Electricity is the presence or flow of charged particles. There are two ($2$) types of electricity:
- Static electricity - this is associated with an excess or lack of charged particles e.g. too many electrons or missing electrons
- Current electricity - this is associated with the movement of charged particles
Current
This is the charge transferred per unit time: $$ \begin{equation}\begin{aligned} I&=\frac{Q}{t}\\ \end{aligned}\end{equation} $$
Thus charge is the product of current and time: $$ \begin{equation}\begin{aligned} Q&=It\\ \end{aligned}\end{equation} $$
A Faraday is the charge associated with discharging one mole of a singly charged species using a mole of electrons: $$ \begin{equation}\begin{aligned} Q&=ne\\ &=6.022\times 10^{23}\text{electron}\times 1.6\times 10^{-19}C/\text{electrons}\\ &=96,352 C\\ \end{aligned}\end{equation} $$
Example Find the amount of charge passed when a current of $4A$ is used for $20s$.
Solution The charge is the product of the current and the time: $$ \begin{equation}\begin{aligned} Q&=It\\ &=4A\times 20s\\ &=80C\\ \end{aligned}\end{equation} $$
Potential difference
Electric potential, $V$ at a point $A$ is the work done per unit charge in moving a charged point from infinity to that point $A$:
$$ \begin{equation}\begin{aligned} V_A&=\frac{E_A}{Q}\\ \end{aligned}\end{equation} $$Because it is impractical to consider moving a charged particle from infinity to a point, we can instead use the difference in the electric potential of two points. Thus the electric potential difference is simply a matter of measuring electric potential of one $B$ relative to another point $A$ rather than relative to infinity: $$ \begin{equation}\begin{aligned} V_A&=\frac{E_A}{Q}\\ V_B&=\frac{E_B}{Q}\\ \therefore V&=V_B-V_A\\ \end{aligned}\end{equation} $$
From the formula for potential difference: $$ \begin{equation}\begin{aligned} V&=\frac{E}{Q}\\ \therefore E&=QV\\ \end{aligned}\end{equation} $$
Recall that $Q=It$: $$ \begin{equation}\begin{aligned} E&=QV\\ &=(It)V\\ &=IVt\\ \end{aligned}\end{equation} $$
Power is the work done or energy used per unit time: $$ \begin{equation}\begin{aligned} P&=\frac{E}{t}\\ \therefore P&=\frac{IVt}{t}\\ P&=IV\\ \end{aligned}\end{equation} $$
Thus electric power is the product of the current flowing and the potential difference across the ends of the conductor.
Electromotive force
Voltages can be either potential differences (PDs) or electromotive forces (EMFs). EMFs are a misnomer as the concept is not at all a force but rather the increase in potential for a circuit.
| Aspect | EMF | PD |
|---|---|---|
| Definition | Max voltage available from a power source | Voltage drop across a device |
| Associated devices | Active devices (add energy to circuit) | Passive devices (use energy in the circuit) |
| Examples of devices | Batteries, generators | Resistors, motors, heating elements, LEDs |
| Effect of resistance | Not affected by resistance | Changes with resistance |
| Energy in circuit | Energy is gained | Energy is lost |
| Change in electric potential | Increases potential | Decreases potential |
| Current | Present regardless of current | No current, no PD |
Series and parallel circuits
A circuit is simply a path for electric current to flow. A series circuit is a circuit where there is only one path for current to flow. A parallel circuit has multiple paths.
| Property | Series | Parallel |
|---|---|---|
| Current | Is the same ($I_1=I_2=…$) | Is additive ($I_T=I_1+I_2+…$) |
| Voltage | Is additive ($V_T=V_1+V_2+…$) | Is the same ($V_1=V_2=…$) |
Electrical appliances are placed in parallel so that they can have the same input voltages. Of course, adaptors are used to break down the standard voltage (e.g. $110V$, $220V$, etc.) to a voltage usable by the appliance.