Electricity Flashcards
(52 cards)
1
Q
What is electric current
A
- the flow of electrical charge
- current can only flow if theres a source of potential difference
- unit is A, ampere
2
Q
What is current like in a single closed loop
A
Same value everywhere in circuit
3
Q
What is potential difference
A
- voltage
- the driving force that pushes the charge around
- until: volt, V
4
Q
What is resistance
A
Anything that slows the flow down
- unit: ohm
5
Q
Relationship between resistance and current
A
The greater the resistance across a component, the smaller the current that flows (for a given potential difference across the component)
6
Q
What does total charge in a circuit depend on
A
- current and time
- Q= It
- this formula is used when current flows past a point in a circuit for a length of time then the charge that has passed
- more charge passes around the circuit when a larger current flows
7
Q
What is charge flow measured in
A
Coulombs, C
8
Q
Lean circuit diagram symbols
A
- cell: big line small line and +
- battery: double of cell
- switched open/closed
- filament lamp: circle with cross
- fuse: rectangle with line through it
- LED: circle, two arrows points diagonally away from it, sideways triangle and vertical line inside of circle
- resistor: rectangle
- variable resistor: rectangle with diagonal arrow through it
- ammeter
- voltmeter
- diode: circle, sideways triangle and vertical line inside
- LDR: circle, square inside, two diagonal arrows pointing at it
- thermistor: rectangle with hockey stick line in it
9
Q
Investigation of factors affecting resistance
A
- Attach crocodile clip to the wire level With 0cm on the ruler
- Attach the second crocodile clip to the wire, eg. 10cm away from the first clip. Write down length of wire between clips
- Close the switch, the record the current through the wire and pd across it
- Open switch, move second crocodile clip, eg. Alter 10cm along the wire, close switch again, then record the new length, current and pd
- Repeat this for number of different lengths of the test wire
- Use measurements to calculate resistance (R=V/I)
- Plot graph and draw line of best fit
- Should be a straight line through the origin meaning resistance is directly proportional to length
10
Q
Relationship tween resistance and length of wire
A
Directionally proportional
11
Q
What happens with resistance of ohmic conductors
A
- doesn’t change with current
- at a constant temperature, the current flowing through an ohmic conductor is directly proportional to the potential difference across it
12
Q
What happens with resistance in filament lamps
A
- when an electrical charge flows through, the filament transfers some energy to the thermal energy store of the filament, which is designed to heat up
- resistance increases with temperature so as the current increases, the filament lamp heats up more and the resistance increase
13
Q
What happens with resistance in diodes
A
- the resistance depends on the direction of the current
- happy to let current flowing through an in one direction but have a very high resistance if it is reversed
14
Q
Graph of ohmic conductor
A
- current is directly proportional to pd, so straight line through origin
15
Q
Graph of filament lamp
A
- as current increases, so does temperature so resistance increases
- this means less current can flow per unit pd
- graph: goes through origin but has a curve, like an f
16
Q
Graph of a diode
A
- current will only flow through a diode in one direction
- graph: curve in positive axis
17
Q
Resistance of LDR
A
- light dependent resistor
- its a resistor that’s dependent on the intensity of light
- in bright light, resistance falls
- in darkness, resistance is highest
- they conclude things like: automatic night lights, outdoor lighting, and burglar detections
18
Q
Resistance of a thermistor
A
- temperature dependent resistor
- in hot conditions, the resistance drops
- in cool conditions, the resistance goes up
- they include: temperature detectors, car engine temperature sensors, electronic thermostats
19
Q
What are sensing circuits used for
A
- used to turn on or increase power to components depending on the conditions hat they’re in
20
Q
Series circuit
A
- the different components are connected in a line end to end, between the + an - power supply (expect voltmeters which are always connected in parallel)
- if you remove or disconnect one component, the circuit is broken and they all stop
21
Q
Example of sensor circuit
A
- Fixed resistor and fan will always have same potential difference across them (connected in parallel)
- Pd of power supply is shared out between thermistor and the loop made up of the fixed resistor and the fan according to their resistance (the bigger the resistance, the more of the pd it takes)
- as room gets hotter, resistance of thermistor decreases and it takes a smaller share of pd from power supply
- so pd across fixed resistor and the fan rises, making the fan go faster
22
Q
Potential difference in series circuit
A
- total pd is shared between various components
- pd around a series circuit always add up to equal source of pd
- Vtotal = V1 + V2…
23
Q
Current in series circuit
A
- same current flows through all components
- the size of current is determined by the total pd of the cells and the total resistance of the circuit
- I1= I2 = I3…
24
Q
Resistance in series circuit
A
- the total resistance of 2 components is just the sum of their resistances
- this is because by adding a resistor in series, the 2 resistors have to share the total pd
- the current is the same everywhere so the total current in the circuit is reduced wen resistor is added, this means the total resistance of thermistor decreases circuit increases
- the bigger a components resistance, the bigger its share of total potential difference
- Rtotal= R1+R2
25
Cell potential difference in series
- there is a bigger pd when more cells are in series, if they’re all connected the same way
- eg. If there’s two cells that have a pd of 1.5V are connected in series they supply 3V between them
26
Parallel Circuits
- each component is separately connected to the + and - of the supply (except ammeters which are always connected in series)
- if you remove or disconnect one of them, it will hardly affect the others at all
- this is how most things today are connected, for example in cars or household electronics
27
Potential difference in parallel circuits
- all components get the full source of pd, so the pd is the same across all components
- V1 = V2 = V3…
- this means that identical bulbs connected in parallel will all be at the same brightness
28
Current in parallel circuits
- the total current flowing around the circuit is equal to the total of all currents through the separate components
- there are junctions whee the current either splits or rejoins. The total current going into a junction has to equal the total current leaving
- if 2 identical components are connected in parallel, then the same current will flow through each component
- Itotal = I1+I2…
29
Resistance is parallel
- if you have 2 resistors in parallel, their total resistance is less than the resistance of the smallest of the 2 resistors
- in parallel both resistors have the same pd across them as a source
- this means the ‘pushing force’ making the current flow is the same as the source of pd for each resistor that you add
- but by adding another loop, then the same current has more than one direction to go in
- this increases the total current that can flow around the circuit. Using V=IR , an increase in current means a decrease in the total resistance of the circuit
30
Investigating resistors in series
1. Use 4 resistors in
2. Build a circuit using one resistor. Kk a note of pd of the battery
3. Measure current through circuit using ammeter. Use this to calculate the resistance of the circuit using R=V/I
4. Add another resistor in series with the first
5. Again measure the current and use equation again
6. Repeat steps until all resistors are added
7. Plot a graph of the number of resistors against total resistance of circuit
8. Graph should be straight diagonal line not crossing origin because as number of resistors increase, as does resistance
31
Investigating resistors in parallel
- Build series with resistor
- measure total current and pd and use equation to find resistance
- replete by adding another resistor in parallel
- measure total current and pd and find overall resistance
- repeat until all resistors are 4 resistors are added and you know the resistance each time
- plot graph of number of resistors against total resistance
- graph should be a decreasing curve
- this is because, the total current through circuit increases, so total resistance of the circuit has decreased
- the more resistors you add, the smaller the overall resistance becomes
32
What is DC
- direct current
- current that is always flowing in the same direction. Created by direct voltage
33
Frequency and pd of ac in the uk
- 230V
- 50Hz
34
What is ac
- alternating current
- produced by alternating voltages in which the positive and negative ends keep alternating
35
Live wire
- brown
- live wire produces alternating pd (at about 230 V) from mains supply
36
Neutral wire
- blue
- completes the circuit
- when the appliance is operating normally, current flows through the live and neutral wires.
- it is around 0V
37
Earth wire
- green and yellow
- its for protecting the wiring, and for safety
- it stops the appliance casing from becoming live
- it doesn’t usually carry a current, only when theres a fault
- its also at 0V
38
Why can the live wire give you electric shock
- our bodies are at 0V
- this means that if you touch the live wire, a large pd is produced across your body and current flows through you
- this causes a large electric shock that could injure or even kill you
- even if a plug socket or a light switch is turned off there is still a danger of an electric shock. A current isn’t flowing but theres still a pd in the live wire
- if you made contact with the live wire, your body would provide a link between the supply and the earth, so a current would flow through you
- any connection between live and earth ca be Angelou’s. If the link create a low resistance path to earth, a huge current will flow through you, which could result in a fire
39
How to kettles transfer energy
- energy is transferred electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle
40
How do fans transfer energy
- energy is transferred electrically from the battery of the handheld fan to the kinetic energy store of the fans motor
41
What is the power rating
- appliances are labelled with the maximum safe power that they can operate at
- the power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use
- but a higher power doesn’t necessarily mean that it transfers more energy usefully, an appliance may be more powerful but less efficient meaning that it might still only transfer the American amount of energy or less to useful stores
42
Why is a power rating helpful
Helps customers choose between models, the lower the power rating, the less electricity an appliance uses in a given time and so the cheaper it is to run
43
Why is potential difference, energy transferred per charge passed
- when electrical charge goes through a change in pd, then energy is transferred
- energy is supplied to the charge at the power source to raise it through a potential
- the charge gives up this energy when it falls through any potential drop in opponents elsewhere in the circuit
- that mans that a battery with a bigger pd will supply more energy to the circuit for every coulomb of charge which flows round it because the charge is raised up higher at the start
44
What is the national grid
A giant system of cables and transformers that covers the UK and connects power stations to consumers
- it transfers electrical power from power stations anywhere on the grid (the supply) to anywhere else on the grid where its needed (the demand)
45
Why does electricity production have to met demand- what happens?
- throughout the day, electricity usage changes. Power stations have to produce enough electricity for everyone to have it when they need it
- power stations often run a well bellow their maximum power output, so theres a spare capacity to cope with a high demand, even if there’s an unexpected shut down of another station
- lots of smaller power stations that can start up quickly are also kept in standby just in case
46
When does demand for electricity change
- when people get up in he morning
- come home from school or work
- when it starts to get dark or cold
- popular events like a sporting final being shown on tv could also cause a peak in demand
47
What type of power does national grid need
- high pd and low current
48
Why des national grid not use high current
- you lose loads of energy as the wires heat up ad the energy is transferred to the thermal store of the surroundings
- its much cheaper to boost the pd really high (400,000V) and keep current s low as possible
49
Why is national grid so efficient
- for a given power, increasing pd decreases the current which decreases the energy lost by the wires and the surroundings. This makes the national grid an efficient way of transferring energy
50
What are transformers
- used to get the potential difference to 400,000 V for efficient transmission
- all have two coils, a primary coil ad a secondary coil, joined with an iron core
51
How is potential difference increased in national grid
- increased using a step up transformer
- they have more turns on the secondary coil than the primary coil.
- as the pd is increased by the transformer the current is decreased
- the pd is then reduced again at the local consumer end using a step down transformer (current is therefore increased by this transformer)
- they have more turns on the primary coil than the secondary
52
What is power of a primary coil
- power of a primary coil is give by power = pd x current
- transformers are nearly100% efficient so the power in primary coil= power in secondary coil
