Physics U10

Coulomb’s law is completely analogous to Newton’s law of gravity. Both are “inverse square” laws. Explain what this means.

An inverse square law means that force decreases with the square of the distance between two objects.

Compare contrast Protons and electrons in terms of a) size b) mass c) charge

• Protons are much larger and heavier than electrons.

• Electrons have nearly negligible mass compared to protons.

• Protons have a positive charge, while electrons have an equal but opposite negative charge.

Electrical field strength is completely analogous to Gravitational field strength. Both are inverse square relations, and both are independent of test objects. Explain what this means.

Both electric field strength and gravitational field strength follow an inverse square law and are independent of test objects, meaning their values depend only on the source creating the field, not on any object placed within the field.

Under what circumstances is g constant? Not Constant? Under what circumstances is E constant? Not constant?

g is constant near Earth's surface or in uniform fields but varies with distance and mass distribution. E is constant between parallel plates but varies around point charges or irregular distributions. Both follow inverse square laws.

Electric Potential Energy is completely analogous to Gravitational Potential Energy. Both depend on strength of Field, and on magnitude of the test object. Consider a single proton, in a uniform Electric Field of 10 N/C, at a height of one meter. About how much stronger would g on Earth have to be so that, for this proton, Ug = Ue?

Earth's gravitational field would need to be about 1×10^8 times stronger for Ug=Ue for the proton.

Don’t confuse “Potential Energy” with “Potential.” What is the difference?

• Potential Energy (U) depends on the object's charge or mass.

• Potential (V or φ) is a property of the field itself, independent of the test object.

Electric Field strength can have units of V/m (Volts per meter). Show that these units are equivalent to

N/C (Newtons per Coulomb).

What is the relationship between the following variables? Answer with direct, inverse, or not related: Magnitude of Charge and Electric Force

Direct

Distance and Field Strength for a single source charge

Inverse

Distance and field Strength inside a capacitor

Not Related

Spacing between Equipotential Lines and strength of Field

Inverse

Spacing between Field Lines and Strength of Field,

Inverse

Distance and Potential Energy for a uniform E

Direct

Distance and Voltage for a uniform E

Direct

Field Strength and Voltage for a uniform E

Direct

Electron-Volts, eV, are the units of Energy most useful for work with subatomic particles. Convert each of the following chunks of Energy in eV into the equivalent amount of Joules:

(Recall 1eV = 1.6x10-19 J)