Fields
Physics: AQA A Level, topic 7.1.1
Force Fields
Fields are a convenient mathematical idea for representing how forces and potential energies change in a region of space around an object. Forces arise from the interactions between objects.
Gravitational force
- Gravitational force arises because of the interaction between two masses.
- We map gravitational fields by considering the effect of that field on a point mass.
Electrostatic force
- Electrostatic force arises because of the interaction between two charges.
- We map electric fields by considering the effect of that field on a point positive charge.
Magnetic force
- Magnetic force arises, fundamentally, because of the interaction between charges that have relative motion between them.
- When we map magnetic fields, we simplify matters by considering the effect on a small bar magnet, or magnetic dipole.
Comparing Gravitational and Electrostatic Forces
Gravitational and electrostatic forces have similarities and differences.
Similar ideas
- If we consider both as fields, we can apply similar ideas.
- We use field lines to represent the direction of the force on a point particle.
- We use the concept of equipotential lines and surfaces.
Similarities
- When considering point objects, both forces have inverse-square laws.
- The field line patterns around point objects are radial - like spokes on a bicycle wheel.
- The field lines get further apart as the distance increases from the point object. This indicates that the force on a test particle gets smaller.
Similarities 2
- Equipotential lines/surfaces around point objects are concentric circles or spheres.
- Those equipotential lines get further apart for equal changes in potential between lines.
- You can add field strengths from two or more particles vectorially. This means you can determine field line patterns from complex arrangements of particles.
Differences
- The force between masses is always attractive.
- Field lines always point towards masses.
- Potential-distance graphs for gravitation are always negative.
- The force between charges can be either attractive or repulsive.
- Field lines always point away from positive charges and towards negative charges.
- Potential-distance graphs for a point charge can either be positive or negative.
Test yourself on Fields
Which force arises due to the interaction between masses?
Which force arises due to the interaction between static charges?
Which force arises due to the interaction between charges moving relative to one another?
Which of the following are types of fields?
Which of these is NOT true for gravitational fields?