Circuits

Diagramming Circuits 1. conductor 2. light bulb 3. switch 4. battery 5. resistor 6. ammeter 7. voltmeter

Resistance (R) = measure of how easily charges flow in a material The unit for resistance is the Ohm (Ω) The gauge of the wire determines its resistance; larger gauge = more resistance

The resistance of a conducting wire depends on four main factors: Length – R increases as the length of wire increases (more time for electrons to collide with other atoms) Cross sectional area – R increases as area decreases (wider wire = less resistance) Resistivity – material dependent (low resistivity are silver, copper, aluminum) Temperature – R increases as temperature increases (increased kinetic energy increases electrons collisions with other atoms)

German physicist studied the resistance of different materials and in 1826 published his findings which stated that: V = IR Since voltage encourages the movement of electrons and resistors slow the movement of electrons, the combined effect of these two quantities in a circuit can give us the ratio: Ohm’s Law

When a steady current is flowing through a load such as a resistor, it dissipates energy The energy “lost” by the electrons is equal to the energy gained by the resistor (this energy is used to do work) Due to this relationship, we can derive some equations for Power P = VI = I 2 R = V 2 /R Combining the Ideas:

Example A 541 Watt toaster is connected to a 120 V electrical outlet. What is the resistance of the toaster? P = VI = I 2 R = V 2 /R P = V 2 /R WHAT DO WE KNOW? P = 541 Watts V = 120 Volts R = ? So we need to use the parts of the equation that contain P, V, and R and we can omit the other parts!

Example A 541 Watt toaster is connected to a 120 V electrical outlet. What is the resistance of the toaster? P = VI = I 2 R = V 2 /R P = V 2 /R Rearrange to: R = V 2 /P R = (120) 2 /541 R = 26 Ω WHAT DO WE KNOW? P = 541 Watts V = 120 Volts R = ?