Electricity Flashcards
What is meant by superconductivity and what are the conditions for a material to
become superconducting
Zero resistivity
As resistivity decreases with temperature a material becomes superconducting when you reach the critical transition temperature
What is current
Rate of flow of charge
What is the charge of a single electron
1.6x10-19C
What is the unit of charge
Coulombs
What is potential difference
The work done per unit of charge
How do you measure the potential difference
Using a voltmeter, this has to be placed in parallel with the component being measured
What is resistance measured in
Ohms
What does ohms law state
the current through an ohmic conductor is directly proportional to the potential difference across it as long as the physical conditions such as temperature around it stay constant
What does an IV graph show
how current flowing through a component changes as the potential difference is increased, the graph follows ohms law.
The shallower the gradient – the greater the resistance of the component
What assumption can you make about voltmeters and ammeters
- voltmeters are assumed to have infinite resistance so no current can flow through them and ammeters are assumed to have no resistance so potential difference will not flow through them
What are filament lamps and explain the shape of the graph
- filament is a thin coil of metal wire
- when a current flows through the filament in a filament lamp some of the electrical energy is converted into heat energy and causes the metal to heat up. The extra energy causes particles in the metal filament to vibrate more so it is harder for the charge carrying electrons to pass through the resistor
- the current cant flow as easily and resistance increases
Graph appears like two opposite curves
What is a diode
- designed to let the current flow only in one direction
- most diodes require a voltage of 0.6 volts before they will conduct known as the threshold voltage
- in reverse bias the resistance of the diode is very high and current that flows is very small
What is resistivity
The resistance of a 1m length with a 1m ^2 cross-sectional area.
measured in ohm-metres
What three factors effect the amount of current that passes through a wire
- Length – the longer the wire the more difficult it Is to make a current flow through it
- Area – The wider the wire the easier it will be for electrons to pass along it
- Resistivity – measure of how much a particular material resists a current flow
– for example the type of the material and environmental factors
What are semi-conductors
– group of materials that aren’t as good at conducting electricity as metals because they have far fewer electrons available. However when energy is supplied more electrons are released and the resistivity of the material decreases . Examples of this is a thermistor and a diode
What is a thermistor
component with resistance that depends on its temperature
What is the NTC
negative temperature coefficient – resistance decreases as temperature increases
- Warming a thermistor means more electrons are available, so the resistance is lower. This makes them really good temperature detectors
How could you do an experiment to investigate the resistance of a thermistor
- Place thermistor in a beaker and pour enough boiling water into the beaker to cover the thermistor
- Measure and record the temperature of the water using a digital thermometer and current through a circuit – the potential difference across the circuit must be kept constant
- Continue to record the current and temperature for every 5 degree drops in temperature
- Use recorded values for current and potential difference to calculate the resistance of the thermistor at each temperature
- As temperature decreases you should find resistance increases
What is power
Defined as the rate of energy transfer
What is the unit of measurement for power
watts
What is e.m.f
- The amount of electrical energy the battery produces and transfers to each coulomb of charge is called its electromotive force – measured in volts
explain internal resistance
resistance comes from electrons colliding with atom’s and losing electrons
- in a battery, chemical energy Is used to make electrons move and as they move they collide with atoms inside the battery – so batteries must have resistance.
- This is called internal resistance and is what makes batteries heat up when they are used
- load resistance of all the components in the external circuit.
What is Kirchoffs first law
The total current entering a junction = the total current leaving it
What is Kirchoffs second law
The total sum of e.m.f around a series circuit=the sum of p.d.s across each component- fundamentally a statement of the conservation of energy
What is a potential divider
A combination of resistors in series connected across a voltage source (to produce a
required pd)
Explain the graph for a diode
A diode only lets the current flow in one direction.
When the diode is facing the direction of the current flow it is said to be in forward bias. Even in forward bias it will not conduct until a voltage of around 0.6 volts has passed across it.
Called the switch on voltage
After this switch on voltage is reached, resistance decreases as current rises.
What is a voltage drop
A voltage drop means the charge has lost energy
What is fundamental about the potential difference in a series circuit
V= V1 + V2 ETC.
How can we measure internal resistance
- Using an oscilloscope, as they have effectively and
infinite resistance
-graphical method
How can we measure internal resistance using the graphical method
- If we replace R with a variable resistor we can get a
range of results for V and I - This gives us a V as our Y variable and I as our X
variable - compare this with Y=mx+c we get a straight line plot of V vs I with a gradient of -r (the internal resistance) and a Y intercept of E
How does resistance relate in series circuits
R=R1+R2+R3
define resistance
Resistance is defined as the ratio of the voltage across a component to the current through it.
define internal resistance
The internal resistance is the opposition to current flow charge experiences inside a cell
Under what conditions are the emf and terminal p.d equal
When no current is drawn from the cell
What fundamental laws are Kirchoffs laws based upon
conservation of charge and energy
What is meant by a non ohmic conductor
no resistance
define e.m.f
work (done)/energy (supplied) per unit charge (by battery) !
1 (or pd across terminals when no current passing through cell or open
circuit)
State and explain why it is important for car batteries to have a very low internal resistance
need large current/power to start the car ! (or current too low)
2 internal resistance limits the current/wastes power(or energy)/reduces
terminal pd/increases lost volts !
