Simple electric circuits

• Objectives: draw circuits conventionally using standard symbols

and straight lines for the connecting wires learn how to construct circuits from circuit diagrams discuss the meaning and results of a circuit diagram using their own ideas for circuits- predict how each

circuit will behave, giving a reason, before trying it out

Current flows due to

a.There must be an energy supply capable

doing work on charge to move it from a low

energy location to a high energy location and

thus establish an electric potential difference

across the two ends of the external circuit.

b.There must be a closed conducting loop

Quantitative investigation of voltage, current, and Ohm's law

Electric current is the flow rate of electric charge and is measured in amperes.

The current transports electrical energy along conductors.

Voltage (V) is a measure of energy per unit charge between two points in the circuit. One may think of voltage as the effective "pressure difference" which causes the current to flow.

• Resistance (R) is the opposition to current flow and is measured in ohms. In practice, resistors take the form of light bulbs, toasters, heaters and other devices which use electrical energy to perform useful tasks as well as the undesirable form of resistance in electrical wiring that transports the electrical energy to you.

NotesYou need a continuous circuit to make the lamp light

• the shape of the circuit does not matter if there is only one single loop (this is important if Worcester circuit boards are used) • with more than one cell then the relative directions of the cells matter • the lamps in a series circuit are all the same brightness • more lamps in a series circuit means dimmer lamps • the same number of cells and lamps in a series circuit means that each lamp is at normal brightness • one cell with two lamps connected in parallel with it shows that both lamps have the same brightness (if 'high power, or low internal resistance cells are used) Two lamps in series with a cell will run for longer than two similar lamps in parallel with a cell, though students may not discover this during one lesson.

Circuit symbols & diagrams

In this circuit, there is only one source of voltage (the battery, on the left) and only one source of

resistance to current (the lamp, on the right).

Predict and explain

1. Circuit will be altered by adding a light bulb in parallel. Predict & explain changes in brightness of bulbs and current supplied by battery.

2. The circuit will be altered by adding a light bulb in series. Students will predict & explain changes in brightness of light bulbs and current supplied by the battery

Which is most dangerous, current or voltage?

• A high voltage is not necessarily dangerous if it cannot deliver substantial current.

• current is the killer not volts• Static discharge to car doors in winter can

involve voltages as high as 20,000 V• you can have thousands of volts but without

flow (current) the danger is minimal• but if you have voltage its very easy to induce

current so be careful and let a professional take care of it if you do not know what you are doing.

How voltage, current, and resistance relate

• voltage -the measurement of how much potential energy exists to move electrons from one particular point in that circuit to another particular point. Without reference to two particular points, it has no meaning

• motivate electrons from one point to another. Before we can precisely define what a "volt" is, we must understand how to measure this quantity we call "potential energy."

Coulomb- is a measure of electric charge proportional to the number of electrons in an imbalanced state. One coulomb of charge is

equal to 6,250,000,000,000,000,000 electrons.

International safety symbol "Caution, risk of electric shock" (ISO 3864), also known as high

voltage symbol

•electrocution danger is mostly determined by the low electrical conductivity

•Accidental contact with high voltage supplying sufficient energy will usually result in

severe injury or death. This can occur as a person's body provides a path for current

•Low-energy exposure to high voltage may be harmless(spark touching a doorknob,

• it should not be assumed that being insulated from earth guarantees that no

current will flow to earth as grounding, or arcing to ground, can occur in unexpected

ways

•High-frequency currents can cause burns even to an ungrounded person (touching

a transmitting antenna is dangerous for this reason, and a high-frequency Tesla Coil

can sustain a spark with only one endpoint).

• The means by which the resistors are

connected will have a major affect upon the

overall resistance of the circuit, the total

current in the circuit, and the current in each

resistor.

Series Circuits

• charge passes through every light bulb.

• As more and more light bulbs are added, the

brightness of each bulb gradually decreases.

• This observation is an indicator that the

current within the circuit is decreasing.

• decrease in current is consistent with the conclusion that

the overall resistance increases.

In order for the devices in a series circuit to work, each device must work.

• If one goes out, they all go out.

• Suppose that all the appliances in a household kitchen were all

connected in series.

• In order for the refrigerator to work in that kitchen, the toaster

oven, dishwasher, garbage disposal and overhead light would

all have to be on.

• If current is cut from any one of them, it is cut from all of them.

Quite obviously, the appliances in the kitchen are not

connected in series.

in parallel

• a single charge passing through the

external circuit would only pass through

one of the light bulbs.

• The light bulbs are placed within a

separate branch line, and will pass through

only one of the branches during its path

back to the low potential terminal.

a study of the overall current for parallel connections requires the

addition of an indicator bulb.

• An indicator bulb placed outside of the branches allows one to observe the affect of additional resistors upon the overall current.

• The bulbs in parallel branches only provide an indicator of current through that branch.

• So if investigating the affect of the number of resistors upon the overall current and resistance, one must make careful observations of the indicator bulb, not the bulbs that are placed in the branches.

For parallel circuits, as the number of resistors increases, the overall current

also increases.

• This increase in current is

consistent with a decrease in

overall resistance.

• Adding more resistors in a

separate branch has the

unexpected result of

decreasing the overall

resistance!

The affect of adding resistors is quite different if added in parallel compared to adding them in series.

• Adding more resistors in series means that there is more overall resistance;

• Adding more resistors in parallel means that there is less overall resistance.

• You can add more resistors in parallel and produce less resistance

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

connections. Explain each choice.

Wires have little

resistance

• The electrical resistance is greater for a longer wire, less for a wire of larger cross sectional area, and would be expected to depend upon the material out of which the wire is made

• Resistance also depends on temperature, usually increasing as the temperature increases.

If the connecting wires resistance is very little or none, we can regard the

connected points in a circuit as being

electrically common

• That is, points 1 and 2 in the circuits may be physically joined close together or far apart, and it doesn't matter for any voltage or resistance measurements relative to those points. The same goes for points 3 and 4. It is as if the ends of the resistor were attached directly across the terminals of the battery

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.

In series? ___________________ In parallel? _________________

• As the number of resistors (light bulbs) increases, what happens to the overall current within the circuit?

• As the number of resistors (light bulbs) increases, what happens to the overall resistance within the circuit?

• If one of the resistors is turned off (i.e., a light bulb goes out), what happens to the other resistors (light bulbs) in the circuit? Do they remain on (i.e., lit)?

Current is the rate at which charge crosses a point on a circuit.

Drift speed

1 ampere = 1 coulomb / 1 second

Batteries are not rechargeable

An electrochemical cell supplies the energy needed to move a charge from a

low potential location to a high potential location

The charge that flows through a circuit originates in the wires of the circuit. The

charge carriers in wires are simply the electrons possessed by the atoms that

make up the wires.

Charge moves abnormally slowly - on average, about 1 meter in an hour - through a circuit

The rate at which charge flows is everywhere the same within an electric circuit. The rate at which

charge flows into a light bulb is the same as the rate at which charge flows out of a light bulb.

An electrical appliance such as a light bulb transforms the electrical energy of moving charge into

other forms of energy such as light energy and thermal energy. Thus, the amount of electrical

energy possessed by a charge as it exits an appliance is less than it possessed when it entered the

appliance

Electric circuits are all about energy, not charge.

• When a battery no longer works, it is out of energy.

• A battery (or single cell) operates by packing a collection of reactive

chemicals inside. These chemicals undergo an oxidation-reduction

reaction that produces energy. This energy-producing reaction is

capable of pumping the charge through the battery from low energy

terminal to high energy terminal and establishing the electric

potential difference across the external circuit. And when a battery no

longer works, it is because the chemicals have been consumed to the

point that the ability of the battery to move the charge between

terminals has been severely diminished.

• When a battery no longer works, it is because the conversion of

reactants to products have occurred to the extent that the energy-

producing reaction is no longer able to do its job of pumping charge.

• By placing the cell into a so-called recharger, the

energy of a household electrical circuit can be

used to drive the reaction in the reverse direction

and transform the chemical products back into

chemical reactants. This reverse process requires

energy; it is the recharger which supplies the

energy. With reactants replenished, the cell can

now be used again to power the electric circuit.

Ohm's Law

• If you know 2 pars, and wish to determine another, just eliminate it from the picture and see what's left:

REVIEW:

• With resistance steady, current follows voltage (an increase in voltage means an increase in current, and vice versa).

• With voltage steady, changes in current and resistance are opposite (an increase in current means a decrease in resistance, and vice versa).

• With current steady, voltage follows resistance (an increase in resistance means an increase in voltage).

REVIEW:

• Devices called resistors are built to provide precise amounts of resistance in electric circuits. Resistors are rated both in terms of their resistance (ohms) and their ability to dissipate heat energy (watts).

• Resistor resistance ratings cannot be determined from the physical size of the resistor(s) in question, although approximate power ratings can. The larger the resistor is, the more power it can safely dissipate without suffering damage.

• Any device that performs some useful task with electric power is generally known as a load. Sometimes resistor symbols are used in schematic diagrams to designate a non-specific load, rather than an actual resistor

What is the amount of current (I) in this circuit?

• What is the

amount of

current (I) in

this circuit?

What is the amount of voltage provided

by the battery?