Practical skills Flashcards
Describe an experiment used to show how mass, length and tension change the resonant frequencies of a string. (6)
- 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
What are the factors that you can keep and one you can change?
choose 1 to change and keep the rest the same:
mass (per unit length), the length or the tension; of the string
Which factors during the stationary wave experiment may affect the resonant frequencies? (3)
- 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)
How can the length of the vibrating string in the stationary waves experiment be varied?
- Keep the type of string and tension the same
- Move the vibration transducer towards or away from the pulley
How can the tension in the string in the stationary waves experiment be varied?
- Keep the string type and length the same
- Add or remove masses to vary tension
How can the string type in the stationary waves experiment be varied?
- Keep the vibrating string length and tension the same
- Use different string samples to vary μ (different masses of string with the same length)
How does string length affect the resonant frequency in the stationary wave experiment?
- The longer the string, the lower the resonant frequency
- Because the half wavelength at the resonant frequency is longer
How does the type of string affect the the resonant frequency in the stationary wave experiment?
- 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.
How does tension affect the the resonant frequency in the stationary wave experiment?
- 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.
What is the advantage of using a long piece of string when measuring the mass per unit length?
lower percentage uncertainty in the measurement
When vibrating in its fundamental mode, what is the wavelength relative to the oscillating string?
λ = 2L
How can wave speed be calculated from the string’s tension and mess per unit length
What graph is plotted in Stationary Waves on a String
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
In the Stationary Waves on a String experiment, what must you add to the clamp stand to carry out this experiment safely?
A counterweight to produce a counteracting moment that prevents the stand from toppling over
How are stationary waves formed on a string?
- 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
standing/stationary
wave
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.
How can you perform the YDS experiment?
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.
How does the interference pattern form from the YDS experiment ?
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.
What are the safety precautions to be followed while using lasers? (3)
● Wear laser safety goggles
● Don’t shine the laser at reflective surfaces
● Never shine the laser at a person
what does Young’s double slit experiment provide evidence for?
wave nature of light because
diffraction and interference are wave properties, and so proved that EM radiation must act as a
wave
Node
A point of zero amplitude along a stationary wave caused by destructive interference.
Antinode
A point of maximum amplitude along a stationary wave caused by constructive interference.
When does the greatest diffraction occur?
when the gap is the same size as the wavelength.
What is the interference pattern formed by monochromatic light diffracted through a single slit?
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.
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
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
What happens to the central maximum if you increase the light wavelength
central maximum becomes wider and its
intensity decreases.
What happens to the central maximum if you Increasing the slit width
central maximum
becomes narrower and its intensity increases.
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
What can increase the pitch of a note on a guitar string?
- ↑ tightness/tension
- ↓ length of string
- ↓ thickness of string
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
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
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 ↑)
What happens to diffraction when the gap width ↓?
Diffraction ↑
What is diffraction grating
A set of slits containing many equally spaced slits very close together for light waves to pass through
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
What do you use to measure the slit separation
Vernier Calliper
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
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.
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.
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.
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.
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
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.
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
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
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).
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
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
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.
Describe the relationship between T and m in a mass-spring system and how this can be shown graphically.
- T ∝ √m
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.
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.
Describe the relationship between T and A in a pendulum and how this can be shown graphically.
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
Describe the relationship between T and k in a mass-spring system and how this can be shown graphically.
- T ∝ √(1/k)
Describe the relationship between T and A in a mass-spring system and how this can be shown graphically.
- There is no relationship.
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.
Describe the I-t graph for a capacitor charging.
- Starts at positive y-intercept
- Current decreases at decreasing rate
- Eventually reaches 0
Describe the V-t graph for a capacitor charging.
- Starts at origin
- P.d. increases at decreasing rate
- Eventually reaches peak p.d.
Describe the Q-t graph for a capacitor charging.
- Starts at origin
- Charge increases at decreasing rate
- Eventually reaches peak charge
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
Describe the I-t graph for a capacitor discharging.
Describe the Q-t graph for a capacitor discharging.
Describe the V-t graph for a capacitor discharging.
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
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.)
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.
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°.
