electricity Flashcards
what is electric current
the flow of electrical charge
state the equation linking charge, current and time
Q = it
state the units for charge for Q=IT
charge
current
time
what can be said about the value of current at any point in a single closed loop
current is the same at all points in a closed loop
what 2 factors does the current in a circuit depend on
- potential difference
- resistance
what is an ohmic conductor
a conductor for which current and potential difference are directly proportional
resistance remains constant as current changes
what is a condition required for a ohmic conductor
temp must be constant
list 4 components for which resistance is not constant as current changes
- lamps
- diodes
- thermistors
- light dependent resistors
what happens to the resistance of a filament lamp as temp increases
resistance increases
ions in metal have more energy so vibrate more causing more collisions with electrons as they flow through metal creating greater resistance to current flow
what is different about current flow through a diode
the current only flows in one direction
resistance is very high in the other direction, preventing current flow
state what happens to the resistance of a thermistor as temp increases
the thermistors resistance decreases
give 2 examples of when a thermistor may be used
- in a thermostat to turn a heater on below a certain temp
- in a freezer to turn on a cooler when the temp becomes to high
state what happens to the resistance of a LDR as light intensity increases
the LDR’s resistance increases
give an application for a LDR
street lights often use LDR’s
when light levels become too low, the light gains sufficient current to turn on
what are the 2 ways that a component can be connected in a circuit
- series
- parallel
how does the potential difference across components vary when connected in series and parallel
series: total P.D is shared between each component
parallel: P.D across each component is the same
if 2 resistors are connected in parallel, what can be said about their combined total resistance
their total resistance is less than the smallest of the two individuals
if 2 resistors are connected in series, what can be said about their total resistance
their total combined resistance is equal to the sum of the two individual resistances
describe the current in a series circuit
in a series circuit, the current is the same at all positions since the charge only has one path to flow through
describe the current in a parallel circuit
in a parallel circuit, the current is shared between the different branches. when the charge reaches a junction it splits
how should you connect an ammeter in a circuit to measure current
ammeters should be connected in series with the component that they are measuring current through
how should you connect a voltmeter in a circuit to measure potential difference
voltmeters should be connected in parallel to the component that they are measuring the potential difference of
why is it an advantage to connect lamps in parallel
if one lamps blows, the rest will be unaffected and can still receive current
is mains electricity an alternating current or a direct current supply
alternating current
what is a.c
alternating current
what is d.c
direct current
define alternating current
current that continuously changes direction at a specific frequency
define direct current
one directional current flow
what is the frequency of the uk mains electricity supply
50 Hz
what is the voltage of the uk mains electricity supply
230 V
name the wires in cables connecting electrical appliances
- live wire
- neutral wire
- earth wire
state the insulation colour used on the earth wire
green and yellow stripes
state the insulation colour used on the live wire
brown
state the insulation colour used on the earth wire
blue
explain when the earth wire does and doesn’t carry a current
- under normal circumstances, no current flows through the earth wire
- if a fault occurs in the appliance, the current will flow to the ground
what potential is the neural wire at
0 volts
state the potential difference between the live and earth wires
230 volts
what is the purpose of the neutral wire
to complete the circuit by connecting the appliance back to the mains supply
for metal appliances, where is the earth wire connected
- earth wire is connected to the metal casting of the appliance
- if a live wire becomes loose and touches the casing, the current will flow through the earth wire, preventing electrocution
what are the two main factors does the amount of energy transferred by an appliance depend on
- how long the appliance is being used for
- the power of the appliance
describe the energy transfers in a battery powered torch
- battery converts chemical energy into electrical energy
- bulb converts electrical energy into light as well as waste energy in the form of heating
describe the energy transfers in a better powered motor
- battery converts chemical energy into electrical energy
- motor converts electrical energy into kinetic energy as well as waste energy in the form of heating due to friction
what are 3 things that determine the power of a circuit device
- the potential difference across the circuit
- the current through the circuit
- the amount of energy transferred in a given time
what is the purpose of the national grid
to link power stations to consumers so that they have access to a source of electricity
what are two types of transformers used in the national grid
- step-up transformers
- step-down transformers
what are step-up transformers found in the national grid
step up transformers are used when connecting power stations to transmission cables
the increase the potential difference
what are step-down transformers found in the national grid
step-down transformers are used in connecting transmission cables to domestic buildings (houses)
the decrease the potential difference
why do transmission lines transfers electricity at high potentials
at high potential, results in a low current
the lower the current, the less energy that is wasted as heat
therefore it is more efficient
why does the potential need to be decreased between transmission lines and houses
lower potential are safer for domestic use and rescues the likely hood of severe electrocution
applicants are designed for 230 V
what can happen when insulating materials are rubbed together
they become electrically charged
why can insulators become electrically charged when rubbed together
electrons are rubbed from one material onto the other
the material gaining electrons becomes negatively charged
the material losing electrons becomes equally positively charged
what happens when two electrically charged objects are brought close together
they exert a force on each other
what happens when two identically charged objects are brought close together
they exert a repulsive force on each other and repel
what happens when two apishly charged objects are brought close together
they exert an attractive force on each other and attract
give an example of a non-contact force
the repulsive or attractive force acting between two electrically charged objects
what is an electrical field
a region in which a charged object will experience a non-contact electrical force
when can electrical field be found
surrounding any charged objects
describe the electric field around a charged particle
- strongest closest to the object
- decreases in strength as you move away from the object
what happens to the force between two charged objects when they are moved closer together
the force between them becomes stronger as the separation reduces
in situations where sparks are unwanted, what precaution must be taken to prevent the build up of static charge
any surfaces that are rubbing against each other should be earthed to allow the charge to flow off the materials
