Charge (Oxford AQA IGCSE Combined Science Double Award)

Revision Note

Charge

  • Charge is a property of matter

    • Charge is measured in units called coulombs, C

  • Objects can be positively charged, negatively charged or neutral (i.e. not charged)

    • Objects are neutral if the positive charge and negative charge cancel each other out

    • For example, a neutral atom has an equal number of protons (positively charged particles) and electrons (negatively charged particles

  • If a neutral object gains electrons, it becomes negatively charged

  • If a neutral object loses electrons, it becomes positively charged

  • When two charged objects are brought close together, there will be a force of attraction or repulsion between those objects

Positive and negative charges

Two positive charges repel each other. A positive charge is attracted towards a negative charge, for IGCSE & GCSE Physics revision notes
Like charges repel; opposite charges attract
  • Remember:

    • Opposite charges attract

    • Like charges repel

  • In some objects, the electrical charges can move easily around the object

    • For example, in metals such as copper, electrons can move easily around the metal

Potential difference

  • For an electrical charge to flow through a closed circuit, it must include a source of potential difference (p.d.)

    • This is often called voltage or e.m.f.

  • Sources of potential difference include:

    • A cell

    • Batteries (multiple cells)

    • Electrical generator

  • The terminals in a cell make one end of the circuit positive and the other negative

  • This sets up a potential difference across the circuit

    • The symbol for the potential difference is V

  • In a closed circuit with a source of potential difference, electrons flow through the metal wire from the negative end to the positive end

The flow of electrons

Electrons free from their metal ions move from the negative side of a wire to the positive side
In metals, the current is caused by a flow of electrons
  • The potential difference across a component in a circuit is defined as

The energy transferred per unit charge flowing from one point to another

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