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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

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current and resistance

Current and resistance

Principles of Physics

moving vs nonmoving charge
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 charge1
Moving vs. Nonmoving Charge
  • Charged objects are not useful unless moving
  • They move if there is a voltage


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 (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/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


simple circuit
Simple Circuit

Voltage supply (power supply)






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 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


*Always add x to the side with less current

4 A = 3.0 A + x

3.0 A

controlling current
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
  • 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