# Circuit Symbols and Conventions

## Circuits model electrical systems

• Voltage is work done per unit charge
• Potential difference- difference in electrical potential between two points in an electric field
• A force used to move charge between two points in space

• Moving charges produce an electric current
• Moving charges can do electrical work the same way moving objects do mechanical work

• Electrical energy is the capacity to do electrical work
• Electrical power is the rate at which work is done

### Resistance

• Resistance is the opposition to the flow of current
• Ohm's Law:

• Resistance is also proportional to the Resistivity of the material
• and are the length and area of the conductor, respectively.

## Sources and Nodes

Everything in a circuit can be modelled as either a source, or a node.

### Voltage Sources • DC and AC voltage sources
• DC source has positive and negative terminals
• Ideal voltage source has 0 internal resistance (infinite conductance)
• Supplies constant voltage regardless of load
• This is an assumption, is not the case in reality

### Current Sources • Ideal current source has infinite resistance (0 conductance)
• Supplies constant current regardless of load
• Also an assumption
• In reality, will have some internal resistance and therefore a maximum power limit

### Dependant sources • Diamond-shaped
• Sources depend on values in other parts of the circuit
• Model real sources more accurately

### Nodes All passive elements: generate no electrical power.

• Resistors provide resistance/impedance in Ohms ()
• Inductors provide inductance in Henries ()
• Capacitors provide capacitance in Farads ()

The voltage rise across an impedance conducting current is in opposition to the flow of current in the impedance.

## Basic Conventions

Electrical current always flows from high to low potential.

• If the direction of the current in a circuit is such that it leaves the positive terminal of a voltage source and enters the negative terminal, then the voltage is designated as negative
• If the direction of the current is such that it leaves the negative and enters the positive, then the voltage is positive
• The sign of the loop current is the terminal that it flows into

The power absorbed/produced by a source is .

• A voltage source is absorbing power if it is supplying a negative current
• A voltage source is producing power if it is supplying a positive current

The power dissapated in a resistor is .

## Resistors in series and parallel Resistors in series:

Resistors in parallel:

Resistors dissipate electrical power, so there is a drop in voltage accross them, in the direction of current flow. Therefore, the voltage rise is in opposition to the direction of current

## Voltage dividers Using two resistors to divide a voltage

In the general case:

## Current Dividers

Similar deal to voltage divider 