Physics - Electric Fields

Created: March 01, 2022

Flashcards

2022-03-01

What is electric field strength?

The force per unit positive charge.

What is the formula for electric field strength, $E$?
\[E = \frac{F}{Q}\]

What is confusing about $E$ in electric field equations?

It represents electric field strength, not energy.

What are the units of electric field strength?

Newtons per Coulomb, $NC^{-1}$

What assumption are you making for point charges?

That the charge of a sphere can be considered to act from a central point.

In what direction do electric field lines get drawn?

From positive to negative.

What’s true about the field lines on an electric field diagram and a gravitational field diagram?

Electric field lines point outwards, gravitational field lines point inwards.

What are equipotentials?

A region in space where every point in it has equation electric potential energy.

How are equipotentials like contour lines on a map?

You don’t go up or down following contour lines.

What is Coulomb’s law for force $F$?
\[F = \frac{Qq}{4 \pi \epsilon_0 r^2}\]

How does Coulomb’s law

\[F = \frac{Qq}{4\pi \epsilon _ 0 r^2}\]

deal with different charges attracting or repelling one another?

$Qq$ being positive or negative depending on whether $q$ and $Q$ are negative or positive.

What is the name for the constant $\epsilon _ 0$?

The permittivity of free space.

What is the formula for electric field strength $E$ using Coulomb’s law?
\[E = \frac{Q}{4\pi \epsilon_0 r^2}\]

What is always true about electric field lines in relation to the conductor?

They are at right angles.

What is always true about the position of equipotentials in relation to the electric field lines?

They are at right angles.

2022-03-02

What are the two types of electric field under consideration in the electric fields topic?
  • Radial fields
  • Uniform fields

What type of electric field is Coulomb’s law

\[F = \frac{Qq}{4\pi \epsilon _ 0 r^2}\]

relevant to?

Radial fields.

If $\text{work done} = VQ$ but also $\text{work done} = Fd$, how can you come up with an equation for $\frac{F}{Q}$ (which is equivalent to $E$, electric field strength)?
\[\frac{F}{Q} = \frac{v}{d}\]

If

\[\frac{F}{Q} = \frac{v}{d}\]

then what is a nice formula for electric field strength $E$?
\[E = \frac{v}{d}\]

What is an alternative unit for electric field strength given that $E = \frac{v}{d}$?
\[\text{V}\text{m}^{-1}\]

How is an electron in a uniform field (such as the one constructed by two oppositely charged metal plates) like a projectile?

It experiences constant downward acceleration due to the electric field.

How could you work out the formula for the force on a charge in a uniform electric field given that $\frac{F}{Q} = \frac{V}{d}$?
\[F = \frac{QV}{d}\]

2022-03-11

What is electric potential?

The work done to bring a positive charge from infinity to a point.

What is electric potential at some imaginary point at infinity?
\[0V\]

What are the units for electric potential?
\[\text{volts}, V\]

What is the formula for electric potential in terms of electric potential energy?
\[V = \frac{\text{electric potential energy}}{q}\]

What is the formula for electric potential $V$ in terms of $Q$ and $r$?
\[V = \frac{Q}{4\pi \epsilon_0 r}\]

What is the formula for capacitance $C$ in terms of charge stored $Q$ and potential difference $V$?
\[C = \frac{Q}{V}\]

What are the units for capacitance, $C$?
\[CV^{-1} = F = \text{farad}\]

How can you derive the formula for the capacitance of a sphere?

Substitute $V = \frac{Q}{4\pi\epsilon _ 0 r}$ into $C = \frac{Q}{V}$.

What is the formula for the capacitance of a sphere?
\[C = 4\pi \epsilon_0 r\]

What is the formula for electric potential energy $U _ E$ and charges $Q, q$?
\[U_E = \frac{Qq}{4\pi \epsilon_0 r}\]

How can you think about the formulas for electric and potential energies from the inverse-square force law that governs them?

They’re multiplied by $r$ and so don’t have $r^2$ in the denominator.



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