Current and resistance

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Current and resistance - PowerPoint PPT Presentation

Current and resistance. Principles of Physics. Moving vs. Nonmoving Charge. So far, we’ve talked about non-moving charge We know that charged objects : exert an electrostatic force on each other move when the force is unbalanced

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Current and resistance

Principles of Physics

Moving vs. Nonmoving Charge

So far, we’ve talked about non-moving charge

We 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 source

Must 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

-

+

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)

-

+

Junction Rule

Junction: where multiple wires (paths) meet

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

Current flowing in:

1.5 A + 2.5 A = 4 A

1.5 A

2.5 A

1.0 A

Current flowing out:

3.0 A

IN = OUT

*Always add x to the side with less current

4 A = 3.0 A + x

3.0 A

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

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