Level+2+Physics+June+2007

**November 2012 Current Electricity and Resistances**
I've been doing some past papers for physics and I just cannot work this one out. I've typed out the question and the answer given. What do you think? Amara

Anna is a portrait photographer. Before she takes her photographs she uses a hand-held meter to measure the light intensity. The circuit meter shown is a basic voltage divider, the lower resistor is 100 ohm and the upper resistor is a LDR. The battery is 9.0V. When Light intensity is low (50 lux), the LDR has a resistance of about 2600 ohm. When the light intensity is high (1000 lux) it has a resistance of about 100 ohm. (i.e. resistance decreases when light intensity increases) The meter connected across the 100 ohm resistor is essentially a voltmeter and has a very high resistance. It has been recalibrated so that the readings are in lux rather than volts.
 * Question **

Q) Is the light intensity becomes greater than 1000 lux, the meter goes off the scale. When this happens, Anna switches to another scale which can handle hight values. The second scale is a second set of numbers on the same voltmeter. When she switches to this, one change is made to the circuit. Suggest what this change might be.  A) A very high resistor is switched into the circuit in series with the meter. (the resistor has the same resistance as the meter.)

I simply do not understand this answer. If anything, I would have thought that the answer was to put another resistor in series with the LDR, not the voltmeter.

A Possible Answer

Putting the second resistor in series with the LDR would not affect the range of readings on the voltmeter, just the overall voltage across the 100 ohm resistor.

What is needed is to change this range and that means adjusting the current through the voltmeter itself. All voltmeters have a very high resistance so that they do not draw much of the current from the circuit. But their range is affected by their total resistance. Adding another resistance in series with the voltmeter makes a change in the overall resistance of that branch of the circuit and thus reduces the current through the added resistor and the voltmeter. This will affect the range of the voltmeter.

I have to admit I did this one by first drawing the circuit and then putting in the second resistor in the voltmeter branch was more or less instinctive. Tish

I am currently working through PX201 – Motion and am up to activity 7a. I was just wondering if you could please explain to me how you decide which of the four equations you are supposed to use? e.g. v2=v1+at, d=1/2//x//(v1+v2)//x//t , d=v1t+1/2a(t)^2 and v2^2=v1^2+2ad Thanks, Jamie What you should do is list the quantities you have been give and the one you have to find. Check out this problem: - Problem: A train is travelling at 40 m/s and decelerates at 2.5 m/s2. What distance does it take to stop? Hint – remember ‘to stop’ means a velocity of 0 m/s. Solution v1 = 40 m/s v2 = 0 m/s a = - 2.5 m/s2 d = ? Look to see which of the formulae has these four quantities in it. In this case it is v22 =v12 + 2ad The method will work with other problems as well Tish
 * Wednesday 16 Feb 2011**

hi. activity 2B, question 2e. In the answers it show v1 to be 0 but the question asks for the time from point A to point C, the velocity at point A is 8 m/s so wouldn't v1 be 8? Also where does sound energy end up? Thanks Alex Bradford #07056109

I guess you mean book Px203. This question is related to projectiles and is a somewhat less than gentle introduction to them! It takes the ball the same time to go from A to C as it would take it to drop from A straight down to the ground. This is one of the properties of projectiles. I always think of hitting a golfball off a tee on a cliff edge.

This means that although you don't know the horizontal distance to C, you CAN work with the vertical part of the motion. In the vertical plane, the initial velocity is zero, the distance 45 m and the acceleration is 10 ms-2 which means you can use d = v1t + 1/2at2 to find t. Have a look at these two sites [|http://www.physicsclassroom.com/mmedia/vectors/bds.html] [|http://www.fearofphysics.com/XYIndep/xyindep.html] Posted by Tish at [|9:17 PM] [|0 comments]

[|Help with Px 201 to Px204]
Mechanics is the forces and motion part of Physics. So it deals with things like acceleration, projectiles, balance and collisions.

To Te Kura's OTLE

Vectors are special numbers which have a size or 'magnitude' AND a direction. Things like force, momentum, velocity and acceleration.

Posted by Tish at [|1:56 PM] [|0 comments]    Labels: [|Mechanics help intro] [|July 2007] [|Home] Subscribe to: [|Posts (Atom)]

Tish Glasson
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I’m Tish Glasson and I’m one of a team of Physics teachers in our Science ropu at Te Kura/The Correspondence School of New Zealand [|View my complete profile]