Electric Circuits

What is Electric Current?• Current in a river

– The movement of water molecules• Higher current = more water passing a given point

each second.

• Current in a pipe (flow rate)– How fast water is flowing in the pipe.

• What is needed for water to flow?– A path for flow– Pressure to move the water

• Electric Current– The movement of charge past a given point.

• Higher current = more charge passing a given point each second.

• What is needed for electric current in a wire?– A closed path for flow called a circuit– “Pressure”, called a potential difference (V)

What is Electric Current?

What is Electric Current?• How does a power source work?

– Creates a potential difference• Current flows from high potential to low potential• Electrons flow from negative to positive

• Battery– Symbol = – Potential Difference measured in Volts (V)

Conductivity (Solids)

• The ability of a material to conduct electricity depends on the following:– The number of free charges per unit volume.– The mobility of charges

• Conductor, insulator, and semiconductor– Conductor: Many free electrons (Metals)– Insulator: Few free electrons (Non-metals)

– Semiconductor: In between a conductor and insulator.

Conductivity (Liquids)

• Electrolytes– Separate into positive and negative ions

• Ions are free to move• Salt (NaCl)

Na+ and Cl

Conductivity (Gases)

• How do gases conduct electricity?– Ionized to form a plasma– Caused by high energy radiation, electric fields, or

collisions.

• Remember– A path for flow called a circuit– “Pressure”, called a potential difference (V)

Conditions necessary for current flow

• What supplies potential difference in a circuit?– Battery or Generator

• How can we measure it?– Voltmeter– Measured across device

Voltage

V

Ex) 6.0 joules of work are done to move 2.0 coulombs of charge from point A to point B. Determine the potential difference between points A and B.

1. 6.0 v

2. 2.0 v

3. 3.0 v

4. 12 v

Voltage

Current• What is current?

– The rate of electron flow

• What units do we use?– Ampere (A) or Amps

• How is it measured?– In Series (ammeter)– Current through a

device

A

Current

Ex) Find the current in a wire, if 14c of charge pass by in 7s?

1. 2 A2. 7 A3. 14 A4. 21 A

Resistance

What is Resistance?

• The opposition that a device or conductor offers to the flow of electric current.– Measured in Ohms (Ω)

What variables determine resistance in a wire?

– Length of wire (L) in meters.– Area (A) in meters2.– Resistivity of material () in ohmsmeters.

• What is resistivity?– How well a certain material “resists” the flow of

electrons.

The Laws of Resistance

Length: Length

Area: Area

Temperature: Temp

Resistance

Resistance

Resistance

3.14x10-6 m2

R = 2.19 x 10-2 Ω

Sample ProblemDetermine the resistance of a 4.00 m length of

copper wire having a diameter of 2.00 mm. Assume a temperature of 20c.

Known Unknown

L = 4.00 m

A =

= 1.72 x 10-8 Ωm

R = ?

Material Resistivity

Aluminum 2.82 x 10-8

Copper 1.72 x 10-8

Gold 2.44 x 10-8

Nichrome 150 x 10-8

Silver 1.59 x 10-8

Tungsten 5.60 x 10-8

3.14 x10-6 m2

R = 3.04 x 10-2 Ω

Sample ProblemDetermine the resistance of a 6.00 m length of silver

wire having a cross-sectional area of 3.14 x10-6 m2. Assume a temperature of 20c.

Known Unknown

L = 6.00 m

A =

= 1.59 x 10-8 Ωm

R = ?

Material Resistivity

Aluminum 2.82 x 10-8

Copper 1.72 x 10-8

Gold 2.44 x 10-8

Nichrome 150 x 10-8

Silver 1.59 x 10-8

Tungsten 5.60 x 10-8

Superconductivity

• Cool a conductor below a critical temperature – Resistance decreases to zero.– Magnetic field lines will not penetrate.