Welcome to nothingnerdy&#x2019;s presentation on Everything Electric. Part 2: Vaguely Voltage\n
An electric circuit has three elements. The source (such as an electric cell) provides electrical energy to the moving charges by converting it from other energy types. The components, such as the bulb here, convert electrical energy to other forms, such as heat and light. Between the different components and the source there will be conducting connections, such as wires which make a complete loop.\n
Here are three examples of sources converting other forms of energy to electrical.\n
There are many different components which convert electrical energy to other forms when connected in an electric circuit.\n
Electric current is one of the quantities we often measure in electric circuits. It is defined as the rate of flow of electric charge and is measure in amperes. Charge is measured in coulombs. When one coulomb passes a point in a circuit in one second, this is one ampere. When the time is measured in seconds, we can use the formula Charge=Current * time or Q = It\n
We know that current in a wire is the flow of electrons and that these are negative charges. They flow from the negative terminal on a battery round the circuit to the positive terminal losing their electrical energy on the way. Long before the electron was discovered, scientists guessed that current flows from + to -. They were wrong but it was too late. When we draw arrows to show the direction of current, it is from + to -, the opposite direction to the flow of electrons. Sorry, that&#x2019;s just the way it is.\n
Here are many of the names and symbols of the most common electrical components.\n
Parallel circuits are connected like a ladder which means that you can remove or add new components, like rungs in a ladder, without affecting the other components. to this simple circuit, we add bulbs in parallel and note that all of the bulbs have the same current in them. It is the battery which provides more and more current.\n
The components in a series circuit are joined like a chain. If one component fails, the whole circuit stops working. Let&#x2019;s see what happens when the number of bulbs is changed...\n
When there are fewer bulbs but the same number of cells, the remaining bulbs are brighter because there is now more current.\n\nLet&#x2019;s now see what happens if the number of cells is changed.\n
When we reduce the number of cells but keep the bulbs the same, the bulbs are now dimmer. This is because the current is lower.\n
As well as current in a circuit, we also measure voltage. Voltage (aka PD) is the amount of energy transferred for every unit of charge which passes. If one coulomb of charge receives 6 joules of energy from a battery, then the voltage of the battery is 6 volts. You can measure the voltage across a source or across a component.\n
Current in amps is measured by an ammeter; voltage in volts is measured by a voltmeter. They are connected in different ways. The current passes through the ammeter so it is connected in series. The voltmeter measure the voltage across the component, so it is connected in parallel.\n
This is an important circuit in which the voltmeter and ammeter are connected to measure the relationship between V and I for a component such as a bulb. The voltage is changed by adding new cells, and each time the new voltage and current are measured.\n
Here are the results of two real experiments measuring V and I for a bulb and a resistor. We can see that the result for the resistor is a straight line, but for the bulb it curves upwards.\n
If you plot the V-I graph for a diode, the current passes in one direction, when the voltage across the diode is positive, but when the voltage is reversed, the diode blocks the current. The symbol is shown above and the triangle shows which way the current flows.\n
A LED, like a diode, only conducts in one direction. It emits light and can be used to show the presence of a current. But if it is connected in reverse, no current flows. The symbol shows a diode with rays of light coming from it.\n
Resistance measures how much a conductor opposes current. The formula is V=IR as can be seen in the triangle on the slide. For a certain voltage, the current and resistance are inversely proportional. This means that if the resistance is high, the current is low and vice versa. For a component, resistance is calculated by dividing voltage by current. The unit of resistance is the ohm which is equal to 1 volt divided by 1 amp.\n
For certain conductors, when the temperature is constant, the voltage across it is proportional to the current (straight line through the origin). That means the graph of V against I is a straight line. This also means that the resistance is constant. These components, such as resistors, obey Ohm&#x2019;s Law (they are ohmic). Other components have a varying resistance (such as bulbs and diodes) and are called non-ohmic.\n
As the name suggests, LDRs have a resistance which changes with the brightness of the light falling on them. As you can see in the graph, the resistance is low when the brightness is high. This means a high current flows - it could ring a buzzer for instance. You can also see the symbol here with rays of light falling on a resistor symbol.\n
The resistance of a thermistor changes with the temperature. When it is hot, the resistance falls which means that the current is high - it could operate a fan, for example. The symbol is a resistor with a bent line across it as shown on the slide.\n
We can put together all of the ideas about circuits and energy. The battery gives 6J of energy to each coulomb that passes through it. In the wires, almost no energy os transferred. At the branch, three times more energy is converted in the low resistance bulb. After the parallel bulbs, there is still 3J remaining on each coulomb which all gets converted in the last bulb so that when the charge returns to the battery, it has transferred all 6J. The conventional current goes in the opposite direction\n
Vaguely voltage - nothingnerdy igcse physics
presentsa production Everything Electric Episode 2 Vaguely Voltage
Simple circuit Source: chemical to electrical Connection: conduct electricity with almost no energy change Component: electrical to light and heat
Sources of electrical energyThey convert different types of energy to electrical energy Batteries: Chemical to ElectricalImage credit: Ian Britton @ FreeFoto.com Image: US Department of Energy Eel: Atomic power station: Chemical Nuclear to to Electrical ElectricalImage: Aaron Gustafson @ ﬂickr.com
ComponentsThey convert electrical energy to different types of energy produces heat energy produces heat energy produce light and heat energy produces sound, kinetic and heat energy produces kinetic and heat energy
What is Electric Current?Electric current is the ﬂow of electric charge Q Ixt Charge, Q is measured in coulombs (C) Current, I is measured in amperes (A) Time, t is measured in seconds (s) One ampere = One coulomb per secondCurrent ﬂowing = Charge passing ÷ time taken
Conventional current electrons electronselectrons electronsElectrons travel around the circuit from - to + terminals of the battery, but the arrows on the wires for the conventional current are drawn the other way.
SymbolsThey are easy to draw and everyone agrees what they look like
Parallel circuits 2Think of them as Ladder circuits Try this out: http:// phet.colorado.edu/en/ simulation/circuit- construction-kit-ac When a new component is connected in parallel, the current in the existing components does not change, although the current from the battery will increase
Series circuits Think of them as Chain circuits This circuit contains 3 cells and 3 bulbs in seriesWhat happens if you change the number of bulbs?What happens if you change the number of cells?
Q: What happens if you reduce the number of bulbs but keep the cells the same?A: The bulb gets brighter.This means the electric current is higher.
Q: What happens if you reduce the number of cells but keep the bulbs the same?A: The bulbs get dimmer.This means the electric current is lower.
What is Voltage? also known as potential differencevoltage is the energy transferred per unit charge passed 1 volt = 1 joule per coulomb A battery of voltage 6.0 volts gives 6.0 joules of energy to each coulomb which passes Energy, E in joules PD,V in volts E Charge, Q in coulombs Q xV
Ammeters and voltmeters To measure V and I in the bulb... A current ﬂows through the component Ammeter connected in series - you must break the circuit to connect it A voltage is applied across the component Voltmeter connected in parallel - just touches the wires at either side
Measuring V and I Apply different voltages across a component (this one is a bulb) by adding more cells (or using a variable power supply).Circuits created at echalk.co.uk Plot a graph of V against I
Current in bulbs and resistorsResults for actual investigation using variable power supply
Current in diodesImage: Erik Streb @ wikipedia When the voltage is positive, the diode conducts electricity, but when the voltage is reversed, the diode blocks the current. Image: Theresa Knott @ wikibooks
Light emitting diodesWhen the voltage is positive, the diode conducts electricity, butwhen the voltage is reversed, the diode blocks the current When current ﬂows in the LED, it emits light; if it is connected in reverse, no current will ﬂow. Image: Pete Scamper
What is Resistance?Resistance is how much a conductor opposes the ﬂow of current. If the resistance is low, then the current is high. If the resistance is high, then the current is low Resistance, R is measured in ohms (Ω) Voltage,V is measured in volts (V) Current, I is measured in amps (A) V and... R=V/I IxR
Ohm’s LawThe current through a metallic conductor is directlyproportional to the voltage across its ends if thetemperature and other conditions are constant. Image: bbc gcse bitesizeExplore Ohm’s Law with PHET simulation
Light-Dependent Resistors Also known as LDRs or photoresistors When bright light shines on it, the resistance is low and the current is high. Image: ladyada.net Image:Benthewikiman @ wikipedia
ThermistorsAlso known as Negative Temperature Coefﬁcient (NTC) Resistors When the temperature is high, the resistance is low and the current is high. Image: Julong Technology Ltd Image: Arroyo Instruments
How a circuit works When the electrons return to Each coulomb takes 6.00 J ofthe battery, all electrical energy electrical energy from the battery. has been converted. 0.75 J Almost no electric energy change All 6.00 J has to light/ in the wire;been converted heat still 6.00 J/C from electric energy. 3.00 J electric to light/ heat Each coulomb loses 2.25 J electrical energy to light and heat in this bulb
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