• DC stands for Direct Current.

 
• DC current flow is continuous in the same direction.
• Batteries (AA, AAA, C, D etc.) are the simplest form of DC current source.
• A simple DC circuit consists of a current source (e.g. battery) and one or more "loads" (circuit elements).  Each "load" absorbs electrical energy, converting it to some other form of energy, e.g. a light bulb emits heat and light energy, an electric motor performs mechanical work and emits heat.  Each "load" can be represented in the DC circuit by a characteristic resistance.

 


• AC stands for Alternating Current.



• AC current, as its name suggests 'alternates' in direction.
• In its most simple form this alternating behaviour can be represented by a sine (or cosine) wave
• Electricity generated at power stations are AC current sources.
• Electrical outlets in homes and businesses are almost exclusively sources of AC current.
• The theoretical treatment of AC is signifiacntly more difficult than DC.  For this reason in this course we limit our discussion to DC circuits.

 

As indicated above the "loads" in a circuit can be represented by charactersitic resistances (resistors).  These resistors can be connected "in series", "in parallel" or some combination of the two.
A resistor in a circuit is represented by the symbol 
The current source (battery) is represented by the symbol 
 

Resistors in series.

When resistors are connected in series the same current passes through each resistor.  The potential difference (voltage) across each resistor will typically be different; the sum of the potential differences being that of the p.d. of the battery in the circuit.

As far as the current (and power) provided by the battery is concerned, the three resistors in the above circuit can be replaced by an equivalent resistance, R_eq given by,

Resistors in parallel.

When resistors are connected in parallel the potential difference (voltage) across each resistor is the same.  The current through each resistor will typically be different; the sum of the currents being equal to the net current provided by the battery.

As far as the current (and power) provided by the battery is concerned, the three resistors in the above circuit can be replaced by an equivalent resistance, R_eq  given by,

Combination circuits.

In some circuits the resistors are connected in a combination of series and parallel.  For example in the circuit below R₁ and R ₂ are connected in parallel with each other as are R₄ and R₅ .  The equivalent resistance of each of these two parallel combinations is then connected in series with R₃.

Other circuits.

There are some circuits which cannot be reduced to series and/or parallel combinations.  These circuits may have multiple batteries, multiple resistors and multiple loops, see for example below.  They can be analysed by use of Kirchoff's laws, but this is beyond the scope of this course.