Unit 4: Electric Circuits

Current

Current

The continuous flow of charge

Average current

Iavg = change in charge/ change in time

Battery

A battery is a device that maintains an electric potential difference between the two terminals.

Direct current

The flow is from higher potential to lower potential. The electricity also flows in that direction called direct current.

Resistance

It is the impedance to the flow of electricity through a material. Asa charge moves through a material, it eventually hits a non-moving nucleus in the material.

Resistivity

It can be thought of as the density of nuclei the electrons may strike.

R = ρ l / A

• R = resistance of the circuit

• ρ = resistivity

• l = length

• A = cross-sectional area

low resistivity

conductors

high resistivity

insulators

resistors in series

R eq = R1 + R2

resistors in parallel

1/Req = 1/R1 + 1/R2

Ammeter

An ammeter is a device with a very low resistance that measures the current.

Voltmeter

It measures the electric potential called potential drop.

Ohm’s Law

V = IR

• R is the resistance in the circuit.

• V is the potential difference in the circuit

• I is the electric current

Power dissipation

P = VIP = I^2 RP = V^2 IR

• P is the power

• V is the potential difference in the circuit.

• I is the electric current.

Kirchhoff’s rules

• The loop rule states that the voltage drop across any complete loop in the circuit is 0V.

  • This statement follows from the conservation of energy when applied to circuits.

• The junction rule states that the sum of all current flowing into any junction is equal to the current flowing out of the junction.

  • This statement follows from the conservation of charge.

Capacitance

C = QV

• C = refers to the capacitance that we measure in farads

• Q = refers to the equal charge that we measure in coulombs

• V = refers to the voltage that we measure in volts

• Besides, there is another formula that appears like this:

• C = kε0Ad

  • C = refers to the capacitance

  • K = refers to the relative permittivity

  • ε0 = refers to the permittivity of free space

  • A = refers to the surface area of the plates

  • d = refers to the distance between places measured

Capacitors in parallel

Cp = C1 + C2

Capacitors in series

1/Cs = 1/C1 + 1/C2

Name three possible energy sources for a circuit

• Battery

• Photoelectric cell

• Generator

A circuit is opened and closed using a:

Switch

What will happen to the charges in a circuit when a switch is closed?

The charges will flow through the circuit.

What will happen to the charges in a circuit when a switch is open?

The circuit is broken and the charges stop flowing through the circuit.

What is the standard voltage per branch in a home in the United States?

120 V

A switch that automatically opens if the current is too high is a ________________

Circuit Breaker

The symbol used to represent resistance in a schematic diagram is

a zigzag line

A closed circuit is a circuit in which charge

can flow

When two light bulbs are connected in series, the

same amount of current always flows through each bulb

When resistors are put in parallel with each other their overall resistance is

smaller than the resistance of any of the resistors

As more lamps are put into a series circuit, the overall current in the circuit

decreases

As more lamps are put into a parallel circuit, the overall current in the circuit

increases

When one light bulb in a parallel circuit containing several light bulbs burns out, the other light bulbs

burn the same as before

Electrical devices in our homes are connected in

parallel