Measurements

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1.general physics  1.1 length and time  -i-Physical Quantity Physical Quantities A physical quantity is a property of an object that can be measured with a measuring instrument. A physical quantity is usually expressed as “a magnitude ( numerical value)  and a  physical unit or more ” Example :- Normal body temperature of a healthy human being is  37  0 C -ii-Length Measuring Length. Length is “the distance between two points in a space”. According to SI units, the length quantity is expressed in meter. There are several kinds of measuring devices used to measure quantity of length, among others, are as follows. Tools used… a-   Measuring tape. b-   Ruler c-   Vernier callipers. d-    Micrometer Measurement of length Length is measured accurately through many meth...
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Series and parallel circuits

Series and parallel circuits 

  • current at every point in a series circuit is the same 

    Combining Resistors:
    In series:
    Rtotal = R1 + R2 
    In parallel:
    Rtotal = 1/(1/R1 + 1/R2)

    The current splits at each branch in a parallel circuit so the total current is always greater than the current in one branch
    • combined resistance of two resistors in parallel is less than that of either resistor by itself 
      The advantages of putting lamps in parallel are:
      -if one lamp breaks, the other still works
      -each lamp gets maximum PD
      •in series: PD across the supply = PD across all the components combined
      •in parallel: Current across the source = sum of currents in the separate branches

      Action and use of circuit diagrams 

      A potential divider divides the voltage into smaller parts. To find the voltage (at Vout) we use the following formula:
      Vout = Vin x ( R2 / Rtotal ) 


      A variable potential divider (potentiometer) is the same as the one above but using a variable resistor; it acts like a potential divider, but you can change the output voltage.
      •Thermistor: input sensor and a transducer. It is a temperature-dependent resistor. At higher temperature there is less resistance.
      •Light dependent resistor (LDR): input sensor and a transducer. When light intensity increases, resistance decreases.
      •Capacitor: store small amounts of electric charge. If a capacitor has a higher capacitance (in μF microfarads) means they can store more charge. They are used in time-delay circuits.
      •Relay: a switch operated by an electromagnet 


      Diode: a device that has an extremely high resistance in one direction and a low resistance in the other, therefore it effectively only allows current to flow in 1 direction (the arrow on it is pointing in the conventional current direction).Forward bias is when the diode is pointing in the direction of the conventional current and reverse bias is the opposite
      It can be used in a rectifier. A rectifier turns AC current into DC current. 



      In the set up on the right, the transistor will switch on and the bulb will light when the resistance is high in the variable resistor. Using thermistors or a light-dependent resistor instead of the variable resistor, the circuit can act by itself for example a heater can switch on when it gets cold. The transistor will switch on when the voltage exceeds about 0.6V. 


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