OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Charge: (f) conventional current and electron flow
Conventional current is defined as the direction in which positive charge would flow, from the positive terminal of a source, through the external circuit, to the negative terminal.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 6 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Conventional current is defined as the direction in which positive charge would flow, from the positive terminal of a source, through the external circuit, to the negative terminal. In metallic conductors the actual moving charges are electrons, which are negatively charged, so electron flow is in the opposite direction to conventional current. This convention predates the discovery of the electron and is retained because circuit analysis, Kirchhoff's laws and most equations such as I = ΔQ/Δt assume positive charge flow. In an electrolyte, positive ions move in the conventional current direction while negative ions move the other way, so both contribute to the same conventional current. Always state which direction you mean when describing a circuit.
What earns the marks
- Conventional current is the direction of flow of positive charge, from the positive terminal to the negative terminal through the external circuit.
- In a metal, electrons flow from the negative terminal to the positive terminal, opposite to the conventional current direction.
- The conventional direction is a defined convention, not a claim that positive charges physically move in a metal.
- In an electrolyte, positive ions move in the conventional current direction and negative ions move in the opposite direction, both contributing to the same current.
- Circuit equations and Kirchhoff's laws are written for conventional current, so signs must be handled consistently.
Common mistakes and how to put them right
Stating that conventional current is the flow of electrons.
Put it right: Conventional current is the flow of positive charge, and in metals the electrons move in the opposite direction.
Believing that the two directions describe two different currents in a metal.
Put it right: There is one current, and the two descriptions are two ways of labelling its direction.
Forgetting that in an electrolyte the negative ions move opposite to the conventional current while still contributing to it, so the total current is the sum of both ion contributions.
Exam tips
- When a question asks for the direction of electron flow, give the direction opposite to the conventional current arrow.
- Use the conventional current direction when applying Kirchhoff's laws or I = ΔQ/Δt, and keep signs consistent throughout a calculation.
- If a diagram shows an arrow labelled I, treat it as conventional current unless the question explicitly states otherwise.
Try the lesson
The start of the Charge: (f) conventional current and electron flow lesson, made from checked teaching notes for this specification point.
Listen
Conventional current is defined as the direction in which positive charge would flow, from the positive terminal of a source, through the external circuit, to the negative terminal. In metallic conductors the actual moving charges are electrons, which are negatively charged, so electron flow is in the opposite direction to conventional current. This convention predates the discovery of the electron and is retained because circuit analysis, Kirchhoff's laws and most equations such as I = ΔQ/Δt assume positive charge flow. In an electrolyte, positive ions move in the conventional current direction while negative ions move the other way, so both contribute to the same conventional current. Always state which direction you mean when describing a circuit. A strong answer earns marks for these points. Conventional current is the direction of flow of positive charge, from the positive terminal to the negative terminal through the external circuit. In a metal, electrons flow from the negative terminal to the positive terminal, opposite to the conventional current direction. The conventional direction is a defined convention, not a claim that positive charges physically move in a metal. In an electrolyte, positive ions move in the conventional current direction and negative ions move in the opposite direction, both contributing to the same current. Exam tip: When a question asks for the direction of electron flow, give the direction opposite to the conventional current arrow.
See it
Key words
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Conventional current
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Electron flow in metallic conductors
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Ion movement in electrolytes
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Consistency of sign conventions in circuit analysis
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See every definition
- Conventional current
- The direction in which positive charge would flow, from the positive terminal of a source, through the external circuit, to the negative terminal.
- Electron flow in metallic conductors
- In metallic conductors the actual moving charges are electrons, which are negatively charged, so electron flow is in the opposite direction to conventional current.
- Ion movement in electrolytes
- Positive ions move in the conventional current direction while negative ions move the other way, so both contribute to the same conventional current.
- Consistency of sign conventions in circuit analysis
- Circuit analysis, Kirchhoff's laws and most equations such as I = ΔQ/Δt assume positive charge flow, so signs must be handled consistently.
Label the diagram
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1Box 1
2Box 2
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