# 10.1.2 Uniform Electric Fields

### Electric Field Strength

• The electric field strength of the uniform field between two charged parallel plates is defined by: The electric field strength between two parallel plates is the ratio of the potential difference and separation of the plates

• The direction of the electric field is from the plate connected to the positive terminal of the cell to the plate connected to the negative terminal
• The electric fields lines are parallel and equidistant

### Forces on Charges

• The force on a charge in a uniform electric field is the product of the electric field strength E and the magnitude of the charge q Force on a charge in a uniform electric field is in the same direction as the electric field

• A uniform electric field (e.g. between two charged parallel plates) is one where the electric field lines are parallel and equidistant
• The force on the charge is constant
• The force will be in parallel to the direction of the electric field
• If the charge if positive, the force is in the same direction as the electric field
• If the charge is negative, the force is in the opposite direction to the electric field
• q is always the charge of the particle itself, not the charge of a particle creating an electric field

#### Exam Tip

• To remember which way the force acts on a charged particle, remember electric fields are always from positive to negative
• A positive charge will be attracted towards a negative plate (in the same direction as the field)
• A negative charge will be attracted towards a positive plate (the opposite direction as the field)

### Motion of Charged Particles

• A charged particle experiences an electric force F in an electric field E which will cause it to move
• The force on the particle is constant
• When a charged particle is kept still in a uniform electric field, it will move in the same or opposite direction to the electric field lines
• If a charged particle is in motion through an electric field, it will experience a constant electric force and travel in a parabolic trajectory: Charged particles in an electric field follow a parabolic trajectory

• The direction of the parabola will depend on the charge of the particle
• A positive charge will be deflected towards the negative plate
• A negative charge will be deflected towards the positive plate

#### Worked example • The proton has a charge of +1e so is deflected upwards towards the negative plate
• Since a neutron is neutral (it has no charge), it experiences no electric force in the electric field and travels straight through between the plates undeflected ### Author: Ashika

Ashika graduated with a first-class Physics degree from Manchester University and, having worked as a software engineer, focused on Physics education, creating engaging content to help students across all levels. Now an experienced GCSE and A Level Physics and Maths tutor, Ashika helps to grow and improve our Physics resources.
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