OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall that current, I, depends on both resistance, R, and potential…
Full specification wording
Simple circuits: recall that current, I, depends on both resistance, R, and potential difference, V, and the units in which these are measured (to include: the definition of potential difference)
Current in a circuit is not set by the supply alone: it depends on the potential difference across the component and on the resistance of that component.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 6 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Current in a circuit is not set by the supply alone: it depends on the potential difference across the component and on the resistance of that component. Potential difference is defined as the energy transferred per unit charge, measured in volts (V), where one volt is one joule per coulomb. Current is measured in amperes (A) and resistance in ohms (Ω). For a fixed resistance, increasing the potential difference increases the current; for a fixed potential difference, increasing the resistance decreases the current. These relationships are summarised by V = I × R, so a 6 V supply across a 3 Ω resistor gives 2 A.
What earns the marks
- States that current depends on both the potential difference across a component and the resistance of that component.
- Defines potential difference as the energy transferred per unit charge.
- Gives the unit of potential difference as the volt (V), the unit of current as the ampere (A) and the unit of resistance as the ohm (Ω).
- Explains that increasing resistance at constant potential difference reduces the current.
- Applies V = I × R to calculate an unknown current, potential difference or resistance from the other two quantities.
Common mistakes and how to put them right
States that current depends only on the supply voltage.
Put it right: Include resistance as the second factor that sets the current.
Defines potential difference as the total energy supplied rather than energy transferred per unit charge.
Put it right: Give the per-coulomb definition.
Gives resistance in volts or current in ohms.
Put it right: Match each quantity to its own unit, A for current, V for potential difference and Ω for resistance.
Assumes a larger resistance always means a larger current.
Put it right: At constant potential difference a larger resistance gives a smaller current.
Exam tips
- Write the equation V = I × R at the top of your working, then substitute values with their units before calculating.
- Check the unit asked for in the answer line, because a calculation may give a value that must be matched to A, V or Ω.
- Learn the definition of potential difference as a short phrase about energy per unit charge, since recall questions often ask for it directly.
Try the lesson
The start of the Simple circuits: recall that current, I, depends on both resistance, R, and potential difference, V, and the units in which these are measured (to include: the definition of potential difference) lesson, made from checked teaching notes for this specification point.
Have a go first
Which unit is used to measure the potential difference across a resistor?
Listen
Current in a circuit is not set by the supply alone: it depends on the potential difference across the component and on the resistance of that component. Potential difference is defined as the energy transferred per unit charge, measured in volts (V), where one volt is one joule per coulomb. Current is measured in amperes (A) and resistance in ohms (Ω). For a fixed resistance, increasing the potential difference increases the current; for a fixed potential difference, increasing the resistance decreases the current. These relationships are summarised by V = I × R, so a 6 V supply across a 3 Ω resistor gives 2 A. A strong answer earns marks for these points. States that current depends on both the potential difference across a component and the resistance of that component. Defines potential difference as the energy transferred per unit charge. Gives the unit of potential difference as the volt (V), the unit of current as the ampere (A) and the unit of resistance as the ohm (Ω). Explains that increasing resistance at constant potential difference reduces the current. Exam tip: Write the equation V = I × R at the top of your working, then substitute values with their units before calculating.
See it
Key words
Say what each word means before you turn the card.
Dependence of current on potential difference and resistance
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Definition of potential difference
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Units of current, potential difference and resistance
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Using V = I × R
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See every definition
- Dependence of current on potential difference and resistance
- Current depends on both the potential difference across a component and the resistance of that component.
- Definition of potential difference
- The energy transferred per unit charge, measured in volts (V), where one volt is one joule per coulomb.
- Units of current, potential difference and resistance
- Current is measured in amperes (A), potential difference in volts (V) and resistance in ohms (Ω).
- Using V = I × R
- The relationships are summarised by this equation, so a 6 V supply across a 3 Ω resistor gives 2 A.
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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