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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.

Resistivity

  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.

  • Marked by Teachers essays 5
  • Peer Reviewed essays 9
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  1. Marked by a teacher

    Internal Resistance of a cell

    5 star(s)

    through that circuit defines quantitatively the amount of electrical resistance R. Precisely, R = V/I. Thus, if a 12-volt battery steadily drives a 2-ampere current through a length of wire, the wire has a resistance of 6 volts per ampere, or 6 ohms. Ohm is the common unit of electrical resistance, equivalent to one volt per ampere and represented by the capital Greek letter omega, ?. The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area. Resistance also depends on the material of the conductor.

    • Word count: 2385
  2. Peer reviewed

    Measurement of the resistivity of Nichrome

    5 star(s)

    material (measured in ohms, ?); > L is the length of the piece of material (measured in metres, m); > A is the cross-sectional area of the specimen (measured in square metres, m�). From this equation, I can see that the resistivity will be the resistance over length multiplied by the cross-sectional area. I rearranged the equation and substituted into the straight line equation as is shown below. y = mx + c R = ?L /A R?L The value of resistance of the material is depends on the value of the length.

    • Word count: 2614
  3. Investigation On The Resistivity Of Apples. Since we are measuring the resistance of an apple, when the length increases, resistance will increase. Although we are mostly measuring the water content in the apple, we are also measuring the resistance of th

    The only thing is to be careful with the knife and do not play with it or swing it around. Preliminary method 1. Using the ruler and pencil, mark the places of the apple to cut. 2. Cut three 1 x 1 x 1 cm slices of apple with the fruit knife. 3. Repeat step 2, with 2 x 1 x 1 cm, 3 x 1 x 1 cm, 4 x 1 x 1 cm, and 5 x 1 x 1 cm slices of apple.

    • Word count: 2042
  4. Resistivity of Lightbulbs

    Increasing light levels increases output voltage and this is the light sensor. The Diagram below shows this: Choosing the correct Fixed Resistor: It is very important to ensure the correct Fixed Resistor is installed in the circuit. This is due to the ratio between the fixed resistor and the variable resistor, the LDR, needing to be as wide as possible thus giving an accurate sensor reading across the input range that it will be sensing. It is therefore necessary to trial various resistors and pick the resistance which results in the highest potential difference change across the fixed resistor for the sensing spectrum.

    • Word count: 2527
  5. In this experiment, we will measure the e.m.f. and the internal resistance of a dry cell.

    Simply, A Voltage where the charge is gaining energy is an electromotive force. It is total opposite the potential difference (p.d), which is the energy released when a unit of charge passes between two points. Dry cell is very common in daily life and has a wide variety of usages. In an ideal dry cell, there should be no any resistance. However, in the reality, there is no ideal dry cell and it must have an internal resistance. Internal resistance of a dry cell just like that there is a resistor in the dry cell.

    • Word count: 2557
  6. Sensors Project Report

    It is not very accurate because the thickness of paper varies from one side of the paper to another and from one piece of paper to the other. - Using a log Potentiometer to determine the number of sheets of paper in the sensor by the thickness of the paper (number of papers) and that varies the ratio of the resistance in the potentiometer so the number of papers can be determined by the ratio. From all the choices above, the potentiometer is chosen because it is more related to the work in the syllabus and it is one of the most accurate ways to measure among the others that have been suggested.

    • Word count: 2031
  7. Is polymer electronics the future of TV screens

    Each pixel is then lit up in slightly different colours to create a picture. This technology is now starting to become obsolete, as the public are wanting thinner more efficient TVs, that don't have to be the centre piece of a room, but placed on a wall like a picture, and go reasonable unnoticed. People are also seeking a better quality picture, which is like real life. The picture quality of TVs has change hugely since the CRT screens, but people still demand a better picture. The most recent advance in TV screens is the development of polymer electronics, producing TV screens known as OLEDs.

    • Word count: 2290
  8. Light notes

    The colours of the complete spectrum are red, orange, yellow, green, blue, indigo and violet. The atoms in an object control which frequencies are absorbed and which are reflected. There is no colour in objects. Which frequencies of light are reflected by an object controls what colour you see. The colour of a filter depends on which of the frequencies it allows to pass through. We call it red light but it is just the light with the frequency that causes the brain to make you think the object is red. The primary parts of the complete spectrum are red, green and blue.

    • Word count: 2164
  9. Finding the Resistivity of a Wire

    Next I will fix the wire to the ruler. I will start by tightly fixing one end of the wire to one end of the ruler using sticky tape. Then I will ensure the wire is straight and tight before fixing it at a point approximately 0.35m down the ruler. I will repeat this at 0.65m and at the end of the metre ruler. Subsequently, I will connect the circuit as shown in the diagram, with the wire connected between 1.00m and 0.00m.

    • Word count: 2530
  10. Investigating Internal Resistance

    The main variable in this experiment is the external resistance. As the external resistance increases the voltage will decrease. This is because fewer electrons can pass, also this increase in resistance will cause an increase in current. Measurements: In this investigation the voltage and current levels as resistance changes will be recorded. The results for each resistance will be recorded 3 times for both voltage and current to improve accuracy. These results will also be recorded to a two decimal place degree of accuracy. Apparatus...in detail: Power pack: The power pack used was the same throughout.

    • Word count: 2097
  11. A2 Viscosity investigation

    (ensure the first line is at least 3cm from the top of where the syrup is filled to as it needs time to reach terminal velocity) 4. Place the ball bearing into the syrup then get to eye level of the first line to ensure maximum accuracy and then start the timer when the bottom of the ball bearing touches the line previously drawn. 5. Kneel until you are at eye level of the second line and wait until the bottom of the ball bearing touches the line and stop the timer.

    • Word count: 2351
  12. Investigating the Smoothing Effect of a Capacitor on a Resistive Load

    1.2 100 0.05 6.0 6.0 0.300 0.255 100 1.2 100 0.05 3.4 3.4 0.170 0.120 220 1.2 100 0.01 7.5 7.5 0.075 0.055 470 1.2 100 0.005 6.3 6.3 0.031 0.026 1000 1.2 100 0.005 3.0 3.0 0.015 0.012 The experiment was repeated and more readings recorded in order to diminish the possibility of anomalies and errors occurring and to obtain more reliable results. Problems encountered Using the oscilloscope required us to ensure that the trace that appeared on the screen would be appropriate to read from and to calculate such values as the frequency.

    • Word count: 2664
  13. Free essay

    Resistance of a wire

    It will also give clear and concise readings that can be trusted and used for comparing/observing trends. I chose not to measure the width of the wire and presume that the wire is of the stated thickness of 28 swg Nichrome wire. For this experiment I will assume that the wire is perfectly round at 0.38mm diameter. FORMULA'S Charge is measured in coulombs and charge is the amount of current that flows every second. Electric current is measured in AMP's (A), Charge in COULOMBS (C)

    • Word count: 2555
  14. silicon project

    Talk about Silicon picture on board (crystalline type) Slide 3 Here are the contents of my presentation. Slide 4 Carbon, silicon and germanium (germanium, like silicon, is also a semiconductor) have a unique property in their electron structure -- each has four electrons in its outer orbital. This allows them to form nice crystals. The four electrons form perfect covalent bonds with four neighbouring atoms, creating a lattice. In carbon, we know the crystalline form as diamond. In silicon, the crystalline form is a silvery, metallic-looking substance.

    • Word count: 2504
  15. How different factors affect the resistance of a wire

    The limited amount of types of wires will not allow me to have enough readings and therefore restrict the range of my results. I have decided not to investigate the resistance of a wire by varying the cross sectional area as it is extremely difficult to measure the diameter of a wire accurately with the given equipment. However, the widths of the wires are given to us on the packaging. It is written as SWG (Standard Width Gage) and the higher the number, the thinner the wire.

    • Word count: 2414
  16. The varying of the resistance of nichrome wire depending on its length

    Therefore resistance is calculated by dividing the current into the voltage. Hence, the formula would be: Resistance = Voltage/Current =V/I Ohms = Volts/Ampere (Amps) There are four factors which affect resistance. Length: As explained before, as the length increases so does the resistance. This is because of the increased number of atoms, but also because the longer it is the more its width is restricted, which therefore restricts the movement of electrons which carry electric charges. As the electrons move along a lengthy wire, this will cause resistance because they will have to move slowly due to the longer length in which they must travel.

    • Word count: 2597
  17. AimI am trying to find out how the current changes with the area of the solar cells.

    The positioning of the lamp - if this is changed; it could also affect the number of photons hitting the solar cell for each reading. The lamps must point at the solar panel only. 3. The time - this, would have to be accurate i.e. not leave the solar panel under the lamp for too long or it could affect the temperature and the intensity of the light producing photons on the solar cell. 4. The light source - The intensity of the light source will be kept the same to prevent more or less photons hitting the solar cell for each reading.

    • Word count: 2657
  18. Investigating Ohms law

    This means that voltage and current are directly proportional to each other, which is what Ohms law states. So I hypothesise that the metal wire will obey Ohms law. Apparatus I will be using the following apparatus in order to set up my experiment: Connecting wires: I used connecting wires to connect the apparatus I used in the experiment and to provide a path for the current to flow through. Power pack: I will use a power pack instead of batteries as the batteries can be run down and may not provide a constant voltage.

    • Word count: 2105
  19. A coursework on sensing temperature with voltage.

    They do not really alter the results. Rather they tend to move the whole data and the values are not affected singularly. Good sensors should have as little systematic error as possible. However today there are 'smart' systems which process information to compensate for systematic error. Handling data: The data gotten from a good sensor must be adequately processed often with appropriate averaging, analyzed and displayed effectively so that it is generally comprehensible. Creativity: It should be creative, experimental, spontaneous even!

    • Word count: 2213
  20. Investigating Electricity.

    (Voltage = Joules per Coulomb = Volts) * Ohm's Rule: - Ohm's law, named after its discoverer Georg Simon Ohm, states that the potential difference (or voltage drop V) between the ends of a conductor (for example, a resistor R) and the current, (I) flowing through R are proportional at a given temperature: where V is the voltage and I is the current; the equation yields the proportionality constant R, which is the electrical resistance of the device. Aim: My aim in this coursework will be to do some experiments To measure electrical circuits I will be carrying out an

    • Word count: 2351
  21. Physics coursework

    Instead of using a metal I decided to use a thermistor as it can be subjected to different temperatures and I can expect some results Aim: The aim of my investigation is to see whether the resistance of a thermistor (a type of temperature sensitive resistor) is directly proportional to the temperature. I'm anticipating that this experiment will be interesting and relatively challenging because a thermistor is a type of resistors whose resistance changes significantly when its temperature changes, consequently a slight variation in temperature can lead to a vast change in the resistance, therefore I have to be extremely accurate when calculating the temperature as a mistake or inaccurate reading on the thermometer will affect my results considerably.

    • Word count: 2107
  22. Operational amplifiers (opamps)

    A basic opamp with negative feedback is shown below in figure 1: Figure 1: A basic operational amplifier with negative feedback The output voltage of an opamp with negative feedback can be shown to be proportional to its input. However there is a limit to this proportionality as we will see later. For a more detailed explanation of the mechanism of an opamp with negative feedback, please refer to appendix 1. Similarly, an opamp is said to have positive feedback when its output is connected back to the positive input terminal also usually through some intermediary components.

    • Word count: 2142
  23. Using a sensor to measure an angle.

    Really top quality hi-fi equipment sometimes uses a chain of fixed resistors in between gold-plated contacts to beat the noise problem Another use for these devises is in car fuel gauges. In a car the fuel gauge doesn't actually measure the amount of fuel in the tank. Instead, it measures the level of fuel in a tank. A fuel gauge can use a potential divider to detect the position of the float. When petrol is used up the float goes down.

    • Word count: 2164
  24. Experiments with a thermistor

    Since semiconductors generally behave this way, they are used to manufacture thermistors. They can also be manufactured from the oxides of transition metals such as manganese, cobalt, copper and nickel. Thermistors are widely used in potential dividers and in Wheatstone bridge circuits to supply varying potential differences which is done by altering the resistance of the thermistor. OBJECTIVE For this sensor project, I have chosen to acquire a series of measurements using a temperature sensor called a thermistor, and design a circuit for an application of a temperature sensor. Due to availability of various types and capabilities of temperature sensors, I had to perform small experiments to decide on which sensor to use.

    • Word count: 2591
  25. An experiment to investigate how the resistance between two copper plates immersed in copper sulphate solution varies with the surface area of the plates.

    This is also the same as Resistance being directly proportional to the reciprocal of the surface area of the copper plates and hence to the volume of liquid between them. Equipment Here is a list of equipment which should be set up as in diagram 1: * 1 power supply set at 4 volts * 1 beaker * 2 copper plates * 1 ammeter - 1 to 10 amps * 1 voltmeter - 1 to 20 volts * 1 connection block * 1 measuring cylinder * 1 ruler * 5 wires * 1 thermometer Plan For this experiment, different depths of 0.5M copper sulphate solution were put into a beaker with two copper plates.

    • Word count: 2147

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