Practical skills Flashcards
1
Q
Describe an experiment used to show how mass, length and tension change the resonant frequencies of a string. (6)
A
- Measure the mass and length of the string using a mass balance and ruler. Work out the mass per unit length (μ = M/L) in kg/m
- Set up the equipment as shown.
- This involves connecting a vibration generator (connected to a signal generator) to a piece of string attached to a pulley and some masses. Clamp the entire setup to the bench.
- Measure the length (l) of the string between the vibration generator and the pulley.
- Work out the tension in the string using (T = mg) where m is the mass of the masses on the end of the string.
- Turn on the signal generator and adjust the frequency until the first harmonic is found
2
Q
What are the factors that you can keep and one you can change?
A
choose 1 to change and keep the rest the same:
mass (per unit length), the length or the tension; of the string
3
Q
Which factors during the stationary wave experiment may affect the resonant frequencies? (3)
A
- Length of the vibrating string - longer the string the lower the resonant frequency - because the half wavelength is longer ( c=fλ, if λ ↑ , f ↓ for fixed c )
- Tension in the string - waves travel more slowly if the string is loose and there is less tension (lower c = lower f)
- Type of string (different μ) - heavier string (more mass per unit length) - waves more slowly down the string (lower c = lower f)
4
Q
How can the length of the vibrating string in the stationary waves experiment be varied?
A
- Keep the type of string and tension the same
- Move the vibration transducer towards or away from the pulley
5
Q
How can the tension in the string in the stationary waves experiment be varied?
A
- Keep the string type and length the same
- Add or remove masses to vary tension
6
Q
How can the string type in the stationary waves experiment be varied?
A
- Keep the vibrating string length and tension the same
- Use different string samples to vary μ (different masses of string with the same length)
7
Q
How does string length affect the resonant frequency in the stationary wave experiment?
A
- The longer the string, the lower the resonant frequency
- Because the half wavelength at the resonant frequency is longer
8
Q
How does the type of string affect the the resonant frequency in the stationary wave experiment?
A
- The heavier (greater μ) the string, the lower the resonant frequency.
- Because waves travel more slowly down the string. A lower wave speed, c, makes a lower frequency, f.
9
Q
How does tension affect the the resonant frequency in the stationary wave experiment?
A
- The higher the tension, the higher the resonant frequency.
- Because waves travel more quickly on a taut string. A higher wave speed, c, makes a higher frequency.
10
Q
What is the advantage of using a long piece of string when measuring the mass per unit length?
A
lower percentage uncertainty in the measurement
11
Q
When vibrating in its fundamental mode, what is the wavelength relative to the oscillating string?
A
λ = 2L
12
Q
How can wave speed be calculated from the string’s tension and mess per unit length
A
13
Q
What graph is plotted in Stationary Waves on a String
A
Plot a graph of the mean value of f against 1/l and draw a line of best fit. The wave
speed will be two times the gradient. v = 2G
14
Q
In the Stationary Waves on a String experiment, what must you add to the clamp stand to carry out this experiment safely?
A
A counterweight to produce a counteracting moment that prevents the stand from toppling over
15
Q
How are stationary waves formed on a string?
A
- vibrator moves up and down - sends travelling wave down cord
- wave reflected at end, so 2 travelling waves overlap and interfere
- has antinodes and nodes; distance between nodes = 1/2λ
16
Q
standing/stationary
wave
A
Stationary waves are formed when two identical waves travelling in opposite directions meet and superpose. This usually happens when one wave is the reflection of the other. It has no net flow of energy.
17
Q
How can you perform the YDS experiment?
A
use two coherent sources of light or one coherent source and a double slit to form an interference pattern. If you don’t have a coherent source of light, you can use a single slit before the double slit to make the light have a fixed path difference and a filter to make it monochromatic.
18
Q
How does the interference pattern form from the YDS experiment ?
A
Each slit acts as a coherent point source making a pattern of light and dark fringes. Light fringes are formed where the light meets in phase and interferes constructively. Dark fringes are formed where the light meets completely out of phase and interferes destructively.
19
Q
What are the safety precautions to be followed while using lasers? (3)
A
● Wear laser safety goggles
● Don’t shine the laser at reflective surfaces
● Never shine the laser at a person
20
Q
what does Young’s double slit experiment provide evidence for?
A
wave nature of light because
diffraction and interference are wave properties, and so proved that EM radiation must act as a
wave
20
Q
Node
A
A point of zero amplitude along a stationary wave caused by destructive interference.
21
Q
Antinode
A
A point of maximum amplitude along a stationary wave caused by constructive interference.
22
Q
When does the greatest diffraction occur?
A
when the gap is the same size as the wavelength.
23
Q
What is the interference pattern formed by monochromatic light diffracted through a single slit?
A
a pattern of light and dark fringes, with a bright central fringe that is double the width of all other fringes, with alternating dark and bright fringes on either side.
24
What happens when white light is diffracted through a single slit?
the different wavelengths of light are all diffracted by different amounts so you get a spectrum of colour in the diffraction pattern with a central white maximum
25
Using white light instead of monochromatic laser light
gives wider maxima and a less intense diffraction pattern with a central white fringe with alternating bright fringes
26
What happens to the central maximum if you increase the light wavelength
central maximum becomes wider and its
intensity decreases.
27
What happens to the central maximum if you Increasing the slit width
central maximum
becomes narrower and its intensity increases.
28
Why is the diffraction grating more accurate than the double slits?
* double slits - fringes formed are slightly blurred → large errors
* diffraction grating - images are clear and measurements accurate, also final result is an average of several calculations
29
What can increase the pitch of a note on a guitar string?
* ↑ tightness/tension
* ↓ length of string
* ↓ thickness of string
30
Examples of coherent sources?
* light produced by a laser
* sound from two loudspeakers connected in parallel
* light emerging from two apertures illuminated by the same source
31
When are superposed waves easier to ‘see’? (3)
* the waves are of similar amplitude (↑ contrast between maxima and minima)
* the waves have similar frequencies - otherwise the interference patterns create change so fast that they are difficult to detect
* the waves have a constant phase difference i.e. they are phase linked
32
What happens to the double slit interference pattern if both slits are made narrower?
Wider interference so there are more dots, but fainter as there is less light through(x ↑)
33
What happens to diffraction when the gap width ↓?
Diffraction ↑
34
What is diffraction grating
A set of slits containing many equally spaced slits very close together for light waves to pass through
35
Difference between single and double slit pattern?
single slit - central max. fringe that is twice the width of the other fringes* double slit pattern has equally spaced fringes
36
What do you use to measure the slit separation
Vernier Calliper
37
How can Young's double slit experiment be adapted for microwaves? (3)
* Replace the laser and slits with 2 microwave transmitter cones attached to the same signal generator
* Replace the screen with a receiver probe
* Move the probe along where the screen was and you'll get an alternating pattern of strong and weak signals
38
n Young's double slit experiment, what is the easiest way to get an accurate reading for 'w'?
- Measure several fringes and divide by the number of fringe widths between them.
39
In Young's double slit experiment, what must you be careful of when measuring several fringes?
* When dividing to find 'w', remember to divide by the number of fringe WIDTHS between them, not the number of fringes.
* e.g. 10 bright lines only have 9 fringe widths between them.
40
Compare the double slit interference pattern for red and blue light.<
The blue light creates a smaller fringe separation. This makes the pattern appear more compact.
41
In a double slit interference pattern, why does the intensity of the fringes decrease as you get further away from the central maximum?
Because it's multiplied by the single slit diffraction pattern for either of the slits separately.
42
Compare single and double slit diffraction patterns in terms of fringe widths and intensities.
Single slit:
* Widest central maximum + equal outer fringes
* Brightest central maximum + decreasing intensity of outer fringes
Double slit:
* All fringes of equal width
* Decreasing intensity of outer fringes
43
What is the difference between the interference pattern formed by a diffraction grating and a double slit using monochromatic light ?
diffraction grating - pattern is much sharper and brighter than double slit because there are many more rays of light reinforcing the pattern.
44
Describe an experiment to calculate g.
1)Set up a circuit with a switch that controls two parallel circuits: one with an electromagnet and ball, the other with a timer and trapdoor
2) Measure the height from the bottom of the bearing to the trapdoor.
3) Flick the switch to start the timer and release the bearing.
4) Bearing falls, hits trapdoor and stops the timer. Record the time.
5) Repeat 3 times at this height and average the time.
6) Repeat at various heights.
7) Plot a graph of height (m) against time taken squares (s²).
8) a = 2 x Gradient
45
In the experiment to calculate g, how is error reduced?
* Bearing is small and heavy -> Means air resistance is negligible
* Computer releasing and timing fall -> Reduces uncertainty
46
In the experiment to calculate g, what is the biggest source of error?
RANDOM error: The measurement of h (using a ruler = uncertainty of + or - 1mm).
47
What are some advantages of data-loggers over traditional methods of recording data?
1) Data is more accurate - don’t have to allow for human reaction times.
2) Higher sampling rate than humans (for example, ultrasound position detectors can take a reading ten times every second)
3) Data displayed in real time
48
What is free fall?
The motion of an object undergoing an acceleration of ‘g’ (i.e. under gravity and nothing else).
49
In free fall, what is the only force acting on an object?
Its weight.
50
What is the Young Modulus measured in?
Nm⁻² or Pa
51
Brief explanation of experiment to find the Young Modulus of a wire?
* stress → wire with mass attached - measure mass using top-pan balance and use W=mg. measure diameter of wire using micrometer, then calculate area
* then stress = F/A
* strain → measure extension by measuring distance marker moves from original position, and length of wire. calculate strain
* vary mass for range of values - plot stress-strain graph
52
How to improve accuracy in the experiment to calculate the Young Modulus of a wire?
* use long thin wire and heavier weights → greater Δl so smaller % uncertainty
* measure diameter accurately using micrometer
* measure wire by holding ruler as close to the wire as possible
53
In an experiment to calculate the Young Modulus of a wire, how can kinks in the wire be avoided?
Weights are added at the beginning, before length measured
54
In an experiment to calculate the Young Modulus of a wire, how can we make sure there is no thermal expansion?
By comparing the test wire to a control wire
55
Give the equation for the Young modulus.
E = (F x L) / (A x ΔL)
Where:
F = Force (N)
A = Cross-sectional area (m²)
L = Original length (m)
ΔL = Extension (m)
```
56
Describe an experiment to find the Young modulus of a wire. (8)
1) Measure the diameter of a thin wire using a micrometer in several places and take an average.
2) Find the cross-sectional area of the wire using “A = πr²”.
3) Clamp the wire with a clamp at one end and over pulley at the other end, so that weights can be hung on the wire.
4) Align a ruler with the wire and attach a marker.
5) Start with the smallest weight to straighten the wire (but ignore this weight in calculations).
6) Measure the unstretched length of the wire from clamped end of the string to the marker.
7) Add 100g weights to the string and measure the extension.
8) Plot a stress (y) against strain (x) graph of your results. The gradient of the straight part is the Young modulus.
57
Name some ways in which the experiment to find the Young modulus of a wire is made more accurate. (4)
* Using a long, thin wire -> Reduces uncertainty
* Taking several diameter readings and finding an average
* Using a thin marker on the wire
* Looking directly at the marker and ruler when measuring extension
58
Describe how you can calculate the resistivity of a piece of wire. (7)
1) Measure the diameter at at least 3 points along the wire using a micrometre -> Find average -> Divide by two to get radius
2) Area = πr^2
Calculate R/l:
1) Set up a circuit with an ammeter, wire and voltmeter.
2) Attach a test wire along a ruler -> Attach one end where the ruler reads 0cm
3) Move the crocodile clip at the other end to adjust the length of the wire
4) Record the length of the wire and the resistance (R = V/I)
5) Repeat this to find an average resistance for that length
6) Vary the length from 0.10 to 1.00m
7) Plot a graph of resistance (y) against length (x) + draw a line of best fit (should be straight line through origin)
Find the resistivity.
59
When calculating the resistivity of a piece of wire, what is it important to keep constant and how?
* Temperature -> Resistivity depends on it
60
How do you find the resistivity in Determination Of Resistivity Of A Wire experiment
1) The gradient is R/l, so it can be subbed in to the equation p = RA/l by multiplying by the area.
2) Take note to maintain the temperature of the wire constant at all times (since resistivity depends on temperature).
61
What is the easiest way to lower the resistivity of most materials?
Cool them down.
62
63
64
65
What are the independent, control dependent variables in Determination Of Resistivity Of A Wire experiment
Variables:
- Independent variable = Length, L, of the wire (m)
- Dependent variable = The current, I, through the wire (A)
Control variables:
- Voltage through the wire
- The material the wire is made from
66
What are the independent, control dependent variables in Stationary Waves experiment
Variables:
- Independent variable = either length, tension, or mass per unit length
- Dependent variable = frequency of the first harmonic
Control variables:
- If length is varied = same masses attached (tension), same string (mass per unit length)
- If tension is varied = same length of the string, same string (mass per unit length)
- If mass per unit length is varied = same masses attached (tension), same length of the string
67
What are the independent, control dependent variables in G by free fall
Variables:
- Independent variable = height, h
- Dependent variable = time, t
Control variables:
- Same steel ball–bearing
- Same electromagnet
- Distance between ball-bearing and top of the glass tube
68
What are the independent, control dependent variables in YDS Experiment
Variables:
- Independent variable = Distance between slits and screen
- Dependent variable = Fringe width, w
Control variables:
- slit separation
- Laser wavelength
69
What are the independent, control dependent variables in Young Modulus experiment
Variables:
- Independent variable = Force
- Dependent variable = Extension
Control variables:
- The original length of wire
- The thickness of the wire
- The metal used for wire
70
what is emf
71
In the experiment to find the e.m.f. and internal resistance of a cell, how should the V and I values be plotted and why?
start with V= ϵ - Ir equation
rearrange to V= -rI + ϵ
Y=mX+c
the gradient is -r and emf is the y intercept
72
Describe a series circuit in terms of:
* Current
* e.m.f.
* Resistance
```
* V = ε - Ir
Rearranges to:
* V = -rI + ε
* y = mx + c
Therefore:
* Plot V on the y and I on the x
* Gradient = -r
* y-intercept = ε
```
73
Describe an experiment to calculate the e.m.f. and internal resistance of a cell.
1) Connect the cell in series with an ammeter and variable resistor + connect a voltmeter across the cell
2) Vary the current using the variable resistor - start at highest resistance (open the switch and close it again to get two more sets of I and V values and find mean)
3) Record the voltage at each current.
4) Plot a graph of voltage (y) against current (x).
5) y-intercept = ε
Gradient = -r
Make sure external factors are kept the constant like temperature.
74
What is an easy way to measure a cell’s e.m.f.?
* Connect a high-resistance voltmeter across its terminals
75
When connecting a high-resistance voltmeter across a cell to find its e.m.f., what is the error in the results?
* Small amount of current flows through the voltmeter
* So there are some lost volts
* Measured value is very slightly less than the e.m.f.
* But this is negligible
76
Why should you use new cell/battery be used when carrying out emf and IR of a cell experiment?
Run-down cells and batteries have internal resistance that may fluctuate throughout experiement
Using new source will result in a more consistent value = increased accuracy
77
What are the independent, control dependent variables in Internal Resistance and emf experiment?
Variables:
- Independent variable = Voltage ,current
- Dependent variable = Resistance
Control variables:
- E.m.f of the cell
- Internal resistance of the cell
78
Describe the experiment to check the formula for the period of a mass-spring system. (7)
1) Using string, tie a trolley to a spring
2) Put masses in the trolley
3) Place a position sensor in front of the trolley and spring
4) Pull the trolley to one side by a certain amount and let go.
5) The trolley will oscillate back + forwards as the spring pulls and pushes it in each direction
6) you can measure the period, T, by getting a computer to connect to the position sensor and create a displacement-time graph from a data logger.
7) Read off the period, T, from the graph.
79
Describe the relationship between T and m in a mass-spring system and how this can be shown graphically.
* T ∝ √m
80
Describe how factors affecting SHM in a simple pendulum can be investigated. (5)
1) Attach a pendulum to an angle sensor connected to a computer.
2) Displace the pendulum by a small angle (less than 10°) and let it go.
3) The angle sensor measures how the bob’s displacement from the rest position varies with time.
4) Use the computer to plot a displacement-time graph and read off the period, T, from it. Take the average of several oscillations to reduce percentage uncertainty.
5) Change the mass of the pendulum bob (m), amplitude of displacement (A) and length on the rod (l) independently to see how they affect the period. Plot each graph.
(NOTE: Alternatively, you could also measure the period using a stopwatch. Measure several oscillations and divide by the number to get an average.)
81
When measuring the period of a simple pendulum, what reference point should be used to measure the start of each oscillation?
The midpoint of the swing, because:
* This is where the pendulum moves fastest, so it is more clear-cut when it reaches the midpoint
* The clamp from which it is hung may be used as a marker
82
Describe the relationship between T and l in a pendulum and how this can be shown graphically.
* T ∝ √l
| * So a graph of T² against l can be plotted, which shows a straight line of positive gradient.
83
Describe the relationship between T and m in a pendulum and how this can be shown graphically.
* There is no relationship.
| * So a graph of T against A can be plotted, which shows a straight horizontal line.
84
How does a mass on a spring oscillate when stretched and released?
* At its resonant (natural) frequency.
* It is a free vibration, so if no energy is transferred to or from the surroundings, it will keep oscillating with the same amplitude.
85
How to verify Hooke’s law from the mass-spring experiment?
* T² = 4π² x m/k
* graph T²-m
* gradient = T²/m = 4π²/k
* draw graph F=kx and see if gradient = 4π²/m
86
Describe the relationship between T and A in a pendulum and how this can be shown graphically.
87
How could you change the experiment to find the relationship between spring constant and Time period?
Change the spring constant (k) by using different combinations of springs
88
Describe the relationship between T and k in a mass-spring system and how this can be shown graphically.
* T ∝ √(1/k)
89
How can you investigate the relationship between Time period and amplitude of a mass -spring system?
Change the amplitude (A) by pulling the trolley across by different amounts.
90
Describe the relationship between T and A in a mass-spring system and how this can be shown graphically.
* There is no relationship.
91
Describe what graphs can be plotted in an investigation into factors affecting a mass-spring system. What does each illustrate?
* T² against m -> Illustrates T ∝ √m
* T² against 1/k -> Illustrates T ∝ √(1/k)
* T against A -> Shows there is no relationship between T and A.
92
What factors affect the period of a mass-spring system?
* Mass
93
What are the constraints of the formula for the period of a pendulum?
It only works for small angles of oscillation (up to about 10°).
94
What are the independent, control dependent variables in SHM in a Mass-Spring System?
Variables:
- Independent variable = mass, m
- Dependent variable = time period, T
Control variables:
- Spring constant, k
- Number of oscillations
95
What are the independent, control dependent variables in SHM in a simple pendulum system
Variables:
- Independent variable = length, L
- Dependent variable = time period, T
Control variables:
- Mass of pendulum bob, m
- Number of oscillations
96
What could be added to your apparatus to help measure time period more accurately?
A fiducial marker could be added at the center of oscillation to show exactly where an oscillation has been completed
97
What two conditions must be met when carrying out a simple harmonic motion experiment involving a pendulum?
- The amplitude of oscillation should be small
- The pendulum should oscillate in a straight line
98
Describe the circuit used to investigate capacitor charging and discharging. (3)
* Capacitor is in series with resistor, ammeter and power supply
* Voltmeter around capacitor
* Ammeter and voltmeter connected to data logger
99
Describe what occurs when a capacitor starts charging (in terms of charge, current and and pd). (6)
1) When switch is closed, current starts to flow.
2) Electrons flow onto the plate connected to the negative terminal, which causes negative charge to build up.
3) This charge repels electrons off the other plate, causing it to become positive. The electrons are attracted to the positive terminal.
4) Equal but opposite charge on each plate causes a potential difference to be created.
5) As charge builds up on the plates, it becomes harder to more electrons to be deposited due to electrostatic repulsion.
6) When the pd is equal to the power supply pd, the current falls to 0.
100
What happens to current as a capacitor charges?
It slows down until it reaches zero due to the increased electrostatic repulsion making it harder for electrons to be deposited.
101
Describe the I-t graph for a capacitor charging.
- Starts at positive y-intercept
- Current decreases at decreasing rate
- Eventually reaches 0
102
Describe the V-t graph for a capacitor charging.
* Starts at origin
* P.d. increases at decreasing rate
* Eventually reaches peak p.d.
103
Describe the Q-t graph for a capacitor charging.
* Starts at origin
* Charge increases at decreasing rate
* Eventually reaches peak charge
104
Describe how discharging a capacitor can be investigated. (4)
* First, charge the capacitor fully
* Close the switch to complete the circuit
* Let the capacitor charge while the data logged records the potential difference and current
* When the ammeter reading is 0, the capacitor is fully discharged
105
Describe the I-t graph for a capacitor discharging.
106
Describe the Q-t graph for a capacitor discharging.
107
Describe the V-t graph for a capacitor discharging.
108
Why does capacitance affect the time to charge or discharge a capacitor?
Capacitance affects the amount of charge that can be transferred at a one potential difference.
(Since C = Q/V)
109
How can you find the time constant, τ?
Either do τ = RC.
or
Read it from a Q-t or ln(Q)-t graph:
Discharging: 0.37Q₀
Charging: 0.63Q₀
110
Derive which graph should be plotted to find the time constant more accurately when discharging.
* Q = Q₀e^(-t/RC)
* ln(Q) = (-1/RC)t + ln(Q₀)
* This is now in the form y = mx + c.
* Plot ln(Q) against t. The gradient is -1/RC.
(NOTE: This also works for potential difference and current instead of charge.)
111
What are the independent, control dependent variables in Charging And Discharging Capacitors
Variables:
- Independent variable = time, t
- Dependent variable= potential difference, V
Control variables:
- Resistance of the resistor
- Current in the circuit
112
What must always be checked when using an electrolytic capacitor in a circuit?
- Capacitor is polarised and must be connected with correct polarity in the circuit.
- If connected incorrectly, it can overheat and become a safety hazard
113
Describe a experiment to investigate the effect of current on the force exerted on a current-carrying wire in a magnetic field. (7)
1) Set up a top pan balance with a square loop of wire fixed to it, so that it is standing up and that the top of the loop passes through a magnetic field, perpendicular to it.
2) Connect the wire in a circuit with a variable resistor, ammeter and dc power supply. Zero the top pan balance when no current is flowing.
If the reading is negative swap the crocodile clips over
3) Vary the current using the variable resistor. At each current value, record the current and mass. Repeat 3 times at each current value and average.
4) Convert into force using F = mg.
5) Plot a graph of force F against current I. Draw a line of best fit.
6) Since F = BIl, the gradient of the rain is equal to B x l.
7) Alternatively, you could vary “l” by varying the length of wire that is perpendicular to the field or vary “B” by changing the strength of the magnets.
114
In the experiment to investigate F = BIl, how can you vary each of the variables?
What would need to keep constant when investigating length?
* B -> Use different magnets to vary field strength
* I -> Use variable resistor to vary the current
* l -> Use different loop sizes with different lengths perpendicular to the magnetic field (keep I constant with variable resistor, and also B)
115
Why does a current-carrying wire experience a force in a magnetic field?
A force act on the charged particles (electron) moving through it.
116
Describe an experiment to investigate how the angle of a coil in a magnetic field affects the induced emf.
1) Set up solenoid connected to an alternating power supply. This will act to provide an alternating magnetic field.
2) Place a search coil on a podium in the centre. It should have a known area and number of loops. Place the podium on a protractor and connect the search coil to an oscilloscope.
3) Turn the time base off on the oscilloscope so that only the amplitude is shown as a vertical line.
4) Orientate the search coil so that it is perpendicular to the podium and record the emf.
5) Keep rotating the search coil by 10° and record the emf each time. Do this until the coil has been rotated by 90°.
6) The emf should decrease gradually until the search coil is parallel to the field lines.
7) Plot a graph of induced emf against θ, where the emf is at a maximum at 0° and zero at 90°.
117
What type of power source should be used when investigating the effect of current on the force exerted on a current-carrying wire in a magnetic field?
DC so the force is constant and in one direction only
118
What are the independent, control dependent variables when investigating the effect of current on the force exerted on a current-carrying wire in a magnetic field?
Variables:
- Independent variable = Current, I
- Dependent variable = mass, m
Control variables:
- Length of wire, L
- Magnetic Flux density, B
- Potential difference of the power supply
119
What are the independent, control dependent variables in Investigating Flux Linkage on a Search Coil
Variables:
- Independent variable = Angle between the normal to the search coil and the magnetic field lines, θ
- Dependent variable= Induced e.m.f, ε
Control variables:
- Area of the search coil, A
- Number of loops on both coils, N
- Magnetic field strength, B
- Frequency of the power supply, f
