Charge (Oxford AQA IGCSE Combined Science Double Award)

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

• 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

• The potential difference across a component in a circuit is defined as

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

• For more information, read our revision notes about Voltage & Potential Difference