Electric Field and Electric Field Intensity
Simple Explanation
An electric field is the region of influence surrounding a charge, where another charge would experience a force. Electric field intensity (or field strength) quantifies this influence as the force per unit charge — a way of describing the field's strength independent of any particular test charge placed in it.
Why Do We Need It?
The field concept lets physicists describe the 'influence' surrounding a charge even before another charge is placed there to feel a force — this is a powerful, general way of thinking that extends to gravitational and magnetic fields too.
Formula
Electric Field Intensity
E = F / q
Electric field intensity measures the force per unit charge that would be experienced by a small positive test charge placed at a point.
- E
- — Electric field intensity, in N/C (or equivalently V/m)
- F
- — Force experienced by the test charge, in newtons
- q
- — Magnitude of the test charge, in coulombs
When to use it: Use to find the electric field strength at a point from the force it would exert on a known test charge, or to find the force on a charge placed in a known field.
Worked Example
Finding electric field intensity
A test charge of 2 × 10⁻⁶ C experiences a force of 0.4 N at a point. Find the electric field intensity there.
Why Does This Work?
Dividing the force by the test charge's magnitude removes the test charge's own size from the answer, leaving a quantity that describes ONLY the field itself — this means E is a property of the source charge and location, useful regardless of what test charge (if any) happens to be placed there.
Real-Life Example
Lightning and strong electric fields
Lightning occurs when the electric field between storm clouds and the ground becomes extremely strong.
When the electric field intensity in the air becomes strong enough (around 3 million V/m), it overcomes the air's insulating properties and ionises it, allowing a massive discharge of current — lightning — to occur.
Practice
A charge of 5 × 10⁻⁶ C experiences a force of 0.1 N. Find the electric field intensity.
MediumCommon mistake
Thinking electric field intensity depends on the test charge used to measure it — E is a property of the SOURCE charge and location; dividing by q specifically removes the test charge's influence from the result.
Quick Review
- E = F/q defines electric field intensity as force per unit charge.
- The field exists independently of whether a test charge is present.
- Very strong fields (like in a storm) can ionise air, causing lightning.