Electric Current and Ohm’s LawPhysics 12

Clip of the day: Minutephysics…. http://www.youtube.com/watch?v=jN-FfJKgis8

Review: Electric Potential

Electric potential energy has a dependency upon the charge of the object experiencing the electric field, electric potential is purely location dependent.

Electric potential is the potential energy per charge.

Unit = J/C or V

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V Q

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Conductors vs. Insulators

In 1729 Stephen Gray observed the movement of electric charge on some materials while not on others

As a result he classified materials as either:o Conductors – objects that allow electric charge to moveo Insulator – objects that restrict the movement of electric

charge

Beyond Electrostatics: As a result of Gray’s discovery,

scientists began to move beyond electrostatics and start to consider moving electric charges

In 1800, Alessandro Volta added to this new branch of charge theory when he invented the electrochemical cell

His cell was the first voltaic pile (or battery) and consisted of layers of silver, zinc and salt water soaked paper

When the top and bottom contacts were connected by a wire, an electric current flowed through the voltaic pile and the connecting wire.

Voltaic Cell One of the silver, zinc and salt water soaked discs

became known as a voltaic cell It derives electrical energy from spontaneous

redox reaction taking place within the cell. A voltaic cell requires:

o Cathode (positive end)o Anode (negative end)o Electrolyte (to allow the movement of charge)

Again…..ELECTRIC POTENTIAL or VOLTAGE (V)

The energy each electron has is called the

electric potential. Electric potential is commonly referred to as

voltage. A voltmeter is used to measure voltage.

Electric Current Electric current describes the flow of

charge carriers through a conductor Analogy: If we were discussing the flow of

water, it is likely we would consider a flow rate in litres per second; when it comes to charge, we consider charge per second (one ampere is equal to one coulomb per second)

Electric Current: Electric current is a measure of the rate at which

electric charges move past a given point in a circuit.

Symbol for current is (I). Unit for current is Ampere (A) = one coulomb per

second

Electric Circuits A circuit is simply a closed loop through which

charges can continuously move. An electric circuit exists anytime a closed loop

exists that involves a battery and some type of load (light bulb, etc.)

Charge carriers will flow from the battery with high electric potential, give off their electric potential as they cross the load and then return to the battery to regain electric potential

Requirements for a circuit:

There are two requirements that must be met in order to establish an electric circuit:1. There must be an energy supply capable doing work on

charge to move it from a low energy location to a high energy location and thus establish an electric potential difference across the two ends of the external circuit . This is the battery or power supply!

2. There must be a closed conducting loop in the external circuit that stretches from the high potential, positive terminal to the low potential, negative terminal.

Current vs. Electron Flow

By convention, current is assumed to be the flow of positive particles (from cathode to anode)

However, we now know that electrons are the charge carriers and have a negative charge (flow from anode to cathode)

So the direction of current flow and electron flow are in opposite directions

Resistance In a way that is similar to friction opposing motion,

electrical resistance opposes the flow of charge carriers

The ability to impede the flow of electrons in a material is called electrical resistance (R).

Resistance is measured in ohms (Ω) and is a function of the material that is conducting charge carrierso INSULATORS oppose the flow of electric current and have a

high resistance.o CONDUCTORS have a low resistance.

This leads to Ohm’s Law…. IRV

Ohm’s Law Named after the German scientist George Ohm. Ohm figured out a basic law for electricity

IRV

I electricity….

Ohm’s LawV = Volts (electrical potential/voltage) I = Amperes (current)R = Ohms (resistance)

And…….. I = V / RR = V / I

IRV

Ohm’s Law Current is directly proportional to voltage,

therefore, increasing (↑) the voltage increases (↑) the current.

The current is inversely proportional to resistance; therefore, as resistance increases (↑), current decreases (↓).

Sample problem:If a water heater has a voltage drop of 240V

and a resistance of 12.8Ω, what is the current?

I = V / RI = 240V / 12.8ΩI = 18.75A

The water heater has a current of 18.75 amps.

Joke of the day:

Try it : Voltage

o Page 692• 1-3

Currento Page 696

• 4-11 Ohms Law

o Page 714• 21-26

Factors that affect the resistance of wires:

1. The thickness of wires2. The length of wire3. The material of wire4. The temperature of the wire