Understanding ElectricityPhysical Science Chapters 6,7,8,and 9

What is electricity?

A form of energy that exists as an accumulation of charge (static) or as the flow of charge (current).

These charges can be either protons (+) or electrons (-).

VOCABULARY

COULOMB - The unit of electric charge. The quantity of charge conveyed in one second by one ampere.

AMP - One amp is a flow of one coulomb of charge per second

VOLT - One volt is an energy of one joule per coulomb of charge

OHM – The SI unit of electrical resistance. One ohm of resistance exists in a circuit that has one amp of current pushed by 1 Volt.

VOCABULARY

CIRCUIT – A path through which electricity can flow.

CIRCUIT DIAGRAM – A drawing that uses symbols to represent the real circuit.

Circuits

Circuits are paths through which electricity can flow.

Open circuits are broken circuits.

Close circuits are complete circuits.

Circuits, Symbols, and Circuit diagrams

Types of circuits

Series Circuits Only one

possible path for current to flow.

Characteristic of Series Circuit

Types of Circuits

Parallel Circuit More than one

possible path for current to flow.

Characteristic of Parallel Circuits

Series vs. Parallel

PROBLEM 1

Observe the electrical wiring below. Indicate whether the connections are series or parallel connections. Explain each choice.

PROBLEM 1 ANSWER

Diagram A: Parallel

There is a branching location in the circuit that allows for more than one pathway by which charge can flow through the external circuit.

Diagram B: Series

There is no branching in the circuit - that is, no location where one wire leads into a point and branches off into two or more wires at that particular point.

PROBLEM 2

Two electric circuits are diagrammed below. For each circuit, indicate which two devices are connected in series and which two devices are connected in parallel.

PROBLEM 2 ANSWER

In Diagram A:

SERIES: the ammeter and the resistor are in series PARALLEL: the bulb and speaker

In Diagram B:

SERIES: the speaker and the ammeter PARALLEL: the bulb and the resistor

Current

Current is the “flow” of electrons, or electrons in motion.

Current is measured in a unit called amperes or amps for short. The symbol for current is a capital i.

Current is affected by the Voltage (push) and the Resistance.

Increasing the voltage will increase the current in a circuit.

Increasing the resistance will decrease the current in a circuit.

CURRENT IN SERIES CIRCUITS

The current in a series circuit is everywhere the same. Charge does NOT pile up and begin to accumulate at any given location such that the current at one location is more than at other locations.

Total current (I) = I 1 = I 2 = I 3 = ...

KIRCHHOFF’S CURRENT LAW

The same total current flowing into a branch point has to flow out again.

Current in Parallel Circuits.

The current outside the branches is the same as the sum of the current in the individual branches.

I total = I 1 + I 2 + I 3 + ...

Current in Parallel Circuits

OHM’S LAW

OHM’S law states that the current in a circuit is equal to the ratio of battery voltage to total circuit resistance.

OHM’S LAW PROBLEM 1

A light bulb with a resistance of 2 ohms is connected to a 1.5 volt battery as shown. Calculate the current that will flow.

OHM’S LAW PROBLEM 1 ANS

OHM’S LAW PROBLEM 2

A light bulb requires 3 amps to produce light. The resistance of the bulb is 1.5 ohms. How many batteries do you need if each battery is 1.5 volts?

OHM’S LAW PROBLEM 2 ANS

Types of current

Alternating current (AC): Current that flows in more than one direction in a wire. Produced at power plants and sent out to your

home. We use AC adapters to change current into DC.

Direct current (DC): Current that flow in only one direction in a wire. Produced by batteries.

Voltage

Voltage is the “push” that causes electrons to move.

It is the difference in electrical potential energy between the two places where electrons are flowing.

Electric potential is a location dependent quantity which expresses the amount of potential energy per unit of charge at a specified location.

VOLTAGE

The standard metric unit on electric potential difference is the volt, abbreviated V and named in honor of Alessandra Volta.

One Volt is equivalent to one Joule per Coulomb.

If the electric potential difference between two locations is 1 volt, then one Coulomb of charge will gain 1 joule of potential energy when moved between those two locations.

VOLTAGE

if the electric potential difference between two locations is 12 volts, then one coulomb of charge will gain 12 joules of potential energy when moved between those two locations.

BELOW ARE A COUPLE OF EXAMPLES:

VOLTAGE IN SERIES CIRCUITS

VOLTAGE DROP in a series circuit is equal to the total voltage drop across each resistor in the circuit.

ΔV battery = Δ V1 + Δ V2 + Δ V3 + ...

KIRCHHOFF’S VOLTAGE RULE

Around any circuit, the voltage changes must add up to zero.

VOLTAGE IN SERIES CIRCUITS

VOLTAGE IN SERIES CIRCUITS

VOLTAGE IN SERIES CIRCUITS

PROBLEM 3

Determine the voltage drop for the two light bulbs.

RESISTANCE

Resistance is the ability of an object to conduct current.

If objects have a high resistance, they are called insulators.

If objects have a low resistance, they are called conductors.

RESISTANCE

RESISTANCE IN SERIES CIRCUITS

The act of adding more resistors to a series circuit results in the rather expected result of having more overall resistance. Since there is only one pathway through the circuit, every charge encounters the resistance of every device; so adding more devices results in more overall resistance. This increased resistance serves to reduce the rate at which charge flows (also known as the current).

RESISTANCE IN SERIES CIRCUITS

There is a clear relationship between the resistance of the individual resistors and the overall resistance of the collection of resistors. As far as the battery which is pumping the charge is concerned, the presence of two 6-Ω resistors in series would be equivalent to having one 12-Ω resistor in the circuit. The presence of three 6-Ω resistors in series would be equivalent to having one 18-Ω resistor in the circuit. And the presence of four 6-Ω resistors in series would be equivalent to having one 24-Ω resistor in the circuit.

RESISTANCE IN SERIES CIRCUITS

Total Resistance = R1 + R2 + R3 + ...

RESISTANCE IN SERIES CIRCUITS

SHORT CIRCUITS