Certain insulating materials become charged when rubbed together.
Electrons are transferred when objects become charged.
Like charges repel; unlike charges attract.

Every component has its own agreed symbol.

I = Q / T

V = W / Q = E / Q
R = V / I
Ohm's law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.
Reversing the current through a component reverses the potential difference across it.

Filament bulb: resistance increases with increase of the filament temperature.
Diode: 'forward' resistance low; 'reverse' resistance high.
Thermistor: resistance decreases if its temperature increases.
LDR: resistance decreases if the light intensity on it increases.

For components in series:
-the current is the same in each component.
-adding the potential differences gives the total potential difference.
-adding the resistances gives the total resistance.

For components in parallel:
-the total current is the sum of the currents through the separate components.
-the bigger the resistance of a component the smaller the current is.
In a parallel circuit the potential difference is the same across each component.
To calculate the current through a resistor in a parallel circuit use I = V / R.