CURRENT FLOW AND RESISTANCE Previously in Physics: we were concerned with forces between charges and charges in electric fields. Now we consider the motion of charges inside a conductor. Electrons are loosely bound in conductors and move through the atomic structure easily.

CURRENT FLOW AND RESISTANCE CURRENT is the flow of charge and is defined as the amount of charge, q, that flows in time t, or: Back in the day, the international ampere was defined as the current that would deposit grams of silver per second from a solution of silver nitrate in water. At present, it is defined as the amount of current which generates a force of two dynes per centimeter of length between two wires one centimeter apart.

CURRENT FLOW AND RESISTANCE In metallic conductors: electrons flow In electrolytes (salt water): + and - ions flow In a gas (neon, fluorescent): + and - ions, and electrons flow What causes charges to move?

CURRENT FLOW AND RESISTANCE + V b V a An Electric field. The field sets up an "electric potential" (voltage) Negative charges fall into the potential Positive charges get pushed away

CURRENT FLOW AND RESISTANCE If the electric field points in the same direction all the time, we get "DC", or direct current flow…a continual flow of electrons in the same direction If the electric field changes directions periodically then we get "AC", or alternating current…the electrons get pulled back and forth, but make no net motion

CURRENT FLOW AND RESISTANCE As the electrons are accelerated through the wire they collide with fixed particles (atoms) that deflect them, only to have them accelerate again. - +

CURRENT FLOW AND RESISTANCE These collisions results in HEATING…which is why wires heat up if too much "current" flows through them - + As the electrons are accelerated through the wire they collide with fixed particles (atoms) that deflect them, only to have them accelerate again. - +

CURRENT FLOW AND RESISTANCE So we write: The heating is due to "resistance" of the wire to the passage of the electrons. Different metals have different resistance to the flow of electrons.

CURRENT FLOW AND RESISTANCE So that: OHM'S LAW Volts = (Amps) x (Ohms) V = (A) x (  So we write: The heating is due to "resistance" of the wire to the passage of the electrons. Different metals have different resistance to the flow of electrons.

CURRENT FLOW AND RESISTANCE

Math trick #1:

CURRENT FLOW AND RESISTANCE

What’s (q/t)?

CURRENT FLOW AND RESISTANCE

1 Watt = 1 Amp x 1 Volt

CURRENT FLOW AND RESISTANCE

What is the relationship between I and E?

CURRENT FLOW AND RESISTANCE What is the relationship between I and E? What about the dimensions of the conductor?

CURRENT FLOW AND RESISTANCE What is the relationship between I and E? What about the dimensions of the conductor?

CURRENT FLOW AND RESISTANCE What is the relationship between I and E? What about the dimensions of the conductor? What about the conductor material?

CURRENT FLOW AND RESISTANCE What is the relationship between I and E? What about the dimensions of the conductor? What about the conductor material?  = “resistivity”

CURRENT FLOW AND RESISTANCE Next is the first math ‘trick’…

CURRENT FLOW AND RESISTANCE

Now we include some Physics substitutions to include a more practical term, Voltage.

CURRENT FLOW AND RESISTANCE Now we include some Physics substitutions to include a more practical term, Voltage.

CURRENT FLOW AND RESISTANCE Now we include some Physics substitutions to include a more practical term, Voltage. And now another math “trick”…

CURRENT FLOW AND RESISTANCE Now we include some Physics substitutions to include a more practical term, Voltage. And now another math “trick”…

CURRENT FLOW AND RESISTANCE So here we are…one final math ‘maneuver’…

CURRENT FLOW AND RESISTANCE So here we are…one final math ‘maneuver’…

CURRENT FLOW AND RESISTANCE The term in the denominator is called the "Resistance"  = the resistivity of the material (look this up in a table) d = distance, or length of the wire (“L” is often used) A = the area of the wire (the cross-sectional area)

CURRENT FLOW AND RESISTANCE So that: OHM'S LAW Volts = (Amps) x (Ohms) V = (A) x (  Now we can write: