practicals

Question 1

practical 3.1.1d(i) how to determine power and focal length

what will be uncertainties in this power and f values

Answer 1

you can use a small filament lamp as the source of rays, the object, and lens to display the image on an opaque screen,

change and measure the object distance multiple times moving the screen each time until the object is in focus again. this gives you values of v and u

plot 1 / v on the y-axis and 1 / u on the x-axis giving you a straight line, the y-intercept is the power 1 /f of the lens. the reciprocal of this will give you the focal length. you could also add 1/u and 1/v to get 1/f average the values for 1/f and reciprocate for the average power

the uncertainty in the image distance is much more likely to affect the overall uncertainty in power and f compared to object distance as it’s hard to tell when exactly the image is in focus

Question 2

3.1.1d(ii) observing polarisation using microwaves

how to do experiment, why grille and only one grille explain results

Answer 2

place a metal grille between microwave transmitter and receiver, the strength of the microwaves at the detector will vary as you rotate the grill,

you use a grille because the microwave wavelength is too long to observe polarisation using a polarising filter. only need one grille because the transmitter transmits vertically polarised light.

the intensity of microwaves is at a max when direction of vibration for microwaves is at right angle to grille wires, as you rotate the grille intensity decreases to 0 when they’re parallel.

this is because the grille’s electrons are excited by the microwaves vibrating electric field, the energy of the incoming microwaves is absorbed by the grille and reemitted in all directions, but only a few of these are vibrating in the direction of the receiver.
all energy is absorbed when they’re parallel, 0 intensity at receiver, even at 90 degrees, some electrons still excited so there’s a small drop in intensity

Question 3

3.1.1d(ii) observing polarisation with polarising filter

Answer 3

if you line up 2 polarising filters vertically and shine a light through them, the light will be vertically polarised after travelling through the first filter, and all the light will pass through the second one as well.

but as you rotate it, the amount of light passing through varies, if you rotate through to 90 degrees, the intensity will reduce to 0

at 45 degrees to the first one, the intensity passing through the second filter will be exactly half the light getting through the first filter.

Question 4

3.1.2d(i) investigating iv characteristics for ohmic and non-ohmic components
what does circuit look like
how to carry out
how to reduce uncertainties

Answer 4

set up circuit as shown

component can be a filament lamp or copper block

use variable resistor to vary voltage across component, doing so for a large range of voltages
record current at each V 3 times to get a mean current, reducing effect of random errors

make sure to turn off circuit after every reading to prevent heating of components which would affect results
plot a graph of I against V for the component, doing so for each component so you can compare

you can reduce uncertainties by using voltmeter and ammeter with higher resolution

Question 5

finding resistivity (and conductance of a wire)
how to do 
uncertainties

Answer 5

set up circuit as shown, flying lead to be connected to a point 10cm along ruler when increasing L

first measure diameter of wire at 3 different points to get a mean diameter, halve and find C.S.A

attach flying lead to first point along wire (10cm) for L, measure A and V to calc R, repeat for several lengths and then do experiment 2 more times to get an average value for R for each length

now plot R against L to get gradient of resistivity / area, so simply multiply gradient by area to get resistivity

must keep current constant to stop heating, can also disconnect wire to stop it from heating
to find uncertainty % in R, add the uncertainty%’s for V and I
%uncertainty for Area will be 2 * %uncertainty in diameter
so overall %uncertainty in resistivity is adding %uncertainties for A and R

Question 6

determining internal resistance of cell
setup
how to do
how to improve

Answer 6

set up as shown

set variable resistor to max value
close switch and record values of A and V, open switch between readings to prevent heating up of variable resistor

gradually decrease resistance of variable resistor and find new values of V and A

emf = V + r, so V = -r(I) + emf (y =mx +c), plot graph of V against I. gradient is -r and the y-int is emf

calibrate voltmeters and ammeters or use higher resolution ones