WJEC AS Physics PH1 May 19th 2015

quote your answer to the precision of the least precise number in your calculation

anyone help with how to round numbers, because mark schemes decimal places vary as Ive done past exam papers and they are different for different questions
i could write the full calculated number down and then round it, but that would take a lot time
anyone??

I'm not sure if this is the same thing, but could you help me with the conservation of charge and energy please ? (:
I know the definitions from the WJEC website, but in a question when it asks to state what law is used in a circuit, how would I know which one it is ?

anyone help with how to round numbers, because mark schemes decimal places vary as Ive done past exam papers and they are different for different questions
i could write the full calculated number down and then round it, but that would take a lot time
anyone??

I have a question - what do we need to understand Drift Velocity as? For example, in a question asking about how long it would take an electron to travel a said distance of wire, should we just assume the velocity at which the electron travels is its drift velocity (so it would just be distance/speed).

Since current is the amount of CHARGE passing a given point in one second, this concerns the law of conservation of charge.
Since potential difference is the electrical ENERGY converted into other forms of energy per coulomb of charge passing between two points, this concerns the law of conservation of energy.
Hope that helps.

Absolutely dreading tomorrow! Reckon it will be a hard paper as it's the last year they're doing this syllabus?

Absolutely dreading tomorrow! Reckon it will be a hard paper as it's the last year they're doing this syllabus?

EMF of a cell is the electrical energy the cell supplies, converted from its chemical energy, per unit charge passing through the cell. Make sure you learn that definition and UNDERSTAND it, which I guess means that you will automatically have it memorised. That's a great reason to try and understand concepts rather than memorise blindly.

Note that a cell has a thing called internal resistance, so there is resistance to current inside the cell itself. Therefore energy is dissipated (as heat) in the internal resistance and this energy, per unit charge, we informally call the 'lost volts' (make sure you see why). Hence, by V=IR, and if we call the internal resistance r, the 'lost volts' is clearly Ir.

So this means, that if volts are 'lost', the pd that the components in the circuit receive is actually less than the emf! This pd will be E-Ir, where E is the emf.
We call this pd the terminal pd V, so that V=E-Ir (recognise that equation?). The terminal pd is defined as the electrical energy delivered to the external circuit and converted into other forms of energy per unit charge. Understand that too.

That's pretty much that for emf and the like. You'll be surprised how useful this concept is.

Hey, can you help me with the emf question in May 2010 please ?
They are all 1 markers but I don't understand which of the values to use ):

I assume you've got the definitions done at the start of the question.
Ok, so once you've nailed this whole question and understand it, you should be able to do this type of question a lot more easily.
(i) You know that Q=It, so plug in your current and the time (IN SECONDS!!!) to find the charge.
(ii) Emf is defined as the electrical energy supplied to the external circuit by the cell, from its chemical energy, per coulomb of charge passing through the cell.
Key word is chemical. You know that any voltage is the corresponding energy divided by the total charge, and emf is no different.
(iii) In the same way as we did in (ii), use the definition of terminal pd to decide what energy to use (the bulb is getting the energy corresponding to the terminal pd).
(iv) The lost volts are the energy lost per coulomb of charge, informally. How much energy was lost? Remember, they're VOLTS, so divide by charge.
(v) V = E - Ir. That's all I'm gonna say here.
Hope that helped you understand how to answer the question. Any more questions feel free to ask!

Just finished - what did people get for the question about the two wires in series where we had to circle the answers?