Grade 9 · Electricity and Magnetism · Circuits and Current
Current, Voltage and Resistance
Building circuits and using V = I x R with confidence.
Learning objectives
- Describe current as a flow of charge.
- Compare series and parallel circuits.
- Use V = I x R in calculations.
NGSS alignment
HS-PS3-3
Design, build, and refine a device that works within given constraints to convert one form of energy into another form of energy.
MS-PS3-3
Apply scientific principles to design, construct, and test a device that either minimizes or maximizes thermal energy transfer.
MYP criteria
- Criterion A — Knowing and Understanding
- Criterion B — Inquiring and Designing
- Criterion C — Processing and Evaluating
ENGAGE
One bulb dies, the rest stay lit
In some string lights, removing one bulb switches all of them off. In others, the remaining bulbs stay bright.
Think about it
- What must be different about how the bulbs are wired?
- In which arrangement does charge have more than one path?
EXPLAIN
Current, voltage, resistance
- Current is the rate of flow of charge, measured in amperes (A) with an ammeter in series
- Potential difference (voltage) is the energy transferred per unit charge, measured in volts (V) with a voltmeter in parallel
- Resistance opposes the flow of charge, measured in ohms
Ohm's law
V = I x R. A 3.0 A current through a 4.0 ohm resistor needs 12 V across it.
| Feature | Series | Parallel |
|---|---|---|
| Paths for current | One | More than one |
| Current | Same everywhere | Splits between branches |
| Potential difference | Shared between components | Same across each branch |
| Adding more bulbs | All get dimmer | Brightness stays similar |
Common misconception
Current is not used up by a bulb. The same current leaves as enters; it is energy that is transferred.
INVESTIGATION
Resistance of a wire
research question
How does the length of a nichrome wire affect its resistance?
hypothesis
Resistance will be directly proportional to length because a longer wire has more collisions per unit charge.
variables
Independent: length (10, 20, 30, 40, 50 cm). Dependent: resistance calculated from V divided by I. Controlled: wire material and thickness, supply voltage, keeping the wire cool.
method
Connect the wire in series with an ammeter and a low supply voltage, with a voltmeter across the wire. Record V and I for each length, switching off between readings so the wire does not heat.
safety
Use a low voltage. The wire can become hot enough to burn.
analysis
Plot resistance against length; a straight line through the origin supports direct proportionality. Comment on anomalies caused by heating.
Watch
Electric Current: Crash Course Physics
CrashCourse · 10 min
Builds current, voltage and resistance from the ground up.
Before you watch: Is current used up by a bulb?
- State Ohm's law.
- How does current behave in a parallel circuit?
Interactive simulation · PhET
Circuit Construction Kit: DC
Build a series circuit with two bulbs, then rebuild it in parallel and compare the ammeter readings.
While you explore
- How does current compare in series and parallel?
- What happens when you remove one bulb from each circuit?
Key vocabulary
- Resistance
- How much a component slows the flow of charge.
- Current
- The flow of electric charge around a circuit.
Practice questions
0/2 correct
Level 1 · Criterion A
Which instrument is connected in series to measure current?
Level 2 · Criterion A
A 2 A current flows through a 6 ohm resistor. The potential difference across it is:
MYP criterion tasks
Level 3 · Criterion C
A student measures the resistance of a wire and finds it increases at longer lengths but the last point is far above the line. Analyse the result.
Reflect & track
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