Transcript unit61ppt - Macmillan Academy

Voltage current and resistance
Formula Voltage = Current * resistance
V
=
I
* R
p.d units = Volts
Current
=
Amps
Resistance = Ohms
A variable resistor is a length of very thin wire
It can be used to control the current
The higher the resistance the lower the current
Coil of
thin wire
Current travels
only a little way
along the coil
(high current)
Current travels a long
way along the coil (low
current)
Ohmic and none Ohmic resistors
1. Ohmic resistors gave a constant resistance (as long as they don’t heat up)
Voltage
Resistance = change in Voltage
change in current
Current
2 Non Ohmic resistors have a variable resistance, i.e. a filament light bulb
V
I
The gradient increases
because the resistance
is increasing as the
filament gets hotter
Potential divider (splits the voltage)
The resistors are the same size so the voltage is split evenly
12V
6 ohms
6 ohms
V = 6V
0V
The larger one resistor is compared to the other the greater the voltage it will take
12V
12 ohms
6 ohms
0V
V = 4V
Formula to calculate the output voltage
(Given in exam)
V in
R1
R2
V out
0V
V out
=
V in x
R2
(R1 + R2)
Semi conductors
Light dependent
Resistor (LDR)
As the light increases its resistance
decreases
Thermistor
As the temperature increases its
resistance decreases
LDRs and thermistors affecting circuits
As the light increases the
M
decreases
Resistance if the LDR _______
increases
So the current __________
And the motors speed increases
As the temperature increases the
M
decreases
Resistance if the thermistor _______
increases
So the current __________
And the motors speed increases
LDRs in potential divider circuits – automatic night lights
V in
V out – (to the light circuit)
0V
WHY DOESN’T THE LIGHT COME ON DURING THE DAY?
When it is light the LDR has a low resistance so does not
take enough voltage to power the light circuit
When it is darker the LDR’s resistance increases, eventually
it takes enough voltage to power the light circiut
Thermistors in potential divider circuits – temperature
controls
V in
V out – (to the cut off
switch)
0V
When it is cold why doesn’t the cut off switch operate?
When it is cold the thermistor has a high resistance so there
is not enough voltage for the fixed resistor to operate the
switch
When it is warmer the thermistor resistance decreases so it
takes less of the voltage, eventually the fixed resistor takes
enough to operate the switch.