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Activities ACTIVITY 1 Title: Electric Field Patterns Apparatus:Van de Graaff generator, petri dish with oil and seeds, set of shaped electrodes overhead projector, some sawdust Instructions 2-D Patterns Connect up the apparatus as shown and use the Van de Graaff generator to produce a high voltage between two point electrodes in the oil. Draw the electric field pattern present between the two point electrodes as shown by the seeds. Repeat the above but using two plane electrodes, producing a uniform electric field between them. 3-D Patterns While the Van de Graaff is switched on and charging, throw a small amount of sawdust at the dome. Sketch the path taken by the pieces of sawdust after they came into contact with the dome. The sawdust is affected by the radial field around the dome. Explain the behaviour of the pieces of sawdust after they came into contact with the charging dome. ACTIVITY 2 Title: The Cathode Ray Tube (Teacher demonstration) Apparatus:Cathode Ray Deflection Tube and stand, EHT supply (for electron gun and heater), HT supply (for deflection plates) Instructions Work Done by Electric Field Note the value of the EHT voltage used in the electron gun. Calculate the work done on each electron by the electric field in the electron gun. Hence calculate the speed of the electrons as they leave the electron gun and enter the evacuated tube. Deflection Plates Sketch the path followed by the electron beam as it passes between the deflection plates when: (a) bottom plate is positive, top plate is negative (b) top plate is positive, bottom plate is negative. Explain the effect the following changes have on the path followed by the electron beam: (a) decrease EHT voltage on electron gun (b) decrease HT voltage on deflection plates Sketch the paths followed by the electron beam after the above changes. ACTIVITY 3 Title: E.m.f. and Internal Resistance with parallel circuits Apparatus: Bench Power Unit, parallel circuit board, 3 x 12V bulbs, ammeter, voltmeter to measure the terminal potential difference (t.p.d.). V A L1 L2 L3 Instruction Connect up the circuit above, with all lamps unscrewed so that they are off. Copy the table below. Note down the current (I) and corresponding t.p.d. (V) values when no lamps are screwed in, and enter them into the table. Repeat the above for each case as one, then two, then three lamps are screwed in and light up. Using Ohm’s Law, complete the final column of the table by calculating the value of the internal resistance of the battery. No. of bulbs lit 0 1 2 3 Current I (A) t.p.d. V (V) “lost volts” (V) Internal resistance () ACTIVITY 4 Title: Internal resistance of a cell (Outcome 3) Apparatus: Alba Interface, computer, internal resistance board, 3 x connecting wire Instructions Load Internal resistance application (Start, All Programs, Science, Alba, Load an Application, Internal resistance) Follow the on screen instructions and connect the apparatus as shown in the photograph. For each pair of readings determine the lost volts. Calculate the gradient of the graph. Use an appropriate format to determine the internal resistance of the cell. ACTIVITY 5 Title: The Balanced Wheatstone Bridge Apparatus:Wheatstone Bridge board, 1.5 V cell, 20-0-100 µA ammeter, 10kΩ resistor, 30 kΩ resistor, set of unknown resistances A, B and C, decade resistance board. Schematic diagram showing position of resistances Circuit diagram Instructions Part A Connect the 10 kΩ and 30kΩ resistances, decade resistance board and unknown resistance A as shown in the diagram above. By finding the value of resistance of the decade resistance board which exactly balances the Wheatstone Bridge, find the value of resistance A. Repeat to find the value of resistances B and C. Part B Replace R2 with a 10 kΩ resistor. Predict the value of the decade resistance board that will balance the Wheatstone Bridge for each of the resistors A and B with this value of R2. Confirm your predictions by experiment. ACTIVITY 6 Title: The Out-of-Balance Wheatstone Bridge Apparatus: Wheatstone Bridge board, 1.5 V cell, 20-0-100 µA ammeter, 2 x 1 kΩ resistors, 2 kΩ resistor, decade resistance board. R2 R1 1 k 1 k µA Resistor B Resistance Board Instructions Connect the circuit as shown in the diagram making R1 a 2kΩ resistor and R2 and Resistor B a 1 kΩ resistors. Adjust the resistance of the decade resistance board so that the Wheatstone Bridge is balanced. Increase the resistance of the decade resistance board in 10 Ω steps, noting the value of the out-of- balance current each time. Decrease the resistance of the decade resistance board in 10 Ω steps, noting the value of the out-of- balance current each time. Record your results, copy and complete the table. Plot a graph of change in resistance, ΔR (Ω), on the ×-axis and out-of-balance current, I, (µA) on the y-axis. ΔR (Ω) -50 -40 -30 -20 -10 0 10 20 30 40 50 I1(A) I2(A) I3(A) Average I (A) ACTIVITY 7 Title: The Out-of-Balance Wheatstone Bridge - Applications Apparatus:Wheatstone Bridge board, 6 V battery, 20-0-100 µA ammeter, 1 k Ω/10 kΩ resistance boxes, decade resistance board, LDR, thermistor probe, strain gauge (already set up). Instructions Part A - Light Meter Connect the circuit up as shown. Balance the Wheatstone Bridge circuit with the LDR on your bench. There is no need to “remove” the protective resistor as the circuit is quite sensitive. Take care not to cast shadows over the LDR when finding balance. Find the out-of-balance current when the LDR is in a light place then in a dark place. Explain why the measured current is greater than zero for one condition and less than zero for the other. Part B -Thermometer Place the probe in an ice/water mixture in a beaker. (0 °C) Balance the Wheatstone Bridge with the probe in the ice/water mixture. Place the probe in a beaker of boiling water. (100 °C) Measure and record the out-ofbalance current obtained with the probe in the boiling water. Predict the current obtained when the probe is removed and is measuring room temperature. Calculate the value of room temperature from your results. What assumptions are you making about the temperature of the probe, its resistance, and the out-of-balance current? Continued... ACTIVITY 7 (continued) Instructions (continued) Part C - Strain Gauge The hacksaw blade has been fitted with two strain gauges, one on each side of the blade. The resistance of a strain gauge changes when its shape is deformed - either stretched or compressed. When the hacksaw blade is bent, one of the strain gauges will “stretch” while the other one will “compress”. The two strain gauges are connected to one arm of a Wheatstone Bridge circuit. The strain gauge circuit should already be set up for you. Balance the Wheatstone Bridge circuit when the hacksaw blade is straight. Bend the hacksaw blade in one direction. Note the out-of-balance current. Bend the hacksaw blade in the other direction. Note the out-of-balance current. Explain how the out-of-balance current is used to show (a) the amount of bending/strain put on the hacksaw blade (b) the direction of the bending/strain put on the hacksaw blade. Strain gauge (back) Strain gauge (front) µA 680 Resistance Board 4.5 V ACTIVITY 8 Title: Alternating Current – Peak and r.m.s. values Aim: To establish a relationship between peak and equivalent direct (r.m.s.) values of voltage. Apparatus: Bench Power Unit, 6 V battery, oscilloscope, variable resistor (0-22 Ω), 2 × 2.5 V lamps, connecting leads. Circuit 1 Circuit 2 to oscilloscope to oscilloscope variable a.c. supply Instructions Set up Circuit 1. Switch the time-base on the oscilloscope OFF. Adjust the supply and the oscilloscope to give a measured peak alternating voltage of 1 V on the oscilloscope Leave Circuit 1 switched on. Set up Circuit 2. Adjust the variable resistor until the lamp is the same brightness as the lamp in Circuit 1. (Use a solar cell connected to a voltmeter to determine this) Use the oscilloscope to measure the direct voltage across this lamp. Repeat the measurements for peak voltages of 2 V, 3 V, 4 V and 5 V. Plot a graph of direct voltage against peak voltage. Determine the gradient of the graph. State the relationship between Vd.c. and Vpeak using the value obtained from the gradient of the graph. ACTIVITY 9 Title: Calibration of Signal Generator Aim: To calibrate the frequency scale on a signal generator. Apparatus: Oscilloscope, signal generator, connecting leads Instructions Connect the output of the signal generator to the Y-inputs of the oscilloscope as shown. Switch the time-base ON. Set the signal generator to 10 Hz and switch on. Adjust the oscilloscope controls to obtain a recognisable waveform. Calculate the frequency from the trace on the screen. It is useful to record : the timebase setting, divisions for one cycle, and time for one cycle with the frequency in your table of readings. Repeat for other frequency values of 100 Hz, 10000 Hz and 10,000 Hz. Compare the measured and stated values of frequency. Include a column for percentage uncertainty in your table and complete this column assuming the oscilloscope is 100% accurate. State which scale on the signal generator is most prone to uncertainty. ACTIVITY 10A Title: Charge and potential difference for a capacitor (Outcome 3) Apparatus: Electrolytic capacitor (about 5000 µF), coulomb meter, voltmeter, 6 × 1.5 V battery, changeover switch A B V Coulomb meter Instructions Discharge the capacitor by shorting with connecting lead. Connect the circuit and set the switch to charge the capacitor as shown in the diagram. Allow enough time for the capacitor to charge fully. Set the switch to B to fully discharge the capacitor through the coulomb meter. Repeat for other charging voltages. Use an appropriate format to show the relationship between charge and voltage. ACTIVITY 10B Title: Charging and Discharging Characteristics for a Capacitor Aim: To verify the relationship between charge and voltage for a capacitor Apparatus Alba interface, Capacitor investigation board, 3 x connecting wire, bag of resistors and capacitors. Instructions Load Capacitors - Charge-Voltage Relationship (Start, All Programs, Science, Alba, Load an Application, Charge-Voltage Relationship) Follow the on screen instructions and connect the apparatus as shown in the photograph. Once the capacitor is charged open the table and select voltage and charge columns and plot a graph Calculate the gradient of the line. Repeat this for different values of capacitor Use an appropriate format to present your results ACTIVITY 11 Title: Charging and Discharging Characteristics for a Capacitor Aim: To observe the variation of the current through, and the p.d. across, a capacitor during the charge and discharge cycles. Apparatus Alba interface, Capacitor investigation board, 3 x connecting wire, bag of resistors and capacitors. Instructions Load Charge and Discharge application (Start, All Programs, Science, Alba, Load an Application, Capacitors – Charge and Discharge) Follow the on screen instructions and connect the apparatus as shown in the photograph. Hold down the charging button for the first 20 seconds and the discharging button for the last 20 seconds. Print out both graphs marking clearly on each where charging ends and discharging begins. Repeat this process for different values of capacitor and resistor. For the repeated experiments sketch the graphs explaining the difference in each. ACTIVITY 12 Title: Response of resistance in a variable frequency a.c. circuit Aim: To establish a relationship between the current through a resistor and the frequency of the a.c. supply. Apparatus: resistor (20 - 50 Ω), signal generator, a.c. ammeter, a.c. voltmeter V A Instructions Set up the circuit as shown with the supply switched off. Set the frequency of the supply to 300 Hz. Switch on and note the ammeter reading. Repeat in steps of 50 Hz up to 800 Hz, ensuring the p.d. of the supply is kept constant. Comment on the current through the resistor as the frequency is increased State if the resistance of a resistor is affected by the frequency of the a.c. supply. State why the p.d. of the supply must remain constant. ACTIVITY 13 Title: Current and frequency in a capacitive circuit (Outcome 3) Apparatus: Signal generator, a.c. voltmeter or oscilloscope, a.c. ammeter, 4.7 µF capacitor. ~ Instructions Connect the circuit as shown in the circuit diagram. An oscilloscope may be used in place of the voltmeter. Set the output of the signal generator to about 3 V. Vary the source frequency and record readings of current and frequency using a range of 100 Hz to 1 kHz. Ensure that the supply voltage remains constant. Use an appropriate format to show the relationship between current and frequency. ACTIVITY 14 Title: Uses of Capacitors - the photographic flash Aim: To show the principle behind the operation of a photographic flash. In photography, where light has to be supplied by the flash unit, the light has to be supplied in the short period of time that the shutter is open. In this time a large amount of light energy must be emitted. This is stored as electrical energy in a capacitor until it is needed. Apparatus: 1.5 µF capacitor, neon lamp, 100 kΩ resistor, 120 d.c. supply, 1 SPST switch 1 push switch 120 V dc F 100 k S neon lamp Instructions Set up the circuit as shown. To switch on the “flash unit”, close switch F. To simulate the shutter opening for a very short time, close switch S and release quickly. Note what happens. Explain what happens to the p.d. across the capacitor when switch F is closed? If switch F remains closed state what will happen to the capacitor after switch S has been released and the lamp has flashed. The neon lamp requires a p.d. of 100 V across it to make it light. Explain why the lamp is able to light in this circuit. ACTIVITY 15 Title: Uses of Capacitors - d.c. power supply Aim: To show the effect of capacitors in the production of a smooth d.c. supply from an a.c. supply. In the following circuits, the 120 Ω resistor represents the load resistor or device being driven by the supply e.g. a radio. The oscilloscope indicates the form of the output p.d. across the load resistor. Instructions diode 120 12 V a.c. 120 12 V a.c. CIRCUIT 1 CIRCUIT 2 + C 12 V a.c. 120 CIRCUIT 3 C = (a) 5 µF (b) 10 µF (c) 20 µF 120 12 V a.c. + 2200 µF CIRCUIT 4 Set up circuit 1 as shown. Draw the circuit and sketch the waveform displayed on the oscilloscope screen. Set up each of the other circuits, in turn. Draw the circuits and sketch the waveforms produced. Explain whether the waveform produced in circuit 2 is a.c. or d.c. Describe the effect of the capacitor on the waveforms produced in circuit 3. State what effect the size of the capacitance has on the smoothing of the supply. INFORMATION SHEET FOR ACTIVITIES 16, 17, 18, 19, 20, 21 AND 22 The Amplifier Circuit Board A diagram of the Nuffield Operational Amplifier circuit board is shown below. It is important to become familiar with the input potentiometers in order to work successfully with the op-amp circuit board. ACTIVITY 16 Title: Familiarisation - Using The Input Potentiometers Apparatus: Op-Amp Board, Dual Rail -15 - 0 - 15 V Power Supply, Multimeter and leads. Instructions Part A: Positive (+ve) input potential. Connect up the top input potentiometer and voltmeter using the connections shown. Adjust the top input potentiometer to confirm that you can obtain a range of voltages on the voltmeter from 0 to +15 V. Repeat the above but this time connecting up the bottom input control to obtain a range of voltages from 0 to +15 V also. continued.... ACTIVITY 16 (continued) Instructions (continued) Part B: Negative (-ve) input potential. Connect up the top input potentiometer and voltmeter using the connections shown below. Negative supply rail -V s V Zero volt rail 0V Adjust the top input potentiometer to confirm that you can obtain a range of voltages on the voltmeter from 0 to -15 V. Repeat the above but this time connecting up the bottom input control to obtain a range of voltages from 0 to -15 V also. ACTIVITY 17 Title: The Inverting Amplifier Apparatus: Op-Amp Board, Dual Rail -15 - 0 - 15 V Power Supply, Multimeter and leads. The circuit diagram for the circuit and results table are shown below. Rf () R1 () V1 V0 Rf V0 (V) (V) R1 V1 100K 10K 0.5 100K 10K 1.2 100K 10K -0.3 100K 10K -1.0 10K 10K 8.0 10K 10K 4.5 10K 10K -6.0 10K 10K -0.8 Instructions Connect up the top input potentiometer and two voltmeters using the connections shown. The circuit board connections shown above are for positive (+ve) input potentials and a gain of 10, since R1 = 10 kΩ and Rf is set to 100 kΩ. Connect R1 at 10 kΩ and Rf at 100 kΩ. Set V1 to 1.2 V. Measure the output voltage and record in your own table. Repeat this measurement for the other values of V1 and Rf shown in the sample table Complete the last two columns of your table. Write a conclusion to your experiment including a comment on the polarity of V0 compared to V1. ACTIVITY 18 Title: Saturation Apparatus: Op-Amp Board, Dual Rail -15 - 0 - 15 V Power Supply, Multimeter and leads. Rf 100K R1 10K V1 V0 V + V 0V Instructions Connect up the following circuit using the top input potentiometer and two voltmeters from the circuit diagram below. This circuit is identical to the circuit used in Activity 17. Set the value of the input voltage, V1, to the values shown in the tables and record the corresponding value of the output voltage, V0 in your own table. Graph the results of both tables from your experiment on axes similar to those below. State the gain setting of the inverting amplifier used? Describe what happens to the value of the output voltage, V0, as the input voltage, V1, is increased. State the maximum output voltages available from the amplifier. ACTIVITY 19 Title: Square Wave Generator Apparatus: Op-Amp Board, Dual Rail -15 - 0 - 15 V Power Supply, Multimeter and leads. Instructions Connect the op-amp board as an inverting amplifier, using the resistor values shown. Connect the signal generator to the input of the inverting amplifier. Connect the oscilloscope across the inputs of the op-amp, AB. Set the signal generator to approximately 3 V at a frequency of 200 Hz. Adjust the Y-gain and time-base controls of the oscilloscope until you obtain a steady wave pattern. Accurately sketch the wave pattern produced. Now connect the oscilloscope across the outputs of the op-amp, CD. Without adjusting any of the controls on the oscilloscope or signal generator, sketch the wave pattern produced at the output. Accurately sketch the wave pattern produced this time. State how the phase of the output potential, V0, compares to that of the input potential, V1. Compare the frequency of the output potential, V0, to V1. State the gain of the amplifier in this circuit. Hence state the minimum value of V1 that will produce a saturated output potential, V0. As the input potential from the signal generator, V1, is increased, explain what happens to the output potential, V0. ACTIVITY 20 Title: The Differential Amplifier Apparatus:Op-Amp Board, Dual Rail -15 - 0 - 15 V Power Supply, Multimeter and leads. 100 kΩ and 10 kΩ resistor panels. The circuit diagram for the circuit is shown below and the results table is given on the following page. Rf In this circuit, R f and R3 will always be taken as 100 k due to the limitations of the board. R1 - V1 R2 V2 V0 + R3 V 0V Instructions Connect up the two input potentiometers using the connections shown. The circuit board is shown for positive (+ve) input potentials with a gain of 1, but negative (-ve) input potentials are also used as are different settings for the gain. (Continued...) ACTIVITY 20 (Continued) Rf & R3 R1 & R2 Rf R1 V2 (V) V1 (V) () () 100K 100K 6.0 2.0 100K 100K 1.0 4.0 100K 100K 5.0 -2.5 100K 100K -1.5 -3.0 100K 10K 3.5 3.0 100K 10K 1.5 2.5 100K 10K -4.5 -5.0 100K 10K -1.2 -0.8 V2 -V1 (V) V0 (V) Rf R1 (V2 -V1 ) (V) Instructions (continued) Set Rf and R3 at 100 kΩ. For each of the values of R1 and R2, V2 and V1 given on the sample table record the output voltage V0 in your own table. Complete the other columns in your table. Write a conclusion, including a comment on the polarity of V0 compared to (V2-V1). Comment on the maximum gain you could obtain if V1 = 1 V and V2 = 1.5 V. ACTIVITY 21 Title: The Inverting Amplifier used to Control Heavier Loads Aim: To investigate the use of a transistor and relay in a control circuit The circuit below uses a transistor, relay and op-amp to control high current devices. Some devices, such as motors or heaters, require a current which is too large for small transistors to supply. The transistor can, however, be used to energise the coil of a relay which can then switch on a separate supply to the high current device. Apparatus:op-amp board, 2 × 5 V d.c supplies, n-p-n transistor board, relay, 6 V motor, ammeter. +5 V +Vs 100 k Rv 0-10k 1 M V1 M Reed Relay 5V supply + 0V 0V Instructions Connect up the circuit. Gradually increase the input voltage, V1. At some point, the relay contacts should close and the motor will work. Redraw the circuit, marking in the positions of ammeters required to measure (a) the output current from the op-amp (b) the current in the relay coils (c) the current in the motor. Ensure your circuit diagram is correct, then connect up an ammeter in the correct positions and note the three current values above. State the value of the gain of this circuit. If the n-p-n transistor requires 0.7 V to switch on, state the value of V1 required to operate the reed relay and also the motor. Explain the operation of this circuit. ACTIVITY 22 Title: The Differential Amplifier used to Monitor Light Level Aim: To investigate the use of a differential amplifier with a Wheatstone Bridge to monitor light level. The circuit below uses a LDR as a resistive sensor in a Wheatstone Bridge circuit. With the op-amp, it can be used to monitor light level changes. The two potential dividers, R1, R2 and R3, R4 are connected as a Wheatstone Bridge circuit. Any out of balance potential difference from the Wheatstone Bridge is applied across the inputs of the op-amp. The resistance of R4 can be adjusted to balance the bridge at any desired light level. The output of the op-amp is therefore proportional to the change in resistance of the LDR caused by the change in light level. Apparatus: op-amp board, LDR panel, 2 × 10 kΩ resistor panels. +Vs 100 k R1 10 k R3 10 k 10 k - 10 k R2 R4 0-10 k + 100 k 0V V 0V Instructions Ideally, this experiment should be carried out in a darkened room. Connect up the circuit shown. Place a 60 W lamp facing the LDR at a distance of 50 cm, and adjust R4 until the voltmeter reads zero. Move the lamp closer, 5 cm at a time, and record the output voltage readings. Plot a graph of output voltage against distance. Activities