AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Understanding of definition of absolute electric potential, including zero…
Full specification wording
Electric potential (A-level only): Understanding of definition of absolute electric potential, including zero value at infinity, and of electric potential difference.
Absolute electric potential V at a point is the work done per unit positive charge in bringing a small positive test charge from infinity to that point, so V = W/Q. The zero of potential is…
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- 4 key terms
Study notes
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The idea
Absolute electric potential V at a point is the work done per unit positive charge in bringing a small positive test charge from infinity to that point, so V = W/Q. The zero of potential is defined at infinity, where the field is zero. For a point charge Q, V = kQ/r, a scalar that is positive near a positive charge and negative near a negative charge. Electric potential difference between two points is the work done per unit charge in moving a positive charge between them, ΔV = W/Q, and is the quantity measured by a voltmeter.
For example: Moving a 2 C charge through a potential difference of 3 V requires 6 J of work.
What earns the marks
- Defines absolute electric potential as work done per unit positive charge in bringing a small positive test charge from infinity to the point: V = W/Q.
- States that the zero of electric potential is at infinity, where the electric field is zero.
- Uses V = kQ/r for the potential at distance r from a point charge Q, recognising that V is a scalar and can be positive or negative.
- Defines electric potential difference as the work done per unit positive charge in moving between two points: ΔV = W/Q.
- Calculates potential difference from the potentials at two points, ΔV = V₂ − V₁, and uses it to find energy transferred W = QΔV.
- Uses correct units: volts (V) or joules per coulomb (J C⁻¹) for potential and potential difference.
Common mistakes and how to put them right
Treating electric potential as a vector and adding potentials with direction.
Put it right: Potential is a scalar, so add or subtract algebraic values including signs.
Forgetting that the zero of potential is at infinity and using a finite reference point.
Put it right: State that V = 0 at infinity when defining absolute potential.
Confusing potential with potential energy.
Put it right: Potential is energy per unit charge, while potential energy is QV for a charge Q at potential V.
Exam tips
- When calculating potential due to several point charges, sum the scalar values algebraically, taking account of positive and negative charges.
- Use ΔV = W/Q to link potential difference to energy transfer, and rearrange carefully to find W, Q or ΔV as required.
- Check the sign of your answer: potential near a positive charge is positive, and near a negative charge is negative, relative to infinity.
Try the lesson
The start of the Electric potential (A-level only): Understanding of definition of absolute electric potential, including zero value at infinity, and of electric potential difference. lesson, made from checked teaching notes for this specification point.
Have a go first
At which point is the absolute electric potential defined to be zero?
Listen
Absolute electric potential V at a point is the work done per unit positive charge in bringing a small positive test charge from infinity to that point, so V = W/Q. The zero of potential is defined at infinity, where the field is zero. For a point charge Q, V = kQ/r, a scalar that is positive near a positive charge and negative near a negative charge. Electric potential difference between two points is the work done per unit charge in moving a positive charge between them, ΔV = W/Q, and is the quantity measured by a voltmeter. For example, moving a 2 C charge through a potential difference of 3 V requires 6 J of work. A strong answer earns marks for these points. Defines absolute electric potential as work done per unit positive charge in bringing a small positive test charge from infinity to the point: V = W/Q. States that the zero of electric potential is at infinity, where the electric field is zero. Uses V = kQ/r for the potential at distance r from a point charge Q, recognising that V is a scalar and can be positive or negative. Defines electric potential difference as the work done per unit positive charge in moving between two points: ΔV = W/Q. A common slip: Treating electric potential as a vector and adding potentials with direction. Instead, potential is a scalar, so add or subtract algebraic values including signs. Exam tip: When calculating potential due to several point charges, sum the scalar values algebraically, taking account of positive and negative charges.
See it
Key words
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Absolute electric potential
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Zero of electric potential
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Electric potential difference
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Scalar
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See every definition
- Absolute electric potential
- The work done per unit positive charge in bringing a small positive test charge from infinity to that point, so V = W/Q.
- Zero of electric potential
- Defined at infinity, where the electric field is zero.
- Electric potential difference
- The work done per unit positive charge in moving a positive charge between two points, ΔV = W/Q.
- Scalar
- Electric potential is a scalar that is positive near a positive charge and negative near a negative charge.
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