Quiz: Electric Charge and Electric Fields
Test your understanding of electric charge and electric fields with these 10 questions.
1. What is electric charge?
- The speed of movement of electrons
- A fundamental property that determines how objects interact electromagnetically
- The number of atoms in an object
- The voltage applied to an object
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The correct answer is B. Electric charge is a fundamental property of matter that determines how it interacts with electric and magnetic fields. Charge comes in two types: positive and negative. Like charges repel, unlike charges attract. Charge is quantized—it comes in units of the elementary charge (e = 1.6 × 10⁻¹⁹ C).
Concept Tested: Electric Charge
2. What does Coulomb's Law describe?
- The motion of charged particles
- The force between two point charges: F = kq₁q₂/r²
- The creation of electric fields
- The energy stored in a charged object
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The correct answer is B. Coulomb's Law gives the magnitude of the electrostatic force between two point charges: F = kq₁q₂/r², where k ≈ 9 × 10⁹ N·m²/C². Force is proportional to both charges and inversely proportional to the square of separation distance. Like charges repel, opposite charges attract.
Concept Tested: Coulomb's Law
3. Two point charges of +2 C and +3 C are separated by 1 m. What is the force between them? (Use k = 9 × 10⁹ N·m²/C²)
- 5 N
- 6 N
- 5.4 × 10⁹ N
- 5.4 × 10¹⁰ N
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The correct answer is C. F = kq₁q₂/r² = (9 × 10⁹)(2)(3)/(1)² = (9 × 10⁹)(6) = 5.4 × 10¹⁰ N. This is a repulsive force since both charges are positive. The enormous magnitude shows why electrical forces are so strong compared to gravitational forces.
Concept Tested: Coulomb's Law
4. What is an electric field?
- The force exerted by a charge
- The region where a charged object exerts an electric force on other charges
- The energy of a charged particle
- The motion of electrons through a conductor
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The correct answer is B. An electric field is the region around a charge where it exerts forces on other charges. It's a vector field with magnitude E = F/q (force per unit charge). The field direction is defined as the direction a positive test charge would experience a force. Electric fields point away from positive charges and toward negative charges.
Concept Tested: Electric Field
5. What do electric field lines indicate?
- The paths that charges must follow
- The direction of the electric field and the relative field strength
- The shape of a conductor
- The energy stored in the field
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The correct answer is B. Electric field lines show the direction of the electric field at any point. They emanate from positive charges and terminate on negative charges. The density of field lines indicates field strength—closely spaced lines mean strong field, widely spaced lines mean weak field. Field lines never cross.
Concept Tested: Electric Field Lines
6. What is the relationship between electric field and electric potential?
- They are the same thing
- Electric field is the negative gradient of potential: E = -dV/dx
- Potential is a vector while field is a scalar
- There is no relationship between them
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The correct answer is B. Electric potential (V) is the work per unit charge to move a test charge from a reference point. Electric field (E) is related to potential by E = -dV/dx. Potential decreases in the direction of the electric field. Electric field is more fundamental; potential is derived from it.
Concept Tested: Electric Potential
7. What is electric potential difference (voltage)?
- The strength of the electric field
- The work per unit charge needed to move a charge between two points
- The charge on an object
- The current flowing through a circuit
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The correct answer is B. Potential difference (voltage) is the work per unit charge: V = W/q. It's measured in volts (V), where 1 V = 1 J/C. A 12 V battery does 12 joules of work for every coulomb of charge that flows through it. Voltage drives charge flow in circuits.
Concept Tested: Voltage
8. A positive charge is moved against the direction of an electric field. What happens to its electric potential energy?
- It decreases
- It remains constant
- It increases
- It becomes zero
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The correct answer is C. Moving a positive charge against the electric field (opposite to field direction) requires work against the field. This work is stored as increased electric potential energy, similar to lifting an object against gravity. The electric potential increases in this direction.
Concept Tested: Electric Potential Energy
9. What are equipotential surfaces?
- Surfaces where the electric field is perpendicular
- Surfaces at the same electric potential
- The surface of a conductor
- Imaginary spheres around point charges
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The correct answer is B. Equipotential surfaces are locations where the electric potential has the same value. Moving along an equipotential surface requires no work because potential doesn't change. Equipotential surfaces are always perpendicular to electric field lines, just as surfaces of equal height are perpendicular to gravitational field direction.
Concept Tested: Equipotential Surfaces
10. How is charge conserved in an isolated electrical system?
- Charge can be created from energy
- The total charge remains constant; it cannot be created or destroyed, only transferred between objects
- Charge always flows from positive to negative
- Conservation of charge only applies in circuits
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The correct answer is B. Conservation of charge is a fundamental principle: the total electric charge in an isolated system is constant. Charge can move between objects or change form (e.g., creating electron-positron pairs requires energy from outside the system), but the net charge cannot change. This principle is essential for understanding electrical phenomena.
Concept Tested: Conservation of Charge