Unit 2 Practical's that come up in test Flashcards
Investigation of the I–V Characteristics of a Filament Lamp and a Metal Wire at Constant Temperature?
THEORY:
Ohm’s law states that for a conductor the current, I, is directly proportional to the potential difference, V,
provided physical factors such as temperature and pressure remain constant. Therefore by plotting the I–V
characteristic of a metal wire and of a filament lamp, the validity of Ohm’s law, as applicable to each of these
components, can be determined. A graph of I against V is linear for a metal wire and non-linear for a filament
of a lamp
Experimental Method:
The circuit should be set up as shown.
Starting with the output of the variable d.c. voltage supply set to its
minimum value, slowly increase the value of the applied voltage. The
current through the component and the potential difference across the
component should be recorded for a range of values of the applied voltage.
A graph of current against voltage should then be plotted. This procedure
can be repeated for different components.
Determination of the Resistivity of a Metal Wire?
THEORY:
Resistivity, ρ can be found using the equation 𝑅 = 𝜌
𝑙
𝐴
where l is the length of the wire, A the cross-sectional
area and R the resistance. This can be compared with the equation for a straight line 𝑦 = 𝑚𝑚 + 𝑐. A graph
plotted of R (y-axis) against l (x-axis) will be a straight line through the origin of gradient 𝜌
𝐴
. The cross sectional
area can be found using A=πr2 and the resistivity calculated by ρ = gradient × A.
Experimental Method:
Select the image (left) for a larger diagram of the circuit.
Leaving one crocodile clip fixed at one end of the wire, the other clip should
be moved along at suitable intervals, e.g. every 10 cm/20 cm to cover the
whole range of the wire. Readings on the voltmeter and ammeter should be
noted for each length and the resistance determined using 𝑅 = 𝑉
𝐼 .
The diameter of the wire can be found using a micrometer or vernier
calipers and the cross-sectional area determined. Plot a graph of R (y-axis)
against l (x-axis) and calculate the resistivity using: ρ = gradient × A.
Investigation of the Variation of Resistance with Temperature for a Metal Wire?
THEORY:
Resistance increases with temperature for metals in a linear relationship. This practical will enable data to be
obtained to investigate this relationship.
Experimental Method:
The apparatus should be set up as shown.
The water bath should be heated and the water stirred continuously in order
to ensure an even temperature throughout the water bath. Once the
required temperature has been reached then remove the heat and record
the reading of resistance or take the ammeter and voltmeter readings. This
process should be repeated at intervals until the water boils.
Repeat the experiment during cooling. Plot a graph of resistance (y-axis)
against temperature (x-axis). This should be a straight line through the
origin.
An ice water mixture can be used to record the resistance at a temperature
of 0 o
C.
USEFUL INTERACTIVE RESOURCES
WJEC > A Level Physics > Terms, definitions and units bookle
Determination of the internal resistance of a cell?
THEORY:
The equation used for determining the internal resistance is 𝑉 = 𝐸 − 𝐼𝐼 where V is the terminal p.d. of a cell; E
is the emf of the cell; I is the current flowing in the circuit and r is the internal resistance. V = IR and the
equation can be rewritten as 𝑅 = 𝐸
𝐼 − 𝑟. Therefore a graph of R against 1
𝐼
should be linear.
Experimental Method:
The circuit should be set up as shown.
The resistor values should be varied and the current values recorded.
Plot a graph of R (y-axis) against 1
𝐼
(x-axis). The graph should be a straight
line with the intercept on the y-axis which is equal to the value of the internal
resistance.
Measurement of the Intensity Variations for Polarisation?
THEORY:
The light waves in a ray of light from a lamp have vibrations in all planes and directions.The light is
unpolarised. When the light passes through a polaroid filter, the vibrations will be in one plane or direction
only. In the experiment with two pieces of polaroid, the first polarises the light. The light will then not pass
through the second polaroid if the direction in which the second filters polarises light is at right angles to the
polarising direction of the first polaroid.
Experimental Method:
Investigate the variation in intensity by looking through the lamp through both polaroids and rotating one of the
polaroids through 360o
. Note the change in intensity that occurs.
Determination of Wavelength Using Young’s Double Slits?
THEORY:
The fringe spacing, Δ𝑦 is given by the equation ∆𝑦 =
𝜆𝜆
𝑑 where 𝜆 is the wavelength of the light; D is the
distance from the slits to the screen where the fringes are viewed and d is the distance between the slits. A
graph of Δ𝑦 against D should be a straight line and the gradient can be used to determine the wavelength of
the light.
Experimental Method:
The apparatus should be set up as shown.
Measure the fringe spacing Δ𝑦, the spacing between the double slits, d, and
the distance, D, from the slits to the screen using either the ruler or
digital calipers. Vary the distance, D in equal intervals. Plot a graph of the
fringe spacing Δ𝑦 (y-axis) against the slit-screen distance D (x-axis). This
should be a straight line through the origin.
If the fringes are close together; Δ𝑦 can be determined by measuring the
separation of a number of fringes. So determine Δ𝑦 by dividing the distance
by the number of fringes measured.
Determination of Wavelength Using a Diffraction Grating?
THEORY:
The diffraction grating equation is given by 𝑛𝑛 = 𝑑sin𝜃. The spacing between the lines in a diffraction grating
is usually specified or can be found from the grating ruling. By measuring the angle θ, the wavelength of the
light can be determined.
Experimental Method:
The apparatus should be set up as shown.
The value of θ can be determined from tan𝜗 = 𝑥
𝐷.
Using the equation 𝑛𝑛 = 𝑑𝑑in𝜗 then the wavelength can be determined for
various orders of diffraction.
Determination of the Speed of Sound Using Stationary Waves?
THEORY:
When resonance first occurs, the length of air in the tube, l, plus a small end correction, e (to account for the
position of the tuning fork above the tube) will be equal to a quarter of a wavelength. Hence:
4
λ l e =+ but f
c λ = so ef
c l − = 4
If a graph is plotted of l (y-axis) against f
1 (x-axis) it should be a straight line with a small negative y-intercept.
The gradient of the graph equals 4
c , and so the speed of sound, c, can be found. The small negative intercept
will give the end correction.
Experimental Method:
The apparatus should be set up as shown.
Initially place the resonance tube as deep as possible into the water. Then
gradually raise it. As this is being done hold a vibrating tuning fork over the
top. When resonance occurs (a loud sound will be heard), measure the
length of the tube above the water level.
Repeat the above for each of the tuning forks. Plot a graph of length (y-axis)
against frequency
1 (x-axis). Use the gradient to determine a value for the
speed of sound.
EXTENSION:
The resonance tube could be raised past the first resonance point until the second resonance point is reached.
The results could then be used to show that for the two resonant lengths l1 and l2 then:
Speed of sound, c = 2f(l2 – l1).
Measurement of the Refractive Index of a Material?
THEORY:
The refractive index, n, of a material can be determined from the equation sin𝜃𝑖 = 𝑛sin𝜃𝑟 where n = refractive
index, 𝜃R
i is the angle of incidence and 𝜃R
r is the angle of refraction. The above equation assumes that the
incident ray is travelling in air. A graph of sin𝜃𝑖 (y-axis) against sin𝜃𝑟 (x-axis) will give a straight line through
the origin and the gradient is equal to the refractive index, n.
Experimental Method:
The arrangement should be set up as shown.
The angle of refraction 𝜃𝑟 R can be measured by drawing in the line joining
the incident and emergent rays for different values of the angle of
incidence. The angles can be measured using the protractor after drawing
in the normals. A graph of sin𝜃𝑖 (y-axis) against sin𝜃𝑟 (x-axis) can be
plotted, which should give a straight line. A value of n can then be
determined from the gradient.
Determination of h using LEDs?
THEORY:
The Planck constant, h, can be determined by using a light-emitting diode (LED) and measuring the minimum
voltage, Vmin, at which light is just emitted by the diode. The Planck constant can then be determined from the
equation Vmin = ℎ𝑐
𝑒𝑒 where c is the speed of light 3.00 × 108 m s-1 and e is the electronic charge, 1.60 × 10-19 C.
A graph of Vmin against 1
𝜆 should be a straight line with the gradient equal to ℎc/e
Experimental Method:
The voltage should be varied until light is just emitted by the LED. Record
the voltage to which it corresponds, namely Vmin.
The LED should be replaced and the procedure repeated for LEDs with
different wavelengths of light.
Plot a graph of Vmin (x-axis) against 1/lambda (y-axis) and use it to determine a value
for h.
