Force fields arise from the interaction of mass, of static charge, and…

The idea

A field is a region where a suitable test object experiences a force. Mass creates a gravitational field: place a small test mass m at a point and measure the force per unit mass, g = F/m, directed towards the mass. Static charge creates an electric field: a small positive test charge q experiences F = qE, with E = F/q. Field lines map direction and relative strength, and field strength falls with distance. Moving charges interact magnetically: a current-carrying wire in a magnetic field feels F = BIL sin θ, and a single charge feels F = Bqv sin θ, with force perpendicular to both velocity and field. Thus gravitational, electric and magnetic fields all arise from mass, static charge, and moving charge respectively.

What earns the marks

• Defines a field as a region where a test mass, test charge or moving charge experiences a force, and distinguishes the three sources: mass, static charge, moving charge.
• Uses g = F/m for gravitational field strength and E = F/q for electric field strength, stating that both are vector quantities with direction given by the force on a positive test quantity.
• Explains that field lines show the direction of the force on a positive test charge or on a test mass, and that line spacing indicates relative field strength.
• Applies F = BIL sin θ to a current-carrying conductor and F = Bqv sin θ to a moving charge, noting the force is perpendicular to both the field and the velocity or current.
• Describes how field strength decreases with distance for point mass and point charge, linking to inverse-square behaviour, and contrasts this with the uniform field between parallel plates.

Common mistakes and how to put them right

• Treating the electric field direction as the direction of force on a negative charge.

  Put it right: E points in the direction of force on a positive test charge, so a negative charge feels force opposite to E.

• Assuming a magnetic force acts on a stationary charge in a magnetic field.

  Put it right: A magnetic force requires relative motion, so F = Bqv sin θ is zero when v = 0.

• Confusing field strength with potential or with force.

  Put it right: Field strength is force per unit test quantity, whereas potential is energy per unit test quantity, and the two are related but distinct.

• Using F = BIL without the sin θ factor when the conductor is not perpendicular to the field.

  Put it right: Include sin θ, which equals 1 only at 90°.

Exam tips

• When asked to compare fields, name the source, the test quantity and the defining equation before substituting numbers; this makes the physics explicit and avoids vague descriptions.
• For magnetic force questions, sketch the field direction, the current or velocity direction and the force direction, then apply Fleming's left-hand rule to check the direction before calculating magnitude.
• If a question gives a graph of force against distance or against 1/r², state the relationship shown and use the gradient or area only where the axes make that physically meaningful.