Unit 3Simple Circuits

Electric current

Voltage produces a flow of charge, or current, within a conductor.

The flow is restrained by the resistance it encounters.

The rate at which energy is transferred by electric current is power.

Flow of charge

When the ends of an electric conductor are at different electric potentials, charge flows from one end to the other.

The flow continues until both ends reach the same potential

Flow of charge

When there is no potential difference, there is no longer a flow of charge through the conductor.

To attain a sustained flow of charge in a conductor, one end must remain at a higher potential than the other.

Net electric charge on a wire

A current-carrying wire has a net electric charge of zero.

What is a circuit?

An electronic circuit is a complete course of conductors through which current can travel.

Circuits provide a path for current to flow.

4 parts of a circuit1. A power source - device that supplies electric

power to an electrical load.

2. A load – The part or component in a circuit that converts electricity into light, heat, or mechanical motion.

3. The wire – the conducting material that transfers the electricity.

4. The controlling device – the “on” and “off” switch of the circuit.

Types of simple circuits

Series Circuits

Parallel Circuits

Series CircuitsA series circuit is a circuit in which resistors are arranged in a chain, so the current has only one path to take.

The current is the same through each resistor.

The total resistance of the circuit is found by simply adding up the resistance values of the individual resistors:

Equivalent resistance of resistors in series : R = R1 + R2 + R3 + . . .

Series circuits

A parallel circuit is a circuit in which the resistors are arranged with their heads connected together, and their tails connected together. The current in a parallel circuit breaks up, with some flowing along each parallel branch and re-combining when the branches meet again. The voltage across each resistor in parallel is the same.

The total resistance of a set of resistors in parallel is found by adding up the reciprocals of the resistance values, and then taking the reciprocal of the total:equivalent resistance of resistors in parallel: 1 / R = 1 / R1 + 1 / R2 + 1 / R3 +...

What is an electrical currentAn electric current is a flow of electric charge. Electric charge flows when there is voltage present across a conductor. In electric circuits this charge is often carried by moving electrons in a wire.

The symbol for current is I and its unit is ampere.

An ampere is the flow of 1 coulomb of charge per second

What is a voltage?

Voltage, electrical potential difference, electric tension or electric pressure is the electric potential difference between two points, or the difference in electric potential energy of a unit charge transported between two points.

The symbol for voltage is V and its unit is volt.

1 volt is equaled to the 1 joules of energy used to move 1 coulomb of charge.

What is a resistance?

electric resistance: a material's opposition to the flow of electric current

The symbol for resistance is R and its unit is ohms. Ω

Electric resistance is measured in units called ohms.

Georg Simon Ohm, a German physicist, tested wires in circuits to see what effect the resistance of the wire had on the current.

The relationship among voltage, current, and resistance is called Ohm’s law.

Ohm’s Law

ohm

voltampere

current

IRVorR

VI

11

resistance

voltage

Speed of an electron in a circuitWhen you flip on the light switch on your wall and the circuit is completed, the light bulb appears to glow immediately.

Energy is transported through the connecting wires at nearly the speed of light.

The electrons that make up the current, however, do not move at this high speed.

The electrons inside a metal wire have an average speed of a few million kilometers per hour due to their thermal motion.

PowerElectric power is the rate at which electrical energy is converted into another form such as mechanical energy, heat, or light.

Electric power is equal to the product of current and voltage.

electric power = current × voltageP = I V

If the voltage is expressed in volts and the current in amperes, then the power is expressed in watts.

1 watt = (1 ampere) × (1 volt)

PowerThe power and voltage on the light bulb read “60 W 120 V.”

The current that would flow through the bulb is:

I = P/V = (60 W)/(120 V) = 0.5 A.

Power

A lamp rated at 120 watts operated on a 120-volt line will draw a current of 1 ampere: 120 watts = (1 ampere) × (120 volts).

A 60-watt lamp draws 0.5 ampere on a 120-volt line.

A kilowatt is 1000 watts, and a kilowatt-hourrepresents the amount of energy consumed in 1 hour at the rate of 1 kilowatt.

Where electrical energy costs 10 cents per kilowatt-hour, a 100-watt light bulb burns for 10 hours for 10 cents.

A toaster or iron, which draws more current and therefore more power, costs several times as much to operate for the same time.

Electrical Energy

E = VIt

E = electrical energy (joules)

V = voltage (volts)

I = current (amperes)

t = time (seconds)

The difference between Parallel and Series Circuits

Series Circuits Parallel Circuit

Number of pathways for charge to flow

1 More than 1

If one light bulb goes out, the other . . .

Goes out Still lights up

As the number of resistors increased,

the current . . .

Stays the same Splits up

As the number of resistors increased,

the overallresistance…

Increased Decreased

The difference between Parallel and Series Circuits

Equivalent resistance in parallel

The difference between Parallel and Series Circuits

Equivalent resistance in Series

The difference between Parallel and Series Circuits

The current in the series stays constant all the way through the circuit. But in parallel they split up.

parallel

series

14 cars 14 cars

4 cars

4 cars

6 cars

6 cars

If three resistors are placed in parallel branches and powered by a 12-volt battery, then the voltage drop across each one of the three resistors is 12 volts.

The voltage drops across the resistor must equal the voltage of the power source.

To solve compound circuits where you have a parallel resistors connected to another in a series, we solve for the equivalent resistance first then add the resistor in series.