Electricity and Circuits for Singapore O-Level Physics (6091): static electricity and charge, current, voltage and resistance, series and parallel circuits, and electrical energy, power and safety

What this module covers

Electricity and Circuits is one of the largest themes in O-Level Physics (SEAB 6091). It begins with static charge, then develops the flow of charge as current, links current to voltage and resistance through Ohm's law, analyses how series and parallel circuits share current and voltage, and finishes with electrical power, energy and the safety features of mains electricity.

The energy ideas from earlier modules return here (electrical energy transfers to heat, light and motion), and this module leads directly into magnetism and electromagnetism. Each dot point below has full worked answers and practice questions.

Static electricity and charge

Static electricity and charge explains how rubbing transfers electrons between materials, leaving one positively and one negatively charged. Like charges repel and unlike charges attract. This static charge can produce sparks and is the basis of effects such as a charged rod attracting small pieces of paper.

Charge is conserved: rubbing does not create charge, it only moves electrons from one object to the other.

Current, voltage and resistance

Current, voltage and resistance defines current as the rate of flow of charge, voltage (potential difference) as the energy transferred per unit charge, and resistance as opposition to current. Ohm's law links them:

An ohmic conductor at constant temperature gives a straight-line current-voltage graph.

Series and parallel circuits

Series and parallel circuits sets out the rules. In series the current is the same throughout and voltages add; total resistance is the sum of the resistances. In parallel the voltage across each branch is the same and branch currents add; the total resistance is less than the smallest single resistance. Household appliances are wired in parallel so each works at the full mains voltage and can be switched independently.

Electrical energy, power and safety

Electrical energy, power and safety gives the power and energy relationships

and explains mains safety: the earth wire carries fault current safely away, the fuse or circuit breaker cuts the supply if the current is too large, and correct insulation and wiring protect the user.

How this module is examined

• Use $V = IR$ in the right form. Identify the unknown, rearrange, then substitute in consistent units.
• Apply the circuit rules. Same current and adding voltages in series; same voltage and adding currents in parallel.
• Explain safety features by function. Say what the earth wire and the fuse each do during a fault.

Check your knowledge

Recall and calculation questions across the module. Try them, then check the worked solutions.

1. State Ohm's law. (2 marks)
2. A current of $2.0\ \text{A}$ flows through a $6.0\ \Omega$ resistor. Calculate the voltage across it. (2 marks)
3. Two $3.0\ \Omega$ resistors are connected in series. Calculate the total resistance. (1 mark)
4. A $230\ \text{V}$ heater draws a current of $5.0\ \text{A}$. Calculate its power. (2 marks)
5. State the purpose of the earth wire in a mains appliance. (2 marks)