22.1 Objectives

Students define an electric current and the ampere.

Students describe conditions that create current in an electric circuit.

Students draw circuits and recognize they are closed loops.

Students define power in electric circuits.

Students define resistance and describe Ohm’s law.

 

Bill Nye Electric CurrentBill Nye Electric Current

What is Electric Current?What is Electric Current?

Electric Current Electric current is simply the flow of charge

Electrons flowing in a wire constitute a current Measured in Coulombs per second, or Amperes

Colloquially, Amp (A) refers to amount of charge crossing through cross-

sectional area per unit time Electrons have a charge of –1.610-19 Coulombs

so (negative) one Coulomb is 61018 electrons one amp is 61018 electrons per second subtle gotcha: electrons flow in direction opposite to

current, since current is implicitly positive charge flow, but electrons are negative

Electric Current

€

1 A =1C

s=

6 ×1018 e−

second

What is an Electric Circuit?What is an Electric Circuit?

Would This Work?

Would This Work?

Would This Work?

The Central Concept: Closed Circuit

Circuit in Diagram Form_ +battery

bulb

In a closed circuit, currentflows around the loop

Current flowing through thefilament makes it glow.

No Loop No Current No Light

current

electrons flow opposite theindicated current direction!

(repelled by negative terminal)

Current is the Flowrate

It sometimes helps to think of current as flow of water, which is more familiar to

us. High current means lots of water flow per unit time. Low current is more

like a trickle.

In electronics, it is the flow of charge, not water, that is described by the

word current. And it’s always electrons doing the flowing (thus

electronics)

Voltage is the Pressure

It sometimes helps to think of voltage as water pressure. High voltage means lots

of water pressure. If the water faucet valve is closed, no water flows no matte how high a pressure there is. No current

is present if no electrons flow.

In electronics, it is the flow of charge, not water, that is described by the

word current. And it’s always electrons doing the flowing (thus

electronics)

Resistance is the Valve

It helps to think of resistance as the valve. High resistance means the valve is closed and little water flows through. Low resistance means the valve is open

and a lot of water flows through.

Currents Divide and Merge at Junctions

How much would the current through the battery change if I unscrewed one of the 2 bulbs?

How would the brightness of “A” change if I unscrewed “B”?A B

+

Answer

The battery is supplying an equal amount of current to each of the two bulbs. If one of the bulbs is disconnected, the current through the battery will be halved.

Unscrewing “B” would not affect the current through “A” so it will stay the same brightness.

Why wouldn’t more current flow through A?The battery does not supply constant current

(is there current even when the battery is disconnected? NO. But there is voltage!)

Paul Hewitt Video #98Paul Hewitt Video #98

Bill Nye Electric CurrentBill Nye Electric Current

Electric Circuit

Symbols

Series Circuits

A connection or arrangement of devices that provides only one path for a current.

The same current flows through each device (resistance).

Parallel Circuits

A connection or arrangement of two or more devices that provides more than one path for a current.

The current is divided through each path and merges again after each device.

Each device gets that same voltage.

Series and Parallel Circuits

SeriesParallel

Series and Parallel Combo

Series

Parallel

Meters in a Circuit – Ammeter

The ammeter measures current (I) in amperes or amps or Coulombs/second

It must be connected in series. Why? The voltmeter measures potential difference (ΔV) in volts or

Joules/Coulomb It must be connected in parallel. So what is I(ΔV)?

Circuit DiagramCircuit Diagram

This is useful for measuring resistance, flow of charge is impeded by fixed atoms in conductors

R = ΔV/I measured as volts/amp or ohms (Ω)

An iron has a 6.9 A current when connected to a 120 V outlet, what is its resistance? How is this a circuit?

Resistance

Increasing ΔV is like increasing the slope of a water run, it increases current (I), they are directly related

The constant of proportionality is the resistance of the conductor

Obstructions such as rocks act as the resistance, decreasing the current

Resistance in a circuit is due to collisions between the electrons carrying the current with the fixed atoms inside the conductor

30

Georg Simon Ohm

1787 – 1854 Formulated the concept of

resistance Discovered the proportionality

between current and voltages For many materials, including

most metals, resistance remains constant over a wide range of applied voltages or currents

This statement has become known as Ohm’s Law, ΔV = IR

Paul Hewitt Video #100Paul Hewitt Video #100

Ohmic Devices

Ohm’s Law is an empirical relationship that is valid only for certain materials

Materials that obey Ohm’s Law are said to be ohmic

The resistance is constant over a wide range of voltages

The relationship between current and voltage is linear

How do light dimmers work?

Non-ohmic Materials

The resistance increases as the voltage increases

Notice that current is small and resistance is high when reversed

It therefore acts as a one way valve for current

A diode is a common example of a non-ohmic device

Paul Hewitt Video #102Paul Hewitt Video #102

Paul Hewitt Video #99Paul Hewitt Video #99

22.2 Objectives

Students explain how electric energy is converted into thermal energy.

Students determine why high-voltage transmission lines are used to carry electric energy over long distances.

Students define kilowatt-hour in electric energy delivery.

Power

The POWER dissipated in a resistor (or resisting appliance) is proportional to the square of the current that passes through it and to the resistance.

€

P = IV = I(IR) = I2R

Energy

POWER is Electric ENERGY per unit time.

€

E = Pt = IVt = I(IR)t = I2Rt

Resistance Becomes Heat

Electric energy becomes thermal energy when the friction of electrons moving in a wire dissipates heat. Resistors are coils of wire that can dissipate heat.

€

P = IV = I(IR) = I2R

Alternating Current Current comes to your house as a sine wave that

alternates between positive and negative current. The is called alternating current (AC).

This oscillating current has to be converted to direct current (DC = one direction current like a battery).

Wonder how we get direct current?

Rectification of AC Current

We must get the down humps to flip to up-humps by rectification!

Let’s see if Mr. Mosher can Draw!

Mr. Mosher will now explain how diodes can be used to separated the up-humps and down-humps of a sine wave.

A capacitor is used to flip (rectify) the down-humps to up-humps.

The combination is DC! P.S. - Please be patient with Mr. Mosher!

AC/DC Converter A diode is used to capture current in ONE DIRECTION

ONLY. Use another diode to get the OTHER DIRECTION.

Put them together and what have you got? Direct Current!

Now go and play!!!!!!!!

Power Plants and You

Power Plants and You

Electric energy becomes thermal energy due to the friction of electrons. By keeping the current very low and wire resistance minimized.

Some long distance lines use 500,000 volts with low amperes. Current can be increase in your home with transformers.

Power Costs and You

Power at your house is measured in Kilowatt (1,000 Watts) - hours.

€

E = Pt = kilowatt • hr