Electric Current & Resistance

Electric Circuits Charges continually flow through a complete loop, ultimately returning to their original position and cycling through again. What arrangements result in a successful lighting of the bulb? You have a wire, bulb, and battery. Caution the wire may become hot!

In which of the following circuits would charge flow?

Electric Current Current: the rate at which charge flows past a point on a circuit. Current = I = Q/t Ampere is the standard metric unit for current. 1 ampere = 1 coulomb/1 second 1. .5 amp 2. 4 amp

Current Direction & Drift Speed The current in the external circuit is directed away from the positive terminal towards the negative terminal…BUT WAIT... Electrons are negative charges...therefore they move through the wires in the opposite direction. Due to the collision of electrons with atoms in the network of the conductor, drift speed of a is slow ~1 meter/hour. Current remains high, because there are many charges the density of charge passing through the wire in the circuit is high. A .01 cm cross section of 14 g copper wire would have 56 coulombs of charge leading to a large current.

Charge Flow If electrons are so slow, why does the light in a room light immediately after the switch is turned on? The electrons that light the bulb are already present in the filament. Like turning on a faucet, the pipes are already filled with water. There is no place where charge carriers become consumed or accumulate. Rate at which charge enters the external circuit on one end is the same as the rate at which charge exits the external circuit on the other end.

Power The rate at which mechanical work is done. The rate at which electrical energy is supplied to a circuit or consumed by a load. Power = Work Done on Charge = Energy Consumed by Load time time P=ΔE/t The rate at which charge changes its energy. The unit of power is the watt = 1 joule/second Kilowatt hour is = power x time A 60W light bulb 60 j delivered to the light bulb per a second. Kilowatt hour = Energy (j)

Calculating Power P=ΔE/t I=Q/t ΔV= ΔPE/Q P=ΔVQ/t P=ΔVI To determine power multiply the electric potential difference by the current

Resistance As electrons move through the external circuit there are several losses in electric potential. Each of these losses results in a voltage drop. Collisions Light bulbs-release light & thermal energy Resistance: Hindrance to the flow of charge.

Variables that Impact Resistance How do you think each of the following variable impacts the movement of charge throughout a circuit? Length of the wire Cross sectional area of the wire Wire material Longer= more collision Wider = less collision Some metals are more resitive

Resistance & Math YAY! Resistance can be measured mathamatically as R=p (L/A) R=Resistance p = resistivity of the material (Ωm) L= Length of the wire (m) A= cross-sectional area of the wire (m2) Unit is ohms Ω

Ohm’s Law ΔV=IR The electric potential difference between two points on a circuit (ΔV) is equivalent to the product of the current between those two points (I) and the total resistance of all electrical devices present between those two points (R).

Predicting Current Ohm’s Law can be rearranged: I= ΔV/R What happens to current if… Electric potential is doubled?Halved? Resistance is doubled?Halved?

All the math we know so far...

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