P2 - Electricity Flashcards
What colour is the Earth wire?
Green and yellow
What do AC and DC mean?
AC - Alternating current. When constantly swapping directions, electrons bump into each other and pass the charge along (most common)
DC - DIrect current. Friction of electrons trying to get through creates energy.
What is a diode and what does it look like?
A diode only lets electricity pass one way (+ or -). Looks like a skip forwards button in a circle.
Which wire is blue?
The neutral wire
The live wire is blue. True or false?
False - the live wire is brown (just like revision)
The neutral wire is blue
What does potential difference mean? What is its units?
Voltz - V
Q is the symbol for Coulombs, which is the unit for resistance. True or false?
False - Q is the symbol for coulombs, but coulombs are the units for charge
When electricity flows through a resistor, what happens?
It becomes hot - the resistor is making it more difficult for electrons to pass through, so the friction creates thermal energy, which is heat
What are the two formulas for current?
Charge (coulombs) ÷ time (seconds)
OR
Voltz (V) ÷ Resistance (Ohms)
In a series circuit, the current is 10A at component 1
a) What is the current for component 2?
b) What is the voltage at component 3?
(series circuit - 12V, 10A, 3 components)
a) 10A
b) 4V
Complete these sentences about series circuits:
Potential difference is…
Current is the…
Resistance…
Shared (1stV + 2ndV ...= totalV) Same everywhere (1stI =2ndI ...) Adds up (1stR + 2ndR ...= totalR)
Complete the sentences about parallel circuits:
Potential difference is…
Current is the…
Same everywhere (1stV =2ndV ...) Shared (1stI + 2ndI ...= total I)
Why does increasing the amount of resistors in a parallel circuit decrease the resistance?
The total current through the circuit increases, so the total resistance decreases (downwards curve on a graph)
Mains supply is AC
Battery supply is DC
True or false?
True
Energy transferred =
Charge (Coulombs) x Voltage (V)
What does the national grid do?
It is a system of giant cables and transformers that covers the UK, connecting power stations to consumers of electricity.
What happens if one bulb of many stops working in a series circuit? What about in a parallel circuit?
All the bulbs in the circuit turn off as there is no longer a complete circuit through that bulb - as it is broken. However, electrons have a way of getting around a bulb in a parallel circuit, so the entire circuit doesn’t turn off as it is sill a complete circuit
Resitance is directly/ indirectly proportional to the ;length of the circuit (cm)
Directly proportional
The step up transformer increases/ decreases the voltage.
Which is true?
Increases the voltage
Step-down transformers decrease the voltage
What’s the formula for the voltage of the primary/ secondary coil’s voltage?
Primary Voltz Amount of turns on primary coil
———————— = —————————————————–
Secondary Voltz Number of turns on secondary coil
Rearrange the order of these components, and give their rough voltage:
a) National Grid
b) Houses
c) Step-up transformer
d) Power station
e) Factories
f) Step-down transformer
d) Power station - 20,000V (500 Mega Watts)
c) Step-up transformer - 20,000 to 400,000V
a) National Grid - 400,000V
f) Step-down transfrormer - 400,000V to 33,000V
e) More than houses
- Step-down transformers here too
b) Houses/ domestic - 230V
How can power be lost through the wires of pylons and stuff?
Through resistance
Resistance has a heating effect, which increases the current. True or false?
True for some reason
Give the definition of a fuse
A fuse is a safety device which breaks when unsafe.
A hairdryer needs a fuse of 11V. When replacing it, you can have a choice between 3V, 10V, 12V or 24V. Which should you choose?
12V. Slightly over is better, as too little will be unable to power the device and break
Give an example of a device which has an internal step-up transformer, and why
A TV, because the 230V from the mains voltage is not enough to power the TV
What is the definition of current?
The flow of electrical charge. Measured in amps
Electrical charge will only flow round a complete (closed) circuit if there is a potential difference, so current can only flow if there’s a source of potential difference
In a single, closed loop, is the current shared of equal?
It is equal - current has the same value everywhere in the circuit
What is potential difference?
Voltage - the driving force that pushes the charge round. Measured in volts
What is resistance?
Anything that slows the flow down. Measured in Ohms
Complete the sentence:
The greater the resistance across a component…
The smaller the current that flows (for a given potential difference across the component)
What does the current flowing through a component depend on?
The potential difference across it and the resistance of the component
What does the total charge through a circuit depend on?
Current and time
What is the equation for charge?
Charge flow (C) = Current (A) x Time (s) Q = IT
More charge passes through a current when what?
When a larger current flows
A battery charger passes a current of 2A through a cell over 2.5 hours. How much charge is transferred to the cell?
Charge = current x time 2A x (2.5 x 60 x 60 to get seconds) = 18000 C (coulombs)
The size of the current is the rate of flow of charge. When current flows past a point in a circuit for a length of time, then the charge that has passed is given by what formula?
Q = It Charge = current x time (seconds)
What are the circuit diagram symbols for:
a) A cell
b) a battery
c) An open switch
d) A closed switch
e) A filament lamp (bulb)
f) A resistor
g) An ammeter
h) A voltmeter
i) A diode
j) Variable resistor
a) –| i– (can have a + above first long & thin line) NO i DOT
b) –| ii- - -| ii– (can have a + above first long & thin line) NO i DOT
c) –o/ o–
d) –o–o–
e) —(X)—
f) –rectangle–
g) –(A)–
h) –(V)–
i) Skip forward icon
j) –rectangle with / up arrow through
What is the circuit diagram symbol for a fuse?
A rectangle with a horizontal line through it
–[——]——–
What is the circuit diagram symbol for an LED?
Skip forward icon (like diode), but with 2 / up arrows from the top right
What is the circuit diagram symbol for an LDR?
A rectangle in a circle, with 2 \ down arrows onto circle from top left
What is a thermistor circuit symbol?
A rectangle, with a / line through it, and a _ at the start of the bottom of the line
—[_/]–
What formula links potential difference and current?
Potential difference = current x resistance (V= IR)
Volts = amperes x ohms
Name some factors that affect resistance
Series or parallel circuit
Length of the wire (longer wire, greater resistance)
Thickness of the wire
Temperature (Hotter wire, greater resistance)
What is used to measure (and how)
a) Voltage?
b) Current?
a) A voltmeter is placed as a separate parallel circuit around whatever you’re investigating
b) An ammeter is placed as part of the series circuit with whatever your’e investigating
Resistance is directly proportional to the length of a wire. True or false?
True - graph goes through origin, and straight / line
Rearrange these instructions for investigating how the length of a wire affects the resistance
1) Repeat this for a number of different lengths of the test wire
2) Close the switch, then record the current through the wire and the potential difference across it
3) Attach a crocodile clip to the wire level with 0cm on the ruler
4) If your graph doesn’t go through the origin, it could be because the first clip isn’t at exactly 0cm, so all of your length readings are a bit out. This is a systematic error
5) Plot a graph of resistance (y axis) against wire length (x axis) and draw a line of best fit
6) Attach the second crocodile clip to the wire (e.g. 10 cm away from the first clip). Write down the length of the wire between the clips in a results table
7) Your graph should be a straight line through the origin, meaning resistance is directly proportional to the length of a wire. The longer the wire, the greater the resistance
8) Use your measurements of current and potential difference to calculate resistance for each length of wire, using R = V/ I
9) Open the switch, then move the second crocodile clip e.g. another 10 cm along the wire. Close the switch again, and record the new length, current and potential difference
3) Attach a crocodile clip to the wire level with 0cm on the ruler
6) Attach the second crocodile clip to the wire (e.g. 10 cm away from the first clip). Write down the length of the wire between the clips in a results table
2) Close the switch, then record the current through the wire and the potential difference across it
9) Open the switch, then move the second crocodile clip e.g. another 10 cm along the wire. Close the switch again, and record the new length, current and potential difference
1) Repeat this for a number of different lengths of the test wire
8) Use your measurements of current and potential difference to calculate resistance for each length of wire, using R = V/ I
5) Plot a graph of resistance (y axis) against wire length (x axis) and draw a line of best fit
7) Your graph should be a straight line through the origin, meaning resistance is directly proportional to the length of a wire. The longer the wire, the greater the resistance
4) If your graph doesn’t go through the origin, it could be because the first clip isn’t at exactly 0cm, so all of your length readings are a bit out. This is a systematic error
A THIN WIRE WILL GIVE THE BEST RESULTS. MAKE SURE IT’S AS STRAIGHT AS POSSIBLE SO YOUR LENGTH MEASUREMENTS ARE ACCURATE
THE WIRE MAY HEAT UP DURING THE EXPERIMENT, WHICH WILL AFFECT IT’S RESISTANCE. LEAVE THE SWITCH OPEN FOR A BIT BETWEEN READINGS TO LET THE CIRCUIT COOL DOWN
Ohmic conductors have a what?
Constant resistance - it does not change with current.
For some components (like a diode or a filament lamp), as the current through them is changed, the resistance of the component changes well
Why does the resistance of a filament lamp heat up?
When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament, which is designed to heat up.
Resistance increases with temperature, so as current increases, the filament lamp heats up more and the resistance increases
For diodes, how does resistance change?
The resistance depends on the direction of the current. They will happily let current flow in one direction, but have a very high resistance is the current is reversed
The resistors of ohmic conductors (e.g. a wire or a resistor) doesn’t change with the current. at a constant temperature, the current flowing through an ohmic conductor is inversely/ directly proportional to the potential difference across it
Directly proportional
What does the term “I-V characteristics” refer to?
A graph which shows how the current (I) flowing through a component changes as the potential difference (V) across it is increased.
Linear components have an I-V characteristic that’s a straight line (e.g. a fixed resistor)
Non - linear components have a curved I-V chraracteristic (e.g. a filament lamp or a diode)
Describe the I-V characteristics of:
a) Linear components
b) No-linear components
a) Have an I-V characteristic that’s a straight line (e.g. a fixed resistor)
b) Have an I-V characteristic that’s curved (e.g. a filament lamp or a diode)
What experiment can you do to find a components I-V characteristic?
1) Set up a test circuit (battery, variable resistor, component, voltmeter, ammeter)
2) Begin to vary the variable resistor. This alters the current flowing through the circuit and the potential difference across the component
3) Take several pairs readings from the ammeter and voltmeter to see how the potential difference across the component varies as the current changes. Repeat each reading twice more to get an average potential difference at each current
4) Swap over the wires connected to the cell, so the direction of the current is reversed
5) Plot a graph of current against voltage for the component
What is the graph (current against voltage) showing the I-V characteristics for an ohmic conductor
Directly proportional, / through 2 of the 4 axis
The current through an ohmic conductor (at a constant temperature) is directly proportional to potential difference so you get a straight line
What is the graph (current against voltage) showing the I-V characteristics for a filament lamp
Like a very itallics letter s
(
)
As current increases, the temperature of the filament lamp increases, so the resistance increases. This means less current can flow per unit of potential difference, so the graph gets shallower - hence the curve
What is the graph (current against voltage) showing the I-V characteristics for a diode?
Like a j I guess, only on the positive axis (top right quadrant)
Current will only flow in one direction. The diode has a very high resistance in the reverse direction
On any I-V characteristic graph, how can you calculate resistance?
With the gradient, as R = V/ I
What is an LDR?
It is a Light Dependant Resistor - dependent on the intensity of light.
In bright light, the resistance falls
In darkness, the resistance is highest
They have lots of applications including automatic night lights, outdoor lighting and burglar detectors
The circuit symbol is a rectangle in a circle, with 2 \ down arrows onto circle from top left
What is a thermistor?
A temperature dependant resistor
In hot conditions, the resistance drops
In cool conditions, the resistance goes up
Thermistors make useful temperature detectors, e.g. car engine temperature sensors and electronic thermistats
The circuit symbol is a rectangle, with a / line through it, and a _ at the start of the bottom of the line
—[_/]–
How can you use thermistors in sensing circuits?
1) Sensing circuits can be used to turn on or increase the power to components depending on the conditions that they are in
2) A thermistor can be used to operate a fan in a room (cell, thermistor in series, fixed resistor, fan in parallel to fixed resistor)
3) The fixed resistor and the fan will always have the same potential difference across them, (because they’re connected in parallel)
4) The potential difference of the power supply is shared out between the thermistor and the loop made of the fixed resistor and the fan according to their resistances - the bigger a component’s resistance, the more of the potential difference it takes
5) As the room gets hotter, the resistance of the thermistor decreases, so it takes a smaller share of the potential difference from the power supply. So, the potential difference across the fixed resistor and the fan rises, making the fan go faster
Complete the sentence:
The bigger a component’s resistance,
The more potential difference it takes
What can a sensing circuit be used for?
To turn on or increase the power of components depending on the contitions they are in. Thermistors and LDR’s are examples in the book
Fill the blanks:
If you connect a bulb in parallel to an LDR (with a cell and a fixed resistor in ______), the potential difference across both the LDR and the bulb will be high when it’s ____ and the LDR’s resistance is ____. The greater the potential difference across a component, the ____ energy it gets. So a bulb connected across an LDR would get ________ ad the room got ______
Series Dark High More Brighter Darker
In a series circuit, what these across a circuit?
a) Potential difference
b) Current
c) Resistance
a) It is shared - the pd of the all the components added up = the pd at the source
b) It is the same everywhere. The size of the current is determined by the total pd of the cells and the total resistance of the circuit (I = V/ R)
c) It adds up - the total resistance is just the total resistance from all components. This is because by adding a resistor in series, the 2 resistors have to share the total pd. The pd across each resistor is lower, so the current through each resistor is also lower. In a series circuit, the current is the same everywhere so the total current in the circuit is reduced when a resistor is added. This means the total resistance of the circuit increases
Complete the sentence:
The bigger a component’s resistance…
The bigger it’s share of the total potential difference
In a circuit with a cell supplying 20V, with 2 resistors (one 2 ohms, the other 3 ohms), calculate the current passing through the circuit
1) Fist find the total resistance - 2 + 3 = 5 ohms
2) Then I = V/ R - 20/5 = 4A
Cell potential differences add up. Give an example showing this.
When 2 cells each with a potential difference o 1.5V are connected in series, they supply 3V between them
What type of circuits are in series, when this is normally rare as one component breaking will ruin the whole system?
Sensor circuits
Which type of circuit is used more often?
Parallel circuits, because if you remove or disconnect one of them, or if one component breaks, it will hardly affect the others at all
In a parallel circuit, what these across a circuit?
a) Potential difference
b) Current
c) Resistance
a) It is the same across all components. This means that identical bulbs connected in parallel will all be at the same brightness
b) It is shared between branches. The total current flowing around the circuit is equal to the total of all the currents through the separate components. The total current going into a junction is equal to the total current leaving. If 2 identical components are connected in parallel, then the same current will flow through each component
c) Adding a resistor in parallel reduces the total resistance. With 2 resistor in parallel, their total resistance is less than of the smallest of the 2 resistors. Because:
[In parallel, both resistors have the same potential difference across them as the source. This means the “pushing force” making the current flow is the same as the source pd for each resistor you add. But by adding another loop, the 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]
What is the practical for investigating resistance in series?
1) First, you’ll need at least 4 identical resistors.
2) Then, build a series circuit with a battery, resistor and ammeter. Make note of the potential difference of the battery
3) Measure the current through the circuit using the 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 through the circuit and use this and the potential difference of the battery to calculate the overall resistance of the circuit
6) Repeat steps 4 and 5 until all of the resistors have been added
7) Plot a graph of the number of resistors against the total resistance (y axis) - should be directly proportional
What is the practical for investigating resistance in parallel?
1) Start with a battery, ammeter and resistor 1 in series circuit, same as series circuit resistance test so it’s a fair test.
2) Measure the total current through the circuit and calculate the resistance of the circuit with R= V/I (V of the battery)
3) Next add another resistor in parallel with the first resistor
4) Measure the total current through the circuit and use this and the potential difference of the battery to calculate the overall resistance of the circuit
5) Repeat steps 3 and 4 until all (4) resistors have been added
6) Plot a graph of the number of resistors against the total resistance (y axis) - should be 1/x curve (but only positive axis) - like a half curve shape, never touching 0
The more resistors you add to a parallel circuit, the…
The more resistors you add in a series circuit, the…
Smaller the overall resistance becomes
Larger the resistance of the whole circuit
Which of these is AC, which is DC?
a) Mains
b) Battery
a) AC (In UK at least)
b) DC
What is the difference between AC and DC current?
AC supplies have a current constantly changing direction. Alternating currents are produced by alternating voltages in which the positive and negative ends keep alternating
DC current is a current that is always flowing the the same direction. It’s created by a direct voltage
What is the frequency & voltage of the ac mains supply?
50 cycles per second, or 50 Hx (hertz)
230V
Which part of a plug can give you an electric shock?
The live wire
Your body like the earth is at 0V. This means that if you touch the live wire, a large potential difference is produced across your body and a current flows through you.
This causes a large electric shock which can injure or even kill
Even if a plug socket or light switch is turned off (the switch is open), there is still a danger of an electric shock. A current isn’t flowing but there’s still potential difference in the live wire. If you make 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 can be dangerous. If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire
Name the wires of a plug
1) Live wire - brown - provides alternating potential difference (at about 230V) from the mains supply. Causes electric shocks
2) Neutral wire - blue - completes the circuit and caries away current electricity normally flows in through the live wire and out through the neutral wire. It is around 0V
3) Earth wire - green and yellow - protecting the wiring and for safety, it stops the appliance from becoming live. It doesn’t usually carry a current, only when there’s a fault. Also at 0V
Live wire
Brown - provides alternating potential difference (at about 230V) from the mains supply. Causes electric shocks
Neutral wire
Blue - completes the circuit and caries away current electricity normally flows in through the live wire and out through the neutral wire. It is around 0V
Earth wire
Green and yellow - protecting the wiring and for safety, it stops the appliance from becoming live. It doesn’t usually carry a current, only when there’s a fault. Also at 0V
What does energy transferred depend on?
The power - the total energy transferred by an appliance depends on how long the appliance is on for and it’s power (energy transferred = power x time)
What is the power of an appliance?
The energy that it transfers per second. Measured in watts (W)So the more energy transferred in a certain time, the higher its power (Power = energy transferred/ time)
Why does a moving charge transfer energy?
Because charge does work against the resistance of the circuit (Work done is the same as energy transferred)
Work done and energy transferred are the same thing. True or false?
True
What do these appliances transfer the energy received from electricity into?
a) A kettle
b) A handheld fan
a) Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle
b) Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor
What does the power rating of an appliance tell you?
The maximum amount of energy transferred between the stores per second when the appliance is in use
It helps customers choose between models - the lower the power rating, the less electricity an appliance uses in a time and so the cheaper it is to run
But, a higher power doesn’t necessarily mean that it transfers more energy usefully. An appliance may be more powerful than another, but less efficient, meaning that it might still only transfer the same amount of energy (or even less) to useful stores
What is potential difference in terms of charge passed and energy transferred?
Potential difference is energy transferred per charge passed.
When an electrical charge goes through a change in potential difference, 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 components elsewhere in the circuit.
The formula is Energy transferred = charge flow x potential difference (E = QV)
This means that a battery with a bigger pd will supply more energy to the circuit for every coulomb of charge which flows around it, because the charge is raised up “higher” at the start
A battery with a bigger voltage will supply more/ less energy to the circuit for every coulomb of charge which flows around it
More
Give the equation linking energy transferred, charge and potential difference
E = QV Energy transferred (J) = Charge flow (C) x potential difference (V)
What’s the units for energy transferred?
J for Joules
What is the equation that links power, potential difference and current?
Power (W) = potential difference (V) x current (A)
P = VI
What does power depend on?
Current and (times) potential difference Energy transferred and (divided by) time
How is electricity distributed across the country?
Via the national grid
What is the national grid?
A giant web of wires & transformers that covers the whole of Britain, getting electricity from power stations to homes everywhere. Whoever you pay for your electricity, its the national grid that gets it to you
It transfers electrical power from power stations anywhere on the grid (the supply) to anywhere on the grid where it’s needed (the demand) - e.g. homes and industry
Electricity production has to meet what?
Demand - the amount of electricity people need. Power stations must produce enough electricity for everyone to have it when they need it
When does electricity demand increase?
When people get up in the morning
Come home from school/ work
When it’s dark or cold outside
Popular events on TV
How do power stations prepare for working up to a higher demand?
They often run at well below their maximum power output, so there’s 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 n standby just in case
What potential difference and current does the national grid use? (high or low)
High potential difference - to transmit huge power - cheaper both of these
Low current - problem with high current is lose loads of energy as the wires heat up and energy transferred to the thermal energy stores
For a given power, increasing pd decreases the current, which decreases the energy lost by heating the wires and the surroundings.
What is a rough voltage of the national grid - wires?
400,000V
How is the potential difference changed from 400,000V to 230V in homes?
Through transformers - lower voltage is step down transformers, raise voltage is step up transformers
Why are step up transformer there?
For efficient transmission - for a given power, increasing pd decreases the current, which decreases the energy lost by heating the wires and the surroundings.
Then step down to bring to safe. usable levels at the other end
