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AS and A Level: Electrical & Thermal Physics
404 AS and A Level Electrical & Thermal Physics essays
- Peer Reviewed essays 9
- Essay length: 1739 words
- Submitted: 02/02/2004
Response to question - To what extent has the student answered the set question? How explicit is their response? The writer has approached the set title very well and therefore produced…
- Essay length: 2614 words
- Submitted: 20/08/2009
- Reviewed by: cpdavis 18/02/2012
Response to question: The candidate has responded extremely well to the question. In the beginning of the coursework he has clearly stated out what he aims to do and how he…
- Essay length: 1954 words
- Submitted: 04/04/2009
- Reviewed by: shreyasi.hait 01/03/2012
This is a good report but a lack of conclusion and some significant errors in analysis of uncertainties have let it down somewhat. The writer initially makes clear what the task…
- Essay length: 1203 words
- Submitted: 05/03/2009
- Reviewed by: k9markiii 08/03/2012
The candidate gives a thorough response to the set topic and clearly describes the different types of electrical hazards including how a person receives an electrical shock. The information provided…
- Essay length: 738 words
- Submitted: 26/10/2004
- Reviewed by: pictureperfect 02/07/2012
Response to question - This was a generally sound report with obvious understanding of how to determine a cell's internal resistance, however it needs work to improve its evaluation of…
- Essay length: 844 words
- Submitted: 06/11/2009
- Reviewed by: k9markiii 17/02/2012
Response to question - To what extent has the student answered the set question? How explicit is their response? The writer has done a good job of answering the question set…
- Essay length: 1102 words
- Submitted: 05/11/2009
- Reviewed by: cpdavis 17/02/2012
Doing circuit calculations
To find the total resistance of a circuit follow these steps.
1) Replace any parallel network with a single equivalent resistor, REQ using 1/REQ= 1/R1 + 1/R2.
Tip: REQ will be lower than either of the parallel resistors R1 or R2 so you can check your calculation.
2) Add all of the series resistors together (including REQ) to find the total resistance of the circuit RT.
- 2 Calculate the total circuit current, IT using IT = V/RT. This current flows through all of the series resistors so the p.d. across each series resistor is given by V = IT R. The p.d. across any parallel network will be IT REQ.
- 3 A potential divider circuit consists of two resistors in series. Follow the same steps as above to find the p.d. across each resistor. Alternatively, R1/R2 =V1/V2 or V1 = V *R1/(R1 +R2) [V = supply voltage]
Which bulb is brightest?
1) If two bulbs are in series, they have the same current. The brighter bulb is the one with greatest power, P. Use P = I2R. The bulb with largest R is brightest.
2) If two bulbs are in parallel, they have the same p.d. across them. Use P=V2/R. The bulb with the lowest R has the highest power and is therefore brightest.
- 1 Use the correct units. If diameter is given in mm, convert to metres before calculating area, A. e.g. d = 1mm so r = 0.5mm = 0.5 x 10-3 m. So A = x (0.5 x 10-3)2 = 7.9 x 10-7 m2.
- 2 Typical questions involve proportions such as what happens to R if the diameter of the wire is doubled? Doubling the diameter would double the radius. Doubling the radius would quadruple the area. So the resistance would decrease to ¼ of the original resistance. The same argument explains why a thinner wire has a higher resistance.
Applications of resistivity:
1) A rheostat is a resistor made by winding a wire around a cylindrical tube. A sliding contact changes the length of the wire carrying current and therefore changes the resistance, R.
2) A strain gauge, has a resistance that increases when it is stretched because the wire from which it is made increases in length.
3) The battery tester on the side of some AA batteries works by using a shaped conductor. The thin end has lowest A, therefore highest R. Current is the same at all points, the thin end gets hottest (P = I2R) and a thermochromic ink becomes transparent, revealing a display.
- 1 Many students find internal resistance a difficult concept. However the circuit is similar to a potential divider. Think of the circuit as a cell of emf E, in series with an internal resistance, r and an external resistance R. When current, I flows through the circuit, E = Ir + IR. This is Kirchhoff’s 2nd law.
- 2 Using a voltmeter to measure the terminal p.d. V, we can rewrite the equation E = Ir + IR as E = Ir + V and then rearrange to give V = rI + E which is the equation of a straight line. A graph of V against I gives a straight line of gradient -r and intercept E. This is how to find the emf experimentally.
- 3 When the current through the cell is high, there is a large drop in the terminal p.d. The difference between the cell emf and the terminal p.d. is called the ‘lost volts’ and equals Ir.
- 4 Short circuiting the cell will lead to a large drop in external voltage and large amount of power dissipated in the cell as P = I2r.
- 5 A car battery (lead acid) is designed to supply large currents. When switching on the engine the current is large and there will be a large drop in terminal p.d. and this will cause lights to dim momentarily.
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