Topic 2 Electricity GCSE Physics Flashcards
What is electric current?
- flow of electric charge
When will electric charge flow around a complete circuit?
- if there is a potential difference
What is the unit of current?
- ampere
- A
What is the current in a single/closed loop?
- current has the same value everywhere in the circuit
What is potential difference?
- driving force that pushes the charge around
What is the unit for potential difference?
- volt
- V
What is resistance?
- anything that slows the flow down
What is the unit for resistance?
- ohm
What does the current flowing through the component depend on?
- the potential difference across it and the resistance of the component
A greater resistance means what for the current?
- the greater the resistance across a component, the smaller the current that flows
What is the size of the current?
- rate of flow of charge
- more charged passes around the circuit when a larger current flows
What is the charge equation?
charge flow= current X time
LEARN CURRENT SYMBOLS
P 24
How do you calculate potential difference?
potential difference = current X resistance
How can you investigate the factors affecting resistance?
1- attach the crocodile clip to wire level with 0cm on the ruler
2- attach second crocodile clip to the wire (10cm away from the first clip)
3- close the switch then record the current and pd
4- open switch then move the second crocodile clip another 10 cm across the wire, close switch again and record current and pd
5- repeat for different lengths
6- work out resistance
- graph should be a straight line
What is an ammeter?
- measures current (in amps)
- ammeter must always be placed in series
What is a voltmeter?
- measures the potential difference
- must always be placed in parallel around whatever your in investigating
Does the resistance of ohmic conductors change with the current?
- does not change with the current
- at a constant temperature the current flowing through an ohmic conductor is directly proportional to the pd across it
What happens when an electric charge flows through a filament lamp?
- it transfer some energy to the thermal energy store of the filament which is designed to heat up
For diodes, what does the resistance depend on?
- depends on the direction of the current
look at bottom of p26
iv graphs!!
What is an LDR?
- resistor that is dependent on the intensity of light
- in bright light the resistance falls
- in darkness the resistance is highest
What is a thermistor?
- temperature dependent resistor
- in hot conditions the resistance drops
- in cool conditions the resistance goes up
What can sensing circuits be used to?
- to turn on and increase the power to components depending on the conditions they are in
How are the different components connected in a series circuit?
- in a line, end to end
In a series circuit, how is pd shared?
- the total pd of the supply is shared between the various components
- so the pd around a series circuit always adds up to the equal source pd
V total= V1 +V2 + ….
How is the current in a series circuit?
- the same current flows through all components
- the size of the current is determined by the total pd of cells and the total resistance of the circuit
How is the resistance in a series circuit?
- the total resistance of two components is just the sum of their resistances
Why is the total resistance of two components the sum of their resistances?
- because adding by resistor in series, the the two resistors have to share the total pd
How is the pd across each resistor?
- lower
- so the current through each resistor is also lower
How is the current in a series circuit?
- same everywhere
- so the total current in the circuit is reduced when a resistor is added
- so total resistance of circuit increases
How is each component connected in a parallel circuit?
- separately connected to the +ve and -ve of the supply
How is the potential difference in parallel circuits?
- all components get the full source pd
- so pd is the same across all components
(so identical bulbs connected in parallel will be the same brightness)
How is the current in a parallel circuit?
- total current around the circuit is equal to the total of all the currents through the separate components
How is the resistance in a parallel circuit?
- if you have two resistors in parallel, their total resistance is less than the resistance of the smallest of the two resistors (look a bottom of p29)
How can you investigate adding resistors in series?
look at p30
What are the two types of electricity supply?
- alternating current and direct current
How is the current in alternating current supplies?
- current is constantly changing direction
What are alternating currents produced by?
- by alternating voltages
- in which the positive and negative ends keep alternating
What is the ac supply of the uk mains?
230V
What is the frequency of the ac mains supply?
50 cycles per second 50Hz
What do cells and batteries supply?
- direct current
What is direct current?
- a current that is always flowing in the same direction
- created by direct voltage
How are electrical appliances connected to the mains supply?
- connected by three-core cables
- they have three wires inside them (each with copper and coloured plastic coating)
What is the live wire?
- brown
- provides an alternating potential difference (at about 230V) from the mains supply
What is the neutral wire?
- blue
- completes the circuit
- when the appliance is operating normally, current flows through the live and neutral wires
- it is around 0V
What is the earth wire?
- green and yellow
- protecting the wiring and for safety
- stops the appliance casing from becoming live
- does not carry a current (only when there’s a fault)
- 0V
What happens if you touch the live wire?
- a large potential difference is produced across your body and a current flows through you
- causing an electric shock
Why does moving charge transfer energy?
- charge does work against the resistance of the circuit
How does the kettle transfer energy?
- electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle
How is energy transferred in a fan?
- electrically from the battery of a hand held fan to the kinetic energy store of the fans motor
What is the power of an appliance?
- energy it transfers per second
- so the more energy in a given time, the higher its power
What is the energy transferred equation depending on power?
energy transferred= power X time
What does the power rating tell you?
- maximum amount of energy transferred between stores per second when the appliance is in use
What happens when an electrical charge goes through a change in potential difference?
- energy is transferred
What is the energy transferred equation with pd?
energy transferred= charge flow X potential difference
How can you calculate the power of an appliance?
power= potential difference X current
What is the national grid?
- giant system of cables and transformers that covers the UK and connects power stations to consumers
What does the national grid transfer?
- electrical power from power stations anywhere on the grid
When does demand for electricity increase for the national grid?
- predict when most electricity needs to be used
- people getting up in the morning, people getting home from school/work
What does the national grid use?
- high potential difference or a high current
What is the problem with high current with the national grid?
- lose loads of energy as wires heat up and energy is transferred to the thermal energy stores of the surroundings
For efficient transmission with the national grid what is used?
- transformers
- nearly 100% efficient
What do transformers have?
- have two coils ( a primary coil and a secondary coil joined with an iron core)
Why is potential difference increased using a step-up-transformer?
- more turns on the secondary coil than the primary coil
What happens when certain insulating materials are rubbed together?
- negatively charged electrons will be scraped off one and dumped on the other
- leaving the materials electrically charged with static charge on one and an equal negative static charge on the other
As an electric charge builds on an object what happens to the pd?
- pd between the object and the earth (which is at 0V) increases
In a static, what happens if the pd gets large enough?
- electrons can jump across the gap between the charged object and the earth
- this is the spark
What happens when two electrically charged objects are brought close together?
- exert a force on one another
When is an electric field created?
- around any electrically charged object
What happens when a charged object is placed in the electric field of another object?
- it feels a force
- this force causes the attraction or repulsion
- force caused by the electric fields of each charged object interacting
When are sparks caused?
- when there is a high enough pd between a charged object and the earth
