symbol: I

Current = flow of charges unit: Amperes (A)

(The rate at which charge flows by

a given cross section)

I =

Electric Current

1C1A =

1s

symbol: I

Current = flow of charges unit: Amperes (A)

(The rate at which charge flows by

a given cross section)

I =

To have an electric current, you need two things:• A closed circuit / path for the charges• A power supply maintain the potential difference

Electric Current

1C1A =

1s

symbol: I

Current = flow of charges unit: Amperes (A)

(The rate at which charge flows by

a given cross section)

I =

To have an electric current, you need two things:• A closed circuit / path for the charges• A power supply maintain the potential difference

Remember: Current will flow from high potential to low potential, but charges need to be pushed (by an energy source) back from low potential to high potential.

Electric Current

1C1A =

1s

Closed vs. Open CircuitsNo, the switch is open, so the path is not complete

No, the circuit needs to go from + to -

Yes, charge will flow, BUT the light will not light

Maybe! If one battery has higher V than the other

Direction of CurrentCurrent is defined as the direction positive charges would flow

• From high potential to low potential• From + side of battery to – side of battery

Fun FactBenjamin Franklin defined current in this manner long before we knew much about charges.

Now, we know that positive charges stay put and negative charges flow. So, electrons actually flow opposite current.

Calculating CurrentCurrent is the rate of charge flow per unit time

I =

So, if you have 6 C of charges passing through a section of wire every 2 seconds, then your current is:

I = 6C / 2 s = 3 A

An important note about current: Although potential decreases across

a circuit, current is the same everywhere in a circuit!

Circuit Analogy – CFU

Circuit Analogy - CFU

EF

B

C

A

Electrical resistance (symbol R)• Electrons flow due to potential difference. If the potential

difference is removed, then • As electrons move, they bump into other atoms, this slows

them down and impedes their motion.

.

atoms

free electron

(actually positive ions)

path

Resistance (R) is a measure of the degree to which an object impedes the flow of current.Resistance is measured in Ohms ()

OHM’S LAW - Current, Voltage and Resistance

Current = the potential difference applied across a circuit divided by the total resistance of the circuit.

VI =

R• R - resistance• I – current V – potential difference across R

Examples

• If a 3 volt flashlight bulb has a resistance of 9 ohms, how much current will it draw?

• I = V / R = 3 V / 9 = 1/3 Amps

• If a light bulb draws 2 A of current when connected to a 120 volt circuit, what is the resistance of the light bulb?

• R = V / I = 120 V / 2 A = 60

Check for Understanding

Which of the following would cause the current in a circuit to decrease the most?

1) Increased voltage and increased resistance

2) Increased voltage and decreased resistance

3) Decreased voltage and decreased resistance

4) Decreased voltage and increased resistance

Check for Understanding

If the resistance of a circuit were tripled, then the current through the circuit would be ____.

1. one-third as much 2. three times as much3. unchanged4. ... nonsense! There would be no way to make such a prediction.

Just for Fun … Effects of electric current on the BODY- electric shockCurrent (A) Effect

0.001 can be felt

0.005 painful

0.010 involuntary muscle contractions (spasms)

0.015loss of muscle control

0.070if through the heart, serious disruption; probably fatal if current lasts for more than 1 second

human body resistance varies: 100 ohms if soaked with salt water; moist skin - 1000 ohms; normal dry skin – 100 000 ohms, extra dry skin – 500 000 ohms.

What would be the current in your body if you touch the terminals of a 12-V battery with dry hands?

I = V/R = 12 V/100 000 W = 0.000 12 A quite harmless

But if your hands are moist and you touch 12 V battery, how much current would you draw?

I = V/R = 12 V/1000 W = 0.012 A a dangerous amount of current.