electrical resistance Flashcards
what is resitance
a measure of the hindrance of the flow of electrons along a conductor
what resistance does a conductor have if the electrons flow easily
low resistance
what resistance does a conductor have if the electrons do not flow easily
high resistance
what is resistance caused by
collisions between electrons and the metal ions that make up a conductor
what happens as a result of the collisions
- electrons lose energy
- metal ions vibrate more
- conductors become hot
what is the equation for calculating resistance
resistance = potential difference (voltage) / current
what is the symbol equation for calculating resistance
R= V / I
what is ohm’s law
voltage is directly proportional to current providing temperature of component stays constant
what is the unit of resistance
ohm
what is the relationship between resistance and temperature
as temperature increases the resistance also increases
what does a fixed resistor do
used when you need to control currents and voltages but not vary them
what does a variable resistor do
allow you to adjust the current and voltage to the required amount
what do thermistors do
resistors that vary their resistance according to temperature
- if the temperature is warm its resistance is low
- if the thermistor is cold its resistance is high
what do light-dependent resistors (LDRs) do
their resistance decreases in the light and increases in the dark
what do diodes and light-emitting diodes (LEDs) do
they only allow current to flow in a single direction
when current freely flows what is the resistance of the diode
low
when the current is prevented from flowing in a diode what is the resistance
high
describe the I-V Characteristics Experiment
- set up the circuit using a battery, ammeter, variable resistor, bulb, and voltmeter
- use the dial on the power supply to set the potential difference to 2.0V. make sure the ammeter and voltmeter give positive readings
- measure and record the current
- measure the current for a good range
- swap around the power supply leads and take measurements for negative potential difference values
- repeat the whole experiment for a different component (LED or Thermistor) but the ammeter must be replaced with a millammeter
- plot the graphs of current against potential difference
what will the graph show for a fixed resistor
the fixed resistor is constant (straight line). This means the gradient of the I-V graph is constant
what will the graph show for a filament lamp
at higher voltages the filament lamp gets hot so the resistance increases. current is less than expected and slope of graph decreases (s-shape)
what will the graph show for a diode
diode only allows current to flow in one direction
- in the reverse direction the resistance is very high so the current is zero
- in the forward direction the line curves towards the I axis because the resistance decreases
what does the graph show for a thermistor
- unusual resistor, its resistance decreases with increasing temperature (opposite of filament lamp)
what would a graph look like for a LDR
- resistance decreases with increasing light intensity
