DP IB Physics: HL

Revision Notes

Syllabus Edition

First teaching 2014

Last exams 2024


5.1.3 Potential Difference & DC

Test Yourself

Potential Difference

  • Potential difference is a measure of the electrical potential energy transferred from an electron as it moves between two points in a conductor

Potential difference is work done per unit charge 

  • It is also known as voltage
  • Potential difference (pd) is calculated as follows:

  • Where:
    • V = potential difference in volts (V)
    • W = work done in joules (J)
    • q = charge in coulombs (C)

  • From the above equation, one volt is equal to one joule per unit coulomb
    • 1 V = 1 J C–1

The Electronvolt

  • The energy values associated to electrons and other microscopic particles are very small when expressed in SI units
  • For this reason, it is often more convenient to use another unit for energy - the electronvolt (eV)
  • The electronvolt is defined as follows:

The amount of energy needed to move an electron through a potential difference of one volt 

Worked example

Determine the value of 1 eV in joules (J).

Step 1: Recall the definition of electronvolt 

    • One electronvolt is the work W associated to an electron of charge e moving through a potential difference V = 1V

W = eV

Step 2: Look up the charge e of the electron in the data booklet 

    • e = 1.6 × 10–19 C

Step 3: Substitute this and the value of the voltage into the above equation for W

W = (1.6 × 10–19 C) × 1 V

W = 1.6 × 10–19 J

One electronvolt is equal to 1.6 × 10–19 joules

Direct Current

  • The potential difference in a circuit is provided by cells or batteries
    • Each cell has a positive terminal (high potential location) and a negative terminal (low potential location)
    • A battery is a collection of cells arranged positive terminal to negative terminal


A cell and a battery made of three cells


  • When a cell or a battery is connected to a loop of copper wire, a circuit is formed
  • The battery is the source of the potential difference V needed for the electrons to flow
  • Electrons gain electrical potential energy as they move through the battery
  • They then leave the battery and move through the wire
    • A little amount of their energy is transferred to the metal atoms of the wire
    • The flow of electrons is from the negative terminal to the positive one

  • Direct current (dc) flows through the circuit in one direction  
    • The direction of conventional current is from the positive terminal to the negative one 
    • This is opposite to the electrons flow


Direct current flows from the positive to the negative terminal of the battery in a circuit. Electrons flow in the opposite direction


Alternating Current

  • Alternating current (ac) is used instead of dc in high voltage devices (i.e. those typically used in homes and industries)
  • Alternating current flows one way around the circuit and then reverses its flow
  • ac direction usually changes every 0.01 s

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