P2: Electricity Flashcards
state the equation linking charge, current and time. Give the units.
Q=IT
charge: coulombs
current: amperes
time: seconds
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 two factors does the current in a circuit depend on?
- Potential difference (V)
- Resistance (R)
what equation should be used to calculate potential difference if current and resistance are known. state the units.
V=IR
PD: V
current: A
resistance ( OHMS )
what is an ‘ohmic conductor’? State the condition required
- a conductor for which current and potential difference are directly proportional
- resistance remains constant as current changes
- temperature must be constant
list four components for which resistance is not constant as current changes
- lamps
- diodes
- thermistors
- light dependant resistors ( LDRs)
what happens to the resistance of a filament lamp as the temperature increases? Why?
- resistance increases
- ions in metals have more energy, so vibrate more
- more frequent collisions with electrons as they flow through the metal
- so great 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 temperature increases
- the thermistor’s resistance decreases
Give two examples of when a thermistor may be used
- In a thermostat to turn a heater on below a certain temperature
- in a freezer to turn on a cooler when the temperature becomes too high
state what happens to the resistance of a LDR as light intensity decreases.
- the LDR’s resistance increases
Give an application for a LDR
- street lights often use LDRs
- when light levels become too low, the light gains sufficient current to turn on
what are the two ways that a component can be connecting in a circuit?
- series ( same loop)
- Parallel ( adjacent loop)
how does the potential difference across two 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 two 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 individual resistances
if two 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
- eg R1 = 5 , R2=10 total resistance = 15
Describe the current 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
- 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 advantageous to connect lamps in parallel?
- if one lamp blows, the rest will be unaffected and can still receive current
- ( i.e the circuit is still complete)
Define alternating and direct current.
a.c: current that continuously changes direction at a specific frequency
d.c: one directional current flow
What is the frequency and voltage of the UK mains electricity supply?
frequency: 50Hz
Voltage: 230V
How many wires are usually in the cables connecting electrical appliances to the mains? Name these wires
- Live wire
- Neutral wire
- Earth wire
what is A.C supply and a D.C supply? Is mains electricity A.C or D.C
- mains electricity is an A.C supply
a.c : alternating current
d.c: direct current
State the insulation colour used on:
- The Live wire
- The Earth wire
- the neutral wire
live wire: brown
earth wire: green and yellow stripes
neutral wire: blue
Explain when the Earth wire carries a current
- under normal circumstances, no current flows through the Earth wire
- if a fault occurs in the appliance ( such as the surge or the casing becoming live), current will flow to the ground
what is the potential difference is the neutral wire?
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 to? Why?
- Earth wire is connected to the metal casing of the appliance
- if live wire becomes loose and touches the casing
- the current will flow though the Earth wire , preventing electrocution
state two equations for the power of a circuit. give appropriate units
P=IV
P= (current )₂ R
Power ( Watts)
Current ( Amperes)
Potential difference (Volts)
Resistance (Ohms)
State an equation linking energy transferred, power and time
E=Pt
energy = Joules
Power = Watts
Time = Seconds
State an equation linking energy transferred, potential difference and charge
E=QV
potential difference: volts
what 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 battery 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 three things 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 the 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? What do they do?
- Step-Up transformers are used when connecting power stations to transmission cables
- They increase the potential difference
what are step-down transformers found in the National Grid? What do they do?
- Step-Down Transformers are used in connecting transmission cables to domestic buildings ( like houses)
- they decrease the potential difference
Why do transmission lines transfer electricity at high potential difference?
- a high potential difference, results in low current
- the lower the current, the less energy that is wasted as heat
- therefore it is more efficient
What does the potential difference need to be decreased between transmission lines ad houses?
- safer for domestic use
- reduces the likelihood of severe electrocution
How does an atom become charged
- If it loses an electron, it becomes a positively charged ion.
If it gains an electron, it becomes negatively charged ion.
Charged atoms are called ions.
Give 2 properties of an insulator
- electrons cannot flow
- they are in fixed positions
A student rubbed their jumper with a balloon, The balloon became negatively charged and the jumper became positively charged. Explain why the jumper becomes positively charged ( 3)
- negatively charged electrons
- move from the jumper to the balloon by friction
- thus there are more positive charges ( protons) than there are negative charges ( electrons)
A person walks along the carpet in socks and becomes negatively charged. They hold their hand near a metal door handle. Explain why a spark jumps between the person and the door handle ( 3 )
- a spark jumps due to the large potential difference between the person and the door handle
- This causes negative electrons to move through the air
- from the person to the earthed conducting door handle
define the term ‘ electric field ‘
- the region around a charged particle which exerts a force on another charged particle
Describe the electric field around a positively charged sphere
- field lines move away to the negative charge
- electric fields get weaker the further it is from the charged particle
Describe the electric field around a negatively sphere
- field lines move towards the negative charges
- electric fields are weaker the further it is from the charged particle
Describe the field between two parallel metal plates ( of different charges)
- there are uniform field lines where the field strength is the same everywhere
- the field lines are evenly spaced out
what happens when two charged objects are brought together:
- same sign
- opposite sign
- they experience a non contact electrostatic force
- same signs: they will repel
- opposite signs: they would attract
1a) Two positive charges are brought together. Explain what will happen. (2)
1b) The charges are brought even closer. Describe how the strength of the force changes.(2)
a. - they will repel
- as their electric fields interact with each other
b. - the strength of the force increases
- as the distance between the objects decreases ( electric fields get closer)
Explain why the negative balloon sticks to the positive jumper after it is brought close to it (2)
- it sticks to the positively charged jumper as they are opposite charges
- thus, they would attract due to the electrostatic force
Explain why there is a spark between two charged objects ( 3 marks)
- the high potential difference causes a strong electric field between the two objects
- the strong electric field causes electrons in the air particles to be removed (ionization)
- when the air is ionized/charged, it can conduct and so a current flow through it; this is a spark
what is electric current?
- the flow of electric charge
What can happen when insulating materials are rubbed together?
- they can become ( statically) electrically charged
Why can insulators become electrically charged when rubbed together ?
- electrons are rubbed together from one material onto the other
- the material gaining electrons becomes negatively charged
- the material losing electrons becomes positively charged
Give an example of a non-contact force
- the repulsive or attractive force acting between two electrically charged objects
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
