zsin θ AC Circuits Power can be expressed in rectangle form: S = P + jQ P- real power Q–reactive power P=VrmsIrmscos(θ) =I2rmsR Q = VrmsIrmssin(θ) = V2rms/X S2 = P2 + Q2 power factor PF= cos(θ) Example A series RLC circuit has R = 425 Ω, L = 1.25 H C = 3.5 μF. It is connected to and AC source with f = 60 Hz and Vmax = 150 V a. Find the impedance of the circuit. b. Find the phase angle. c. Find the current in the circuit. Example A series RLC circuit has R = 425 Ω, L = 1.25 H C = 3.5 μF. It is connected to and AC source with f = 60 Hz and Vmax = 150 V Calculate the average real and reactive power delivered to the circuit. sin(wt-q) sin(wt) sin(wt+q) Blue leads the red or Red lags the blue sin(wt) sin(wt+q) sin(wt-q) sin(wt) blue argument is always larger than red one Sample Problem Read from the plot: Amplitude of i(t) Io= 50 A Irms=50*0.71=35 v(t) = sin(wt) i(t) = sin(wt-90) current lags voltage by 900 Answer = B Sample Problem Magnitude = 5 from Pythagoram principle Angle (phase) from tan(q)=4/3 q = tan-1(4/3) Tan>1 so angle > 450 Answer = D Sample Problem Information 10 kV power line is useless. It is not the potential difference between two ends of the wire. You must use P = I2R to calculate the power dissipated. Answer = C Sample Problem • For AC circuit with Vrms=115V, Irms = 20.1A and phase constant θ=320, find the average real power and average reactive power drawn by the circuit. P = 115V*20.1Acos320 = 1965 W Q = 115V*20.1Asin 320 = 1217 kVAR ( kilovolt-amps reactive) Sample Problem Answer = A Sample Problem Time constant of the circuit is t = RC = 15 ms. Time constant is the time to charge capacitor to 63%. [1- e-1]. To charge more (80%) you need more time. Answer = D Sample Problem Inductances are like resistors in series and in parallel. Lseries = L1 + L2 Energy stored in an inductor: W[J] = 0.5L*I2 IL = 10 A from current source •Answer = B Sample Problem Average of any sin(wt) = 0 so ignore the AC Source Answer = C For DC current inductor resistance is zero (made of copper wire) the battery and 10 resistor are shorted by the 2 H inductance. The current is Iavg = 12/10 Sample Problem + Two 4 resistors are in parallel = 2 . Then 2 and 2 resistors Are in series. I = 40/4 Answer = C Sample Problem 20 is the amplitude Answer = B Power in AC circuits is calculated using rms values (this is why the rms was introduced) rms value is 20*0.7 = 14. P=I2R = 14250 = 10kW. Sample Problem After t = 5t, the capacitor acts like an open circuit Use Ohm’s Law for Ix Answer = C