Section 6 - Electricity Flashcards
What is electric current?
The rate of flow of charge - amps
What is potential difference?
The work done moving a unit charge between 2 points in a circuit
What is resistance?
- A measure of how difficult it is for current to flow through an appliance
- A component has a resistance of 1 Ω if 1 A flows through it when a p.d. of 1 V is applied across it
What is meant by an ohmic conductor?
A conductor that obey’s Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant
How can you measure the current in a circuit?
An ammeter connected in series with the component
How can you measure potential difference across a component?
Using a woltmeter, connected in parallel across the component being mesured
What does the graph of a current-potential difference graph represent?
Rate of change of current with respect to voltage
What does the current-voltage graph of an ohmic conductor look like?
The line has a constant gradient and passes through the origin - voltage is directly proportional to current
What does a V-I graph look like?
A reflection in the line y=x of an I-V graph
How can you tell that an appliance has a higher resistance than another using I-V graphs?
- Shallower gradient
- A higher voltage is required for the same change in current
What does the I-V graph for a filament bulb look like?
An S shape - as the current increases the resistance also increases - a big increase in the voltage produces only a small increase in current
Why does the current increasing on a filament lamp cause an increase in the resistance?
- As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude
- This impedes the movement of electrons through the lamp as they collide with the ions (resisitance has increased)
What is a diode?
- An appliance that only allows current to flow in one direction - forward bias
- Most require a threshold voltage
- In reverse bias, the resistance of the diode is very high and the current that flows is very tiny
Unless stated in the question, should you assume that voltmeters have zero resistance or infinite resistance? Why?
- Infinite resistance
- Current takes the path of least resistance so, if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through it and all the current will flow through the appliance
Why should you assume that an ammeter has zero resistance unles otherwise stated?
There would be 0 potential difference across the ammeter and no energy is lost across it; it does not affect the circuit
What is a light dependent resistor?
- A semiconductor that is sensitive to light
- LURD - as the light intensity increases, its resistance decreases
How does a thermistor work?
- TURD - as the temperature increases, resistance decreases
- They have a negative temperature coefficient
What is resisitivity?
The resistance of a 1m cylinder with a cross sectional area of 1m^2 - an inherent property of a material
Measured in Ωm
What 3 things determine resistance?
- Length - the longer the wire, the more difficult it is to make current flow
- Area - the wider the wire, the easier it will be for the electrons to pass along it
- Resisitivity - the structure of the material may make it easy or difficult for charge to flow - depends on environmental factors as well such as temperature and light intensity
Describe an experiment to determine the resistivity of a metal
- Measure the diameter of the wire with a micrometer, then calculatet he cross-sectional area = π(d/2)^2
- Set up the circuit
- Vary the wire length and record the voltage and current for each length
- Use R=V/I to work out the resistance
- Plot a graph of resistance against wire length
- The gradient - resistivity/cross sectional area
- Resistivity = gradient x cross sectional area
What is a superconductor?
- A material that has a resisitivity of zero at or below a critical temperature
- The critical temperature is an inherent property of the material
How do you find the total resistance in a series circuit?
Add the individual resistances of each component
How do you find the total resistance in a parallel circuit?
1/Rtot = 1/R1 + 1/R2 +1/R3….
How does the current vary between each component of a series circuit?
The current through all of the components is the same so the current doesn’t vary
Is the current in parallel components the same?
Each branch can have different currents throught them according to Kirchoff’s first law
What is Kirchoff’s first law?
All of the current going into a junction is equal to the current leaving the junction
What is Kirchoff’s second law?
For any path (loop) of a circuit, the sum of all of the potential differences must equal the total emf of the circuit
In a series circuit, if 2 cells are connected negative to negative, would their emf add up or cancel out?
They will cancel out
εtotal = ε1 - ε2
What is power?
The rate of energy transfer in Watts or joules per second
What is the purpose of a potential divider?
- To provide variable potential difference
- To provide a constant specific potential difference
What is emf?
x amount of energy is transferred from chemical energy to electrical energy per coulomb of charge
How can you use a graph to find the emf and internal resistance?
- Rearrange ε=I(R+r) into the form y=mx+c
- V = -rI + ε
What are 2 applications of superconductors?
- Power cables - would reduce energy loss through heating to zero during transmission
- Strong magnetic fields - would not require a constant power source - these could be used in maglev trains, where there would be no friction between the train and rail, and in certain medical applications
What are the emf equations?
ε = I(R+r)
ε = V + v
V = ε - v
V = ε - Ir
V - terminal p.d.
v- lost volts
R - load resistance
r - internal resistance
How can you calculate the emf in series vs parallel?
- Series - add the individual emfs of the cells
- Parallel - every cell has the same emf
What is a use of potential dividers?
To callibrate voltmeters which have a very high resistance
What is a potentiometer?
- Uses a variable resistor to give a variable voltage
- Used if you want to be able to continuously change a voltage, like in the volume control of a stereo
If the question doesn’t mention internal resisitance being negligible what should you probably do immediately?
Write the equation ε = I(R+r)
and find the link to it
Why when the voltmeter is moved across a different resistor in the circuit does the sum of the separate potential differences in the voltmeters not equal to the emf of the battery?
- Current in circuit changes
- Because resistance in the circuit changes
What is a disadvantage of using a high order to determine d or N?
Maximum is wider so less defined
How can you make a component function for a longer time by adding more cells?
- Put cells in parallel
- More energy stored in each cell
- Doesn’t increase terminal p.d.
Why would a fixed resisitor be used in a circuit?
- To limit maximum current
- To prevent overheating
What’s a difficulty of using a superconductor?
- Must be kept below the critical temperature
- Hard to maintain such low temperatures
How do you calculate the emf and internal resistance of a power supply?
- Add a voltmeter in parallel across the power supply and an ammeter in series
- Read the ammeter and voltmeter and take measurements
- Plot a graph of V against I
- emf is the y intercept
- -r is the gradient
- Write emf and rearrange into y=mx+c to show this
