7.1.1

# Fields

Test yourself

## 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.