If you want your boats or anything on your boats, go get them now. Otherwise, just leave them here.

Electric Current and Circuits

HEAT will flow if there is a difference in temperature

WATER will flow if there is a difference in pressure.

Is the pressure inside the jug (due to the weight of the water) the same as the pressure outside the jug (due to the weight of the atmosphere)?

If the pressure outside was the same as the pressure inside, would the water flow out?

At which location is there a larger difference between the pressure inside and outside, near the top of the water level or near the bottom of the water level?

What evidence can you see that demonstrates a larger difference in pressure at the two locations?

Electrons will flow if there is a difference in electric pressure.Electric pressure is called “Potential”, and is measured in Volts.If there is no difference in pressure from one location to another, the electrons will not flow.In other words, if there is no

“Potential Difference, DV” from one location to another, there will be no electric “current”.

Current

CURRENT: a flow of charged particles (electrons) through a conductor

Current, I, is measured in amperes, A, or “amps”.

timecharge

I Andre Ampere

Example:

What is the current, I, if 8 C of charge passes through a wire in 2 seconds?

I = q / t

I = 4 amps

How long would it take for 3.2 x 1018 electrons to pass through the wire?

t = q / I

But… what is the charge, q?

q = (3.2 x 1018 )(1.6 x 10-19C) =

0.512 C

t = 0.512 C / 4 A = 0.128 seconds

The Damage Caused by Electric Shock

1 mA = 0.001 A

Mild shock can be felt

5 mA = 0.005 A

Shock is painful

15 mA = 0.015 A

Muscle control is lost

100 mA = 0.1 A

Death can occur

60W light bulb - 0.5 AStarter motor – 210 AClothes dryer – 18 AIron- 3 A

CircuitsELECTRIC CIRCUIT: Charges

moving in a closed loop • A circuit requires a both a

conductor, usually metal wires, and a “charge pump”.

• CHARGE PUMP: a device that provides a potential difference so that charges keep moving.

Alessandro Volta

The Potential Difference, DV, provided by the charge pump is called its VOLTAGE.

If the voltage of a battery is 9 V, this means there is a difference of 9 V of potential (pressure) between the positive terminal and the negative terminal.

If the voltage of a battery is 1.5 V, this means there is a difference of 1.5 V of potential (pressure) between the positive terminal and the negative terminal.

The pressure DIFFERENCE, the VOLTAGE, is required for charges to flow!So, charge pumps such as batteries, are called voltage sources.

Circuits• This potential difference is

sometimes called the emf, e (electromotive force)

• Examples of charge pumps: batteries, solar cells, generators, power supplies

e

The source of the electrons moving in the circuit is NOT the battery or the wall outlet! The free electrons are contained within the wire itself.An individual electron does not actually travel all the way around a circuit.One electron bumps into the next that bumps into the next that bumps into the next …..It is the ENERGY that gets transferred all the way around the circuit.You are not buying electrons from your electric company- you already have them! You are buying energy!

Resistance all conductors offer some

resistance to the flow of charges, even metal wires.

RESISTANCE =

The unit for resistance is the OHM, W.

This equation is often called OHM’S LAW

IV

currentvoltage

R

George Ohm- first determined the math relationship now called Ohm’s Law

IV

currentvoltage

R

Example

What is the resistance of an appliance if 2 amps of current run through it when supplied with 120 V?

R = V / I

R = 120 V / 2 A

R = 60 W

WATER ANALOGYWater ElectricityFlow of water current flow of chargeWater pump keeps flow going charge pumppsi. pressure

voltagePipes of different resistance wires of

diameter different diameter

Small electrical components called “resistors” are inserted into circuits to control the amount of current flowing through that part of the circuit.

Certain metals offer less resistance to the flow of charges than others.

Example: Copper is a better conductor than iron

The resistance of a wire of length L and cross sectional area A is given by

RESISTANCE, R =

where r is the resistivity of that particular metal.

AL

The resistivity increases as temperature increases. So, a hot wire resists the flow of charges more than a cooler wire.

RESISTANCE, R =

AL