AS and A Level: Electrical & Thermal Physics

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Doing circuit calculations

  1. 1 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. 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. 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]
  4. 4 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. 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. 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.
  3. 3 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.

Internal resistance

  1. 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. 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. 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. 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. 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.

404 AS and A Level Electrical & Thermal Physics essays

  • Marked by Teachers essays 5
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  1. Marked by a teacher

    Internal Resistance of a cell

    5 star(s)

    This is a 5 star piece of work investigating the emf and the internal resistance of an old and a new cell. It also investigates the effect of different combinations…

    • Essay length: 2385 words
    • Submitted: 21/07/2007
    • Marked by teacher: (?) Patricia McHugh 13/05/2013
  2. Marked by a teacher

    Sensing project

    5 star(s)

    This is an excellent piece of work and is five stars rating. Excellent introduction explains all options of investigating this problem and explains the choice of the rotary potentiometer clearly.…

    • Essay length: 1875 words
    • Submitted: 02/02/2005
    • Marked by teacher: (?) Patricia McHugh 13/05/2013
  3. Marked by a teacher

    viscosity of golden syrup

    4 star(s)

    This is a 4-5 star piece of work. It has an excellent plan and lots of detail. The results are excellent and the calculations are very detailed considering all possible…

    • Essay length: 3734 words
    • Submitted: 23/03/2009
    • Marked by teacher: (?) Patricia McHugh 13/05/2013
  4. Marked by a teacher

    Determining Avogadro's Number Lab

    4 star(s)

    This is a 4 star piece of work with excellent calculations and clearly laid out steps to calculate Avogadro's number. The layout of the investigation needs to be improved with…

    • Essay length: 1133 words
    • Submitted: 13/11/2007
    • Marked by teacher: (?) Patricia McHugh 13/05/2013
  5. Marked by a teacher

    Experiment to find the specific heat capacity of an aluminium block.

    4 star(s)

    This is a 4 star piece of work because it is very detailed with excellent scientific calculations used throughout and a good evaluation of results. The reason this is not…

    • Essay length: 616 words
    • Submitted: 22/06/2005
    • Marked by teacher: (?) Patricia McHugh 13/05/2013
  6. I aim to find out how distance affects the light intensity emitted from an LED.

    • Essay length: 1739 words
    • Submitted: 02/02/2004
    • Awarding body: Not known/Not applicable
  7. Peer reviewed

    Measurement of the resistivity of Nichrome

    5 star(s)

    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
  8. Peer reviewed

    Experiment: Decay of Charge in a Capacitor

    4 star(s)

    Layout - All the components of a good report is there, the layout is clear and the section headings helpful; but a better flow from one section to another would…

    • Essay length: 1606 words
    • Submitted: 09/10/2011
    • Reviewed by: (?) superwiseman 01/03/2012
  9. Peer reviewed

    Investgating resistivity - Planning and Implementing

    4 star(s)

    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
  10. Peer reviewed

    investigating the relationship between the diameter and the current in a wire at its melting point

    4 star(s)

    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

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