Test on Electric and magnetic fields

Question 1: If the distance from a point charge Q to point P in the example calculation was 0.60 m instead of 0.30 m, the electric field strength at point P would be 2.5 × 10⁵ N/C.

A. Yes

B. No

Explanation: According to the study materials, the magnitude of the electric field strength for a point charge is given by E = k · |Q| / r². In the example, Q = 5.0 × 10⁻⁶ C and k = 8.9875 × 10⁹ N·m²/C². If r was 0.60 m, the calculation would be E = (8.9875 × 10⁹ N·m²/C²) · (5.0 × 10⁻⁶ C) / (0.60 m)² = (8.9875 × 10⁹) · (5.0 × 10⁻⁶) / (0.36) = (44.9375 × 10³) / 0.36 = 124,826.39 N/C. Rounding to two significant figures, this is approximately 1.2 × 10⁵ N/C, not 2.5 × 10⁵ N/C. The original field at 0.30m was 5.0 x 10^5 N/C, so at double the distance, the field strength would be one-fourth (r^2 in the denominator), which is 1.25 x 10^5 N/C.

Question 2: For a long, straight current-carrying wire, the magnitude of the magnetic field induction at a given point is inversely proportional to the perpendicular distance from the wire to that point.

A. Yes

B. No

Explanation: The study materials state the formula for the magnetic field induction of a long, straight current-carrying wire as B = μ₀ · I / (2πr). In this formula, B is inversely proportional to r (the perpendicular distance from the wire), meaning as r increases, B decreases proportionally.

Question 3: In a simulation designed to observe magnetic fields and the Lorentz force, which of the following actions would lead to a predictable change in the observed magnetic phenomena?

A. Reversing the direction of current in a wire to observe the magnetic field lines changing their orientation.

B. Placing a stationary charged particle in a magnetic field to observe it moving in a circular path.

C. Varying the magnitude of the current in a wire to see how the density of magnetic field lines around it changes.

D. Changing the sign of a moving charged particle to reverse the direction of the Lorentz force it experiences.

Explanation: Reversing the current direction in a wire causes the magnetic field lines to reverse their orientation, as stated in Experiment 2. Varying the magnitude of the current changes the magnetic field strength, which is observed as a change in the density of the magnetic field lines, also from Experiment 2. Changing the sign of a moving charge will reverse the direction of the Lorentz force and the curvature of its path, as described in Experiment 3. However, a stationary charged particle experiences no magnetic force, so placing it in a magnetic field would not cause it to move in a circular path; its velocity must be non-zero for a magnetic force to exist, as highlighted in point 4 under 'Lorentz Force on Stationary Charges'.

Question 4: Based on Experiment 1: Charges and Electric Fields in a simulation, a student would be able to perform all of the following actions or observe all of the following phenomena, EXCEPT:

A. Observe electric field lines forming continuous closed loops around a source charge.

B. Change the magnitude of charges and see how the density of field lines varies.

C. Place a positive test charge to see the direction and magnitude of the force it experiences.

D. Move a test charge to confirm that the force is always tangent to the electric field lines.

Explanation: The study materials state that electric field lines originate from positive charges and terminate on negative charges, and they never cross. The concept of field lines forming continuous closed loops is specifically mentioned for magnetic field lines, not electric field lines. Therefore, observing electric field lines forming closed loops would contradict the information provided for Experiment 1 and the general description of electric field lines.

Question 5: A Lorentz force is exerted on a charged particle when it moves parallel to the magnetic field.

A. Yes

B. No

Explanation: The study materials state that if the charge is moving parallel or anti-parallel to the magnetic field, the force is zero. Therefore, no Lorentz force is exerted in this condition.