electricity Flashcards
what is unit of electric charge
coulombs (C)
how is charge detected
Charge can be detected using a gold leaf electroscope. ○If a positively charged rod is brought close to the disc on top of the electroscope, electrons are attracted to the top of the disc, away from the bottom of the metal stem and the gold leaf. The gold leaf will then be repelled from the metalstem because they both become positively charged.○If someone then touches the disc, electrons flow from the ground into the disc as they are attracted to the rod, and the electroscope now contains a net negative charge. This is called charging by induction.
what is transfer of electrons
When two insulators are rubbed together, electrons move from one to the other and they become charged.
When two materials are rubbed together, eletrons may be transffered from one to the other. One material ends up with more electrons than normal (net negative charge) and the other with less (net postive charge)
This does not make elctric charge it just sepreates charges that are already there
what are electric fields
Charges create electric fields (regions in which an electric charge experiences a force)
what is the direction of electric fields
the direction of an electric field at a point is the direction of the force on a positive charge at that point. ●Electric field lines point away from positive charges and towards negative charges
formation of sphere electric field
The field lines around a charged conducting sphere are as if the charge was concentrated at the centreof the sphere.
formation of two charged plates electric field
he field lines between two charged plates go in straight linesfrom the positive plate to the negative plate and are equally spacedapart.
what is current and how is it measured
CurrentI is measured in amps and is the rate of flow of charge at a point in the circuit.
formula for time/current/charge
charge = current x time
how is current measured
using ammeter in series circuit
what causes current
In metals, current is due to a flow of electrons.Because electrons are negativelycharged, conventional current (which is the rate of flow of positivecharge) is in the oppositedirection to the flow of electrons.
what direction does current flow
negative terminal to positive terminal (electrons are negative so they are repelled by negative charge)
current in a series circuit
the current is the same throughout a series circuit
what is potential difference/energy
the energy of the electrons produced by repelling each other
work done per unit chargein movingbetween two pointsin a circuit.
●It is measured with a voltmeterplaced inparallel across the component.
●The higher the potential difference, the greater the current.
what is voltage
- potential difference
- the higher the voltage, the more energy it gives to the electrons pushed out
what is SI unit of Potential difference
Volts (V)
what is electromotive force
The electromotive force (e.m.f) of an electrical source of energy is measured in volts and is the energy supplied by the source per unit charge in driving the charge round a complete circuit.
potential difference/voltage around a circuit
- moving round a circuit, from one battery terminal to the other, the sum of the p.ds across the components is equal to the p.d. across the battery
- the joules of potential energy are spread across the components and rejoin after leaving
what is resistance
The resistance of a component is given by the potential difference across it divided by the current through it. The greater the resistance, the harder it is for current to flow through the component.
what factors affect resistance
-length: longer = more resistance
-thickness : thinner = more resistance
-material : better conductor = less resistance
-temperature : for metals - colder = less resistance
for semiconductors - hotter = less resistance
unit for ressitance
ohm (horseshoe symbol)
formula for resistance/current/p.d
resistance = p.d. across conductor/current through conductor
what are resistors
resistors are specially made to provide resistance. In simple circuits, they reduce the current. In more complicated circuits (like in computers) they keep
what are diodes
diodes have an extremely high resistance in one direction but a low resistance in the toher. In effect, they allows current to flow in one direction only
electrical working
Energy is transferred from chemical energy in the battery toelectricalenergy used by circuit components and then to the surroundings.
characteristics of series circuits
●Components are connected end to endin one loop
●The same currentflows through every component
●The potential difference is sharedacross each component (i.e. the sum of the p.d.s across the components is equal to the total p.d. across the supply).
●The total resistance is the sum of the resistancesof each component RT= R1+ R2+ …
●The combined e.m.f. of several sources in series is the sum of the individual e.m.f.s
characteristics of parallel circuits
Components are connected to the power supply in separate branches
●The current is shared between each branch (i.e. the sum of the currents in the separate branches is equal to the current through the source)
●The potential differenceis the sameacross everybranch
●The total resistance of two resistors in parallel is less than the resistance of either resistor by itself, and is given by 𝟏𝑹𝑻=𝟏𝑹𝟏+𝟏𝑹𝟐
●Connecting lamps in parallel is advantageous because if one breaks, current can still pass through the rest.
what is a potential divider
A potential dividercircuit divides the source voltage into smaller parts.●The voltage across a certain component is given by𝑉𝑜𝑢𝑡=𝑉𝑖𝑛×𝑅𝑅𝑇where Vinis the source voltage, R is the resistance of the component and RTis the total resistance.
what is a thermistor
Athermistoris a resistor whose resistance decreases as thetemperatureincreases.
what is a light dependent resistor
A light dependent resistoris a resistor whose resistance decreases as light intensityincreases.
what is a relay
A relayis an electromagnetically operated switch. When a small current passes through the electromagnet, it switches on and attracts an iron arm. This arm rotates about a pivot and pushes the contacts in another circuit together. ●They are used to switch on a circuit with a high currentusing a circuit with a small current.
electrical energy and power formula
power = energy transformed/time taken
electircal power/pd/current equation
power = p.d. x current
what is an analogue signal
continuously varying signal and can take any value
what is a digital signal
can only take one of two (discrete) states - commonly referred to as 1s and 0s/highs and lows/ons and off
types of logic gates
not and nand or nor
common hazards of electricity
- damaged insulation
- overheating of cables
- damp conditions
what is a fuse
- protects a circuit and cuts off a circuit from its power supply if the current in the circuit exceeds a certain value
- usually consist of a glass cylinder which contains a thin metal wire
what happens if the current in the wire of a fuse becomes too large
- the wire hreats up and melts
- this causes the wire to break, breaking the circuit and stopping the current
what are fuse ratings
- fuses come in different ratings (1A, 3A, 5A, 13A)
- fuses with a higher current rating are thicker than fuses with a lower current rating.
- the fuse should always have a current rating that is higher than the current needed by the appliance, without being too high, always choose the next up size
- fuse always found on the live wire
what are circuit breakers
- also found on the live wire
- circuit breakers can be resent unlike fuses
- there are essentially resettable fuses
what is earthing
- the hazard is that if the live wire comes in to contact with th emetal case and someone touches the case the person may become electrocuted
- earthing the case provides a low resistance path to the earth
- this causes a short circuit if the live wire touches the case as a result of which the fuse blows
- this cuts off the supply of electricity to the appliance, thus protecting the person using the appliance
- appliances with double insulation do not require earthing
