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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