• All materials have some?resistance?to the flow of charge
• As?free?electrons?move?through a metal wire, they collide with ions which get in their way
• As a result, they?transfer?some, or all, of their?kinetic?energy?on?collision, which causes electrical?heating

Free electrons collide with ions which resist their flow

• ?As temperature increases, the vibrations of the ions in the lattice also increase
  • This increases the chance of collisions between the conduction electrons and the ions
• Since?current?is the?flow?of?charge, the ions resisting the flow of electrons cause?resistance
• Therefore as temperature increases so does resistance
  • At small increases of temperature this increase is linear
• A higher current will cause temperature to rise
  • This is due to more collisions between free electrons and ions
  • The collisions cause the ions to vibrate more

• All solids are made up of vibrating atoms
  • This includes metal solids
• As the temperature in a metal rises, the ions vibrate with a greater frequency and amplitude
  • The electrons collide with the vibrating atoms which impede their flow, hence the current?decreases
  • electric current is the flow of free electrons in a material

Metal atoms and free electrons at low and high temperatures

• Current decreases because the resistance has increased (from?V?=?IR)
  • This is because resistivity has increased
  • This is from ρ?∝?R?(if the area?A?and length?L?is constant)

• For a metallic conductor which obeys Ohm's law:
  • An?increase?in temperature causes an?increase?in resistance and resistivity
  • A?decrease?in temperature causes a?decrease?in resistance and resistivity

• The?I-V?graph for a filament lamp shows this effect

I-V characteristics for a filament lamp

• As the current increases, the number of collisions between free electrons and the lattice of ions increases
  • This increases the temperature of the filament in the lamp
• An increase in temperature:
• • Causes greater vibrations in the lattice of ions
  • ?Therefore increased collisions between free electrons and the ions
  • And so an increased resistance
• Resistance opposes the current, causing the current to increase at a slower rate
  • This is seen as a curve in the graph

Step 1: Consider the effect on rate of electron flow:

• • Rate of flow of electrons increases

Step 2: Consider the effect on number of collisions of conduction electrons with the lattice

• • This increases the number of collisions of conduction electrons with the ions in the lattice

Step 3: Describe what happens to the vibrations of the lattice

• • Therefore vibrations of the lattice ions increase

• The resistivity of a thermistor behaves in the?opposite?way to metals
  • This is because it is a type of semiconductor
  • Semiconductors behave in a different way to metals
• The number density of charge carriers (such as electrons)?increases?with?increasing?temperature
• Therefore, for a thermistor:
  • An?increase?in temperature causes a?decrease?in resistance and resistivity
  • A?decrease?in temperature causes an?increase?in resistance and resistivity

• Thermistors are often used in temperature sensing circuits such as thermometers and thermostats
• A thermistor is a non-ohmic conductor and sensory resistor whose resistance varies with temperature
  • Most thermistors are negative temperature coefficient ntc) components.
  • This means that if the temperature?increases,?the resistance of the thermistor?decreases?(and vice versa)
• The temperature-resistance graph for a thermistor is shown below

• Thermistors are temperature sensors and are used in circuits in ovens, fire alarms and digital thermometers
  • As the thermistor gets?hotter, its resistance?decreases
  • As the thermistor gets?cooler, its resistance?increases

   

The resistance through a thermistor is dependent on the temperature of it

ANSWER:?A

Step 1: Outline the nature of a thermistor

• The resistance of the thermistor increases as the temperature decreases

Step 2: Consider the properties of current in a series circuit

• Since the thermistor and resistor?R?are connected in series, the current?I?in both of them is the same

Step 3: Consider a relevant equation

• Ohm’s law states that?V?=?IR
• Since the resistance of the thermistor increases, and?I?is the same, the potential difference?V?across it increases

Step 4: State the conclusion

• Therefore, statement?A?is correct