Practical Skills Test Flashcards
(19 cards)
- What are the two main types of uncertainty in experimental data?
- How many significant figures a piece of processed data should be given to?
- How do you combine uncertainties (including powers)?
- Random errors and systematic errors.
- Processed data should be given to the same number of significant figures as the raw data, so that the number of significant figures are consistent within a column of data.
- When data is added together, add the absolute uncertainties together. When the data is multiplied or divided, add the percentage or fractional uncertainties together. When data is raised to a power, multiply the percentage uncertainty by the power.
What is parallax error and how do you avoid it?
A parallax error is a measurement error that occurs when the eye is not positioned correctly in relation to a measuring scale. Parallax error is minimised by reading the value on a scale only when the line of sight is perpendicular to the scale readings (i.e. at eye level).
- What is the SI unit of surface area and cross-sectional area?
- How is cross-sectional area related to diameter?
- The SI unit for surface and cross-sectional area is the square meter (m²).
- The formula for cross-sectional area is: (diameter² x Pi) / 4
List the 6 base quantities and units (you can ignore luminous intensity).
Length, measured in metres, has the symbol m.
Mass, measured in kilograms, has the symbol kg.
Time, measured in seconds, has the symbol s.
Electric current, measured in amperes, has the symbol A.
Temperature, measured in kelvins, has the symbol K.
Quantity of matter, measured in moles, has the symbol mol.
What are the base units for the following derived units:
1. Newton
2. Joule
3. Watt
- Weight, measured in newtons, has the derived unit N, and the base unit kgms⁻² (force = mass x acceleration)
- Energy, measures in joules, has the derived unit J and the base unit kgm²s⁻² (energy = 0.5 x mass x velocity^2)
- Power, measured in watts, has the derived unit W and the base unit kg m²s⁻³ (power = work done/ time)
What are the base units for the following derived units:
1. Charge
2. Volt
3. Ohm
- Charge, measured in coulombs, has the derived unit C and the base unit As
- Potential difference, measured in volts, has the derived unit V and the base unit kgm²s⁻³A⁻¹ (potential difference = energy / charge)
- Resistance, measured in ohms, has the derived unit and the base unit measured in volts, has the derived unit V and the base unit kgm²s⁻³A⁻²
How does a micrometer work?
Open the measuring faces with the barrel. Place the object in between the anvil and the spindle. Then turn the ratchet stop to close the anvil and spindle until you hear a small click. This means that the measuring faces are fully closed on the object. Then, to read the value: read the main scale first - each division represents 0.5mm. There are 5 divisions so the main scale reading to the nearest 0.5mm is 2.5mm. Next, read the thimble scale, each division represents 0.01mm. Read the number that aligns with the main scale and multiply by 0.01. For example, 5 x 0.01 = 0.05. Read the number that aligns with the main scale and multiply by 0.01mm. Finally, add the main scale and the thimble reading together to get the final measurement.
- How do you find the uncertainty in a single reading or measurement?
- How do you find the uncertainty in repeated results?
- How do you find the percentage uncertainty in a result?
- The uncertainty in a measurement: at least ±1 smallest division
- The uncertainty in repeated data:
When + or - numbers (eg. adding the length of 2 objects), add the actual uncertainties. When x or / numbers (eg. when finding resistance), add % uncertainties. - Percentage uncertainty: (actual uncertainty / measurement ) x 100. The actual uncertainty is the largest of either the precision of the instrument, or half the range of repeated readings
List the prefixes used in physics and the powers of 10 that they represent
1. Larger than the original unit
2. Smaller than the original unit
- Larger than the original unit:
Peta (P) - 10^15
tera (T) - 10^12
giga (G) - 10^9
mega (M) - 10^6
kilo (k) - 10^3 - Smaller than the original unit:
centi (c) - 10^-2
milli (m) - 10^-3
micro (μ) - 10^-6
nano (n) - 10^-9
pico (p) - 10^-12
femto (f) - 10^-15
- How do you label graphs?
- How do you describe how to get specific values from a graph?
- Mass of a car: 1000kg
- Seconds in a day: 90000 s
- Seconds in a year: 3 x 10^7s
- Speed of sound in air: 300 ms^-1
- Power of lightbulb: 60W
- Atmospheric pressure: x 10^5 Pa
State two common unit conversions
J & eV
A common unit conversion in physics is between Joules (J) and electronvolts (eV)
The electronvolt is derived from the equation work done (or energy transferred) W = qV
1 eV = 1.6 × 10^–19 C × 1 V = 1.6 × 10^–19 J
To convert from J → eV, divide by 1.6 × 10^–19
To convert from eV → J, multiply by 1.6 × 10^–19
J & kW h
Another common unit conversion in physics is between Joules (J) and kilowatt-hours (kW h)
To convert between J and kW h, expand the derived units and re-collect terms as follows:
1 kW h = 3600 kW s (since 1 hour = 3600 s)
3600 kW s = 3 600 000 W s (since 1 kW = 1000 W)
3 600 000 W s = 3 600 000 J = 3.6 MJ (since power = energy / time or 1 W = 1 J s^–1)
To convert from J → kW h, divide by 3.6 × 10^6
To convert from kW h → J, multiply by 3.6 × 10^6
What is order of magnitude?
When a number is expressed in an order of 10, this is an order of magnitude. Example: If a number is described as 4 × 10^8 then that number is actually 4 × 100 000 000. The order of magnitude of 4 × 10^8 is 10^8.
The rules for rounding are: the order of magnitude of 6 x 10^8 is 10^9 as the magnitude is rounded up.
What are the estimates for the following physical quantities?
1. Diameter of an atom
2. Wavelength of UV light
3. Height of an adult human
4. Distance between the earth and the sun
5. Mass of a hydrogen atom
6. Mass of an adult human
- Diameter of an atom: 10^-10m
- Wavelength of UV light: 10nm
- Height of an adult human: 2m
- Distance between the earth and the sun: 1.5 x 10^11 m
- Mass of a hydrogen atom: 10^-27 kg
- Mass of an adult human: 70kg
What are the estimates for the following physical quantities?
1. Mass of a car
2. Seconds in a day
3. Seconds in a year
4. Speed of sound in air
5. Power of lightbulb
6. Atmospheric pressure
- Mass of a car: 1000kg
- Seconds in a day: 90000 s
- Seconds in a year: 3 x 10^7s
- Speed of sound in air: 300 ms^-1
- Power of lightbulb: 60W
- Atmospheric pressure: x 10^5. Pa
- How can you reduce random error?
- How can you reduce systematic errors?
- What is a zero error and how can this be corrected?
- To reduce random error: repeat measurements several times and calculate an average.
- To reduce systematic errors: instruments should be recalibrated (or changed), or corrections and adjustments should be made to the technique.
- A zero error is a type of systematic error which occurs when an instrument gives a reading when the true reading is zero.
This introduces a fixed error into readings which must be accounted for when the results are recorded.
- Define precision
- Define accuracy
- Define repeatability
- Precision is how close the measured values are to each other. Measurements are precise if there is little spread about the mean value.
- Accuracy is how close a measurement is to the true value.
- Repeatability is if there original experimenter can repeat the experiment using the same method and equipment, and obtain the same result.
- Define reproducibility
- Define resolution
- A measurement is reproducible if the experiment can be repeated by another person, or different equipment and techniques, while still obtaining the same results.
- Resolution is the smallest change in quantity being measured that can be seen in the reading.
How do you determine uncertainties from graphs containing error bars?
To calculate the uncertainty in a gradient, two lines of best fit should be drawn on the graph (which contains error bars that show the absolute uncertainty of values):
The ‘best’ line of best fit, which passes as close to the points as possible
The ‘worst’ line of best fit, either the steepest possible or the shallowest possible line which fits within all the error bars.
The percentage uncertainty in the gradient can be found using:
The percentage uncertainty in the y-intercept can be found using:
How do you draw error bars?
Find the absolute uncertainty of each value from the data. Where each value is plotted, draw a straight vertical line going above and below, up to the maximum error uncertainty, which is rounded off with a short horizontal line.Then draw the line of best and worst fit, and calculate the percentage uncertainty using the gradients from the two graphs.
