Resistance and resistivity

• What is resistance measured in?

  OHMS ($\Omega$)
• What happens during collisions of delocalised electrons with metal ions?

  Each collision causes a transfer of energy
• How can resistance be measured?

  Using an ohmmeter or voltmeter and ammeter
• What is the formula for calculating resistance?

  $R=$$\frac{V}{I}$
• What does the ratio of p.d drop across a component to the current represent?

  It represents resistance
• How does resistance change with the length of a conductor?

  Resistance is directly proportional to length
• What happens to collision frequency as the length of a conductor increases?

  Collision frequency also increases
• How does temperature affect resistance?

  Resistance increases as temperature increases
• What effect does increased kinetic energy of ions have on resistance?

  It increases the amplitude of collisions, raising resistance
• How does electron density relate to resistance?

  Higher electron density increases resistance
• Resistivity:

  • Is a constant for a metal but only at a certain temperature.
  • $\rho=$$\frac{RA}{L}$
  • Measured in $\Omega m$
• Ohms Law:

  • In a fixed resistor at a constant temperature, current and potential difference are directly proportional.
  • When (R) is lower, (I) will be higher for each value of (V). Meaning the lines gradient will be higher