Download Instructions - Meldrum Academy

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