Electricity Flashcards
Circuit symbols
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What must there be in a closed circuit for charge to flow?
A source of potential difference
What is current?
The flow of charge - the size of the current is the rate of flow of electrical charge
Measured in amperes (A) by ammeters
What is potential difference/voltage?
Difference in the amount of energy between two points in a circuit
Measured in volts (V) by voltmeters
What is resistance?
How hard it is for current to flow through a component
Equation for charge
Charge (coulombs, C) = current x time
Q = It
Equations for potential difference
Potential difference = energy ÷ charge
V = E/Q
Potential difference = current x resistance
V = IR
Required Practical 3: Resistance
- Set up circuit with cells, an ammeter, a gap and a voltmeter parallel to the gap
- Fill the gap with a wire
- It’s hard to connect the crocodile clip exactly to the ends of the wire so zero errors are possible - this is where the measuring apparatus (in this case ammeter) doesn’t go back to zero when it should. To obtain good results the zero error should be subtracted from all readings - for example, if the ammeter reads 2A when it should be 0A, 2A should be subtracted from all readings taken.
- Calculate resistance by measuring current and potential difference
- Repeat this with wires of different lengths
- Resistance should be directly proportional to length of the wire and a graph should be plotted to show this
- Then put 2 resistors of known value in the gap
- Calculate the resistance - should find that the resistance of the resistors add together, the more resistors the higher the total resistance
- Then put these same 2 resistors in parallel
- Calculate the resistance - should find that the more resistors the lower the total resistance and that the total resistance is lower lower than the resistor with the lowest resistance
- In series:
Total resistance = resistance 1 + resistance 2 + resistance 3 …In parallel:
1/total resistance = 1/resistance 1 + 1/resistance 2 + 1/resistance 3 …
What is Ohm’s law?
Current is directly proportional to potential difference at a constant temperature and resistance
What is an ohmic conductor?
A conductor that obeys Ohm’s law
I / V graphs
http://www.alevelphysicsnotes.com/electricity/images/IV%20graphs.svg
Why is the graph like that for a fixed resistor?
It is an ohmic conductor and obeys Ohms law; therefore the straight line passing through the origin shows the current to be in direct proportion to the potential difference at a constant temperature
Why is the graph like that for a filament lamp?
At lower values of potential difference the lamp behaves ohmically but when the potential difference increases the temperature increases, this increasing resistance, so the current decreases
Why is the graph like that for a diode?
The diode has extremely high resistance one way so that virtually no current gets through, but the other way the resistance suddenly drops to nearly zero, allowing a lot of current to pass through
What is a thermistor?
A resistor sensitive to temperature - as temperature increases resistance decreases
What is a light dependent resistor (LDR) ?
A resistor sensitive to light intensity - as light intensity increases resistance decreases
Where are thermistors used?
- Fire alarms
- Ovens
- Fridges
- Digital thermometers
Where are light dependent resistors used?
- Alarm clocks
- Street lights
- Light intensity meter
- Burglar alarms
How can we find the resistance of a component?
Set up a functioning circuit with the cell(s) and ammeter in series to the component and a voltmeter parallel to it - measure the current and potential difference and calculate the resistance using V = IR
Required Practical 4: I/V Characteristics
- Set up a functioning circuit with 2 cells, an ammeter, a variable resistor, a fixed resistor, and a voltmeter across the fixed resistor
- Measure the current and potential difference
- Change the resistance of the variable resistor, measuring current and potential difference values each time
- Flip the cells around and repeat steps 2 - 3; the current and potential difference values should all be negative
- Don’t keep the circuit connected too long or the resistor will get hot and results won’t be accurate
- Plot the results on a graph
- Repeat steps 1 - 5, but for a filament lamp
- Then repeat 1 - 5 again for a diode
- When investigating I/V characteristics for a diode an extra resistor can be added to protect the diode from high currents and a a milliammeter can be used as it is more sensitive than an ammeter so it can detect the low current
- The graphs should be one straight line through the origin (fixed resistor), one S-shaped graph (filament lamp), and one that looks like an exponential graph (diode)
Current, potential difference and resistance in series
- There is the same current through each component
- The total potential difference shared between the branches based on the resistance of the components
- the total resistance of two components is the sum of the resistance of each component
Current, potential difference and resistance in parallel
- The total current through the whole circuit is the sum of the currents through the separate components
- The potential difference across each component is the same
- The total resistance of two resistors is less than the resistance of the smallest individual resistor
Why does adding resistors in series increase the total resistance whilst adding resistors in parallel decreases the total resistance?
In series the current flows through the whole circuit and passes through each component, so the resistance for each component adds up - the more resistors the current has to pass through the more resistance it faces
In parallel the current is split though the branches based on the resistance of the components on each branch - the branch with lowest resistance will get more current flowing through it and vice versa, so the current faces less resistance when more resistors are added
What is direct current (DC)?
Current in only one direction
What is alternating current (AC)?
Current that periodically reverses direction
Mains electricity
- Alternating current
- Frequency 50 Hz - changes direction 50 times per second
- 230 V potential difference
In a three-core cable which wires are there?
- Live wire
- Neutral wire
- Earth wire
What colour is the live wire, what is its voltage and what does it do?
Brown, 230V
Carries the alternating potential difference - they can be dangerous if a connection is provided between them and the ground as the potential difference will pass through it and 230V can be fatal for a human, and even if the switch is open they are dangerous in the section between the source of electricity and the switch
What colour is the neutral wire, what is its voltage and what does it do?
Blue, 0V or close to 0V
Completes the circuit back to the source of the electricity
What colour is the earth wire, what is its voltage and what does it do?
Green and yellow stripes, 0V
Prevents the appliance from becoming live - if there is a fault and the live wire became loose it could touch the appliance case and the next person who uses the appliance would get electrocuted because they are the current’s path to the ground (0V). The earth wire is connected to the case and has low resistance - this means that the live current will take this path to the ground instead of passing though a person
What is a fuse?
An electrical safety device connected to the live wire that has a thin wire which melts and breaks the circuit - the appliance is no longer live
Equations for energy and power of electrical appliances
Energy transferred = power (Watt, W) x time
E = Pt
Energy transferred = potential difference x charge
E = VQ
Power = potential difference x current P = VI
Power = current^2 x resistance P = I^2R
The National Grid
https://bam.files.bbci.co.uk/bam/live/content/zjd4wmn/large
What do transformers do?
Step-up transformers increase the voltage from 25,000V to 400,000V
Step-down transformers decrease the voltage from 400,000V to 230V
Why are they important?
Step-up transformers are important as they make the National Grid efficient: a high 400,000V voltage and a constant power mean the current is drastically reduced (P = VI) and the low current means that power loss is minimal (P = I^2R)
Step-down transformers are important as they decrease the voltage down to a safe level that is sufficient for homes’ or business’ needs
Production of static electricity
When materials are rubbed against each other they can get charged as electrons can be ‘rubbed off’ from one material to the other - the materials electrically charged
Opposite charges _______ , like charges _______
Opposite charges attract, like charges repel
What happens when a polythene rod is rubbed with a cloth?
The friction causes electrons to gain energy, which ‘rub off’ onto the rod which becomes negatively charged - the cloth, having lost electrons, is now positively charged
What happens when a acetate rod is rubbed with a cloth?
The friction causes electrons to gain energy, which ‘rub off’ onto the cloth which becomes negatively charged - the rod, having lost electrons, is now positively charged
If a cloth rubs a plastic rod and the cloth is pulled away from the rod slightly, will the rod and cloth attract, repel or experience no force at all?
If electrons are rubbed off the cloth and onto the rod they will have opposite charges
If electrons are rubbed off the rod and onto the cloth they will have opposite charges
In both cases the opposite charges will attract
Example of non-contact forces exerted by charged objects
A charged rod can pick up small pieces of paper
A charged balloon can stick to the wall by attraction
A charged rod can pull a stream of water towards it
Electric fields
- A charged object creates an electric field around itself
- The closer to the charged object the stronger the field
- Another object in the field experiences a non-contact force
- They look like this: http://buphy.bu.edu/~duffy/PY106/2e.GIF
- The arrows point in the direction a positively charged object would move in the field
