Electricity Flashcards
What is current?
It is the rate of flow of charge
Explain the water pipe analogy for current
The amount of water that flows depends on the flow rate and the time
What equation links charge, current and time?
Q = IT
What is the charge carried by in an electrical circuit?
Charge is carried through the wires by electrons
What is the unit of charge?
Coulombs
What is 1 coulomb defined as?
The amount of charge that passes in 1 second when the current is 1A
Do you attach an ammeter in series or in parallel in an electrical circuit?
Series
Why do you attach an ammeter in series?
So the current through the ammeter is the same as the current through the component
Which way does conventional current flow (in terms of + and -)?
+ve to -ve
Which way do electrons flow (in terms of + and -)?
-ve to +ve
Why do electrons flow from -ve to +ve?
Because electrons are negative, they are repelled by the negative and attracted to the positive
When a charge flows through a component, what does it do?
It transfers energy to the component
What do you need to do to make electrical charge flow through a circuit?
You need to transfer energy to the charge
Where does the energy needed to transfer energy to the charge come from?
A power source e.g. a battery
In a DC circuit, compare the speeds of the electrical current and the electrical energy
The electricity within the wires flows very slowly; at speeds around inches per minute. At the same time, the electrical energy flows at nearly the speed of light
What happens to the charge when it flows through the power source?
It is ‘raised’ through the potential (meaning it’s voltage is raised) and energy is transferred to the charge as electrical energy
Explain how the power source does work?
It does work to move the charge around the circuit by transferring energy
What is potential difference?
The work done in moving a unit of charge between 2 points
What equation links potential difference, work done and charge?
V = W/Q
What is 1 volt equal to?
1 joule per coulomb
Explain, using the water analogy, potential difference
The p.d. is like the pressure that’s forcing water along the pipe
Do you attach a voltmeter in series or parallel?
Parallel
Why do you attach a voltmeter in parallel?
Because it has a high internal resistance
Why do voltmeters have an almost infinite resistance?
So only a tiny current can flow through it because it only needs a small amount of current to measure the p.d.
What is resistance?
A measure of how difficult it is for current to flow
What is the p.d. and current if a component has a resistance of 1Ω?
A p.d. of 1V making a current of 1A flow through it
What equation links resistance, potential difference and current?
R = V/I
Who did most of the early work on resistance?
Georg Simon Ohm
What is Ohm’s Law?
A rule to predict how the current would change as the potential difference is increased for certain types of conductor
What are ohmic conductors?
Conductors that obey Ohm’s Law
What does Ohm’s Law state?
Provided the physical conditions remain constant, the current through an ohmic conductor is directly proportional to the p.d across it
Give 3 examples of ohmic conductors
Aluminium
Titanium
Gold
Give 2 examples of factors that would significantly effect the resistance
Temperature
Light levels
What does a shallower gradient of a characteristic I-V graph mean?
The greater the resistance of the component
What is a multimeter?
A measuring instrument that can be used to measure current, voltage or resistance
How do you set up a circuit to find the I-V characteristics of a component?
Set up a power pack and connect a variable resistor in series. Then connect the component and connect a voltmeter in parallel. Then connect an ammeter in series and connect back to the power pack
What is an ideal voltmeter?
A voltmeter with a assumed infinite resistance (so no current flows through them)
What is an ideal ammeter?
An ammeter with no resistance (so it will have no p.d. across them)
Describe the I-V characteristic for a filament lamp
A curve that starts steep but gets shallower as the voltage rises
What causes the resistance of a filament lamp to increase with current?
The current flowing through the lamp increases its temperature, which increases the resistance of the lamp
Why does resistance increase when the temperature increases?
When a current flows through a metal conductor, some of the electrical energy is transferred into heat energy and causes the metal to heat up. This extra heat energy causes the particles in the metal to vibrate more (increased lattice vibrations). These vibrations make it harder for the charge-carrying electrons to get through, so the current can’t flow as easily
Why do most resistors have a limit of how much current can flow through them?
Because an increasing current means an increase in temperature, which means an increase in resistance, which means the current decreases again
What are diodes made from?
Semi-conductors
What are diodes designed to do?
Let current flow in one direction only
What is forward bias for diodes?
The direction that the current is allowed to flow
What is the threshold voltage of diodes?
0.6V
What is the threshold voltage?
The voltage needed in the forward direction before they will conduct
In reverse bias, how much current can flow through a diode?
Very little as the resistance is extremely high
What is resistivity defined as?
The resistance of a 1m length of a material with a cross-sectional area of 1m^2
What is the difference between resistance and resistivity?
Resistance is a property of an object and it depends on the dimensions of the object. Resistivity is a property of the material
What 3 things does the resistance depend on?
Length (m)
Area (m^2)
Resistivity (Ωm)
What is resistivity measured in?
Ohm-metres
How does the length affect resistance?
The longer the wire, the more difficult it is to make a current flow through it
What is the relationship between resistance and the length of wire?
They are directly proportional
How does the cross-sectional area affect resistance?
The wider the wire, the easier it will be for the electrons to flow through it
What is resistivity a measure of?
How much a particular material resists current flow
What does resistivity depend on?
The structure of the material as well as environmental factors such as temperature and light intensity
What is the equation for resistivity?
ρ = (RA)/L
The lower the resistivity, the…?
… better it is at conducting electricity
What is one of the most commonly used semi-conductors?
Silicon
What is silicon used for?
Making electrical components such as diodes
What are semiconductors?
Are a group of materials that aren’t as good as conducting electricity as metals, as they have fewer charge-carriers available
What happens if energy is supplied to a semiconductor?
More charge carriers are released and the resistivity of the material decreases
What are semiconductors most commonly used as?
Sensors for detecting change in the environment
Name 3 common semiconductor components
Diodes
Thermistor
LDR
What is doping?
The process for which a semiconductors behaviour can be manipulated by the deliberate addition of impurities
Why do semiconductors become more conductive as energy is supplied?
In a semiconductor, very few atoms are ionised, and so very few electrons can move. However, as the semiconductor heats up, the covalent bonds (atoms sharing electrons, causing the electrons to be relatively immobile) break down, freeing the electrons
How do you ‘dope’ a semiconductor?
Ions are added in small quantities, giving the semiconductor a greater or lesser number of free electrons as required
What is a thermistor?
A component with resistance that depends on its temperature
What does the ‘NTC’ is NTC Thermistor stand for?
Negative Temperature Coefficient
What does NTC mean for the component?
Means as the temperature increases, the resistance decreases
How does a thermistor work?
As temperature increases, more energy is supplied to the electrons in the semiconductor, meaning they can escape. This means there are more charge-carriers available, so the resistance decreases
What happens to the resistance of an LDR as light intensity increases?
Resistance decreases
When investigating the resistance of a thermistor, what is a good way of controlling the temperature of the thermistor?
Put the thermistor in a water bath (if the thermistor is waterproof!!)
What are the 4 steps for the investigation for finding the resistance of a thermistor?
- Place thermistor in a beaker and pour boiling water into the beaker until it covers the thermistor
- Measure and record the temperature of the water using a digital thermometer, and record the current using an ammeter
- Continue to measure and record the temperature and current for every 5°C drop in temperature
- Use your recorded values of current and the p.d. to calculate resistance at each temperature and plot on graph
What is a control variable in the experiment for finding the resistance of a thermistor?
The potential difference across the thermistor
What results would you expect to see in the experiment for finding the resistance of an NTC thermistor?
As the temperature decreased, the resistance increases (therefore current decreases)
How can you lower the resistivity of materials such as metals?
By cooling them down
What is the critical temperature for a material to become a superconductor called?
Transitional temperature
What is a superconductor?
A material that, when cooled down to below a critical temperature, their resistivity disappears and so they have no resistance
What is an advantage of a material having no resistance?
In an electrical circuit, none of the electrical energy is wasted as heat
What is the transitional temperature of most normal conductors?
10 kelvin (-263°C)
What are 3 uses for superconductors?
- Power cables that transmit electricity with no loss of power
- Strong electromagnets
- Electrical circuits that work really fast with minimal energy loss
In the experiment for finding the resistivity of a wire, how do you calculate the cross-sectional area of the wire?
Measure the diameter of the wire in at least 3 different places along the wire using a micrometer. Then find the mean diameter and halve it to find the mean radius. Use equation A = πr^2
How do you set up the electrical circuit for the experiment to find the resistivity of a wire?
Connect to a power supply a switch, followed by a flying lead (wire with crocodile clips on end). Place the length of test wire next to a ruler, a then connect an ammeter in series. Connect a voltmeter in parallel to the test wire. The flying lead completes the circuit when connected to the wire at different points
What are the 5 steps for the experiment to find the resistivity of a wire?
- Attach the flying lead to the end of the test wire and measure the length of the test wire connected to the circuit
- Close the switch and measure the current and the p.d. across the test wire. Then open the switch and calculate the resistance
- Repeat this process at least one more time to calculate the mean resistance for that length of wire
- Reposition the flying lead and repeat steps 2 and 3 to get an average resistance for a range of different lengths of wire
- Plot a graph of average resistance against length and plot a line of best fit
In the experiment to find the resistivity of a wire, once you’ve plotted the graph of average resistance against length of wire, how do you find the resistivity of the wire?
Find the gradient of the graph and multiply this by the cross-sectional area of the wire
What are the 2 main sources of random error in the experiment to find the resistivity of a wire?
The temperature of the wire changing (which is why you open the switch once you’ve taken readings)
Measuring the length of the wire
What is power?
Rate of energy transfer
What is power measured in?
Watts
What is 1 watt equal to?
1 joule per second
What equation links power, energy and time?
P = E/T
What is the formula for power in electrical circuits?
P = IV
What is the power rating of an appliance?
The rate at which it transfers energy
What equation links potential difference, current and resistance?
V = I/R
What equation links power, potential difference and resistance?
P = V^2 / R
What equation links power, current and resistance?
P = I^2 x R
What equation links energy, current, potential difference and time?
E = IVt
What equation links energy, potential difference, resistance and time?
E = (V^2/R)t
What equation links energy, current, resistance and time?
E = I^2Rt
How do you convert kWh to J?
Multiply by 3.6x10^6
What causes resistance?
Electrons colliding with atoms and losing energy
In batteries, what is used to make electrons move?
Chemical energy
What is internal resistance?
The resistance created in a power source when electrons collide with atoms inside the power source and lose energy
What does internal resistance in a battery cause?
Causes the battery to heat up when used
What is load resistance?
The total resistance of all the components in the external circuit
What is load resistance also called?
External resistance
What does e.m.f stand for?
Electromotive force
What is e.m.f?
The amount of electrical energy the battery produces and transfers to each coulomb of charge
What is e.m.f measured in?
Volts (it’s not a force)
What equation links e.m.f, electrical energy and charge?
ϵ = E/Q
How do you show internal resistance in circuit diagrams?
As a tiny resistor in the battery
What is terminal potential difference?
The p.d. between the 2 terminals of a power supply. This is equal to e.m.f when there is no internal resistance
The terminal p.d. across the load resistance is the…?
… energy transferred when 1 coulomb of charge flows through the load resistance
What is lost volts?
The energy wasted per coulomb overcoming the internal resistance
What does conservation of energy tell us for electrical circuits?
Energy per coulomb supplied by the source = energy per coulomb transferred in load resistance + energy per coulomb wasted in internal resistance
What equation links e.m.f, current, load resistance and internal resistance?
ϵ = I (R+r)
R= Load resistance r= Internal resistance
What equation links e.m.f, terminal p.d., and lost volts?
ϵ = V + v
V= Terminal p.d. v= Lost volts
What equation links terminal p.d., e.m.f, current and resistance?
V = ϵ - IR
How do you set up the circuit for the experiment to measure the internal resistance and e.m.f?
Connect to a battery an ammeter (in series) and a voltmeter (in parallel). Connect a variable resistor on 3rd branch parallel to the battery and a switch in series between the 2nd and 3rd branch
What are the 4 steps for the experiment to measure the internal resistance and e.m.f?
- Set the variable resistor (load resistance) to its highest resistance. Then close the switch and record the current and p.d. across the circuit. Open the switch and close it again to get 2 more sets of results. Calculate the mean for both results
- Decrease the resistance of the variable resistor by a small amount and repeat step 1
- Repeat steps 1 and 2 until you have a mean set of results for 10 different load resistances
- Plot a V-I graph for your mean data and plot a line of best fit
Once you’ve plotted a V-I graph for the experiment to measure the internal resistance and e.m.f, how do you find the e.m.f and the internal resistance?
The intercept on the vertical axis is e.m.f, and the gradient is -r (internal resistance)
What does rearranging the equation V = ϵ - IR give you?
V = -RI + ϵ, which is in the form Y-MX+C for the V-I graph
What is conservation of charge?
As charge flows through a circuit, it doesn’t get used up or lost
Whatever current flows into a junction is the same as…?
… the current that flows out of the junction
What is Kirchhoff’s 1st Law?
The total current entering a junction = the total current leaving it
What is energy transferred to a charge known as?
e.m.f.
What is energy transferred from a charge known as?
p.d.
What is Kirchhoff’s 2nd Law?
The total e.m.f around a circuit = the sum of the p.d.s across each component
What is the equation for Kirchhoff’s 2nd Law?
ϵ = ΣIR
In a series circuit, what is current like at all points?
Current is the same, as there are no junctions
In a series circuit, what happens to the e.m.f?
It is split between component
ϵ = V1 + V2 + V3 +…
In a series circuit, what happens to the voltage?
It is split proportionally to the resistance, as V = IR
In a series circuit, what is the resistance like?
Rtotal = R1 + R2 + R3 +…
In a parallel circuit, what happens to the current?
It is split at each junction
I = I1 + I2 + I3 +…
In a parallel circuit, what is the p.d. like?
It is the same across all components
What is the equation for total resistance for parallel circuits?
1/Rtotal = 1/R1 + 1/R2 + 1/R3 +…
