IB DP Physics: SL复习笔记5.1.3 Potential Difference & DC

• 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

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

• 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