Edexcel A Level Physics:复习笔记3.12 Potential Difference & Conductor Length

Potential Difference

• A cell makes one end of the circuit positive and the other negative. This sets up a potential difference across the circuit
• The potential difference across a component in a circuit is defined as;

energy transferred per unit charge

• The energy is transferred is from electrical energy into other forms, depending on the component or device being used
• Potential difference is measured in volts (V) which are equivalent to Joule per coulomb (J C−1)
• The potential difference of a power supply connected in series is always shared between all the components in the circuit

The potential difference is the voltage across each component in a circuit

• Another description of energy transfer is work done
• Therefore, potential difference can also be defined as the work done per unit charge

Potential difference is the work done per unit charge

Measuring Potential Difference

•  Potential difference or voltage is measured using a voltmeter
  • A voltmeter is always set up in parallel to the component being measured

Potential difference can be measured by connecting a voltmeter in parallel between two points in a circuit

Conductor Length

• The equation for resistivity is

• 
•  Where:
  • R = resistance (Ω)
  • ρ = resistivity (Ω m−1)
  • l = length (m)
  • A = area (m−2)

• Therefore, as the length of a uniform conductor at constant temperature increases, resistance also increases
• Voltage and current are linked by Ohm's Law
• 
• Where
  • V = potential difference (V)
  • I = current (A)
  • R = resistance (Ω)
• Therefore, as R increases, so must potential difference across the wire
  • Potential difference increases uniformly with length