Unit 2: Engineering Science Unit code: L/601/1404 QCF Level: 4 Credit value: 15 ASSIGNMENT 3.1 RESISTANCE IN ELECTRIC CIRCUITS NAME: Date Issued I agree to the assessment as contained in this assignment. I confirm that the work submitted is my own work. Signature Date submitted Learning outcomes On successful completion of this unit a learner will: L03 Be able to apply DC theory to solve electrical and electronic engineering problems Assessment criteria for pass The learner can: 3.1 solve problems using Kirchhoff s laws to calculate currents and voltages in circuits 3.2 solve problems using circuit theorems to calculate currents and voltages in circuits 3.3 solve problems involving current growth/decay in an L-R circuit and voltage growth/decay in a C-R circuit Achieved Feedback Comments: This assignment brief has been internally verified. Grade Awarded: Tutor Signature Date: Script verification I.V. Signature Date:
PART 1 RESISTANCE NETWORK This part must be written up as a separate report. The student has a circuit board with a suitable resistance network with the facility for safely measuring the voltage and current at key points. A typical circuit is shown. A safe source must be used and it is advisable to use a perspex cover with holes for inserting probes. TASK 1 Examine the network fixed on the board and sketch the circuit. Deduce the total resistance. Measure the total resistance and comment on the accuracy of the two figures. TASK 2 Measure the total current from the source and the source voltage at the same time. Calculate the theoretical current and compare the theoretical and practical values. TASK 3 Calculate and check the voltage across a designated resistor. Comment on the two figures.
PART 2 TEST QUESTION ON RESISTOR NETWORK Solve the question below and hand in for marking. 1. Calculate the total resistance of the network shown below. 2. Calculate the total current. 3. Calculate the power dissipated as heat in the circuit. 4. Calculate the current in R4. 5. Calculate the current in the R1. 6. Calculate the voltage across R1. STUDENT R1 R2 R3 R4 V 1 220 470 680 330 48 2 470 330 220 100 24 3 1000 470 470 200 100 4 680 470 330 220 50 5 2k2 3k3 3k3 1k0 120 6 4k7 2k2 2k2 1k2 50 7 2k2 4k7 6k8 4k7 240 8 870 680 680 330 48 9 100 220 220 100 12 10 1000 680 680 220 48
PART 3 PRACTICAL CAPACITOR - RESISTANCE TIME CONSTANT Write up this part as a separate report that should be concise and accurate using appropriate technical language and terms and presented in an appropriate manner. Set up the circuit shown with a suitable resistor and capacitor. Connect the terminals marked V C to a suitable recording device. A high speed digital recorder connected to a computer would be best so that you can print out the graph. Suggested values are 10 Volt supply, 6 kω resistor and 20 μf capacitor. If a slower system is used use a larher capacitor. Charge the capacitor by putting the switch to position 1 then discharge it by moving the switch to position 2. Record and plot the graph of V C against time. A typical charging graph is shown. Calculate the theoretical timer constant based on R and C. Determine the time constant from the graph using two methods as a check for accuracy. Compare the theoretical and actual time constant. Make suitable observations and comments about the results.
PART 4 PROBLEMS SOLVING Solve the following problems showing your calculations in full and the method used. 1. Calculate the time constant for an RC circuit with a resistance of 220 and capacitance of 470 nf in series. 2. Calculate the time constant for a series R L circuit with an inductance of 6 μh and resistance 0.02 Ω. 3. A capacitor of 2000 μf is charged to 12 V and then a resistor of 5 Ω is connected across it. Calculate the charge stored. Calculate the energy stored. Calculate time taken to discharge to 1 V 4. An inductor with inductance 60 mh and resistance 0.7 Ω suddenly has 2V connected across it. Calculate the steady state current. Calculate the energy stored and power dissipated. Calculate the time taken for the current to rise to 0.5 A. 5. The voltage across the inductor shown is initially zero. Show that when the switch is closed that the voltage across the inductor will be v = E e -t/τ where t is the time elapsed after the switch is closed. Determine an expression for the time constant τ in terms of R and L.