Electricity Flashcards
How does conventional current flow?
Positive to negative
Current in a series circuit
Current is the same everywhere in the circuit
What is current?
The rate of flow of electrons around a circuit
Current and charge equation
Q=IT
Voltage and charge equation
V=W/Q
Kirchhoff’s 2nd law
In a series circuit the sum of the voltages of each component must equal the voltage across the power supply
EMF
Energy transferred by the battery per unit charge
Resistance in series
Rtotal=R1+R2+R3….
Ideal circuit
-Wires have no resistance
-Ammeters are an series and have no resistance
-Voltmeters should be in parallel and have infinite resistance
Voltage in parallel circuits
The same across the branches
Kirchoff’s 1st law
The current going into a junction must equal the current leaving the junction
Resistance in parallel circuits
1/Rtotal=1/R1 + 1/R2 + 1/R3 …
Ohmic conductor
A component that obeys Ohms law
Ohms law
The voltage across a component is directly proportional to the current flowing through it. Provided physical conditions stay constant
What is the gradient of an IV graph
1/R
How to check if a component obeys ohms law
-Reverse current direction by reversing connections on battery
-If line on graph remains straight then it’s an ohmic conductor because R is constant
Variable resistors
Long coil of wire current flows in one end and out through slider
Slider changes length of wire so current can flow through
Experiment to test if something is an ohmic conductor
-Use a variable resistor to change I and V
-Collect 7 sets of data
-Must reverse current flow at some point by flipping battery
-Collect results and plot a graph
Examples of non ohmic components
-Bulb
-Thermistor
-LDR
-Diode
Explain the shape of filament bulb IV graph
-Initially wire in bulb is cold with low resistance
-Electrons bump into atoms in the wire transferring energy heating up wire
-Lattice of atoms vibrate more so it’s harder for electrons to come through
-So R increases
Why do filament bulbs blow?
-Wire starts cold so R is low
-This allows very high current
-This heats wire up rapidly
-Quick change in temperature can break wire
Examples of semi conductors
Thermistor
LDR
Insulators
Very few charge carriers (electrons) available so current can’t flow easily
Semi conductors
Somewhere inbetween conductors and insulators
How do semiconductors work
If you supply energy to a semiconductor in the form of heat/light you can release extra charge carriers
So R will change depending on Light/Tenperature
Diodes
-Only lets current flow in one direction
-When voltage is higher than threshold (0.6 to 0.7 V)
Disadvantage of series circuit
If one element breaks none of the others will work
Why is it an advantage for a rechargeable battery to have a low internal resistance?
-The current will be higher
-Because resistance opposes current
Why is a filament lamp likely to fail when it’s switched on?
-It’s initially cold with a low resistance
-This allows for a large current at the start
-This causes a rapid change in temperature which can break the lamp
Practical use of diodes
Protects against current surges
Power equations
P=E/t
P=VI
P= I²R
P=V²/R
EMF equations
ℰ=I(R+r)
ℰ=V+Ir
Internal resistance for cells in series
Internal resistance adds together
Internal resistance for cells in parallel
Internal resistances don’t add together
Why does terminal voltage fall as current increases?
When the variable resistor has a high resistance it makes the current smaller.
It uses a larger share of the EMF
Potential divider equation
Vout= R2/(R1+R2) x Vin
Variable resistor vs potentiometer
Variable resistor:
-Can vary current but not as sensitively
-Cannot get 0
-Easier to connect
Potentiometer
-Can vary sensitively
-From 0V to EMF
-Harder to connect
.
.
Why does the resistance of an NTC thermistor decrease when temperature increases?
-As it heats up more charge carriers become available (electrons)
-The increased internal energy liberates electrons
-More charge can be carried through meaning the resistance has decreased
-More electrons is more significant than the additional vibration of lattice ions
What causes internal resistance inside a battery?
The electrons collide with atoms inside the battery
Superconductor
A component with no resistance below or at critical temperature
Uses of superconductors + reason
-Electromagnets
-Generators
-Very high magnetic field strength
-Less energy loss
Define potential divider
A set of 2 or more resistors connected across a voltage source used to split EMF
Explain why resistance increases as voltage increases
As voltage increases, more electrons move through the wire
More collisions between these electrons and ions in the lattice
Vibration of lattice ions increases
So number of collisions per second increases
Increasing resistance
