Example 1: parallel currents
A battery supplies 1.5 A to three identical lamps in parallel. What is the current in each lamp?
- The branch currents add up to 1.5 A.
- Each lamp: 1.5 ÷ 3 = 0.5 A.
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IB DemystifiedMYP Sciences
Simple circuits
Every torch, phone and house light works because charge flows around a circuit. Understanding circuits explains why house lights can be switched on one at a time, why fuses protect us, and why some strings of lights all go out when one bulb breaks.
Recommended for MYP 2 · About 3 lessons · Criteria A, B, C and D
On this page
- Learning objectives
- Before you start
- Key vocabulary
- Understanding the ideas
- Series and parallel at a glance
- Circuits in the real world
- Worked examples
- Assessment tips
- Check your understanding
- Practice questions
- Investigation
- Criterion-linked questions
- Challenge questions
- Topic check
- Review your mistakes
- Your progress
Learning objectives
By the end of this topic you should be able to:
- draw and read circuit diagrams using standard symbols
- identify conductors and insulators
- describe current and voltage in series circuits
- describe current and voltage in parallel circuits
- explain electrical safety devices and hazards
- plan fair tests and analyse circuit data
Before you start
You will use these skills. If any feel shaky, review them first.
- forces and energy (see Forces and their effects)
- reading scales and tables
- simple division
Key vocabulary
- Current
- The flow of electric charge, measured in amps (A) with an ammeter.
- Voltage
- The push from the power supply, measured in volts (V) with a voltmeter.
- Series circuit
- Components connected in one loop, one after another.
- Parallel circuit
- Components connected on separate branches.
- Conductor
- A material that lets current flow easily, such as copper.
- Insulator
- A material that does not let current flow, such as plastic.
Understanding the ideas
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What is it?
An electric current is a flow of charge. It needs a complete circuit and a source of energy, such as a battery. Conductors, such as metals and graphite, let current flow; insulators, such as plastic and glass, do not.
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Why does it happen?
In a series circuit, there is one path: the current is the same everywhere, and the battery's voltage is shared between the components. In a parallel circuit, there are several paths: the currents in the branches add up to the current from the battery, and each branch gets the full voltage.
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How do we know?
We measure current with an ammeter connected in series and voltage with a voltmeter connected in parallel. Scientists such as Alessandro Volta and André-Marie Ampère, after whom the units are named, discovered how currents behave.
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Why does it matter?
Circuits are designed for safety: house lights are wired in parallel so each can be switched separately, and fuses and circuit breakers switch off dangerous currents.
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What does it connect to?
Circuits link to energy and power, magnetism and electronics in physics, and to nerve impulses in biology.
Series and parallel at a glance
- Series: one loop; same current everywhere; voltage shared; if one lamp breaks, all go out; adding lamps makes them dimmer.
- Parallel: separate branches; branch currents add up; each branch gets the full voltage; one lamp can fail without affecting the others.
- Ammeters go in series; voltmeters go in parallel.
- Current is not used up: components transfer energy, not charge.
Circuits in the real world
Houses are wired in parallel so each light and socket works independently. Fuses and circuit breakers prevent fires from overloaded extension boards. Solar lanterns use simple circuits with a battery, a switch and LEDs.
Worked examples
Example 2: series voltage
A 9 V battery is connected to three identical lamps in series. What is the voltage across each lamp?
- The voltage is shared equally.
- 9 ÷ 3 = 3 V.
Assessment tips
Circuit questions often use diagrams with meters. Expect to:
- Read and draw circuit diagrams with symbols.
- Calculate currents and voltages in series and parallel circuits.
- Explain why components behave as they do.
- Plan fair tests and spot anomalous readings.
Common mistakes: saying current is used up; putting a voltmeter in series; forgetting that branch currents add up; and mixing up conductors and insulators.
Check your understanding
Quick questions on the ideas above. Try each one before using a hint.
Practice questions
Investigation: which liquids conduct?
Partially guided investigation · about 40 minutes · pairs
- Research question
- Which liquids conduct electricity: distilled water, tap water, salt water, sugar water, lemon juice or vegetable oil?
- Scientific background
- Some liquids contain charged particles (ions) that can carry current. Pure water contains very few, so it conducts poorly.
- Hypothesis
- Write your own prediction, with a scientific justification.
- Variables
- Identify your independent, dependent and control variables, and explain how you will control them.
- Apparatus
- Battery pack (maximum 6 V), LED or low-voltage lamp, ammeter, two carbon or stainless-steel electrodes, connecting wires, small beakers, the liquids.
- Method
- Build a circuit with the battery, ammeter, lamp and the two electrodes.
- Dip the electrodes into each liquid to the same depth and the same distance apart.
- Record whether the lamp lights and the ammeter reading.
- Rinse and dry the electrodes between liquids.
Safety. Use only low-voltage batteries, never mains electricity. Wipe up spills and keep liquids away from sockets. Wash hands afterwards.
Then evaluate: why was it important to keep the electrodes the same distance apart?
Criterion-linked questions
Criterion B: inquiring and designing
Criterion C: processing and evaluating
Criterion D: reflecting on the impacts of science
Challenge questions
Harder problems in unfamiliar contexts. Plan before you calculate.
Topic check
Five questions picked at random from the whole topic. Take a new set whenever you like.
Review your mistakes
Questions you got wrong on this device appear here so you can try them again. Answer one correctly and it leaves the list.
Your progress
Tracked separately for each skill, on this device only.
| Skill | Correct | Status |
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