Electric Fields

What is an electric field?

A region of space where charged particles experience an electrostatic force.

How can static charge be produced?

By friction, e.g. rubbing glass with silk.

What happens when glass is rubbed with silk?

Electrons move from glass to silk.

What charge does the glass gain?

Positive (loss of electrons).

What charge does the silk gain?

Negative (gain of electrons).

What is induced charge?

Charge separation in a neutral object caused by a nearby charged object.

Does induction change total charge?

No, the object remains neutral overall.

Why are neutral objects attracted to charged rods?

Because induced dipoles create electrostatic attraction.

What is a point charge?

A charge assumed to exist at a single point in space.

What is the field shape around a point charge?

Radial.

Field direction around a positive charge?

Outwards.

Field direction around a negative charge?

Inwards.

What determines electric field line density?

Field strength.

How does field strength vary with distance from a point charge?

It decreases with distance.

Why can charged spheres be treated as point charges?

Because their external field is radial.

What do electric field lines show?

Direction of force on a positive test charge.

Where do field lines start and end?

Start at positive, end at negative.

How do field lines show attraction?

Lines join between opposite charges.

How do field lines show repulsion?

Lines do not join between like charges.

Define electric field strength.

Force per unit charge.

Electric field strength formula?

E = F/Q

Unit of electric field strength?

N C⁻¹ or V m⁻¹

Is electric field strength scalar or vector?

Vector.

State Coulomb’s law.

Force ∝ product of charges and inversely ∝ distance².

Coulomb’s law formula?

F = kQq/r²

Value of Coulomb constant k?

8.99 × 10⁹ N m² C⁻²

Alternative form using permittivity?

F = Qq/(4πɛ₀r²)

Permittivity of free space value?

8.85 × 10⁻¹² F m⁻¹

What does permittivity describe?

Ability of a medium to polarise/store charge.

Electric field of a point charge formula?

E = Q/(4πɛ₀r²)

How is point charge field derived?

From Coulomb’s law and E = F/Q.

Property creating gravitational field?

Mass.

Property creating electric field?

Charge.

Are gravitational fields attractive or repulsive?

Always attractive.

Are electric fields attractive or repulsive?

Both.

Gravitational field strength formula?

g = F/m

Electric field strength analogue?

E = F/Q

Shape of gravitational point field?

Radial.

Shape of electric point field?

Radial.

What is a uniform electric field?

A constant field independent of position.

Where is a uniform field produced?

Between parallel plates.

Force on a charge in uniform field?

Constant.

Define work done.

Force × distance moved in force direction.

Work done formula?

W = Fd

Define potential difference.

Work done per unit charge.

Potential difference formula?

V = W/Q

Define electric potential.

Work done per unit charge from infinity.

Potential at infinity convention?

Zero.

Relation between field and potential in uniform field?

E = V/d

When is E = V/d valid?

Only in uniform fields.

Unit equivalence for E?

N C⁻¹ = V m⁻¹

Motion of charged particle in uniform field?

Projectile-like motion.

What experiment measured electron charge?

Millikan oil drop experiment.

How were oil drops charged?

By gaining electrons.

For hovering drops, what forces balance?

Electric force and (weight).

Balance equation in Millikan experiment?

mg = EQ

Charge quantisation formula?

Q = ne

Elementary charge value?

1.602 × 10⁻¹⁹ C

Why were only certain voltages observed?

Charge exists in integer multiples.

Force between point charges graph vs distance?

Inverse square decrease.

Work from force–distance graph?

Area under graph.

Electrical potential energy of point charges?

Ee = Qq/(4πɛ₀r)

Electric potential of a point charge?

V = Q/(4πɛ₀r)

Sign of potential energy for opposite charges?

Negative.

Why is it negative?

Energy released as charges attract.

Energy needed to separate opposite charges?

Equal to magnitude of negative potential energy.