Electricity Flashcards
Define Current
Current is the rate of flow of charge
What direction does conventional current flow and what is this the opposite to?
Conventional current flows from positive to negative and this is the opposite direction to electron flow
What is the coulomb the unit of?
Charge
Define a coloumb
One coulomb is defined as the amount of charge that passes in 1 second if the current is 1 ampere
How can you measure the current flowing through a part of a circuit?
By using an ammeter
How should the ammeter be connected and why should it be connected in this way?
The ammeter should be connected in series to make sure that the current flowing through the ammeter is the same as the current flowing through the component.
What needs to be done to make electric charge flow through a conductor?
Work needs to be done on the conductor
Define potential difference
Potential difference is the work done per unit charge moved
How can you measure the potential difference across a component?
By using a voltmeter
How should a voltmeter be connected and why should it be connected in this way?
A voltmeter should be connected in parallel with the component as the potential difference across components in parallel is the same
What is the definition of the volt?
The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component
How can the volt be explained in terms of other units?
One volt is equal to one joule per coulomb
What happens if you put a potential difference across an electrical component?
A current will flow
What does the amount of current you get for a particular potential difference depend on?
The resistance of the component
How can you think of a component’s resistance?
You can think of a components resistance as a measure of how difficult it is to get a current to flow through it
Unless told otherwise what can you assume about the resistance of voltmeters and ammeters?
Voltmeters are infinitely resistant
Ammeters have no resistance
What unit is resistance measured in?
Ohms
Define the ohm
A component has a resistance of 1 ohm if a potential difference of 1V makes a current of 1A flow through it
What can be said about the resistance of an ohmic conductor?
It is constant
Define Ohm’s Law
Provided the physical conditions such as temperature remain constant the current through an ohmic conductor is directly proportional to the potential difference across it.
What type of graph is the I/V graph for an ohmic conductor at a constant temperature?
A straight line graph through the origin
What do I/V graphs show?
They show how resistance varies
Define the term ‘I/V Characteristics’
The term IV Characteristics refers to a graph of I against V which shows how the current flowing through a component changes as the potential difference across it is increased.
What type of line on an I/V graph for an ohmic conductor shows low resistance?
A steep line
What type of line on a V/I graph for an ohmic conductor shows low resistance?
A shallow line
What is the I/V graph for a filament lamp?
A curve that starts steep but gets shallower as the voltage rises
What is the filament in a lamp?
The filament in a lamp is just a coiled up length of metal wire. It does not have the same characteristic graph as a metallic conductor as it gets hot. Current flowing through the lamp increases its temperature.
When does the resistance of a metal increase?
The resistance of a metal increases as the temperature increases
Describe the V/I graph for a filament lamp
The V/I graph for a filament lamp is a curve that starts shallow and gets steeper as the current and voltage increase.
What are semiconductors used in?
Sensors
Why are semiconductors not as good at conducting electricity as metals?
As there are far fewer charge carriers available
Why do semiconductors make excellent sensors for detecting changes in their environment?
As if energy is supplied to the semiconductor more charge carriers can be released
Name two types of semiconductor components?
Thermistors and diodes
What is a thermistor?
A thermistor is a resistor with a resistance that depends on its temperature
What does NTC stand for in the term ‘NTC Thermistor’?
Negative temperature coefficient
What does ‘NTC’ mean for a thermistor?
This means that the resistance decreases as the temperature goes up
Describe the I/V characteristic graph for an NTC thermistor
The I/V characteristic graph for an NTC thermistor curves upwards
What effect does warming a thermistor have?
Warming the thermistor gives more electrons enough energy to escape from their atoms. This means that there are more charge carriers available so the resistance is lower.
What are diodes?
Diodes including LEDs are designed to let current flow in one direction only.
What is forward bias?
Forward bias is the direction in which the current is allowed to flow in a diode
What is the threshold voltage most diodes require in the forward direction before they will conduct?
0.6V
What is reverse bias?
In reverse bias the resistance of the diode is very high and the current that flows is very tiny
How can you tell which direction a diode will let current flow in?
Diodes will let current flow in the direction that the triangle in the circuit symbol points
What three factors does the resistance of an object depend on?
- Length
- Cross sectional area
- Resistivity
Explain a wires length in relation to its resistance
The longer the wire the more difficult it is to make a current flow
Explain the area of a wire in relation to its resistance
The wider the wire the easier it will be for the electrons to pass along it
What does the resistivity of an object depend on?
The resistivity of an object depends on its material. The structure may make it easy or difficult for charge to flow. In general resistivity depends on environmental factors as well such as temperature and light intensity
Define resistivity
The resistivity of a material is defined as the resistance of a 1m length with a 1m^2 cross - sectional area. It is measured in ohm-metres
How big are typical values for the resistivity of conductors?
They are typically really small
Before you start the finding the resistivity of a wire practical what do you need to do?
You need to find the cross sectional area of the wire and to do so you need to assume that the wire is cylindrical and so the cross section is circular.
How do you find the cross sectional area of the test wire in the finding the resistivity of a wire practical?
Use a micrometer to measure the diameter of the test wire in at least three different points along the wire. Take an average value as the diameter and divide by 2 to get the radius, make sure this is in m. Plug it into the equation Pi*R^2
Explain the practical for finding the resistivity of a wire
1) The test wire should be clamped to a ruler with the circuit attached to the wire where the ruler reads zero
2) Attach the flying lead to the test wire - the lead is just a wire with a crocodile clip at the end to allow connection to any point along the test wire
3) Record the length of the test wire connected in the circuit, the voltmeter and ammeter reading.
4) Use your readings to calculate the resistance of the length of wire using R=V/I
5) Repeat this measurement and calculate an average resistance for the length
6) Repeat for several different lengths for example between 0.10 and 1.00m
7) Plot your results on a graph of resistance against length and draw a line of best fit. The gradient of the line of best fit is equal to R/I = P/A. So multiply the gradient of the line by the cross sectional area of the wire to find the resistivity of the wire material.
What needs to be done during the finding the resistivity of a wire practical to limit random errors?
The temperature of the test wire should be tried to be kept constant for example by only having small currents flow through the wire
Define power
Power is defined as the rate of transfer of energy. It is measured in watts where 1 watt is equivalent to 1 joule per second
What happens as charge flows through a circuit?
As charge flows through a circuit it doesn’t get used up or lost. This means that whatever charge flows into a junction will flow out again
What is the relationship between current and charge flowing through a circuit?
Since current is the rate of flow of charge whatever current flows into a junction is the same as the current flowing out of it
What is Kirchhoff’s first law?
The total current entering a junction is equal to the total current leaving it
What happens when energy flows through a circuit?
Energy is conserved. In electrical circuits energy is transferred round the circuit. Energy transferred to a charge is electromotive force and energy transferred from a charge is potential difference. In a closed loop these two quantities must be equal for energy to be conserved which it is.
What is Kirchhoff’s second law?
The total electromotive force around a series circuit is equal to the sum of the potential differences across each component
When answering a question about resistors in series or parallel what can you assume?
You can ignore internal resistance unless told otherwise
What are the three rules about current, potential difference and resistance in series circuits?
- The current in a series circuit is the same at all point of the circuit as there are no junctions.
- Electromotive force is split between components so Emf = V1 + V2 +V3…
- Provided current is constant - RTotal = R1 + R2 + R3…
What are the three rules about current, potential difference and resistance in parallel circuits?
- Current is split at each junction in a parallel circuit so I = I1 + I2 + I3…
- There is the same potential difference across all components in a parallel circuit. Within each loop the emf is equal to the sum of the individual potential differences
- 1/RTotal = 1/R1 + 1/R2 + 1/R3…
What is the formula for calculating energy transferred linking potential difference, current and time taken?
E=ItV
Define the resistance of a component
The resistance of a component is the ratio of the potential difference across it to the current flowing through it
Do batteries have resistance?
Yes
What causes resistance?
Resistance comes from electrons colliding with atoms and losing energy to other forms
What is internal resistance?
In a battery chemical energy is used to make electrons move. As they move they collide with atoms inside the battery so batteries must have resistance, this is called internal resistance
What is the relationship between the temperature of batteries and cells and their internal resistance?
Internal resistance is what makes batteries and cells warm up when they are used
Define load resistance
Load resistance is the total resistance of all the components in the external circuit. It may also be called external resistance
Define electromotive force (Emf)
The electromotive force is the amount of electrical energy a battery produces for each coulomb of charge.
What is electromotive force measured in?
Volts
Define terminal potential difference
The terminal potential difference is the potential difference across the load resistance which is the energy transferred when one coulomb of charge flows through the load resistance
What is the relationship between the terminal potential difference and the internal resistance?
If there was no internal resistance the terminal potential difference would be the same as the electromotive force. However in real power supplies there is always some energy lost in overcoming the internal resistance
Define lost volts
The lost volts is the energy wasted per coulomb overcoming the internal resistance.
Explain internal resistance in terms of the conservation of energy
Energy per coulomb supplied by the source = Energy per coulomb transferred in load resistance + energy per coulomb wasted in internal resistance
How many emf and internal resistance equations are there?
4
What are the four emf and internal resistance equations?
- ε = V + v
- ε = I(R + r)
- V = ε - v
- V = ε - Ir
What does the symbol ε mean?
Electromotive force
What does the symbol V mean?
Terminal potential difference
What does the symbol v mean?
Lost volts
What does the symbol I mean?
Current
What does the symbol R mean?
Load resistance
What does the symbol r mean?
Internal resistance
How can you work out the total emf of multiple cells in series?
- You can calculate the total emf of the cells by adding their individual emfs
- εtotal = ε1 + ε2 + ε3 …
How can you work out the total emf of multiple cells in parallel?
- For cells in parallel the total emf of the combination of cells is the same size as the emf of each of the individual cells
- εtotal = ε1 = ε2 = ε3 = …
Why is the total emf of cells in parallel the same as each of the individual cells?
This is because the current will split equally between identical cells. The charge only gains emf from the cells it travels through so the overall emf in the circuit doesn’t increase
Why is the total emf of cells in series the sum of all the individual emfs?
This is because each charge goes through each of the cells and so gains emf from each one
Explain the method of the practical for investigating the internal resistance and emf of a cell
1- Vary the current in the circuit by changing the value of the load resistance using the variable resistor. Measure the pd for several different values of current
2- Record the data for V and I in table and plot the results in a graph of V against I
3- To find the emf and internal resistance of the cell start with the equation V = ε - Ir
4- Rearrange the equation to give V = -rI + ε
5 - Since ε and r are constants that’s just the equation of a straight line
6- So the y-intercept is ε
7 - The gradient is -r
8 - Make sure to also sketch a graph with a negative gradient with V against I with a negative gradient in questions
What is an easier way of measuring the emf of a power source?
An easier way to measure the emf of a power source is by connecting a high-resistance voltmeter across its terminals. But a small current flows through the voltmeter so there must be some lost volts, this means you measure a value very slightly less than the emf
What is a potential divider used for?
You can use potential dividers to supply a potential difference (Vout) between zero and the potential difference across the voltage source. This can be useful if you need a varying pd supply or one that is at a lower pd than the voltage source
What is a potential divider?
At its simplest, a potential divider is a circuit with a voltage source and a couple of resistors in series
What is the relationship between the potential difference across the source voltage and the resistance of the resistors in a potential divider?
The potential difference across the voltage source is split in the ratio of the resistances
What ratio of the total pd across the voltage source would a 2 ohm resistor and a 3 ohm resistor get?
The 2 ohm resistor would get 2/5 of the pd across it and the 3 ohm resistor would get 3/5 of the pd across it
What is a typical potential divider circuit used for?
This circuit is mainly used for calibrating voltmeters which have a very high resistance
What can a variable resistor be used for in a potential divider?
To vary the voltage
What can an LDR or thermistor be added to a potential divider for?
An LDR or thermistor can be added to a potential divider for a light or temperature sensor
Explain how and LDR or thermistor can be added to a potential divider as a light or temperature sensor
- A light dependent resistor has a very high resistance in the dark but a lower resistance in the light
- An NTC thermistor has a high resistance at low temperatures but a much lower resistance at high temperatures.
- Either of these can be used as one of the resistors in a potential divider giving an output voltage that varies with the light level or temperature
What is a potentiometer?
A potentiometer has a variable resistor replacing R1 and R2 of a potential divider but it uses the same idea, its even sometimes called a potential divider
How does a potentiometer work?
You move a slider or turn a knob to adjust the relative sizes of R1 and R2. That way you can vary Vout from 0V up to the source voltage. This is handy when you want to be able to change a voltage continuously like in the volume control of a stereo
What limits the minimum size of a resistor?
- Resistors must be large enough to be able to lose heat to the surroundings
- It would also be difficult to make connections to a small resistor
Explain how the emf and internal resistance may be obtained from a graph of V against I
- Re-arrange the equation ε = V + Ir to give V = -Ir + ε
- Relate V = -Ir + ε to the equation of a straight line, y = mx+c
- This shows that the y - intercept of the graph is ε and the gradient of the graph gives -r
Why does the resistivity of metals increase as temperature increases?
As the temperature of a metal increases, the metal ions gain kinetic energy and so vibrate more. This means that there are more collisions between electrons flowing through the metal and the ions and so the rate of flow of electrons is less. This means that the current is less and that the resistivity increases
What is the relationship between superconductors and resistivity?
Superconductors have zero resistivity
What is the relationship between all materials and resistivity?
Normally all materials have some resistivity. That resistance means that whenever electricity flows through them they heat up and some of the electrical energy is wasted as thermal energy
How can you lower the resistivity of materials?
You can lower the resistivity of many materials by cooling them down
What are superconductors?
Superconductors are conductors with zero resistivity/resistance
How can some materials be converted into superconductors?
If you cool some materials down to a temperature equal to or below their critical temperature their resistivity disappears entirely and they become a superconductor
As superconductors have zero resistance what does this mean about the electrical energy flowing through them?
Without any resistance, none of the electrical energy is turned into heat so none of it is wasted. That means you can start a current flowing in a circuit using a magnetic field, take away the magnet and the current would carry on flowing forever
What is the problem incurred when trying to convert most conductors into superconductors?
Most normal conductors have critical temperatures below 10 kelvin (-263 degrees). Getting things that cold is hard and very expensive
What is the critical temperature of a superconductor?
The critical temperature of a superconductor is the temperature at or below which the resistivity of the material is zero
What are 3 uses of superconductors?
Using superconducting wires you could make:
1- Power cables that transmit electricity without any loss of power
2- Really strong electromagnets that don’t need a constant power source (for use in medical applications and Maglev trains)
3- Electronic circuits that work really fast because there’s no resistance to slow them down
