ELECTRIC CURRENT,

VOLTAGE, AND RESISTANCE

ELECTRICAL PRESSURE

• It is a flow of charged particles.• Charges flow from high voltage to low voltage• Voltage = electrical pressure that pushes

charge• Voltage difference (V) is the push that causes

charges to move • Voltage is measured in volts• Flow of current stops when the differences

are equal. • How could you keep flow moving?

FLOW OF CHARGE

• Charge will flow when there is a potential difference between the ends of a conductor

VOLTAGE

• Provides the “electric pressure” to move the e- between terminals in a circuit• If there is a voltage

difference ACROSS a wire, then charge will flow THROUGH it.• If voltage difference

increases, current increases

BATTERIES

• Dry Cell:• Produces voltage

difference between positive and negative terminals• The voltage

difference between the terminals causes current to flow though a closed circuit

• Wet Cell:• Contains two

connected plates consisting of metallic compounds in a conducting solution

RESISTANCE:

• The tendency for a material to oppose the flow of electrons.• This happens because electrical energy is

changed into thermal and light energy. • Causes electric current to lose energy as it moves

through the filament• Resistance is measured in ohms (Ω)• Electrical conductors have a lower resistance

than insulators.

RESISTANCE (CONT.)

• The size, temperature, and length of the wire affects resistance• Electrons move more efficiently through thick wires than

thin wires (the thicker the wire, the less resistance).• The longer the wire, the greater the resistance because

more collisions occur.• The resistance increases as temperature increases.

ELECTRIC RESISTANCE

• Resistance decreases the amount of e- that can flow through a conductor• Increase resistance = decrease current• Factors affecting resistance:• Conductivity of material• Thickness of wire• Length of wire• temperature

ELECTRIC CURRENT:

• The rate of flow of electric charge• Symbolized by: I (capital i)• Measured in coulombs per second• 1 coulomb per second is an ampere (A)• 1 C/s = 1 A

• We can measure it with a device called an ammeter.

CLOSED CIRCUITS

• A circuit is closed, conducting path• Electric Current is the flow of charges through a

wire or any conductor• Current (I) is measured in Amperes (A)• Electrons are the charges that move because

they are not locked in the nucleus but move around the atom

OHM’S LAW:

Current = Voltage Difference Resistance

I (A) = V(V) R(Ω)

OHM’S LAW AND ELECTRIC SHOCK

• Touching surfaces of differing potential you become a pathway for current• Electric shock can overheat (cook) tissues or

disrupt nerve functions = disrupt breathing…remove with non-conducting material and apply artificial respiration

FUSES:

• When a fuse is blown, a small piece of metal melts if the current becomes too high. • This causes a break in the circuit, thus stopping

the flow of current through the overloaded circuit. • Circuit breakers have a piece of metal that bends

instead of melting and can be reset by changing it back to the on position.

ELECTRICAL POWER

• The rate at which electrical energy is converted to another form of energy• Power = current x voltage• Watts = Amps x volts

CALCULATING ENERGY:

• Depends on two things:• Power required • How long they piece of equipment is used

• Energy = power x time• E (kWh) = P (kW) x t(h)• Unit is kilowatt-hr (kWh)

ELECTRIC CIRCUITS:

• Series:• The current has only one

loop to flow through.• Flashlights and some

holiday lights• Amount of current is the

same everywhere. • When any part of a

series circuit is disconnected, no current flows through the circuit (open circuit).

• Parallel:• Contain two or more

branches for current to move through.

• The current splits up to flow through the different branches.

• Voltage difference is the same in each branch.

• More current flows through the branches with lower resistance

CIRCUIT DIAGRAMS (SCHEMATICS)

SWITCH RESISTORBATTERY/

VOLTAGE

SOURCE

PROBLEMS

• A 110 volt wall outlet supplies power to a strobe light with a resistance of 2200 ohms. How much current is flowing through the strobe light?• 0.05 A

• A CD player with a resistance of 40 ohms has a current of 0.1 amps flowing through it. Sketch the circuit diagram and calculate how many volts supply the CD player.• 4.0 V

• A 120-volt power source supplies a lamp with a resistance of 192 ohms. What is the current flow of the circuit?• 0.625 A

PROBLEMS:

• What is the resistance of the circuit conductors when the conductor voltage drop is 3 volts and the current flowing through the conductors is 100 amperes?• 0.03 Ω

PROBLEMS:

• In a series circuit with two 3-ohm light bulbs powered by a 6 V battery:• What is the resistance in the circuit?• What is the current in the circuit?• What is the voltage drop across each

resistor?• Which bulb is brightest and why?• What would happen to the following if I

added a 6-ohm light bulb after the second 3-ohm bulb?• The resistance• The current in the circuit• The voltage drop across each resistor• Which bulb is now brightest?

PROBLEMS:

• Consider a circuit with a 2-ohm, 4-ohm and 8-ohm resistor in parallel to each other, being supplied by a 24V battery:• What is the voltage drop across each resistor?• What is the current in each branch?• 2-ohm• 4-ohm• 8-ohm

• Find the current in the circuit• What is the equivalent resistance of the circuit?

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