42 Physics Equations They Don't Tell You in A Level Exams

[@AlanKey86]

A = πd²/4 Mechanics s = vt s = vt - at²/2 KE = mv²/2 GPE = mgh v = root(2gh) v = root(2GM/r) [escape velocity] W = mg m1u1 + m2u2 = m1v1 + m2v2 [conservation of momentum] Material Properties EPE = kΔL²/2 stress = F/A strain = ΔL/L Young's Modulus = FL/AΔL Thermal Physics n = sample mass/relative molar mass N = nN_A (where N_A = Avagodro's Constant) Uncertainty %uncertainty = absolute uncertainty/value × 100 absolute uncertainty = 1/2 range/mean absolute uncertainty = |G_max - G_min|/2 [where G is gradient] total absolute error if quantities are added (or subtracted) = sum of absolute errors total % error if quantities are multiplied (or divided) = sum of % errors Electricity V1/V2 = R1/R2 [potential dividers] v = root(2E/m) [velocity of accelerated charge] qV = mv²/2 V = V_0 e^(-t/RC) [voltage across discharging capacitor] V = (1 - V_0) e^(-t/RC) [voltage across charging capacitor]

[@zumzuo]

Last one is V = V_0(1- e^(-t/RC) )

[@Fyr35555]

Useful to know that sometimes with escape velocity they don't give you "M" and you have to derive a different expression with the data given that can be rewritten as =GM allowing you to solve the equation. So they might give you the radius of the orbit of a satellite above earth and you need to use the value for the force per unit mass on earth due to gravity to derive an expression for GM. This might sound confusing because I don't remember exactly how to do it, just worth knowing you might not be given M in the question.

[@tomdavid1459]

​@@zumzuo i think they're the same equations except his is expanded but Ty for clarifying because it's harder to understand the expanded version👍🏼

[@zumzuo]

@@tomdavid1459 if you expand his you get a different one, yw btw

[@joelcairns8375]

Impressed at the effort that has gone into printing a backwards t-shirt so its the right way round!

[@PhysicsOnline]

Thanks!

[@DynestiGTI]

Phase difference (rad) = 2π * path difference (m) / λ (m) Constructive interference: Path difference = nλ Destructive interference: Path difference = (n + 1/2)λ

[@DynestiGTI]

Einstein energy-mass equation for photon (m=0): E² = m²c⁴ + p²c² = p²c² This is useful if they ask you to find the momentum of a photon given the energy.

[@Fyr35555]

Its so useful to be able to derive some of these (equating circular motion with gravitational force to find Kepler's 3rd law or using coulombs law to derive other equations for charged particles like in a velocity selector, equating GPE with KE to find the equation for escape velocity, deriving the pV = (1/3)NMC^2 equation from the ideal gas model using conservation of momentum in an elastic collision and modelling 1/3 of particles travelling in each axis etc.)

[@vg6761]

Can you do my exam?

[@shahd888]

you are an actual physics god thanks sir!

[@puggam3erp]

Could you use instead +- 273.15 for converting between degrees and kelvin?

[@henryj.walker2025]

I can't find anything in the AQA spec but we've been taught to use 273.15 and on all the revision sites it says the same so I would (and will) use it with the .15.

[@fahrenheit2101]

I've never had to - should be safe though, if it's accurate.

[@PhysicsOnline]

You could. I’ve always found that the numbers are so large, and the temperature your converting from is usually given to a whole number, that using this figure to 5sf is a bit unnecessary.

[@lustredream]

A small correction, for dicharging the capacitor, the equation for current should include a negative sign (I=-Ioe^-t/rc) since the polarity has changed. Other than that these are quite helpful, thank you!

[@fahrenheit2101]

True, but I don't think I've ever seen an exam question get too concerned about polarity. Maybe that's an exam board difference idk

[@spoonky7243]

thanks man, you should do a collaboration with science shorts, he is also insane at physics

[@eagle1341]

Absolutely Beautiful! You are amazing. I just want to ask 3 simple but very important questions to clear some confusion: 1 - When finding answers to certain degrees/figures, can we write our degree in the answer box next to the answer (e.g. Answer........40..(2.s.f).. N) 2 - Do you always give your answers to the same degree of figures as the values that they give you? I have been checking mark schemes and they sometimes give answers to different degrees. 3 - Does your answer always have to be EXACTLY like the mark scheme answer? Or is there a level of tolerance for every questions? Thank you so much Physics digita...I mean Online!

[@eagle1341]

Sorry I should have said, this isn't really related to your video but just some general questions regarding papers and practice. Thanks!

[@zaidm1134]

1) ? 2) Yes you always give your answer to the same sig f as the numbers in the question unless they specifically say so. The Mark Scheme rarely ever let's you off and you wont know when it will 3) You can use a different method if it is "Correct Physics". Sometimes people use incorrect physics, e.g. When working out the Gravitational Potential for a mass outside of earth in field, you can't use mgh. Let's say you were told to calculate efficiency of a system and you calculated it using "Power" instead of "Energy", this is fine even if the markscheme used "Energy" to do this

[@ukdrillnews8894]

I think he’s talking about when it’s open questions and you have to explain something. Does it have to be exactly as the mark scheme or can it be along the lines of it(with main concept and main definitions in the answer)

[@fahrenheit2101]

A general rule of thumb is to use the LOWEST number of sig fig out of any of the numbers you used in the calculation. E.g. if you're doing F = ma, with a given mass of 2800 kg and acceleration of 1.25ms^-2, it's safest to use 2sf, as 2800kg is given to 2sf, so you can't be assured of any precision beyond that. This doesn't matter too much for most questions, but if they ask for an appropriate number of sf, this is the sort of thing they mean. And I don't think they really mind if you put the degree of rounding in the answer box, but I generally just leave it there in the working out, and then put the rounded answer alone in the box. And there's generally some tolerance, yes. If your answer is to a somewhat appropriate number of sf (3's always a good shout), and rounds to the one they have, that should be alright I reckon. Some questions are harsher about this. A good skill to have is knowing how to store numbers on your calculator - this reduces the risk of any significant rounding errors during working, as you shouldn't really be rounding early.

[@Hasnu777]

For Oxford AQA, if you are asked to show something is approximately a value e.g. show the resistance is approximately 53 ohms, your final value should be at least 1 sig fig more than what the question has shown. For the above example, if your final answer is 52.918182… then round to at least 3sf

[@abdulqodirmir]

and also example or explanation of more usage of the formulae for the waves would be much appreciated (even a link to the related videos would be useful)

[@pratyasha279]

11:02 was going to ask - takes a sheer amount of potential to write the other way around at that speed sir, quite admirable ahah - I was humbled real quick!!

[@jameskonefal3453]

Another useful one when thinking about stationary waves is density (kgm^-3) = mass per unit length (kgm^-1) / cross-sectional area (m^2).

[@PhysicsOnline]

Of course!

[@peter_ade]

Another good one would be the equation for GPE in a radial field: Ep = - G M m / r It isn't on the AQA equation sheet

[@PhysicsOnline]

Is it not? I missed that!

[@peter_ade]

@@PhysicsOnline Credits to my teacher mentioning it the other day!

[@kingzmash9124]

11:00 I have wanted to know this for CENTURIES 😅😂

[@kingzmash9124]

can we get a video on this because i still dont understand how the camera sees you. i feel a lot of other students would be interested in it as well

[@henryj.walker2025]

@@kingzmash9124 At 11:07 when he flips it, that is what the camera sees. The writing looks so bright because put simply, its glass/pastic which as light shining into the side of it from all angles. I don't think there is much else to it except that.

[@kingzmash9124]

@@henryj.walker2025 cheers buddy

[@pratyasha279]

Sir which of the equations do we need to know how to derive? (Year 1 and 2 of OCR A-level Physics)?

[@MdNabilIslam-ko1ui]

Could you please tell me that when do we use the reading furthest from the mean to calculate absolute uncertainty?

[@fahrenheit2101]

Pretty helpful, as usual. Is there a video on this channel going over uncertainties in more depth? Also, I've got a q. I could've sworn I saw something somewhere which said the uncertainty in the gradient is the difference between the 2 worst fit lines divided by 2, and the same for any intercept. Is this also valid? Also, I feel like the MS also said something about estimating the actual gradient as the average of the 2 worst fit lines (instead of eyeballing an actual best fit line ig). Is this fine, too?

[@PhysicsOnline]

Try these videos - www.alevelphysicsonline.com/uncertainty

[@manbha6817]

Look at pmt for uncertainties it is very good

[@TheObloINATOR777]

great video, thanks !

[@abdulqodirmir]

A bit more explanation and example on this would be much appreciated: %U=(half the range)/(mean value)

[@jw5545]

I have to say brightons scouts are next level

[@clytiec08]

Not doing a levels yet, but that marker is so good

[@vitaszernys2893]

He even has his shirt printed the other way around. What a boss.

[@engineeringmadeasy]

Which exam do you teach?

[@pomalositer3243]

Hi there, your videos are super helpful in my a level revision so thank you much. PS I believe that the diffraction grating equation dsin θ = n λ is not on the formula book but is very important. PPS sorry if this is in the video and I just missed it but as far as I can tell it isn't

[@sweepyb]

That equation is in the OCR formula booklet.

[@user-ph9pm6fc7g]

it's also given in AQA, too

[@skcreations8494]

Thank you sir! Btw thumbnail is crazy 😂

[@reality4845]

Is white marker availbale

[@pahcreates378]

in edexcel they dont give these two equations: Diffraction Grating: λ = ax/D Polaruzation: I = Io * Cos^2 * θ

[@jw5545]

How are some comments days old when there video only came out 8 hours ago

[@PhysicsOnline]

It has been on the front page of my website for a few day! ALevelPhysicsOnline.com and I also sent a link out to all my email subscribers.

[@jadefang1436]

I am not the only one wondering how on Earth he is writing backwards right?? like he has to be writing the mirror image of what we are seeing from the way he is writing right? or where is the camera positioned to be able to film this if he is in fact writing it normally??

[@jadefang1436]

oh lol right after I asked the question he explains in the video... I need to be more patient sometimes

[@Samld1200]

Are you writing backwards for all of this?

[@laurenellison7592]

He mirrors the video when editing I think, so doesn't need to write backwards

[@PhysicsOnline]

If you skip forward to the section on uncertainties I explain all.