AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Electric field strength (A-level only): Electric field strength.
Electric field strength E is the force per unit positive charge at a point in a field, E = F/Q, measured in N C⁻¹ or equivalently V m⁻¹.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 2
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Electric field strength E is the force per unit positive charge at a point in a field, E = F/Q, measured in N C⁻¹ or equivalently V m⁻¹. It is a vector, directed along the force on a positive test charge. For a point charge Q, E = kQ/r² where k = 1/(4πε₀); for a uniform field between parallel plates, E = V/d. For example, midway between two equal positive charges, the fields cancel to zero; near a single positive charge, E points radially outward and falls as 1/r². Always state direction and use consistent units.
What earns the marks
- Defines electric field strength as force per unit positive charge: E = F/Q, with E in N C⁻¹ or V m⁻¹.
- States that electric field strength is a vector quantity and gives its direction as the direction of the force on a positive test charge.
- Applies E = kQ/r² for the field of a point or spherical charge, using k = 1/(4πε₀) and r as distance from the centre.
- Uses E = V/d for a uniform field between parallel plates, recognising that V is the potential difference across the plates and d is their separation.
- Combines electric fields from two or more charges by vector addition, including cases where fields are equal and opposite and cancel to zero.
- Uses correct units and prefixes, converting distances to metres and charges to coulombs before substitution.
Common mistakes and how to put them right
Treating electric field strength as a scalar and adding magnitudes without direction.
Put it right: Resolve fields as vectors and state the resultant direction.
Using the distance between two charges instead of the distance from the charge to the point when applying E = kQ/r².
Put it right: Identify r as the separation between the source charge and the point where E is required.
Confusing electric field strength with electric potential or force.
Put it right: E is force per unit positive charge, while potential is energy per unit charge, and force is QE.
Exam tips
- Write the defining equation E = F/Q before substituting values, and include the direction of E in your final answer where the question asks for it.
- For point charges, check whether the field is due to one charge or several; if several, draw arrows to show each field direction before adding them.
- Keep units consistent: convert cm to m, μC to C, and nC to C before calculating, and give the unit N C⁻¹ or V m⁻¹.
Try the lesson
The start of the Electric field strength (A-level only): Electric field strength. lesson, made from checked teaching notes for this specification point.
Have a go first
Which row gives the correct SI unit for electric field strength?
Listen
Electric field strength E is the force per unit positive charge at a point in a field, E = F/Q, measured in N C⁻¹ or equivalently V m⁻¹. It is a vector, directed along the force on a positive test charge. For a point charge Q, E = kQ/r² where k = 1/(4πε₀); for a uniform field between parallel plates, E = V/d. For example, midway between two equal positive charges, the fields cancel to zero; near a single positive charge, E points radially outward and falls as 1/r². Always state direction and use consistent units. A strong answer earns marks for these points. Defines electric field strength as force per unit positive charge: E = F/Q, with E in N C⁻¹ or V m⁻¹. States that electric field strength is a vector quantity and gives its direction as the direction of the force on a positive test charge. Applies E = kQ/r² for the field of a point or spherical charge, using k = 1/(4πε₀) and r as distance from the centre. Uses E = V/d for a uniform field between parallel plates, recognising that V is the potential difference across the plates and d is their separation. A common slip: Treating electric field strength as a scalar and adding magnitudes without direction. Instead, resolve fields as vectors and state the resultant direction. Exam tip: Write the defining equation E = F/Q before substituting values, and include the direction of E in your final answer where the question asks for it.
Key words
Say what each word means before you turn the card.
Electric field strength
Try answering aloud or in your head before turning the card.
Point charge field
Try answering aloud or in your head before turning the card.
Uniform field
Try answering aloud or in your head before turning the card.
See every definition
- Electric field strength
- The force per unit positive charge at a point in a field, E = F/Q, measured in N C⁻¹ or equivalently V m⁻¹.
- Point charge field
- Given by E = kQ/r² where k = 1/(4πε₀) and r is the distance from the centre.
- Uniform field
- Between parallel plates, given by E = V/d, where V is the potential difference across the plates and d is their separation.
There is more to this lesson
The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge
Test your understanding of Electric field strength (A-level only): Electric field strength.
Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.
A week of Premium from your first lesson · Then the free plan continues · No card, no charge