Electricity Flashcards
Define the kilowatt hour (kWh)?
A unit of energy equal to 3.6MJ or 1kW for 1hour.
What is meant by a ‘230V, 25W’ for an appliance?
When connected to the 230V supply, the power (energy per second) is 25W.
Define intensity?
Intensity is the incident energy per unit area per second.
What is a current?
A flow of charge.
In a metal due to flow of electrons.
In an electrolyte due to flow of +ve and -ve ions.
Direction of conventional current?
Positive to negative
Direction of electron flow?
Negative to positive
What is a coulomb?
One coulomb is the charge which passes a point when a current of 1A flows for 1s
Charge =
Current x time
What is the elementary charge?
Minimum charge something can have - 1.6x10^-19C
What is does Kirchhoffs first law state?
That the sum of the current entering any point in a circuit is equal to the sum of the currents leaving that same point.
Current in a conductor:
I = ?
Anev
A is area
n is the number density
e is elementary charge
v is mean drift velocity
How should you attach an ammeter?
In series
How should you connect a voltmeter?
In parallel
What is 2.00mm^2 in metres?
2x10^-6
What is the shape of a diode?
Triangle with flat line on pointy end
If a thermistor heats up, what happens to the resistance through it?
Temperature rises, resistance falls, current rises
After x amount of time, resistance will become constant because thermal equilibrium will be reached
How do LDR’s work?
As light intensity increases, resistance falls
Define emf?
The energy transferred per unit charge when another type of energy is converted into electrical energy
How do you work out the power dissipated by a resistor?
Current (through resistor) x voltage (across resistor)
What is mean drift velocity?
The average velocity that an electron attains when an electric current flows.
Giving examples, what are the differences between conductors, insulators and semi conductors?
Conductors (eg. Copper) contain a high number density of conducting electrons.
Insulators (eg. Rubber) contain very few or no conducting a electrons
Semiconductors (eg. Silicon) materials that have increasing conductivity with temperature, so a medium number density
Define potential difference?
The electrical energy transferred per unit charge when electrical energy is converted into another form of energy
Define the volt?
One volt is 1joule per coulomb
W=
Voltage x charge
W is work done
Define resistance?
The opposition to a flow of charge
Define the ohm?
The resistance when 1volt produces 1amp of current
What is ohms law and how can you tell if it is obeyed in a circuit?
Electrical current is proportional to voltage.
Straight line graph through origin
Benefits of LEDs?
Long lifespan
Energy efficient
Only works one way round (positively biased)
Has a threshold voltage
Lots of different colours that require different voltages
What happens to the resistance of s material when it’s temperature increases?
Resistance increases
How is an NTC thermistor affected by temperature?
When temperature increases, resistance decreases.
How does a fuse work?
A fuse is fitted in an electrical circuit to protect wiring from excessive current (and therefore helps to prevent excessive heat and fires).
Inside the fuse cartridge is a thin wire which gets hot and melts when the current gets too high, thus breaking the circuit. Each fuse has a max current it will permit.
What is Kirchhoffs second law?
The sum of all of the emfs around a circuit are equal to the sum of all the pds around the same circuit.
What is terminal pd?
The pd across the external resistor connected to an emf source.
