What is an Electric Field?
An electric field is the region around a charged object where another charge would feel a force. It describes how a source charge influences the space around it, independent of any test charge placed there.
The electric field of a point charge Q at distance r is E = kQ/r², measured in newtons per coulomb (N/C); it points away from a positive charge and toward a negative charge.
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Step-by-step worked examples
Find the electric field 2 m from a +6 µC point charge.
E = k Q / r² E = (8.99×10⁹)(6×10⁻⁶) / (2)² E = 53940 / 4 ≈ 13485 N/C
A test charge of 2 µC feels a force of 0.02 N in a field. What is the field strength?
E = F / q E = 0.02 / (2×10⁻⁶) = 10000 N/C
How far from a +4 µC charge is the field equal to 9000 N/C?
E = k Q / r² → r² = k Q / E r² = (8.99×10⁹)(4×10⁻⁶) / 9000 r² = 35960 / 9000 ≈ 3.996 r ≈ 2 m
Flashcards
Quick quiz
Q1.The formula for the electric field of a point charge is:
Q2.What is the SI unit of electric field?
Q3.If the distance from a charge doubles, the field becomes:
Q4.Field lines around a negative point charge point:
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Common mistakes
Confusing electric field with electric force. — Correct: Force depends on a test charge (F = qE); field exists independently of any test charge.
Thinking field strength is constant with distance. — Correct: Field strength falls off as 1/r² for a point charge.
Mixing up field direction for positive and negative charges. — Correct: Field points away from positive charges and toward negative charges.
Forgetting to convert µC to C before calculating. — Correct: Always convert charge to coulombs (×10⁻⁶ for µC) before applying E = kQ/r².
FAQ
What is an electric field?
An electric field is the region around a charged object where another charge experiences a force; its strength is E = kQ/r² for a point charge.
What is the electric field formula?
E = kQ/r², where k ≈ 8.99 × 10⁹ N·m²/C², Q is the source charge, and r is the distance from it.
What are examples of an electric field?
The field around a charged balloon, inside a capacitor, and surrounding a charged atomic nucleus are all electric fields.
How do you calculate an electric field?
Divide Coulomb's constant times the source charge by the square of the distance: E = kQ/r², or use E = F/q if you know the force on a test charge.