Electric Fields

Electric Fields

High School

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Learning Objectives

Key Vocabulary

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Electric Field

An invisible force field around a charged object that exerts force on other charges.

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Test Charge

A small, positive charge used to determine the strength and direction of an electric field.

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Field Strength

A measure of the intensity of an electric field at a particular location in the field.

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Grounding

The process of discharging a charged body by allowing excess electrons to flow into the Earth.

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What Is an Electric Field?

• An electric field is an invisible area of influence around a charged object.

• It exerts an electric force of attraction or repulsion on other nearby charges.

• Electric field lines show the direction of the force on a positive charge.

• These lines point away from positive charges and towards negative charges.

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Solved Example 1
A small positive test charge of 2.0 x 10^-9 C is placed in an electric field where it experiences a force of 4.0 x 10^-3 N. What is the electric field strength?

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Step 1: Analyze and Sketch the Problem

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Solved Example 1
A small positive test charge of 2.0 x 10^-9 C is placed in an electric field where it experiences a force of 4.0 x 10^-3 N. What is the electric field strength?

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Step 2: Solve for the Unknown

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Solved Example 1
A small positive test charge of 2.0 x 10^-9 C is placed in an electric field where it experiences a force of 4.0 x 10^-3 N. What is the electric field strength?

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Step 3: Evaluate the Answer

Multiple Choice

Which statement correctly describes the electric field lines around a single, isolated negative charge?

• They point radially outward from the charge.

• They form concentric circles around the charge.

• They point radially inward toward the charge.

• They do not exist for negative charges.

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Electric Field Strength and Distance

• Electric field strength measures the intensity of an electric field at a point.

• ​Field line spacing shows the electric field's strength; closer lines mean a stronger field.

• As distance from a charge increases, the electric field becomes much weaker.

• Consequently, electrical force is strongest when charges are very close to each other.

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Solved Example 2
A point charge of +3.0 µC creates an electric field. Using Coulomb's constant (k = 8.99 x 10⁹ N·m²/C²), what is the electric field strength at a distance of 0.50 m from the charge?

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Step 1: Analyze and Sketch the Problem

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Solved Example 2
A point charge of +3.0 µC creates an electric field. Using Coulomb's constant (k = 8.99 x 10⁹ N·m²/C²), what is the electric field strength at a distance of 0.50 m from the charge?

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Step 2: Solve for the Unknown

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Solved Example 2
A point charge of +3.0 µC creates an electric field. Using Coulomb's constant (k = 8.99 x 10⁹ N·m²/C²), what is the electric field strength at a distance of 0.50 m from the charge?

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Step 3: Evaluate the Answer

• The unit of the result is Newtons per Coulomb (N/C), which is the correct unit for electric field strength.

• Since the source charge is positive, the electric field vector points radially outward from the charge. The magnitude is a large number, which is expected given the large value of Coulomb's constant.

Multiple Choice

If you move a test charge to a location where the electric field lines are much farther apart, what does this imply about the electric field strength at the new location?

• The field direction has reversed.

• The field strength is weaker.

• The field strength has not changed.

• The field strength is stronger.

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Calculating Electric Field Strength

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Multiple Choice

A test charge of 2 C experiences an electric force of 10 N. What is the magnitude of the electric field at this point?

• 0.2 N/C

• 12 N/C

• 5 N/C

• 20 N/C

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Uniform Fields and Conductors

Uniform Electric Fields

• ​A uniform electric field has the same magnitude and direction at every point in space.

• ​​It is created between two large, parallel conducting plates with opposite electrical charges.

• ​The electric field lines point away from the positive plate towards the negative plate.

Charges in Conductors

• ​In any closed conductor, electric charges are mobile and free to move around.

• ​​These charges repel each other and spread out to rest on the outer surface.

• ​The electric field is zero everywhere inside a closed, charged conductor, whether solid or hollow.

Multiple Choice

Where would you find a region with a zero electric field?

• Very close to a single positive charge.

• At the center of a hollow, charged metal sphere.

• Exactly halfway between two parallel charged plates.

• On the sharp point of a charged conductor.

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Energy, Sparks, and Grounding

• Energy is stored in an electric field, giving charges potential energy.

• ​A very strong electric field can create a visible spark in air.

• Grounding is a safety process to discharge an object to the Earth.

• It provides a path for excess electrons to flow away safely.

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Multiple Choice

What is the primary purpose of grounding a charged electrical object?

• To safely discharge excess charge into the Earth.

• To strengthen its electric field.

• To increase its potential energy.

• To create a powerful spark.

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Common Misconceptions: Electric Fields

Misconception	Correction
Electric field lines show the actual path a charged particle will follow.	They show the direction of force on a positive charge at a single point.
Electric fields only exist when there are two or more charges.	A single, isolated charge creates its own electric field in the space surrounding it.
The electric field inside a charged solid conductor is non-zero.	The net electric field inside any charged conductor in equilibrium is always zero.

Multiple Choice

An engineer is designing a device and needs the strongest possible electric field and charge concentration at the surface. Based on the principles of charge distribution on conductors, which shape should the conductor have?

• A large, smooth sphere.

• A hollow cube.

• A flat, wide plate.

• A conductor with several sharp points.

Multiple Choice

If an electric field has a strength of 500 N/C, what is the magnitude of the force it would exert on a particle with a charge of 3.0 x 10^-6 C?

• The force cannot be determined without knowing the distance.

• 1.67 x 10⁸ N

• 6.0 x 10^-9 N

• 1.5 x 10^-3 N

Multiple Choice

A hollow, uncharged conducting sphere is placed in a strong, uniform external electric field. Analyze the resulting electric field inside the sphere.

• The field inside points opposite to the external field but is weaker.

• The field inside is equal to the external field because the field lines pass through the conductor.

• The field inside depends on the size of the sphere.

• The field inside is zero because the charges on the conductor's surface rearrange to cancel the external field.

Multiple Choice

A scientist measures the force on a test charge at a certain distance from a source charge. If the source charge is then doubled, what is the expected outcome for the force on the test charge at the same distance?

• The force will be quartered.

• The force will be halved.

• The force will be doubled.

• The force will be quadrupled.

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Summary

Poll

On a scale of 1-4, how confident are you about the concepts covered in today's review?

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