CURRENT AND RESISTANCEPrinciples of Physics

Moving vs. Nonmoving ChargeSo far, we’ve talked about non-moving chargeWe know that charged objects : exert an electrostatic force on each other move when the force is unbalanced gain and lose energy as they move toward

and away from other charged objects Moving toward a like charge → increase in PE Moving away from like charge→ decrease in PE

We also know that a voltage exists between charged objects

Moving vs. Nonmoving Charge Charged objects are not useful unless moving They move if there is a voltage

Circuit A system in which charged objects move in response to a voltage sourceMust include: voltage source (battery)

closed path for charges to follow (wires)

method of controlling rate of flow (resistor)

Current

Current (I): flow of electrons (negative charge)

In circuits charge is moving in solid materials (copper wires)

Only electrons can move in solids

(Positive charge may flow in liquids or gases)

Current

Current/electron flow is calculated by

I = Current q = charge (C)t = time (s)

Current units: 1 C/s = 1 Ampere = 1 Amp = 1 A

I = q t

Simple Circuit

Voltage supply (power supply)

Resistor

Wire

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+

Current

Electrons flow from the negative side of the voltage supply (the short side)

Conventional current flows from the positive side of the voltage supply (the long side)

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Junction Rule

Junction: where multiple wires (paths) meet

All current flowing into a junction = all current flowing out of a junction.

1.5 A

1.0 A 2.5 A

3.0 A

Current flowing in:1.5 A + 2.5 A = 4 A

Current flowing out:3.0 A

IN = OUT*Always add x to the side with less current

4 A = 3.0 A + x

Controlling Current

When one electron leaves from the negative side of the voltage supply another one enters the positive side

The rate that electrons leave is current How fast they leave depends on

resistance in the circuit

Resistance

A material with high resistance typically causes the electrons to experience more collisions as they move through

RESISTOR

Controlling Current

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Resistance, R, is related to:

•Type of material• Some materials have more resistance than others

•Length• Electrons will experience more collisions

as they travel through the longer resistor•Cross sectional area• Increasing cross sectional area decreases

resistance because more electrons can get through at one time

• Temperature• In metallic conductors, as temperature

increases, resistance increases