Electricity Flashcards
(Static Electricity) Why does negatively charged electrons get dumped onto the material and not positively charged protons ?
Only the electrons move- never the positive charges
both positive and negative charges are produced by the movement of electrons.
positive charge- when electrons move elsewhere
(Static Electricity) how do the materials repel/attract eachither
when two electrically charged objects are brought close together they exert a force on one another.
opposite electric charge = attract
same electrical charge= repel
Forces get weaker the further apart they get
How is a built up of static electricity caused
by friction.
- when some insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other
so one will have a positive static charge and the one with dumped electrons will have a negative static charge.
The way the electrons are transferred depends on the materials
- These electrically charged objects will then attract small objects placed near them
Eg polythene and acetate rods being rubbed by a cloth duster (rod becomes positive)
(Static) Do conductors make chargers hard or easy to travel through
electrical charges move easily through some materials, known as conductors
metals are good conductors
what does current mean
current is flow of electrical charge round the circuit. Unit= A ampere
it will only flow if there is a potential different across that component
What is potential difference
Potential difference is the driving force that pushes the current round (Unit: V) volts
what is resistance
resistance is anything that soles the flow down Unit: Ohm (horseshoe shape)
The greater the resistance across a component, the smaller the current that flows (for a potential difference across the compound)
how do you calculate the total charge through a circuit
CURRENT AND TIME
charge Q Current=--------- I =-----
time T
More charge passes around a circuit when a bigger current flows
how do you calculate the potential difference that passes through two points in an electrical circuit
potential difference across an electrical component is ther amount of energy transferred by that electrical unit per unit of charge
work done W P.D.= ------------- or -----
Charge V x Q
LEARN THE CIRCUIT SYMBOLS FOR
a cell a battery a open switch a closed switch a filament lamp (bulb) a fuse a LED a resistor a variable resistor an ammeter a voltmeter a diode a LDR a thermistor
What is the standard test circuit (learn what it looks like)
used for testing resistance of a components (by measuring current and potential difference) and for getting V-I graphs
the component, the ammeter and the variable resistance or are all in series (so they can be put in any order in the main circuit).
The voltmeter has to be put parallel around the component under test
- As you vary the variable resistor it alters the current flowing through the circuit
This allows you to take several pairs of readings from the ammeter and voltmeter, which you can then plot on a VI graph and find the resistance
How do you measure the current
Ammeter measures the current in amos
it must be placed in series, and placed anywhere within the series
How do you measure the potential difference
Potential difference is measured using a voltmeter (in volts)
it must be placed in parallel around the component under test
What do VI graphs show
VI graphs show how the current varies as you change the potential difference
What are the three important Potential difference-Current Graphs
Different resistors- directly proportional to PD and different resistors have different slopes
Filament Lamp- As the tempreture of the filament increases the resistance increases (so like a s curve)
Diode- current will only flow through a diode in one direction. A diode has a very high resistance in the opposite direction
Why does resistance increase with tempreture
- when electrical charge flows through resistor some energy is transferred as heat so resistor gets hot
- this heat energy causes ions in the conductor the vibrate more so it’s more difficulty for charge carrying electrons to get through so the current can’t flow as easily and the resistance increases
- Most resistors have a limit to the amount of current that can flow. More current=increase in temperature= increase in resistance =current decreases again
- This is why the graph for the filament lamp levels off at high currents
What is the equation for resistance, current and PD
Potential difference= current x resistance
V
—–
I x R
The steeper the graph the lower the resistance. If the graph curves it means the resistance is changing. In this case R can be found by taking a pair of bales from the graph and putting them into his equation
What’s a diode
A diode is a special device made from a semiconductor material e.g. silicon
- used to regulate the potential difference in circuits
- lets current flow freely through it in one direction but not in the other where it has a very high resistance
What are light-emitting diodes
Light emitting diode (LED) emits light when a current flows through it in a forward direction
LEDs are used for lighting as they use a much smaller current than other forms of lighting.
LEDS indicate the presence of a current in a circuit (to show TV appliances are switched on, traffic lights, digital clocks).
What is a light-dependant resistor (LDR)
- LDR is a resistor dependant on the intensity of light
- in bright light resistance falls
- in darkness resistance is highest
used for automatic night lights, outdoor lighting and burglar detectors
What is a thermistor
a thermistor is a temperature dependant resistor
- hot conditions resistance drops
- cool conditions resistance goes up
make useful temperature detectors eg car engine temperature sensors and electronic thermostats
what are the rules of series circuits
- if you remove one component the circuit is broken and they all stop
- potential difference is shared between the various components so all voltages around a circuit add up to equal the source voltage
- the same currents flows through all parts do the circuit A1=A2
- the total resistance is the sum of all the resistances, more resistance =higher PD
- the more cells connected the same way, the bigger the PD
what are the rules of parallel circuits
- each component is separately connected to the +ve and -ve supply
- if a component is remove it hardly affects the others
- All components get the full source of PD, so voltage is same across all components
- Current through the battery is the same as the sum of the other currents in the branches A= A1 +A2…
- there are junctions where the current either splits or rejoins. total current going into junction=total current leaving
What are the Voltmenters and ammeters exceptions to the rule
ammeters always connected in series - even in a parallel circuit
voltmeters always connected in parallel with a component- even in a series
