IB DP Physics: SL复习笔记5.3.1 Primary & Secondary Cells

Primary & Secondary Cells

Simple Cells

  • A simple cell is a source of electrical energy
  • The simplest design consists of two electrodes made from metals of different reactivity immersed in an electrolyte and connected to an external voltmeter by wire, creating a complete circuit
  • A common example is zinc and copper
  • Zinc is the more reactive metal and forms ions more easily, readily releasing electrons
  • The electrons give the more reactive electrode a negative charge and sets up a charge difference between the electrodes
  • The electrons then flow around the circuit to the copper electrode which is now the more positive electrode
  • The difference in the ability of the electrodes to release electrons causes a voltage to be produced
  • The greater the difference in the metals reactivity then the greater the voltage produced
  • The electrolyte used also affects the voltage as different ions react with the electrodes in different ways

5.2.1-How-Reactivity-Affects-Voltage-in-Cells-1

5.2.1-How-Reactivity-Affects-Voltage-in-Cells-2

Simple cell made with Cu and Mg. These metals are further apart on the reactivity series than Cu and Zn and produce a greater voltage

Batteries

  • Electrochemical cells include the familiar batteries used in everyday appliances and cars
  • Batteries work by connecting two or more cells in series, which combine to give a larger overall voltage
  • Over time the electrodes degrade as the reactions that occur there are irreversible
  • Cells produce a voltage only until one of the reactants is used up and when this occurs the battery dies or goes flat
  • The products formed cannot be reverted back into reactants as the reaction is irreversible and the battery must be replaced
  • This happens in non-rechargeable batteries such as alkaline batteries
  • In rechargeable batteries the reactions are reversed by connecting the cells to an external electrical supply
  • This reverses the chemical reactions taking place allowing the cycle to be repeated

Primary Cells

  • Cells that are non-rechargeable are known as primary cells
    • Primary cells include AA batteries (known as dry-cells) common in many small devices
  • Primary cells are by definition only able to be used once as the chemicals within them are used up
  • During normal operation of a primary cell, the electrons flow from the negative plate to the positive plate of the cell

Secondary Cells

  • Cells that are rechargeable are known as secondary cells
    • Secondary cells include:
      • Lithium-ion batteries used in laptops and other larger modern devices
      • Lead-acid batteries such as those used in cars and other motor vehicles
  • Secondary cells can be used many times as they are attached to a charger and the chemical reaction is reversed allowing the cells to store energy for use once again
  • When recharging a secondary cell, the electrons are forced from the positive plate to the negative plate by an external current

 

 

 

 

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