OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Charge: (b) the coulomb as the unit of charge
The coulomb (symbol C) is the SI unit of electric charge.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The coulomb (symbol C) is the SI unit of electric charge. It is defined as the amount of charge transferred by a current of 1 ampere in 1 second: 1 C = 1 A × 1 s. This means that if a steady current of 1 A flows for 1 s, the total charge that has passed is 1 C. For example, a current of 2 A flowing for 5 s transfers 10 C of charge. The coulomb is a derived unit, defined in terms of the ampere and the second. In calculations, charge is often expressed in coulombs, and it is important to convert units such as millicoulombs (1 mC = 1 × 10⁻³ C) or microcoulombs (1 μC = 1 × 10⁻⁶ C) when necessary. The coulomb is used to quantify the charge on particles, in capacitors, and in current flow.
What earns the marks
- The coulomb is the SI unit of charge, symbol C.
- 1 C is the charge transferred by a current of 1 A in 1 s.
- The coulomb is a derived unit: 1 C = 1 A s.
- Charge can be measured in coulombs, millicoulombs, microcoulombs, etc.
- The coulomb is related to the elementary charge: 1 C ≈ 6.25 × 10¹⁸ e.
Common mistakes and how to put them right
Thinking the coulomb is a base unit.
Put it right: 1 C = 1 A s.
Confusing coulomb with ampere.
Put it right: Current is the rate of flow of charge, measured in amperes.
Forgetting to convert units.
Put it right: Convert to SI base units before calculation.
Exam tips
- Memorise the relationship 1 C = 1 A s and use it to check unit consistency.
- In multiple-choice questions, watch for options that confuse charge and current units.
- When converting from millicoulombs or microcoulombs, multiply by 10⁻³ or 10⁻⁶ respectively.
Try the lesson
The start of the Charge: (b) the coulomb as the unit of charge lesson, made from checked teaching notes for this specification point.
Listen
The coulomb (symbol C) is the SI unit of electric charge. It is defined as the amount of charge transferred by a current of 1 ampere in 1 second: 1 C = 1 A × 1 s. This means that if a steady current of 1 A flows for 1 s, the total charge that has passed is 1 C. For example, a current of 2 A flowing for 5 s transfers 10 C of charge. The coulomb is a derived unit, defined in terms of the ampere and the second. In calculations, charge is often expressed in coulombs, and it is important to convert units such as millicoulombs (1 mC = 1 × 10⁻³ C) or microcoulombs (1 μC = 1 × 10⁻⁶ C) when necessary. The coulomb is used to quantify the charge on particles, in capacitors, and in current flow. A strong answer earns marks for these points. The coulomb is the SI unit of charge, symbol C. 1 C is the charge transferred by a current of 1 A in 1 s. The coulomb is a derived unit: 1 C = 1 A s. Charge can be measured in coulombs, millicoulombs, microcoulombs, etc. A common slip: Thinking the coulomb is a base unit: it is derived from the ampere and second. Instead, 1 C = 1 A s. Exam tip: Memorise the relationship 1 C = 1 A s and use it to check unit consistency.
Key words
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SI unit of charge
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Definition of the coulomb
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Derived units
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Unit conversions
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See every definition
- SI unit of charge
- The coulomb, symbol C, is the SI unit of electric charge.
- Definition of the coulomb
- The amount of charge transferred by a current of 1 ampere in 1 second, so that 1 C = 1 A × 1 s.
- Derived units
- The coulomb is a derived unit, defined in terms of the ampere and the second.
- Unit conversions
- Charge is often expressed in coulombs, and units such as millicoulombs (1 mC = 1 × 10⁻³ C) or microcoulombs (1 μC = 1 × 10⁻⁶ C) must be converted when necessary.
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