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Module 4 - Chapter 10 - Electrical Circuits II - Coggle Diagram
Module 4 - Chapter 10 - Electrical Circuits II
Sensing circuits
Producing a varying V out
Using a pair of fixed resistors in series in a potential divider has the effect of splitting the pd
Replacing one of the fixed resistors with a variable resistor can vary V out
Temperature sensing circuits
Using a thermisotr allows V out to vary depending on the temperature of the surroundings
As temperature increases, the resistances of the thermistor decreases
Light sensing circuits
Using an LDR allows V out to vary depending on the light intensity of the surroundings
Increasing light intensity decreases the resistance of the LDR so pd across it decreases
If R1 is the LDR, then increasing light intenstiy means that R2 receives a greater proportion of the pd and V out increases
Potentiometer
Potentiometer can create a varying pd
Its a variable resistor with three terminals and a sliding contact
Moving the contant varies the pd between two of the terminals, giving a variable V out
Moving the contact towards A increase V out until it equals V in
Moving the contact towards B decreases V out until at B where it is zero
A dial could be used rather than a slider to make the potentiometer even more compact
Internal resistance
Whenever there is current, work has to be done by the charges as they move through the power source
Pd measured at the terminals of the power sources is less than the actual emf, this is lost volts (Energy per unit charge lost across the power supply)
If a large current is needed, a power source with a small internal resistance is required
Equations
Lost volts = I x r (internal resistance)
EMF = V + Ir
EMF = IR + Ir
EMF = terminal pd + lost volts
Increasing current means more charge travels through the cell each second and more work is done by the charges, increasing the lost volts. EMF doesn't change
if r is constnat, the current in the power source is directly proportional to the lost volts
EMF is always more than the terminal pd unless there is no current
Connecting cells
Connecting cells in series increase the available emf, but also increases internal resistance
Same cells connected in parallel produce the same emf as one cell, but have a much smalled internal resistance, so provide a greater current
Investigating
V = -rI + E
As the current increases, the terminal pd drops and the lost volts increases
Graph of V against I
Doubling the EMF and keeping the internal resistance the same will double the y intercept
Having the same EMF and halfing the internal resistance will half the gradient
High or Low
High voltage power supplies have high internal resistances to prevent the power supply from delivering a fatal current
Small internal resistances allows device to be charged without overheating or wasting alot of energy
Resistance within a power source due to the materials and components which make it up
