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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.

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  1. Gcse Physics Course Work (Resistance)

    I also have found some useful formula such as this should help me through out my final experiment. AIM: My aim of this project is to find out more about resistance and if it reacts to electricity being past through the paper and if width and length affect it in any way HYPOTHESIS: My hypothesis is that the resistance will obey ohms law and I believe the graph and the results will show that the resistance paper obeys ohms law the graphs will tend to suggest that the paper does obey ohms law.

    • Word count: 1236
  2. Investigating LDRs

    We will be repeating each concentration three times so all together we will have 24 sets of results. This will make it a fair test. I will be changing the amount of light going to the LDR by moving the LDR further away from the light that means that I will be changing the distance. I will by measuring the current that flows thought the circuit by using a milliammeter. To get a reading the light has to shin on the LDR (semi conductor)

    • Word count: 770
  3. The factors affecting the current flow through a conductor at a constant temperature.

    Voltage is the amount of energy transferred by each coulomb of charge as it passes round the circuit. Resistance affects the flow of electrons in a circuit. If you increase the resistance in a circuit it is more difficult for the electrons to flow. It is almost like an athlete running a track, he runs at a constant speed until he reaches some hurdles, this slows him down because they are obstacles to avoid, it is exactly the same in a circuit for the electrons. The components in a circuit can also increase resistance. The electrons have to flow through more hurdles/ resistance, slowing it down and decreasing the reading on the ammeter.

    • Word count: 1759
  4. Find out the factors affecting the strength of an electromagnet.

    These place were the magnetism appear to be concentrated are called the poles of the magnet. INDUCED MAGNETISM (1) IF a piece of ferromagnetic substance is made to touch or brought near to the pole of a magnet, it becomes a magnet itself. This is called induced magnetism. This end of the substances nearest to the inducing pole has an opposite pole inducing in it. A chain of pin can be attached to a bar magnet, with each induced pole inducing and opposite pole next to it. If the South Pole of a magnet is brought near the bottom of the lowest pin in fig1.2, the pin will be repelled showing it is a South Pole.

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  5. Investigate methods of finding and comparing the e.m.f and internal resistance of different cells and power supplies and factors that can affect these values.

    Method 1. Set up the equipment as shown above, use a milliammeter. 2. Ensure you set resistance to highest value. 3. Record the terminal p.d and current for different value resistance. 4. Display results in a suitable form (terminal p.d against current) 5. Repeat experiment with another power source. Results Source1: Mitsubishi R20-1.5V Resistance/? Current/mA Voltage1/V Voltage2/V Average Voltage/V 500 2 1.36 1.36 1.36 450 3 1.36 1.36 1.36 400 4 1.36 1.36 1.36 350 4.5 1.35 1.36 1.355 300 5 1.35 1.36 1.355 250 6 1.35 1.35 1.35 200 7 1.35 1.35 1.35 150 10 1.34 1.35 1.345 100 14 1.33 1.34 1.335 50 26 1.32 1.32 1.32 Source2: Sony R6-1.5V Resistance/?

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  6. You are to investigate the factors that affect the current through a light bulb.

    Potential difference is measured in volts (symbol V). A voltmeter measures potential difference. Variables: Independent variable: This will be the Voltage as this is the factor that I intend to change. Dependant variable: The Current as this will change because of this. Control Variable: Keeping this the same throughout the experiment will result in a fair test. They are the Resistance, and the length/thickness of the wire, and length of time. Hypothesis: My hypothesis is that the independent variable, the voltage, will change the dependant variable, the current, as voltage is the charge flowing and hence is related to the amount of current.

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  7. To measure the resistance of a current, flowing through different lengths of Nichrome Wires

    At a given temperature, the resistance is proportional to the object's resistivity and length, and inversely proportional to its cross-sectional area. Usually, a material's resistance increases with increases in temperature. Some Electronic circuits are varied in resistance using variable resistors (rheostat). These are used in lighting circuits (dimmer switches) and hi-fi systems (volume control) to vary the current flow. I will use one in my experiment because without one the amperes are too high and the reading goes of the scale Factors Affecting Resistance Length Cross-Sectional Area Materials - The atom structure is different so it easier for electrons to move is some the others.

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  8. Diode Application in Rectifier Circuits

    The AC on each side of the center-tap is ½ of the total secondary voltage. Only one diode will be biased on at a time the prose to be center tap full-wave rectifier is like below. Last is about full-wave bridge rectifier circuit, The Bridge is shown in the figure below. The circuit has four diodes connected to form a bridge. The ac input voltage is applied to the diagonally opposite ends of the bridge. The load resistance is connected between the other two ends of the bridge.

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  9. Energy Efficiency Experiments

    = 9171.998J Useful energy output= Mass x Specific heat capacity x Temperature change Useful energy output = 9.0414J Electrical energy → Heat energy Efficiency = 9171.998 ÷ 11940 x 100 = 77% Useful energy output ÷ energy input x 100 Energy input= Electrical. C:\Users\Hello\Pictures\TPhoto_00007.jpg Output = heat. Useful= heat. Wasted= heat. Energy→ heat. Method –Experiment 3 In this experiment we timed how long the glider went past the light gate. We measured the mass of the glider, height of the ramp at highest end, length of the ramp, measured the length of the card and recorded the time.

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  10. Physics Case Study Solar Electric Panels

    The specific energy level that is required signifies the ?band gap? between the valence and the conduction band. These bands are ranges of energy levels in which electrons rest at when under no excitation. When a photon has a higher energy level than the band gap hits an electron, it will excite it to the conduction band. In this band, electrons have enough energy to dissociate from its original atom and form free electron gas. Each material will have different bands and band gaps, and some don?t even have a band gap. The process of creating an electron-hole pair is most notable in semiconductors as a band gap exists, but is also relatively small.

    • Word count: 819
  11. Geothermal energy is not easily accessible with our current technology. Our main focus was to figure out methods of harnessing this energy with new innovations based off of current technologies

    Each of these processes has its own set of limitations. Basaltic magma is the most common type of magma and it has a low viscosity, which means it is able to flow freely. This low viscosity allows dissolved gasses to escape which prevents gas pressure from accumulating in magma chambers. Because of this, basaltic magma would be the most logical to generate and utilize. Raising the geothermal gradient would increase the formation of magma. As mentioned before, it could be created by radioactive heat, frictional heat, and decompression due to convection.

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  12. The objectives of this experiment are to Verify Kirchhoffs laws Determine the equivalent resistance of a network & Study the maximum power transfer theorem.

    Procedure: The experimental board was connected to the DC power supply as follow 2. The DC supply was adjusted to keep the voltage across Nodes A and C to 5V 3. The voltage across the branched that connect to Node B were measured, the data we took Voltage(V) Current entering Node B(A) =2.925 =0.1330 =-0.796 =-0.0398 =-1.872 =-0.0936 ++=0.1330+(-0.0398)+(-0.0936)=-4xA 4. Step 3 was repeated for Node M, the data we took Voltage(V) Current entering Node B(A) =3.72 =0.06643 =-1.078 =-0.1078 =-0.799 =0.03995 ++=0.06643+(-0.1078)=0.03995=-1.42xA The results we took both are very close to zero.

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  13. Electricity generation from renewable and non-renewable sources.

    Oil forms underground in rock such as shale, which is rich in organic materials. 1. High net energy yield 2. Easily transported within and between countries 3. Low land use 1. Causes air pollution when burned 2. Releases CO2 when burned 3. Increases dependency on countries that are major oil producers Natural Gas Natural gas is a mixture of gases, the most common being methane. It also contains some ethane, propane, and butane. Natural gas is usually not contaminated with sulphur and is therefore the cleanest burning fossil fuel. 1. Less air pollution than fossil fuel 2.

    • Word count: 888
  14. Electrical Energy Questions and Answers

    The charge for one proton is +1.602 X 10-19 C 1. Describe the behaviour of electrostatic charges and the properties of the fields associated with them 2. Define the electric field as a field of force with a field strength equal to the force per unit charge at that point E=Fq 1. An electric field is a region in which a charged particle will experience a force 2. An electric field is a vector quantity, which means it must have both magnitude and direction 3. Strength of an electric field at any point is defined as the size of the electric force action on a unit of charge 4.

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  15. The Purpose of my sensing circuit is to regulate the temperature in a Steam Sauna which operates between the temperatures 30C - 60C

    A rise in temperature thus increases the overall conductance of a Negative Temperature Coefficient thermistor. A simple model is shown below At 30°C the NTC thermistor has few mobile charge carriers At 60°C some atoms ionise and increase mobile charge carrier density of the NTC thermistor KEY Potential Divider A potential divider can be made by connecting at least two resistors in series across a source electromotive force in a circuit. In a potential divider circuit the ratio of resistances determines the potential difference across a resistor. The larger the resistance, the greater the potential difference across the resistor and the smaller the resistance the smaller the potential difference across it.

    • Word count: 3975
  16. As-Physics Investigation, Geo-Physics. the two main principles I will cover are, GPR- or Ground Penetrating Radar and EM- or ElectroMagnetic induction.

    reflected will continue until it is lost or absorbed by other objects.[1] Once the signal has been returned the computer, or a very clever person can calculate the distance and size of the object using the following equation: [2] X) The distance you are calculating. V) The velocity of the wave which can be calculated by: [2] T) The time for the signal to return. An image showing GRP in use to find a pipe [3] Once the computer has gathered all the distances of an object it can product an image that might look a bit like this one: An image showing the computer generated image of the GPR scans [4] -EM (Electromagnetic induction)

    • Word count: 883

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