electricity Flashcards
charging by friction
- friction causes displacement of electrons
- When 2 insulators move relative to each other, friction can result in electrons being transferred from one to the other which can result in both objects becoming electrically charged.
- One of the objects (the one that lost electrons) has +ve charge, and the other (the one that gained electrons) has -ve charge, depending on the nature of the materials
- e.g. plastic being rubbed by a duster; hair being combed; an aeroplane moving through the air.
induction
- when a neutral object becomes charged by being placed near a charged object
- the neutral object is oppositely charged to the already charged object
repulsion vs attraction
- like charges repel each other
- unlike charges attract each other
- the larger the charges, the larger the force
- the larger the distance of separation, the smaller the force
conductors and charge
- it is possible for 2 conductors to charge each other by friction. However, the fact that electrons in them are free to move means that any charge transfer between them will instantly return, given that the objects must be in contact for friction to occur between them.
- they will only retain that charge if they are insulated from their surroundings. If they are not insulated, then any charge that builds up will leak away
photocopier
- The scanning process results in charge being placed on the paper at the locations where the image is to be printed
- The paper is then exposed to toner powder, which ‘sticks’ to the paper at those locations as a result of electrostatic induction
- The paper is then heated so that the toner powder melts and then re-solidifies on the paper
aircraft refuelling
- When aircraft are refuelled, large volumes of fuel flow through the pipe very quickly creating large amounts of friction
- This results in the fuel and the pipe becoming charged
- Sparking presents a significant risk of explosion of the fuel in the fuel tank, so, the tank and pipe are always earthed beforehand.
- This earthing prevents the build-up of charge and so eliminates the risk of explosion.
direct current (DC)
- a current that is always in the same direction
- cells/batteries are sources of dc
alternating current (AC)
- a current that repeatedly changes direction, usually very rapidly
- Generators in power stations produce ac
- its waveform must include a graph line that is sometimes above the axis and sometimes below it
diode
- A diode only allows current in one direction (the direction of the arrow on the symbol)
- The output from a power supply from mains electricity can be converted from ac to dc using diodes as a ‘rectifier’
- for current to flow the diode arrowhead must point from the +ve terminal of the battery to the -ve terminal
conductors
- material that allows flow of charge
- e.g: all metals (because of the presence of delocalised electrons which can flow through the metal), particularly copper, gold and silver, carbon (in the form of graphite), ionic solutions
insulators
- material in which electric current does not flow freely
- e.g: most non-metals, particularly plastics, rubber, dry wood, air, vacuum
water
Water, unless extremely pure, is a conductor, so wet or damp materials are not good insulators
cables
An electric cable usually uses an insulator on the outside and a conductor on the inside
directions of flow
- current flows from the +ve end of a conductor to the -ve end
- electrons flow from the -ve end of a conductor to the +ve end
electrolytes
- a substance that conducts electricity when dissolved in water
- this is because it contains charged ions
- If two electrodes are placed in the liquid and a voltage is applied across them, the cations move towards the -ve cathode, and the anions move towards the +ve anode
voltmeters
- connected in parallel with a component
- has very high resistance (infinite), otherwise it would ‘short circuit’ the component across which it was connected (because there would be a significant amount of current in the voltmeter instead of in the component)
ammeter
- connected in series with a component
- has very low resistance (zero), otherwise it would tend to reduce the amount of current that it was being used to measure
Ohm’s Law
-a current flowing through a conductor is directly proportional to the voltage, given the temperature of the conductor remains constant
constant resistance
- A fixed resistor at constant temperature has a constant resistance
- This means that a graph of 𝑉 against 𝐼 will be a straight line passing through the origin
- The resistance is given by: Δ𝑉 / Δ𝐼 (or 1/grad)
varied resistance
- in lamps, the filament heats up as more current passes through it
- the heat generated increases the resistance of the filament (resistance is no longer constant)
- This means that a graph of 𝑉 against 𝐼 will be a curve (steep gradient then levels off)
- The resistance is given by dividing one value of 𝑉 by the corresponding value of 𝐼 (or 1/grad)
thermistor
- A thermistor is a resistor with a resistance that depends on its temperature.
- as its temperature increases, its resistance decreases (TURD)
light dependant resistor (LDR)
- An LDR is a resistor with a resistance that depends on the intensity (brightness) of light that falls on it
- As the light intensity increases, the resistance of the LDR decreases (LURD)
battery symbol
-long line is +ve end and short line is -ve end
series circuits
- the current in each component is the same
- total voltage across the components = sum of the voltages across each individual component
parallel circuits
- the voltage across each component is the same
- total current moving into the branch = total current moving out of it
current definition
the flow of charge passing a point in the circuit per unit time
voltage defintion
the energy required to move a unit of charge between two points
resistance in series
-The combined resistance 𝑅𝑇 resistors connected in series is the sum of the individual resistances:
𝑅𝑇=𝑅1+𝑅2+……
-𝑅𝑇 > 𝑅1 /𝑅2
resistance in parallel
- for 2 resistors in parallel: RT=(R1xR2)/(R1+R2)
- for 3 or more resistors in parallel: 1/RT=1/R1 + 1/R2 +….
- RT < R1 / R2
fixed resistor
the resistance will not change no matter what the voltage across it or the current flowing through it
voltage formula
voltage = energy / charge (V=E/Q)
power formulae
𝑃 = 𝑉𝐼 = 𝐼squared𝑅 = 𝑉squared/𝑅
E=Pt
E=IVt
voltage-resistance relationship in series circuits
R1/R2 = V1/V2
short circuits
-when the charge bypasses a component by taking the path of least resistance
household electricity
- voltage = 230V
- frequency = 50Hz
- ac
resistance in wires
- doubling the length of the wire doubles the resistance
- halving the diameter/radius of the wire quadruples the resistance
power energy formula
E=Pt OR P=E/t