You are Here: Home >< Physics

# Edexcel Physics Unit 2 "Physics at work" June 2013 watch

• View Poll Results: The last question - Does resistance increase or decrease?
It increases ( using V=IR or some other method)
70.73%
It decreases using the 'lattice vibrations' theory
29.27%

1. Hey! In May 2012, does any one know how to calculate the total circuit resistance in Q11 a)?
2. does someone know how to work out question 4 on jan 2013 paper. the one with refracted angle
3. Can somebody please explain to me what internal resistance is and how to use the formula and work it out?
4. (Original post by maryam1996)
Hey! In May 2012, does any one know how to calculate the total circuit resistance in Q11 a)?
I'll try write my answer in a methodological steps;

1) A open. B open

no current will flow through the resistor next to the switch A, the current must flow through the 2 resistors on the edge of the circuit, they are in series, therefore the total resistance will be 2R.

2) A closed. B closed.

Current will now flow through the resistor next to switch A as there is a complete path for the current to flow down, however only some current will flow down A, at the junction current will split and some current will flow through the top right resistor on the edge of the circuit, however the resistor on the bottom on the right hand side will not be considered as no current will flow through that resistor, the current will go will flow through switch B and evade the resistor ( current flows through paths of least resistance).

Thus, we must calculate a parralel resistance, 1/[1/R + 1/R] = R/2 (Ohms)

3) A closed. B Open.

Some current will flow through the resistor next to switch A, also the current will flow through both resistors on the edge of the circuit as switch B is open, there is no way to evade the resistors. Thus we have 2 series resistors on the RHS ( R + R ), which is in parralel with the resistor next to switch A.

Hence forth; 1[1/2R + 1/R] = 2R/3 (Ohms)
5. (Original post by alygirl)
Can somebody please explain to me what internal resistance is and how to use the formula and work it out?
Internal Resistance is the resistance inside a source of electromotive force. Such as a battery or cell.
The formula for it can be written as any of these:
ε=V + v
ε=I(R + r)
V=ε - v
V=ε - Ir

V is Terminal Potential Difference
v is Lost volts
ε is Electromotive force
I is Current
r is Internal Resistance
6. Guys I know that for car headlights, they dim at the start because the huge surge in current causes a drop in the voltage across the internal resistance, but why do they go on to become brighter? Please, any help would be great.
7. (Original post by gurpz)
does someone know how to work out question 4 on jan 2013 paper. the one with refracted angle
The angle of incidence is measured from the normal, hence the angle of incidence is (90-55) 35 degrees. The angle of refraction can clearly be identified as 90 degrees. [This means 35 degrees is the critical angle don't worry about this]

Using snell's law. n1Sini = n2Sinr

were n1 and n2 is the refractive index of glass and air respectively, and i and r are the angle of incidence and refraction.

n1 = n2sinr/sini

n1 = 1sin90/sin35

n1 = 1.74...
8. (Original post by ArcRaman)
I'll try write my answer in a methodological steps;

1) A open. B open

no current will flow through the resistor next to the switch A, the current must flow through the 2 resistors on the edge of the circuit, they are in series, therefore the total resistance will be 2R.

2) A closed. B closed.

Current will now flow through the resistor next to switch A as there is a complete path for the current to flow down, however only some current will flow down A, at the junction current will split and some current will flow through the top right resistor on the edge of the circuit, however the resistor on the bottom on the right hand side will not be considered as no current will flow through that resistor, the current will go will flow through switch B and evade the resistor ( current flows through paths of least resistance).

Thus, we must calculate a parralel resistance, 1/[1/R + 1/R] = R/2 (Ohms)

3) A closed. B Open.

Some current will flow through the resistor next to switch A, also the current will flow through both resistors on the edge of the circuit as switch B is open, there is no way to evade the resistors. Thus we have 2 series resistors on the RHS ( R + R ), which is in parralel with the resistor next to switch A.

Hence forth; 1[1/2R + 1/R] = 2R/3 (Ohms)
why is the final combo not [1/r + 1/r]^-1 + r = 3/2 R??
9. June 12 15 (c) people???
10. can someone explain january 13 q8, answer is C but not sure why
11. (Original post by stealth_writer)
why is the final combo not [1/r + 1/r]^-1 + r = 3/2 R??
Because the last two resistors on the edge of the circuit are in series which you can model as a single resistor as 2R. Which will be in parralel with the resistor at A.

So 2R and R are in parralel so it must be [1/R + 1/2R]^-1
12. (Original post by ArcRaman)
I'll try write my answer in a methodological steps;

1) A open. B open

no current will flow through the resistor next to the switch A, the current must flow through the 2 resistors on the edge of the circuit, they are in series, therefore the total resistance will be 2R.

2) A closed. B closed.

Current will now flow through the resistor next to switch A as there is a complete path for the current to flow down, however only some current will flow down A, at the junction current will split and some current will flow through the top right resistor on the edge of the circuit, however the resistor on the bottom on the right hand side will not be considered as no current will flow through that resistor, the current will go will flow through switch B and evade the resistor ( current flows through paths of least resistance).

Thus, we must calculate a parralel resistance, 1/[1/R + 1/R] = R/2 (Ohms)

3) A closed. B Open.

Some current will flow through the resistor next to switch A, also the current will flow through both resistors on the edge of the circuit as switch B is open, there is no way to evade the resistors. Thus we have 2 series resistors on the RHS ( R + R ), which is in parralel with the resistor next to switch A.

Hence forth; 1[1/2R + 1/R] = 2R/3 (Ohms)
Heya, so I agree with you for number 1 as in by agreeing I got there too however 2 and 3 perplex me.

For 2 I thought okay A is closed and so is B meaning some current will flow through to all the resistors right?
I thought the same for 3. However when I'm working it out, I think there's something wrong with my method. I thought the total of the two resistors in series is 2R and then so 1/2R + 1/R = 3/2R? Can you please explain what I'm doing wrong?
13. (Original post by stealth_writer)
June 12 15 (c) people???
c) The smallest detail that can be seen on the image is half the length of the ultrasound pulse. The thumbnail on the fetus is 0.50mm thick. The speed of ultrasound in the thumbnail is 2000ms^(-1). Calculate the maximum pulse duration if the thumbnail is to be seen on the image.

v=s/t
t=s/v
=0.0005m/2000ms^(-1)
=2.5x10^(-7)s

Because the smallest detail can be seen at half the length of the ultrasound pulse.

2.5x10^(-7)s x 2= 5x10^(-7)s
14. (Original post by stealth_writer)
June 12 15 (c) people???
So the image which is the thumbnail is half as long as the length of the ultrasound pulse. The thumbnail is 0.50mm so 0.5 x 2 = 1mm
1mm = 0.001 m

Using speed = distance/time

speed of ultrasound is 2000ms

therefore time = distance/speed

so 0.001/2000 = ANS
15. (Original post by stealth_writer)
June 12 15 (c) people???
okay so I did this in my way. U keep resistance same that is 52.9. take length half of previous answer u found that becomes 3.13. use resistivity formula to find the area of cross section. from area you use formula A=3.14*diameter^2/4 . subtitue value of A and get diameter.
17. (Original post by YA BISH)
can someone explain january 13 q8, answer is C but not sure why
This is a combination of resistance in parallel and potential dividers equation.

1/R1=1/R.1 + 1/R.2
=1/6000Ohms +1/6000Ohms
=1/3000Ohms
R1=3 kiloOhms

Vout=Vin x R2/(R1+R2)
=12V x 6000Ohms/9000Ohms
=2/3 x 12V
=8V
18. Guys can anyone explain properly Q18 part a(1) and part b. please
19. (Original post by CharlieTT)
This is a combination of resistance in parallel and potential dividers equation.

1/R1=1/R.1 + 1/R.2
=1/6000Ohms +1/6000Ohms
=1/3000Ohms
R1=3 kiloOhms

Vout=Vin x R2/(R1+R2)
=12V x 6000Ohms/9000Ohms
=2/3 x 12V
=8V
thank you
20. (Original post by jollygood)
Guys can anyone explain properly Q18 part a(1) and part b. please
June 2012?

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: December 18, 2013
Today on TSR

### Buying condoms for the first time - help!

Discussions on TSR

• Latest
Poll
Useful resources

Can you help? Study help unanswered threadsStudy Help rules and posting guidelinesLaTex guide for writing equations on TSR

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE