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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
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    viscosity of golden syrup

    4 star(s)

    multiplying it by gravity and then the density of steel to calculate the weight of the ball bearing and calculate the downward force on the object. This came about as m=?V and W=mg. Substituting in W= ?Vg. The volume of a sphere = 4/3?r3. Substituting in again, W=4/3?r3?steelg. r= radius of sphere (m) ?steel=density of steel (7.8 g/cm3) g=gravity (9.81m/s2) However, there is more than this acting on the ball bearing. Viscous drag is a force opposing the weight of the object and this is calculated by using Stokes' law.

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  2. The aim of the experiment is to verify the maximum power theorem and investigate the efficiency with which energy is transferred from a source of e.m.f to a load resistance.

    In the above cases, we know that friction can be mainly divided into two types, which are static friction and kinetic friction. Static friction occurs when two surfaces are in contact but without any relative motion. It always acts in a direction along the surface and tends to oppose the relative motion. The magnitude of static friction increases with the applied force to oppose the star of motion, until reaches a certain value called limiting friction. Therefore, the applied force must be same as or larger than the limiting friction in order to make the object to move.

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  3. The potato - a source of EMF

    What is EMF? EMF is the electrical pressure that pushes current around the circuit and is measured in Volts. Although EMF is measured in Volts it is not the same as Potential difference, which is the electrical pressure difference across an electrical component. Some of the energy given by the potato cell is used up in the cell itself through having to push the electrons through the system; this is called internal resistance. I imagine that the greater the distance between the two electrodes the greater the internal resistance as more energy will be needed to push the electrons a further distance through the system.

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  4. Sensors cwk. The aim of this coursework is to construct a potential divider circuit with a light dependent resistor (LDR), and observe how light intensity affects the voltage output

    and the resistance decreases as the brightness of light falling on the LDR increases. There are two main types of light dependent resistors, the negative co-efficient, which causes the resistance of the device to fall as the light falling onto it increases. Whereas, some other LDR's work in the opposite way i.e. their resistance increases with light, called positive co-efficient LDR's. For, the purpose of this coursework we are only going to need to now about the negative co-efficient. If light falling on the negative co-efficient device, is of a high enough frequency, photons absorbed by the semiconductor gives bound electrons enough energy to jump into the conduction band.

    • Word count: 4605
  5. resistivity if a nichrome wire

    1 Nails N/A N/A I will use these to hold the nichrome wire to the meter stick. 2 Dependant and independent variables Independent variable- The independent variable throughout this investigation will be the length of the nichrome wire. I will change the lengths to determine the voltage for each length. These results will therefore aid me in resolving what the resistance in the wire is. Controlled variables All there variables must stay constant in order to prevent them from affecting the dependant variables, and therefore creating anomalous results. What to be kept constant Why is it kept constant How it's going to be kept constant.

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  6. physics sensor coursework

    As a result, my prediction is that as the light intensity increases, the voltage output will decrease. Methodology The investigation will be carried out in the laboratory. This is so that external factors - except for the independent variable - remain relatively constant. Such factors that need to be controlled include the temperature of components and humidity of external environment, although the Wheatstone Bridge arrangement already overcomes these. I will take a range of seven values between 0 lux and 60 lux.

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  7. Using an LDR to detect the intensity of plane polarised light allowed through a Polaroid.

    Since a Polaroid plane polarises the wave, I will be able to use that. Unfortunately, this has the effect of halving the intensity of the light; increasing the need for a more sensitive resistor. There were two other methods I considered: Lasers are already polarised, so I could have used a laser to pass through a rotating Polaroid. However, the problem with this is that although I am not losing any of the original light by passing it through a Polaroid, the laser's initial intensity is much lower than the intensity of the original light bulb.

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  8. Physics - Resistivity

    The thickness of the wire would also affect the resistance of wire, a thick wire of the same material, length and temperature would have less of a resistance of a thinner wire, this is because there is more room for more electrons to be able squeeze together, creating more 'lanes' for the electrons to be able to pass through, this increases the current, which for if the voltage was the same, would lower the resistance, we can see this using the simple formula: Where V is voltage and I is current.

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  9. Investigating the effect of 'length' on the resistance of a wire

    I chose this because it is the most practical to carry out within the laboratory. Not a lot of equipment is needed and it can be controlled without complications as I would have to use one wire and just keep changing the length by connecting one end of the circuit to the wire at the length required. It will also produce reliable results where trends can be observed easily. Had I chosen any other variable, for example the diameter (or cross-sectional area) I would have had to use wires of different diameter, this can be very expensive and impractical.

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  10. Choosing a light source

    They are more expensive to buy but last up to two thousand hours. They can be either 240V bulbs, which are usually tubular and often used in up lighters and outdoor floodlights, or low voltage bulbs typically used in down lighting. All halogen lamps require special light fittings. Advantages: Low voltage halogen lamps are slightly more efficient than normal bulbs of the same wattage, but they use a transformer that can consume from 10 to 30 percent of the bulb energy, reducing the efficiency gain. More efficient electronic transformers are available which reduce transformer losses.

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  11. Electronics CW2

    With the numbers showing faster than the human eye can see to choose them. Therefore making it impossible to choose numbers. So it must count faster than .. I will research this later... Must operate from a 9V battery, as these are commonly available. Must consume as little power as possible, otherwise battery life will be short. So have current consumption of less than 200mA. Possible Solutions: My specification can be achieved in a few ways... I) A button is pressed, which triggers 7 numbers each on there own 7 seg display. Each number would have to be own timed.

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  12. I am going to investigate what the resistivity is of a pencil lead. Pencil lead is made from a combination of finely ground graphite and clay, mixed with water and pressed together at high temperatures into thin rods

    If this were to happen I would use the results from the point (0,0) up until the line starts to curve. This would mean that I could take the gradient from the straight line as if it was an ohmic conductor and use it for the resistance. Safety Electricity: - Electricity is dangerous at any time and especially near water. For this reason I will keep the experiment away from any source of water and turn off the power when not in use. Temperature of the pencil lead: - during the experiment the pencil lead is likely to become hot so I will be careful when touching the pencil lead and keep objects away from it.

    • Word count: 4209
  13. Find The Internal Resistance Of A Power Supply

    Any power supply will have internal resistance because of the material it is made from. This material (conductor), under normal circumstances will have resistance, also called internal resistance (r). The internal resistance of the power supply obeys Ohm's law, which states that for an ohmic conductor the potential difference is proportional to current. Therefore as a current flows through the circuit, there is a drop in potential difference across the internal resistance, which is called the lost volts. The potential difference across the power supply is known called the terminal potential difference (Ir).

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  14. Investigating the monitoring systems used on modern day large A/C for detection of specific variables.

    These problems can be overcome by the selection of a protective sheath that does not react with the atmosphere or fluid. Although they do give a direct voltage, this is generally small - in the order of mill-volts - and often requires amplification. Advantages of using thermocouples are that temperature at localised points can be determined, because of the small size of the thermocouple, and they are robust, with a wide operating range from -250�C to 2600�C. The most suitable instrumentation output for Exhaust Gas Temperature would be an analogue display unit.

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  15. How does the mass of copper plated in the electrolysis of copper sulphate solution vary with the current used?

    I also found out that the bigger the current used the more mass deposited on the cathode. Furthermore, I found out that that the amount of substance formed is directly proportional to the size of the current and for how long it flows. From this experiment I got a good idea of what the values I am going to use and how I should control my other factors and how important they are. Also I am going to use my preliminary experiment to compare it to the real one and the repeat.

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  16. To Investigate How the Resistance of the Light Dependent Resistor Depends On the Current In the Light Bulb?

    Power is the rate of transfer of electrical energy, it tells you how quickly the energy is used up. The units are measured in Watts (w). Electrical power is also related to current and resistance, as Power = Current� * resistance As the resistance of the light bulb doesn't change too much (except the temperature of the light bulb may affect the resistance, but we do not really need to take account this in this research), the power is mainly dependent on the current.

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  17. Investigation into the resistance of a filament lamp.

    Incandescence is defined as the emission of visible light by a hot body. Any hot object gives off incandescent light. The higher the temperature of the object, the brighter the light given off. Examples of incandescent objects include hot coals in a campfire or barbecue grill, the sun, light bulb filaments and the burners on electric stoves, which glow dull red when their temperature is on high. (harison-toshiba.com) Resistance Definition: The resistance between two points in a circuit is the ratio of the potential difference between the points to the current flowing between them.

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  18. Experimenting with Thermocouples.

    Accordingly, the other junction that actually measures the temperature is commonly referred to as the "hot junction". When there is a difference in temperature between the junctions, a very small voltage is produced and a very small current will flow. Figure 1 A Copper-Constantan thermocouple I decided to analyse the relationship between the temperature difference across the junctions of a thermocouple, and the magnitude of the current produced. My aim is to find the formula linking current (in Amperes) to the temperature difference (in centigrade1). From this, I can determine the sensitivity of the thermocouple, measured in Amps/ oC.

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  19. Investigating the E.m.f and Internal Resistance of 2 cells on different circuit Structures.

    This causes the charge circulating in the circuit to dissipate some electrical energy in the power supply itself. The power supply becomes warm when it delivers a current. Diag 3: The circuit above shows a power supply which has e.m.f (E) and internal resistance (r). It delivers a current I when connected to an external resistor of resistance R, also called the load. Vr is the potential difference across the internal resistance. Kirchoffs second law states: E = Vr + VR The potential difference VR across the load is thus given by VR = E - Vr VR is called the terminal potential difference.

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  20. To investigate the factors which may affect the resistance of resistance putty.

    I also predict my graph will look like this: I don't think I will get results which are directly proportional (This is when one of the quantities is doubled, and the other quantity also doubles. E.g. If Resistance is doubled, length would also be doubled). I also don't think I will get results which are inversely proportional (This is when one of the quantities is doubled, the other quantity halves. E.g. If length is doubled, the resistance would half). Instead, I think I would get results which are neither of these but have a linear relationship (i.e.

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  21. Characteristics of Ohmic and non-Ohmic Conductors.

    As the number of free electrons increase, resistance decreases. Ohmic Conductors Metallic Conductors A metal contains a large number of almost 'free' electrons. This property makes metals extremely good conductors of electricity. But as the temperature of the metal increases the ions in the metal vibrate vigorously, colliding with the free flowing electrons, obstructing the flow of electricity, thus as temperature increases, resistance increases as well. Metallic conductors are Ohmic Conductors. In ohmic conductors, Current is always proportional to the voltage.

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  22. Build a successful sensor that will measure the proximity of a light source.

    Also if a different box was used it may be a different colour or shade so would absorb or reflect a different amount of light from the light source, meaning the photodiode would detect more or less light. To limit this interference of light absorption or reflection of light, the same box would have to be used throughout. After discussing my equipment list with the technicians, they do not have a completely opaque, light stopping box, but do have a spare dark room, or an opaque, telescopic black tube in which a light source can be placed.

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  23. Characteristics of Ohmic and Non Ohmic Conductors.

    The free electrons are those electrons that are transferred from a positive to a negatively charged ion. This is called the free electron theory. Electricity is basically the transfer of electrons from one atom to the other. In conductors, the atoms are moving in different directions. But when the potential difference is applied between the ends of the conductor, the atoms line up and move to one direction. Electric conductivity means that a certain substance can carry an electric current. Current is a flow of electrons. So the material that can conduct electricity has to have free electrons to carry this current.

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  24. Single Phase Transformer (Experiment) Report.

    for distribution to industrial consumers and finally to 230V for domestic purposes. Power system transformers are usually 3-phase devices, but their basic concepts are most easily demonstrated using a single-phase transformer. Transformers change voltage through electromagnetic induction; i.e. as the magnetic lines of force (flux lines) build up and collapse with the changes in current passing through the primary coil, current is induced in another coil, called the secondary. The secondary voltage is calculated by multiplying the primary voltage by the ratio of the number of turns in the secondary coil to the number of turns in the primary coil, a quantity called the turns ratio.

    • Word count: 3561
  25. To investigate how the temperature affects the resistance of a thermistor.

    The voltage in a circuit gives the electrons the energy to move and as more voltage is applied and the potential difference across the atoms increases it gives the atoms that are not in the outer shells the energy to push against the force from the nucleus and effectively jump into the band of moving electrons in the outer shells. This applies to the band theory which explains why metals conduct electricity so easy, why insulators conduct virtually no electricity whatsoever and the behaviour of a thermistor.

    • Word count: 3302

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