# Physics - Electric Fields

> Source: https://ollybritton.com/notes/a-level/physics/topics/electric-fields/ · Updated: 2022-03-01 · Tags: school, physics, year-2, electric-fields

## 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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