Unit 1: Section 2 Sensing

A summary of chapter 2

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  • Created by: R_Hall
  • Created on: 20-03-13 13:17

Charge, Current and Potential Difference

  • Electronic sensors (eg. microphones, thermistors and electron microscopes) are designed to sense things we cannot. A change in the stimulus will change the current in the connected circuit. The current is processed to give a reading
  • Current is the rate of flow of charged particles- flows from + to - (opposite to electron flow)
  • One coulomb (C) is defined as the amount of charge that passes in 1 second when the current is 1 ampere
  • To measure current, an ammeter needs to be attached in series (so current in ammeter= current through component)
  • Potential difference (or voltage) is defined as the energy converted per unit charge moved
  • Power is defined as the rate of transfer of energy (the rate of work done). It's measured in watts (W), 1 watt= 1 joule per second
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Resistance and Conductance

  • If you put potential difference across an electrical component, a current will flow. How much current you get depends on the resistance of the component
  • Resistance is a measure of how difficult it is to get a current to flow through it. It is measured in ohms.
  • Power dissipation is the rate that a component converts electrical energy into other types of energy eg. heat
  • The I/V characteristic refers to a graph which shows how current flowing through a component changes as potential difference is increased. The shallower the gradient, the greater the resistance. A curve shows a changing resistance
  • Ohm's law- Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it
  • Conductors which obey this rule are Ohmic conductors
  • Doubling pd doubles the current -> resistance is constant. Factors such as light and temperature effect resistance- must be constant
  • A thermistor is a resistor with a resistance that depends on its temperature. Temperature increases - resistance decreases
  • Sensitivity of any sensor is the change in output value measured from a given change in the input variable
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E.m.f.and Internal Resistance

  • Resistance comes from electrons colliding with atoms and losing energy
  • Internal resistance comes from the collision of electrons with atoms inside the battery. It is what makes them warm up when they are used
  • Electromotive force (e.m.f.) is the amount of electrical energy the battery produces for each coulomb of charge. Measured in volts
  • Terminal p.d. (V) is the p.d. across the load resistance is the energy transferred when one coulomb of charge flows through the load resistance
  • If there was no internal resistance, the terminal p.d. would = the e.m.f.. There is always some energy lost though
  • The energy wasted per coulomb is called the lost volts
  • Most high current power supplies lead a low internal resistance
  • High voltage power supplies have very high internal resistances- when short circuited, only a small current can flow- safer
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Conservation of Energy and Charge in Circuits

  • As a charge flows through a circuit, it doesn't get used up or lost- the charge is conserved
  • Energy is conserved too. Energy transferred to a charge is e.m.f. and the energy transferred from a charge is potential difference. These two quantities must be equal for energy to be conserved (which they are)
  • Series circuits- same current at all points, e.m.f. split between components
  • Parallel circuits- current is split at each junction, same p.d. across all components
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The Potential Divider

  • At its simplest, a potential divider is a circuit with a voltage source and a couple of resistors in series
  • The potential of the voltage source is divided in the ratio of the resistances eg. if you had a 2 ohm resistor and a 3 ohm resistor, you'd get 2/5 of the p.d. across the 2 ohm and 3/5 across the 3 ohm
  • You can chose the resistances to get the desired voltage across one of them.
  • This circuit is used for calibrating voltmeters, which have a very high resistance
  • An LDR (light dependent resistor) has a very high resistance in dark, but low in light
  • An NTC thermistor has a high resistance at low temperatures, and a low resistance at high temperatures
  • Either can be used in a resistor in a potential divider, giving an output voltage that varies with light/ temperature. Add a transistor to make a switch
  • A potentiometer uses a variable resistor to give a variable voltage (eg like in the volume control of a stereo)
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