More on simple battery circuits

Review

• A simple voltaic cell diagram shown at right

• What direction do the electrons flow? Do they change at all?

• If the electrons only flow in one direction, this is a called a Direct Current

Series circuits

• Batteries can be connected in series to increase voltage

• For any circuit, the total voltage is the sum voltage of the batteries connected in the series

More about Direct Current

• The current flowing from a battery through a circuit only flows in ONE direction

• The current is described as Direct Current (or DC) power

• Voltage of a battery diminishes as the chemical reactions driving the battery run out of reagents (run out of chemical potential)

What is happening in an electrical circuit?

• First – what does “circuit” mean? – What is a circuit workout? – What is a racing circuit– In an electrical circuit,

“charge” flows in a circular pathway

• The energy of electrons may be transformed multiple ways throughout their journey

Anode – where electrons are

generated

This switch is closed, meaning it is connected and charge is flowing

through it

Here, kinetic energy of electrons is put to work, in this case powering a light

The reason electrons are flowing – there is a

positive charge at the cathode they are attracted

to

Things that affect flow of charge

• Voltage – symbol is V (sometimes written as E)• Amperage – symbol is I• Ohms (resistance) – symbol is R (sometimes shown as

Ω)

Ohm’s law

• Ohm’s law relates how Voltage, current and resistance relate to each other

• For any electrical circuit, ΔV = I • R• Put into words, the current flow depends on the Voltage

divided by the resistance• Given constant resistance, to get more flow what must be

changed?• The voltage must be increased!• Given constant voltage, to get more flow what must be

changed?• The resistance must be lowered!

What about Watts?

• Simply put, Wattage (power) is Voltage times current, or

• W = I x V• So electrical devices can be described in terms of

either the current they like to take (because Voltage in American electrical outlets is always the same), or –

• They may be described in terms of their wattage• In American homes, electrical outlets are standard at

120 V• European systems are standard at 220 V

Parallel circuits and current

Capacity of a battery is how long it can generate a current

• Current (measured in Amperes or “Amps”), uses symbol I

• A measure of the amount of electrical charge passing through a point in a circuit in a given time

• Sometimes this is described in terms of “Amp-hours”

Gift wrap available!

Increasing capacity

• The capacity of a battery (amount of current it generate in a unit time) is fixed

• But when batteries are joined in parallel, the capacities add up

• A parallel connection links the battery’s anodes to each other, and the battery’s cathodes to each other

• Voltage is unchanged, capacity increases

Series versus Parallel

• In a series connection, voltages add, but capacity is not changed

• In parallel connections, voltages remain the same, but capacity increases

Series connection – only Voltage increases

Parallel connection – only capacity increases

Series versus parallel

Sometimes a circuit can be both

What is Voltage/Capacity of this system?

Series connection

Series connection

Parallel connection

Parallel connection

Resistance…

…is futile

Resistors

• A resistor is a device to increase the resistance at a particular point in a circuit by reducing the current that can flow through a point

• Think of a valve in pipe, in our “water analogy”• Unit is the Ohm• Resistors are used to generate heat, lower

voltage, produce light, etc.• Essentially, a light bulb can be thought of as a

resistor

Resistors are color-coded

Resistors in series

• When resistors are in series – the total resistance is the sum of the individual resistor’s value in Ohms

• So it works just like batteries connected in series

Resistors in parallel

Practice problems

• Three resistors are connected in a series. The value of each resistor is 10 Ω. What is the resistance of the circuit?

• 30 Ω• The same three resistors are now assembled

in parallel. What is the resistance of the circuit?

• 3.33 Ω

Conclusion - resistance

• All electrical circuits will have some resistance• Even though Copper is a good conductor, good is not

“perfect”• So all circuits will lose some of their electrical energy as

heat (electronics get warm when you use them because of resistance in the circuits)

• With heat – resistance increases!• A superconductor is a material that can carry current with

little – or even no – resistance! But the material often has to be cooled to very low temperatures to become superconductive

Super-cooled substances show strange effects