Unit 1: Section 2 Sensing

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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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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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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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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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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