PAG Flashcards
1
Q
Free fall experiment
A
- Measure the height (h) from the bottom of the ball bearing to the trapdoor.
- Simultaneously start the timer and disconnect the electromagnet by flicking the switch, releasing the ball bearing.
- The ball bearing falls, knocking the trapdoor down, breaking the circuit, and stopping the timer. Record the time (t) shown on the timer.
- Repeat the experiment three times and average the time taken to fall.
- Use the equation h = 0.5 * g * t2 (from page 50) to calculate the acceleration due to gravity (g), where t is the time measured by the timer and h is the height fallen.
- Repeat the experiment for several different heights, calculating a value of g for each height, and average these values to find the final value of g.
2
Q
Young’s Modulus (PAG)
A
- Use a thin and long test wire for more accurate measurements, reducing uncertainty.
- Find the wire’s cross-sectional area by measuring its diameter with a micrometer and using the formula for the area of a circle.
- Clamp the wire to the bench and hang weights off one end, starting with the smallest weight necessary to straighten the wire.
- Record the starting position of the marker and measure the unstretched length from the fixed end of the wire to the marker.
- Increase the weight in steps and record the marker reading each time to determine the extension.
- Utilize the experiment results to calculate stress and strain of the wire and plot a stress-strain curve.
3
Q
Resistivity (PAG)
A
- Calculate the cross-sectional area of the wire by measuring its diameter in at least three places and finding the mean diameter.
- Halve the mean diameter to find the mean radius and use it to calculate the cross-sectional area assuming a circular cross-section.
- Set up the experiment by clamping the wire and attaching the flying lead.
- Close the switch and measure the current and potential difference across the test wire to calculate its resistance.
- Repeat the measurement at least once to find the mean resistance and reposition the flying lead for different wire lengths.
- Plot a graph of average resistance against length and use the gradient of the graph to calculate the resistivity of the wire metal by multiplying it by the cross-sectional area.
4
Q
Potential divider (PAG)
A
- A light-dependent resistor (LDR) has high resistance in the dark but lower resistance in the light, while an NTC thermistor has high resistance at low temperatures and lower resistance at high temperatures.
- By using these components in a potential divider, the output voltage can vary with light or heat, making them useful as light or heat sensors.
- Figure 3 illustrates a circuit for a heat sensor using a thermistor.
- To create a heat sensor, place the thermistor in a beaker of ice water, measure the initial temperature, and record the voltage across the thermistor.
- Heat the beaker gradually, recording temperature and voltage at regular intervals over a suitable range.
- Plot a graph of voltage against temperature, observing that as the temperature increases, the voltage decreases due to the thermistor’s decreasing resistance, allowing less share of the total potential difference.
5
Q
Diffraction grating (PAG)
A
- Monochromatic light passing through a diffraction grating creates a pattern of bright lines (maxima) on a dark background due to constructive and destructive interference.
- The central line of maximum brightness is the zero-order line, while the lines on each side are called first-order lines.
- The angle of the first-order line relative to the zero-order line can be calculated using the small angle approximation, given the fringe width (x) and the distance to the screen (D).
- Knowing the slit separation (d), the order of the maximum (n), and the angle between the maximum and the incident light (θ), allows for the calculation of the wavelength of the incident light.
- White light passing through a diffraction grating splits into its component wavelengths, creating a spectrum from red on the outside to violet on the inside for each order in the pattern.
- The zero-order maximum remains white since all wavelengths produce a maximum when θ = 0.
6
Q
Plank constant (PAG)
A
- Use the threshold voltage of LEDs to determine the Planck constant, employing monochromatic LEDs emitting a single wavelength of light.
- 𝑒𝑉 =
ℎ𝑐/𝜆 (Energy of electron = Energy of photon) - Start by setting the variable resistor to its maximum resistance to prevent current flow.
- Adjust the resistor until the LED just begins to light up, recording the threshold voltage (V₀) and the wavelength of the emitted light.
- Repeat the experiment multiple times, averaging the results for V₀, and perform the experiment for a range of LEDs.
- Plot a graph of threshold voltages against the inverse of the wavelength (1/λ) to obtain a straight line of best fit, with the gradient representing V₀λ.
- Substitute the gradient into the equation to calculate the Planck constant (h).
7
Q
Internal resistance (PAG)
A
- Set the variable resistor to its highest resistance.
- Record current (I) and potential difference (V) across the circuit.
- Repeat measurements for multiple load resistances, calculating mean values for each.
- Plot a V-I graph with mean data points and draw a line of best fit.
- Keep all variables constant, including temperature.
- Analyze results by determining the gradient (-r) and intercept (E) from the graph.
(V = E -Ir)
8
Q
Oscilloscope (PAG)
A
- A cathode ray oscilloscope (CRO) displays voltage over time from a signal generator, known as a trace.
- The type of trace depends on the source connected; for example, an AC supply produces a trace that alternates between positive and negative patterns.
- Sound waves converted into electrical signals by a microphone can also be observed on an oscilloscope.
- The screen is divided into squares called divisions, with the vertical axis representing volts and the horizontal axis representing time.
- The volts per division and seconds per division are controlled by the gain and timebase dials, respectively.
- Oscilloscope traces can be used to calculate wave properties such as frequency by using frequency = 1/period over the time base.
9
Q
Polarising filters for light (PAG)
A
- Polarisation is observed by shining unpolarised light through two polarising filters aligned vertically and rotating the second.
- Rotating the second filter reduces the light intensity as its transmission axis deviates from vertical.
- At a 45-degree angle between transmission axes, the intensity passing through the second filter is half that of the first; at right angles, no light passes through.
- Further rotation causes intensity to increase until the axes realign after a 180-degree rotation, allowing all light through.
- Polaroid sunglasses use polarising filters to block partially polarised light, reducing glare.
10
Q
Polarising filters for Microwave (PAG)
A
- Metal grilles work like polarising filters for microwaves.
- Microwaves have longer wavelengths than visible light and are a type of electromagnetic wave.
- Polarisation direction is about the direction of the electric field, not the magnetic field.
- Metal grilles absorb electric fields aligned with their orientation using free electrons.
- Horizontal electric fields pass through vertically positioned grilles, and vice versa, unlike polarising filters.
11
Q
Speed of sound (PAG)
A
- Place a hollow tube in water to create a closed-end pipe.
- Note the frequency of a tuning fork and hold it above the tube.
- Tap the tuning fork gently to produce sound waves that travel down the tube and reflect at the air/water surface.
- Adjust the tube’s height until the sound resonates the loudest, indicating the lowest resonant frequency of the closed tube.
- The tube’s length at this frequency is a quarter of the sound wave’s wavelength.
- Use the frequency and wavelength to calculate the speed of sound in air using the equation v = fλ.
12
Q
Semi circle light rays (PAG)
A
-Place the glass block on paper and shine light from the ray box into the
curved surface of the block.
-Rotate either the ray-box or the block until the refracted ray from the glass
block makes an angle of 90°.
-Use a protractor to measure the angle of incidence of the ray of light within
the block; this is the critical angle c.
-The refractive index n is calculated using n = (sin c)
–1
13
Q
Center of mass (PAG)
A
- If you hang an object from a single point, the centre of mass will always lie directly below that point
- So hang an object from two different points and draw a line straight down the object (the plumb line)
- The point at which these lines intersect is the centre of mass
14
Q
Number of Photons equation
A
Energy of wave supplier ÷ energy of one photon
15
Q
Lightgates
A
- Light gates measure time accurately in labs.
- A light gate starts a timer when an object passes through, blocking a light beam.
- A second light gate stops the timer, measuring the time taken.
- Single light gates measure speed by timing how long the light is blocked.
- Speed is calculated using the length of the flag and the time it blocks the light.
