Electric CircuitsElectric Circuits

Conventional Current Flow &Ohm’s Law

Conventional Current Flow &Ohm’s Law

Electricity Water AnalogyElectricity Water Analogy

Current ElectricityCurrent Electricity• Electrostatics: Non-moving electricity• Current Electricity: Moving electricity

Electric Current• Flow of charge (usually electrons) in a

conductor (usually metal) past a point in one second

• Measured in Amperes (A)I = Current (A)

Q = Charge (C)t = Time (s)

• Electrostatics: Non-moving electricity• Current Electricity: Moving electricity

Electric Current• Flow of charge (usually electrons) in a

conductor (usually metal) past a point in one second

• Measured in Amperes (A)I = Current (A)

Q = Charge (C)t = Time (s)

I Q

t

Current ElectricityCurrent ElectricityExample: My 50A speaker is going full blast for

20minutes, how many electrons pass through it?

• I = Q/t• Q = I*t• Q = (50A)(20min)(60s/min)• Q = 6x 104 C

• Q = N*e• N = Q/e• N =(6x104C)/(1.6x10-19C/electron)• N = 3.75x1023electrons

Example: My 50A speaker is going full blast for 20minutes, how many electrons pass through it?

• I = Q/t• Q = I*t• Q = (50A)(20min)(60s/min)• Q = 6x 104 C

• Q = N*e• N = Q/e• N =(6x104C)/(1.6x10-19C/electron)• N = 3.75x1023electrons

Current ElectricityCurrent

Electricity

What actually flows?• Electrons:

• What truly happens• Protons - hard to move• Electrons - easy to move

• Positive Charge:• Historically what people thought• Conventional Current Flow• Assume current flows out of the positive

terminal

What actually flows?• Electrons:

• What truly happens• Protons - hard to move• Electrons - easy to move

• Positive Charge:• Historically what people thought• Conventional Current Flow• Assume current flows out of the positive

terminal

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ElectronFlow

PositiveCharge

Electrons Do Work

We are usingConventional

Does the battery carewhat we hook up to it?

• No, Voltage stays the sameWhat affects the amount of

current flowing?• Voltage of battery• Size of resistances hooked up to the battery

I = Current (A)V = Voltage (V)

R = Resistance (Ω)

Does the battery carewhat we hook up to it?

• No, Voltage stays the sameWhat affects the amount of

current flowing?• Voltage of battery• Size of resistances hooked up to the battery

I = Current (A)V = Voltage (V)

R = Resistance (Ω)

Ohm’s LawOhm’s Law

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Current

ΔV

V IROhm’s Law

Example: What current will flow from a 6.0V battery when a 50.0Ω light bulb is attached?

• V = I*R• I = V/R• I = (6.0V)/(50.0Ω)• I = 0.12A

Example: What current will flow from a 6.0V battery when a 50.0Ω light bulb is attached?

• V = I*R• I = V/R• I = (6.0V)/(50.0Ω)• I = 0.12A

Ohm’s LawOhm’s Law

Example: What is the minimum resistance a 120V house circuit with a 15A breaker can withstand?

• V = I*R• R = V/I• R = (120V)/(15A)• R = 8ΩIf we hooked the same resistance to a

240V dryer plug, will its 15A breaker turn off?• I = V/R• I = (240V)/(8Ω)• I = 30A

Example: What is the minimum resistance a 120V house circuit with a 15A breaker can withstand?

• V = I*R• R = V/I• R = (120V)/(15A)• R = 8ΩIf we hooked the same resistance to a

240V dryer plug, will its 15A breaker turn off?• I = V/R• I = (240V)/(8Ω)• I = 30A

Ohm’s LawOhm’s Law

The circuit breaker willopen because 30A > 15A