Section 2: Electric Current

• This section discusses electrical pressure

• Charges flow from high voltage areas to low voltage areas

• Voltage—like an electrical pressure that pushes charge

(Section 2 cont’d)• Just as water or air must have a

pressure difference to flow, voltage difference must be present for electric charges to flow

• Voltage difference—the push that causes charges to move and is measured in volts (V)

(fig. 11 pg. 202)

Closed Circuits

• The flow of charges through a wire or any conductor is called electric current

• The electric current in a circuit is measured in amperes (A)

• Circuit—a closed, conducting path for the flow of electrons

(Section 2 cont’d.)• Current is almost always the flow

of electrons• In order to keep current moving

continuously through a circuit a power source (voltage source) is needed

• One common source--battery

Dry-Cell Batteries• Individual batteries used in flashlights,

etc.—dry cell batteries• Made of a zinc container that surrounds

a moist chemical paste w/a solid carbon rod suspended in the middle

• Provides a voltage difference between +/- terminals

(Dry-cell batteries cont’d)

• When the two terminals of a dry cell battery are connected in a circuit, a reaction occurs.

• The voltage difference between these 2 terminals causes current to flow through a closed circuit

Wet-cell batteries• Wet-cell battery—contains two

connected plates made of different metals or metallic compounds in a conducting solution– Ex: car batteries (lead plates in sulfuric acid)

– The chemical reaction in ea/cell provides a voltage difference

More on Batteries

• In addition to batteries a voltage difference is provided at electrical outlets

• Household devices are designed for 120 V (standard)

• Some wall sockets supply 240 V – Ex: electric ranges, clothes dryers,

etc.

Resistance

• Resistance—the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light

• All materials have some resistance• Resistance is measured in ohms

*type of material & wire size effect resistance

Controlling the flow of electricity

• So far: Voltage difference causes charges to flow

• An electrical resistance restricts the movement of charges

ex: water flowing in a pipe

Ohm’s Law:

• Current = Voltage difference / resistance

• OR• I(A) = V (V) / R (ohms)• OR• V = I R

• Ohm’s Law Diagram—pg. 207