Electrical quantities are the physical quantities (the measurable properties of a system or object) related to the study of electricity. Some important electrical quantities are:
- current ($I$)
- voltage ($V$)
- resistance ($R$)
- power ($P$)
Charge can be quantized
Elementary charge (a.k.a. quanta) refers to the charge of an electron ($1.6\times 10^{-19}$). We can thus express the charge on an object in terms of the elementary charge:
$$Q=\pm Ne$$ where $N$ is the number of electrons present.
Note that the number of electrons must be a whole number
Challenge
1) Electrical charge can be found in discrete packets.
- False
- True
These discrete packets are called quanta.
The Coulomb
This is the amount of charge carried by unit current for unit time: $$ \begin{equation}\begin{aligned} Q&=It\\ 1C&=1A\times 1s\\ \end{aligned}\end{equation} $$
It is the SI unit for electric charge.
Challenge
2) The Coulomb is equivalent to the
- Volt per second
- Joule per Coulomb
- Volt per Joule
- Joule per Volt
Hint: Recall that $W=QV$ where $W$ is work.
Potential difference
Electric potential is the work required to move unit positive charge from infinity to a point. Potential difference is thus the work required to move unit positive charge from one point to another:
$$V=\frac{W}{Q}$$
The SI unit is the Volt: $$1V=\frac{1J}{1C}$$
The Volt is therefore the work required to move unit positive charge between two points.
Electrical resistance
This is the ability of a material to resist the flow of an electric current. It is measured in Ohms ($\Omega$).
Ohm’s Law
The current flowing through a resistor/conductor is directly proportional to the voltage across its ends, given that the temperature remains constant.
$$V=IR$$
Challenge
3) A wire being heated will always have the same resistance.
- False
- True
Resistance varies with temperature.
Other formulae
Some commonly used formulae are:
- $P=IV$
- $P=I^2R$
- $P=\frac{V^2}{R}$