Electric Fields

Definition

An electric field is an area where a charged particle will experience a force.

Charges with the same polarity repel, and charges with opposite polarities attract.

A charged object of any polarity will attract an uncharged object, due to the movement of electrons in the uncharged object. Objects with more free-moving electrons will create a stronger attraction.

Electric Field Diagrams

An electric field diagram shows the influence of charged objects on an electric field. Field lines show the path which a positive test charge, and more field lines signifies a stronger electric field.

Common Mistakes

Field lines not making contact with charges
Incorrectly labelled arrows
Field lines touching each other
Uneven spacing of field lines around charges
Field lines not perpendicular to charges

Voltage

When a charged particle is in an electric field, it will experience a force which gives it the potential to move. When a charged particle moves in an electric field, its potential changes as well. This is called potential difference (p.d.) or voltage.

Voltage is the work done per coulomb of charge.

Formula

W = Q V

Variable Key

$W$ is the work done (energy), in joules.
$Q$ is the charge, in coulombs.
$V$ is the potential difference or voltage, in volts.

The Electronvolt

Energy is sometimes measured in electronvolts instead of joules, usually when talking about very small amounts of energy relevant to charged particles. One electronvolt is one volt times the charge of an electron, about $1.6 \times 10^{- 19} C$ (provided on the SQA exam data sheet).

Electron Guns

An electron gun is a device with an anode and heated cathode, with a high voltage across them. This causes electrons to leave the cathode and accelerate very rapidly towards the anode, firing the electron out of the device.

The speed of the fired electrons can be calculated using the kinetic energy formula, $E_{k} = \frac{1}{2} m v^{2}$ . The kinetic energy is equal to the work done from $W = Q V$ , and the mass of an electron is approximately $9.11 \times 10^{- 31} kg$ (also provided on the SQA exam data sheet).