P4 Flashcards
Electric circuits
What is static electricity ?
2+ insulators becoming charged due to friction with one another
How does static electricity occur ?
- electrons cannot move through insulators
- insulators have outer layers of electrons
- when two insulators rub together, friction makes some electrons move from one insulator to the other
[] this leaves the item with the most electrons negatively charged overall, and the other with an overall positive charge - because they are insulators and not conducting materials, electrons cannot move back to restore a neutral overall charge
[] hence being called static electricity
Describe the flow of electrons and end overall charges when a polythene rod is rubbed with a dry cloth
- electrons go from the cloth to the rod
- rod is negatively charged after - cloth is positively charged after
Describe the flow of electrons and end overall charges when a perspex/acetate rod is rubbed with a dry cloth
- electrons go from the rod to the cloth
- rod is positively charged after
- cloth is negatively charged after
What is potential difference ?
the difference in electrical charge between two points
- 1 volt shows that 1 joule of energy is transferred for each coulomb of charge moving through the circuit
What are sparks and how do they occur ?
electric sparks happen when the potential difference between two objects is so different that the electrons from the negatively charged one can jump across the gap
- NEGATIVE electric charge accumulates in an object and creates a very strong electric field
- this electric field pushes and rips an electron from the nearest atom (usually in air) turning it into an ion
- this electron is propelled by the strong electric field away and knocks into other atoms, knocking out some of their electrons and causing them to do the same
What do like charges do ?
repel
What do opposite charges do ?
attract
What is the name of the non-contact force exerted on each other by two charged objects ?
electrostatic force
- force exerted by the electrical fields of the two charged objects on each other grow stronger with LESS/DECREASING distance
How can electrostatic sparks be dangerous and how is this danger prevented ?
- in refuelling vehicles, electrostatic charge may build up between the fuel and the pipes, causing a spark
[] this would ignite the fuel, and cause it to explode - to prevent this (for example when fuelling an aeroplane) both the fuel storage/pipes and the vehicle are earthed (made to 0 volts, preventing charge from building up)
How can electric fields be represented in a diagram ?
- circle drawing of positive or negative subatomic particle representing charge of the field
- lines outwards perpendicular to the circle
- if positive, arrows away from centre
- if negative, arrows towards centre
What is current and what does its size mean ?
the flow of electrical charge, usually through a circuit but not constrained to one
- size of current means the RATE of flow of electrical charge
Give an equation for current (with units)
current (A (amperes)) = charge flow (C (coulombs))/time (s)
What is a battery, and what is its symbol (draw it) ?
- two or more cells joined together
- double check using the internet
What is a switch, and what is its symbol (draw it) ?
- enables current in a circuit to be switched on/off
- use the internet
What is an indicator/bulb, and what is its symbol (draw it) ?
- designed to emit light as a signal when current passed through it
- use internet
What is a diode, and what is its symbol (draw it) ?
- only allows current to pass in one direction
[] low resistance in one direction, and HIGH resistance in the other - very useful to control the flow of current in a circuit
- use internet
What is an LED, and what is its symbol (draw it) ?
- light emitting diode
- emits light when current passes through
- only allows current in one direction
[] low resistance in one direction, and HIGH resistance in the other - very energy-efficient light source so useful in displays, lightbulbs etc.
- use internet
What is an ammeter, and what is its symbol (draw it) ?
- measures electric current in amperes/amps/A
- use internet
What is a fixed resistor, and what is its symbol (draw it) ?
- limits the current in a circuit by a specific amount and this cannot be changed
- use internet
What is a variable resistor, and what is its symbol (draw it) ?
- limits the current in a circuit, but this limitation can be varied/changed
- use internet
What is a fuse, and what is its symbol (draw it) ?
- has a thin wire at its centre that is designed to melt and break the circuit if the current is too high
[] means that can simply replace the fuse if damaged instead of the whole circuit - use internet
What is a heater, and what is its symbol (draw it) ?
- transfers energy from an electric current to the surroundings as thermal energy
- use internet
What is a voltmeter, and what is its symbol (draw it) ?
- measures potential difference in a circuit in volts
- use internet
What is an LDR, and what is its symbol (draw it) ?
- light dependent resistor
- varies resistance in a circuit based on the amount of incoming light
[] high resistance in low light
[] low resistance in high light levels - used in streetlamps/automatic lights
- use internet
What is a thermistor, and what is its symbol (draw it) ?
- temperature dependent resistor
- used in (domestic) automatic heating/thermostats and laptop cooling fans
- low resistance in high temperatures
- high resistance in low temperatures
- use internet
What is a bell, and what is its symbol (draw it) ?
- designed to convert electrical energy to sound energy
- use internet
What is a coil, and what is its symbol (draw it) ?
- coil of wires used in electrical transformers to change voltage and current
- use internet
Draw two circuit diagrams with a battery and two bulbs (one parallel circuit and one series)
- parallel
[] battery at top
[] one bulb underneath
[] the other bulb underneath the first
[] two loops of wire
[] charge from positive terminal to negative terminal - series
[] battery at top
[] both bulbs underneath and next to one another
[] one loop of wire
[] charge from positive terminal to negative terminal
Describe why two bulbs connected in parallel are brighter than two bulbs connected in series
- when released from the battery, electrons have maximum voltage
- when passing through a bulb, these electrons lose voltage before travelling back to the battery again
- in parallel, electrons split BEFORE they get to the bulbs, so any electrons passing through both bulbs in a parallel circuit are at maximum voltage and thus the bulbs are at maximum brightness
- HOWEVER, in series, once the electrons have already passed through the first bulb and thus lost voltage, they have to travel through the second bulb
- they thus have less voltage to give the second bulb and so the bulbs are dimmer and not at maximum brightness
- voltage is SPLIT in a series circuit, but is at MAXIMUM in a parallel circuit
How should an ammeter be connected to a circuit ?
in series with the bulb
How should a voltmeter be connected to a circuit ?
in parallel with the bulb
[] measures voltage ACROSS the bulb
How does current change in a series circuit vs a parallel circuit ?
- series
[] CONSTANT - never changes - parallel
[] split between the branches
[] total current in branches = total current leaving the battery/cell (add up current of each branch to get total current of circuit)
Give the equation for potential difference (with units)
p.d (V (volts)) = energy (J)/charge (C(coulombs))
Give the equation for energy transferred using charge flow and p.d (with units)
energy transferred (J) = charge flow (C) x voltage (V)
Give the equation for resistance
resistance (Ω) = voltage (V)/current (A)
What is resistance ?
The potential difference required to drive a current through a component
- more resistance = more energy transferred/converted
What is ohm’s law ?
the current through a resistor/ohmic conductor AT A CONSTANT TEMPERATURE is DIRECTLY PROPORTIONAL to the potential difference
What is an ohmic conductor ?
a resistor whose resistance STAYS CONSTANT regardless of the current used as long as the TEMPERATURE is CONSTANT
What does reversing the p.d across a resistor do ?
reverses the current through it
Describe an investigation into the effect of the changing length of a wire on its resistance
- gather equipment
[] ammeter
[] voltmeter
[] battery
[] wire - with tape, attach a length of wire (straight) to a metre rule
[] use crocodile clips (one as close to zero as possible, and one at the desired length (at start, should be 0m so just one clip close to 0 on the ruler) - connect the crocodile clips attached to the wire to the circuit, with the ammeter in series and the voltmeter in parallel so that it reads the p.d across the wire
- turn on power supply AT A LOW VOLTAGE
- take the first reading and plot it on a graph (using the equation R = V/I to get R)
[] length of wire in m should be on x-axis, with resistance on y-axis - repeat, sliding the second crocodile clip up the metre rule by 10cm each time, thus lengthening the wire by 10cm each time
- SHOULD FIND THAT longer wire = more resistance, and that the LENGTH OF WIRE IS DIRECTLY PROPORTIONAL TO ITS RESISTANCE AT A CONSTANT TEMPERATURE
NOTE: turn off power supply between readings, as leaving it on can make the wire hot and thus increase its resistance
Describe the errors/problems associated with the length of wire practical (resistance)
ZERO ERRORS
- due to resistance between the crocodile clip and the wire itself as well as the fact it is difficult to place the clip exactly at 0m on the ruler, there will be a 0 error on the first reading
[] TO NEGATE THIS, subtract the zero error from all readings
[] first reading in this practical should always be 0 if starting at 0m of wire
HEATING ERRORS
- if the wire heats, its resistance will increase
[] this introduces a new independent variable which strips the test of validity
- reduce by turning power off when not making a reading from the volt/ammeters, and also using a low potential difference from the power pack
Explain how a variable resistor works
- has length of coiled wire inside
- has clips that can move to change the length of the wire
- thus, changing the length of the wire changes the resistance in the circuit
Why do filament lamps get so hot ?
- filament is very tightly coiled wire
[] long length of wire - more length of wire = more resistance = energy lost as heat
Describe the relationship between the resistance of a filament lamp and the p.d used
as p.d increases, resistance increases
[] this is because an increase in p.d means more heat, and thus metal atoms vibrate more, so electrons collide with the metal atoms more frequently, increasing resistance
- they are NOT directly proportional
[] this means that a filament lamp is not an ohmic conductor
Draw the shape of a current-p.d graph for a filament lamp
the weird stretchy s-shape (use internet)
Explain how to gauge the total resistance of 2+ resistors in a SERIES circuit and why you can do this
- add the resistance of each
[] eg. 3 resistors with 40 ohms resistance each means a total of 120 ohms of resistance in the whole circuit - this is because the current has to pass through each resistor in turn, without bypassing any
[] due to equivalent resistance, thus replacing all the resistors in series with one resistor of the total resistance would do the same job as all [insert number]
Give the rule for the total resistance of two resistors in parallel and why
total resistance of two resistors in parallel is LESS than the individual resistor with the smallest individual resistance
- eg., if there were two resistors, one with 50 ohms and one with 40 ohms, the total resistance would be less than 40 ohms
- this is because resistors in parallel increases the current flowing through the circuit (multiple pathways) but NOT the potential difference, so resistance decreases
Describe the method for an investigation into how the current through an electrical component depends on the p.d across it
- gather equipment
[] diode
[] lightbulb/filament lamp
[] resistor
[] variable resistor
[] voltmeter
[] ammeter
[] wires (to connect circuit)
[] battery
FIRST SET UP: RESISTOR
- make a circuit by connecting an ammeter, resistor and variable resistor in series, with a voltmeter in parallel across the resistor
- connect the circuit to a power pack
- set the power pack to 3 volts and turn it on
- take readings of the p.d and current through/across the resistor and record these results in a table
[] then change the p.d by +2V using the variable resistor and record the new readings
[] do this a further two times
- reverse the battery so that current flows in the opposite direction and repeat measuring steps
[] these results should be negative values
- plot graph; should be a straight line through 0 if the resistor’s temperature was kept constant, showing that it is an ohmic conductor
SECOND SET UP: BULB
- make a circuit by connecting an ammeter, filament lamp and variable resistor in series, with a voltmeter in parallel across the filament lamp
- connect the circuit to a power pack
- set the power pack to 3 volts and turn it on
- take readings of the p.d and current through/across the resistor and record these results in a table
[] then change the p.d by +2V using the variable resistor and record the new readings
[] do this a further two times
- reverse the battery so that current flows in the opposite direction and repeat measuring steps
[] these results should be negative values
- plot graph; should be the weird s-shape, showing that the current is NOT directly proportional to the p.d due to the increased resistance due to heating
- even if p.d is reversed, the shape of the graph overall will be the same
THIRD SET UP: DIODE
- make a circuit by connecting an extra resistor, milliammeter, diode and variable resistor in series, with a voltmeter in parallel across the diode
[] THE EXTRA RESISTOR IS NEEDED HERE AS DIODES ARE VERY EASILY DAMAGED BY TOO MUCH CURRENT
[] due to the low current the ammeter must be SENSITIVE, hence using a milliammeter instead of a regular one
- connect the circuit to a power pack
- set the power pack to 3 volts and turn it on
- take readings of the p.d and current through/across the resistor and record these results in a table
[] then change the p.d by +2V using the variable resistor and record the new readings
[] do this a further two times
- reverse the battery so that current flows in the opposite direction and repeat measuring steps
[] these results should be 0 for current
- plot graph; should be a straight line along y = 0 until reaching 0.6-0.7 volts, and from there current rises sharply
[] NO current if p.d is reversed due to very high resistance in opposite direction
Describe the method for an investigation into the total resistance of two resistors in series vs. in parallel
SERIES
- gather equipment
[] two resistors with a known resistance (50 ohms each for example)
[] a battery
[] ammeter
[] voltmeter
[] wires
- connect the battery, then the ammeter, then the two resistors in series (with the voltmeter across BOTH resistors)
- take readings from the ammeter and voltmeter
- calculate the resistance of the circuit using R = V/I
- total resistance should be 100 ohms if using two 50 ohm resistors, otherwise simply the total of both resistors’ individual resistance
PARALLEL
- gather equipment
[] two IDENTICAL resistors with a known resistance (50 ohms each for example)
[] a battery
[] ammeter
[] voltmeter
[] wires
- make a circuit by connecting the battery and ammeter in series with the first resistor, the second resistor in parallel to the first and the voltmeter in parallel to the second
- take readings from the voltmeter and ammeter
- calculate the resistance of one resistor using R = V/I
[] this result should be (nearly) half of that resistor’s total individual resistance, in this case 25 ohms
[] since both are identical, this is also the resistance of the other resistor
- use 1/R (total) = 1/R1 + 1/R2 to find the total resistance of the circuit
