OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Calculate the currents, potential differences and resistances in d.c.…
Full specification wording
Simple circuits: calculate the currents, potential differences and resistances in d.c. series and parallel circuits (to include: components such as wire of varying resistance, filament lamps, diodes, NTC thermistors and LDRs)
In a series circuit the current is the same at every point, the supply potential difference is shared between components, and resistances add: R_total = R₁ + R₂ + ….
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 12 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In a series circuit the current is the same at every point, the supply potential difference is shared between components, and resistances add: R_total = R₁ + R₂ + …. In a parallel circuit the potential difference across each branch equals the supply value, the branch currents add to give the total current, and the combined resistance is less than the smallest branch resistance. Use V = IR on each component or branch, then apply the series and parallel rules. Non-ohmic components change resistance with conditions: a filament lamp's resistance rises as it heats, a diode conducts in one direction only, an NTC thermistor's resistance falls as temperature rises, and an LDR's resistance falls as light intensity rises. Wire of greater length or smaller cross-section has more resistance.
What earns the marks
- Applies V = IR to each component or branch before combining values with the series or parallel rules.
- Adds resistances directly for components connected in series.
- States that the current is the same at all points in a series circuit.
- Recognises that the potential difference across each parallel branch equals the supply potential difference.
- Adds branch currents to find the total current in a parallel circuit.
- Identifies that the combined resistance of a parallel network is smaller than the smallest branch resistance.
- Explains that a filament lamp's resistance increases as its temperature rises.
- Describes a diode as conducting in one direction only.
- Explains that an NTC thermistor's resistance decreases as temperature increases.
- Explains that an LDR's resistance decreases as light intensity increases.
- Relates the resistance of a wire to its length and cross-sectional area.
Common mistakes and how to put them right
Adds currents at different points of a series circuit.
Put it right: Keep the current the same throughout a series loop.
Treats the combined resistance of a parallel network as the sum of the branches.
Put it right: Expect a value smaller than the smallest branch resistance.
Assumes a filament lamp obeys Ohm's law at all currents.
Put it right: Allow its resistance to rise as it heats.
Reverses the thermistor response.
Put it right: State that an NTC thermistor's resistance falls as temperature rises.
Reverses the LDR response.
Put it right: State that an LDR's resistance falls as light intensity rises.
Ignores the diode's one-way conduction.
Put it right: Allow current only in the forward direction.
Exam tips
- Write the series or parallel rule you are using before substituting numbers, so the method is visible.
- Check each answer for physical sense: a parallel combined resistance must be below the smallest branch value.
- Read the component named in the question and apply its specific behaviour rather than assuming constant resistance.
Try the lesson
The start of the Simple circuits: calculate the currents, potential differences and resistances in d.c. series and parallel circuits (to include: components such as wire of varying resistance, filament lamps, diodes, NTC thermistors and LDRs) lesson, made from checked teaching notes for this specification point.
Have a go first
A 12 V battery is connected to two resistors in series: a 4 Ω resistor and a 2 Ω resistor. What is the current in the circuit?
Listen
In a series circuit the current is the same at every point, the supply potential difference is shared between components, and resistances add: R_total = R₁ + R₂ + … In a parallel circuit the potential difference across each branch equals the supply value, the branch currents add to give the total current, and the combined resistance is less than the smallest branch resistance. Use V = IR on each component or branch, then apply the series and parallel rules. Non-ohmic components change resistance with conditions: a filament lamp's resistance rises as it heats, a diode conducts in one direction only, an NTC thermistor's resistance falls as temperature rises, and an LDR's resistance falls as light intensity rises. Wire of greater length or smaller cross-section has more resistance. A strong answer earns marks for these points. Applies V = IR to each component or branch before combining values with the series or parallel rules. Adds resistances directly for components connected in series. States that the current is the same at all points in a series circuit. Recognises that the potential difference across each parallel branch equals the supply potential difference. Exam tip: Write the series or parallel rule you are using before substituting numbers, so the method is visible.
See it
Key words
Say what each word means before you turn the card.
Series circuit
Try answering aloud or in your head before turning the card.
Parallel circuit
Try answering aloud or in your head before turning the card.
Non-ohmic components
Try answering aloud or in your head before turning the card.
Diode
Try answering aloud or in your head before turning the card.
See every definition
- Series circuit
- A circuit in which the current is the same at every point, the supply potential difference is shared between components, and resistances add.
- Parallel circuit
- A circuit in which the potential difference across each branch equals the supply value, the branch currents add to give the total current, and the combined resistance is less than the smallest branch resistance.
- Non-ohmic components
- Components that change resistance with conditions, such as a filament lamp, a diode, an NTC thermistor and an LDR.
- Diode
- A component that conducts in one direction only.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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