Field Interactions: Attraction and Repulsion

This section explores the fundamental differences in interactions between gravitational, electric, and magnetic fields, focusing on attraction and repulsion.

1. Fundamental Forces and Fields

• Fields: Regions of space where an object experiences a force. We consider three fundamental fields:

  • Gravitational Field: Exists around objects with mass.
  • Electric Field: Exists around objects with electric charge.
  • Magnetic Field: Exists around magnets and moving electric charges (currents).

• Force: An interaction that, when unopposed, will change the motion of an object.

2. Gravitational Fields

• Source: Mass.
• Interaction: Always attractive.
• Nature:
  • Masses only attract each other. There is no such thing as “negative mass” that would cause repulsion.
  • The gravitational force is described by Newton’s Law of Universal Gravitation:
    $$F = G \frac{m_1 m_2}{r^2}$$
    where:
    • $F$ is the gravitational force between two masses.
    • $G$ is the gravitational constant (\$6.674 \times 10^{-11} \, \text{N m}^2 \text{kg}^{-2}$).
    • $m_1$ and $m_2$ are the masses of the two objects.
    • $r$ is the distance between the centers of the two masses.

KEY TAKEAWAY: Gravity is always attractive, acting between any two objects with mass.

3. Electric Fields

• Source: Electric charge (positive or negative).
• Interaction: Can be attractive or repulsive.
• Nature:
  • Like charges repel each other (positive-positive or negative-negative).
  • Opposite charges attract each other (positive-negative).
  • The electric force is described by Coulomb’s Law:
    $$F = k \frac{q_1 q_2}{r^2}$$
    where:
    • $F$ is the electric force between two charges.
    • $k$ is Coulomb’s constant (\$8.9875 \times 10^9 \, \text{N m}^2 \text{C}^{-2}$).
    • $q_1$ and $q_2$ are the magnitudes of the two charges.
    • $r$ is the distance between the charges.

EXAM TIP: Remember to consider the sign of the charges when determining the direction of the electric force. Positive force implies repulsion, negative force implies attraction.

4. Magnetic Fields

• Source: Moving electric charges (currents) and magnetic dipoles (North and South poles).
• Interaction: Can be attractive or repulsive.
• Nature:
  • Like poles repel each other (North-North or South-South).
  • Opposite poles attract each other (North-South).
  • Current-carrying conductors:
    • Parallel currents in the same direction attract each other.
    • Parallel currents in opposite directions repel each other.
  • The force on a current-carrying conductor in a magnetic field is given by:
    $$F = nIlB \sin{\theta}$$
    where:
    • $F$ is the force on the conductor.
    • $n$ is the number of wires.
    • $I$ is the current in the conductor.
    • $l$ is the length of the conductor within the magnetic field.
    • $B$ is the magnetic field strength.
    • $\theta$ is the angle between the direction of the current and the magnetic field.

COMMON MISTAKE: Failing to consider the direction of current flow when determining the direction of the magnetic force using the right-hand rule.

5. Comparison Table

STUDY HINT: Create flashcards with the source, interaction type, and governing equations for each field to aid memorization.

6. Summary

• Masses always attract each other due to the gravitational force.
• Electric charges can attract or repel depending on their sign (positive or negative).
• Magnetic poles can attract or repel depending on their polarity (North or South). Current carrying conductors can attract or repel if the currents are parallel.

REMEMBER: Gravitational force: Always attractive. Electric and magnetic forces: Attractive or repulsive.

7. Force on a Charged Particle in a Magnetic Field

The force on a single charged particle moving in a magnetic field is given by:

$$F = qvB \sin{\theta}$$

where:

• $F$ is the magnetic force on the charge.
• $q$ is the magnitude of the charge.
• $v$ is the velocity of the charge.
• $B$ is the magnetic field strength.
• $\theta$ is the angle between the velocity vector and the magnetic field vector.

If the velocity and the magnetic field are perpendicular ($\theta = 90^\circ$), the force is maximum: $F = qvB$. If they are parallel ($\theta = 0^\circ$), the force is zero.

APPLICATION: Particle accelerators use electric and magnetic fields to accelerate and steer charged particles. Understanding the attractive and repulsive forces is crucial for their design.

8. Visual Representation (Description)

A diagram could show:

• Two masses attracting each other with gravitational force lines.
• Two positive charges repelling each other with electric field lines.
• A positive and a negative charge attracting each other with electric field lines.
• Two bar magnets with like poles repelling and unlike poles attracting, showing magnetic field lines.
• Two parallel wires, both carrying current in the same direction, attracting each other.

VCAA FOCUS: VCAA often tests your ability to compare and contrast the three fields. Pay attention to their similarities (e.g., field model) and differences (e.g., attractive vs. repulsive interactions).