Topic 2 Electricity Flashcards
What is current?
Flow of electrical charge
What must a closed circuit have in order for current to flow?
Source of potential difference
What 2 things does the current through a component depend on?
- Potential difference across it
- resistance of component
How does a greater resistance across a component affect the current (for a given potential difference)?
Current is smaller
What is the size of electrical current?
Rate of flow of charge
What is the value of the current in a single closed loop?
same at any point
What is potential difference?
Driving force that pushes charge round
What is resistance?
Anything that slows flow down
What does the resistance of an ohmic conductor do when the current changes?
Doesn’t change with the current (remains constant)
What are 2 examples of ohmic conductors?
- Wire
- Resistor
Through what component is the current (at a constant temperature) directly proportional to the potential difference across it?
Ohmic conductor
What are 4 components where the resistance does change with the current through them?
- diode
- filament lamp
- thermistor
- LDR
What does the resistance of a filament lamp increase with?
As temperature of filament increases
What happens when the current through a filament lamp increases?
Filament lamp heats up more and so resistance increases
What does the resistance depend on in a diode?
Direction of current
Where does the current through a diode flow?
In one direction only
When will a diode have a very high resistance?
If the flow of current is reversed
What is the resistance of an LDR dependent on?
Light intensity
What happens to the resistance of an LDR when light intensity increases?
Resistance decreases
What happens to the resistance of an LDR when light intensity decreases?
Resistance increases
What are 3 examples of applications of LDRs?
- automatic night lights
- outdoor lighting
- burglar detectors
What is the resistance of a thermistor dependent on?
Temperature
What happens to the resistance of a thermistor when the temperature increases?
Resistance decreases
What happens to the resistance of a thermistor when the temperature decreases?
Resistance increases
What are 2 examples of applications of thermistors as temperature detectors?
- electronic thermostats
- car engine temperature sensors
What happens if you disconnect/remove one component in a series circuit?
Circuit is broken and all components stop
What is the rule for potential difference in series circuits?
Pd is shared between the components
What is the rule for current in series circuits?
Same current through each component
What is the size of the current in a series circuit determined by?
Total pd of cells and total resistance of circuit
What is the rule for resistance in a series circuit?
total resistance of two components is the sum of their resistances
In series circuits, why is the total resistance of two components, the sum of the resistance of each component?
By adding resistor in series, two resistors have to share total pd
What are the 2 types of electricity supply?
- alternating current (ac)
- direct current (dc)
What is alternating current produced by?
Alternating voltages
What is alternating current?
Current that regularly changes direction
What would alternating current appear as on a voltage-time graph?
curve alternating between positive and negative voltages
What type of supply is mains electricity?
ac (alternating current)
What voltage does the UK mains supply (electricity in the home) have?
230V
What is direct current?
Current that flows in only one direction
What is direct current produced by?
Direct voltage
What would direct current appear as on a voltage-time graph?
straight horizontal line at constant voltage
What type of current is supplied by cells and batteries?
Direct current
What is the frequency of the UK mains supply (electricity in the home)?
50Hz
What are most electrical appliances connected to the mains supply by?
three core cables
What does a three-core cable contain?
three wires, each with copper core and coloured plastic coating
How are the wires in a three-core cable made easily identifiable?
Insulation covering each wire is colour coded
What colour is the live wire?
Brown
What colour is the neutral wire?
Blue
What colour is the earth wire?
Green and yellow stripes
What are the 3 wires inside a three-core cable called?
- live wire
- neutral wire
- earth wire
What does the live wire do?
Carries alternating pd from mains supply
What does the neutral wire do?
- completes circuit
- when appliance operates normally, current flows through live and neutral wires
What does the earth wire do?
- safety wire
- stops appliance from becoming live
What is the voltage of the live wire?
230V (mains supply)
What is the voltage of the neutral wire?
Close to 0V
What is the voltage of the earth wire?
0V
When is the only time the earth wire carries a current?
If there’s a fault
What voltage is the body and earth at?
0V
What happens if you touch a live wire?
large pd produced across your body and current flows through you (large electric shock)
What is another way of saying a plug socket/light switch is turned off?
Switch is open
Why is a live wire still dangerous even when the switch is open?
Current isn’t flowing but there’s still a pd in live wire
When you make contact with a live wire, what is your body providing a link between?
Supply and the earth
What does the earth wire provide?
Low resistance path to earth (instead of passing through a person)
What are electrical appliances designed to do?
Transfer energy to components in circuit when current flows
What 2 things does the amount of energy transferred by an appliance depend on?
How long appliance is switched on for and power of appliance
When is work done/energy transferred in a circuit?
When charge flows
What is the power of an appliance?
Energy it transfers per second
What does a kettle transfer energy from?
Mains ac supply
What does a kettle transfer energy to?
Thermal energy store of heating element inside kettle
What does a handheld fan transfer energy from?
Battery
What does a handheld fan transfer energy to?
Kinetic energy store of fans motor
What are appliances labelled with that is their maximum operating power?
Power rating
What does the power rating on an appliance tell you?
Maximum amount of energy transferred between stores per second when appliance is in use
What does a lower power rating on an appliance tell you?
Uses less electricity in a given time- cheaper to run
What is the National Grid?
System of cables and transformers linking power stations to consumers
What does the National Grid do?
Transfers electrical power from power stations to consumers
What do power stations often do so that there’s spare capacity to cope with high demand?
Run well below their maximum power output
Why is the national grid an efficient way of transferring energy?
Uses high pd while keeping current low (decreases energy lost by heating wires and surroundings)
What are used to increase the pd from power station to transmission cables in the national grid?
Step-up transformers
What are used to decrease the pd (to a much lower value) for domestic use in the national grid?
Step-down transformers
How do step-up transformers work?
Increases pd which decreases current- reducing power lost due to heating in cables
What is static electricity?
When certain insulating materials are rubbed together, electrons are transferred, they become electrically charged
What happens when certain insulating materials are rubbed together?
Negatively charged electrons are rubbed off one material and on to other
What happens to the material that gains electrons (static electricity)?
Becomes negatively charged
What happens to the material that loses electrons (static electricity)?
Left with equal positive charge
What happens when 2 electrically charged objects are brought close together?
exert a force on each other
What type of force is electrostatic attraction or repulsion?
Non-contact
What can forces of attraction and repulsion cause objects to do?
Move
What is a method to see objects exert forces of attraction or repulsion on each other?
- suspend rod (with known charge) from piece of string (free to move)
- placing object with same charge will repel rod (move away from object)
- oppositely charged object - rod move towards object
What happens when electric charge builds on an object?
Pd between object and earth increases
How are sparks caused?
When pd between charged object and earth gets high enough
What is an electric field created around?
Any electrically charged object
Where is an electric field stronger?
Closer to charged object
What happens to a charged object when placed in the electric field of another object?
experiences a force
What does a high pd cause between a charged object and an earthed object?
Strong electric field
How are sparks produced?
If electric field is strong enough, charges can be forced though insulators (air) and spark will occur
How do we represent an electric field?
Using field lines
In an electric field diagram, what do the arrows on field lines show?
What force would be on positive charge, if placed in field
When the lines are closer together on an electric field diagram, is the field stronger or weaker?
Stronger
When the lines are further apart and you get further from charge on an electric field diagram, is the field stronger or weaker?
Weaker
What happens if you increase the distance between charged objects?
Strength of field decreases and force between them gets smaller
What is the force between charged objects caused by?
Electric fields interacting
What will the potential differences around a series circuit always add up to?
Total (source) pd
How does adding resistors in series affect the total resistance?
Increases
In a series circuit, if a component has a bigger resistance, is its share of the pd bigger or smaller?
Bigger
Why does adding a resistor to a series circuit increase the total resistance (3 points)?
Pd across each resistor is lower
Current through each resistor is lower
Total current reduced as current same everywhere
What happens to the cell pd if cells are connected in series?
Cell pd adds up, bigger pd
What are ammeters always connected in?
Series
What happens if you remove a component from a parallel circuit?
Hardly affect other components
What is the rule for pd in parallel circuits?
Same across all components
What is the rule for current in parallel circuits?
Total current = sum of currents through separate components (current shared between branches)
In a parallel circuit, what does the total current going into a junction have to equal?
Total current leaving
What is the rule for resistance in parallel circuits?
If 2 resistors in parallel, their total resistance is less than resistance of smallest resistor
What does adding a resistor in parallel do to the total resistance?
Decreases
Why does adding a resistor in parallel reduce the total resistance (3 points)?
Current has more routes- easier to flow
Increases total current
What can dc series circuits be designed and used for?
Measure quantities and test components
