4.2 Energy, Power And Resistance Flashcards
Define emf.
Of a supply, the energy gained per unit charge by charges passing through the supply, when a form of energy is transferred to electrical energy carried by the charges. Measured in v or jc
Define pd.
Measured across a component, energy transferred per unit charge by the charges passing through the component.
Define Resistivity.
The resistance offered by a unit length of wire made of a material having a cross sectional area of one unit.
=RA/l and define terms.
Define power.
The rate at which work is done.
Define electron drift velocity.
The average displacement travelled of the electron along the wire per second. On average they move slowly in one direction through the metal lattice, when a pd is applied because they collide constantly.
Define elementary charge.
Charge on a single electron, -1.6x10^-19
Conventional current
The direction of flow of current from positive to negative, opposite to the flow of electrons, original idea behind flow of charge.
Diode
Allows current to only flow in one direction
Forward bias
Forward bias is when current flows in the direction allowed by the diode.
Reverse bias
Flow of current against the direction of the diode
Internal resistance
The resistance within a voltage source that causes a lower volte supply that expected when current flows
Current
Rate of flow of charge, the charge flowing through a given point per second
What is Kirchhoff’s first law
As charge is always conserved, the sum of charges flowing into a junction must be equal to the sum of the charge leaving it, the same is true for current
Equation for drift velocity and factors in equation?
I=nAve
Conductors
Larger number of free charger carriers per unit volume
Insulators
Very few free charge carriers per unit volume
Ohms law
The current through a conductor is directly proportional to the potential difference across it, provided the temperature is constant
Factors that affect electrical resistance
- Material of wire
- Length
- Cross sectional area
- Temperature of wire
How does length of wire affect resistance?
p.d is more spread amongst electrons
How does cross sectional area affect resistance?
More electrons to flow at same drift velocity
How does temperature of wire affect resistance?
Resisting ions have more energy to vibrate
I-V characteristics of a resistor
Directly proportional
I-V characteristics of a filament lamp
Low current= ohmic conductor
High current, resistance increases due to more collisions and higher temperature
I-V characteristics of a diode
Require a threshold voltage for conduction to occur, current only in forward bias.
Define resistivity.
The measure of the resistance of a material for its cross-sectional area per unit length of wire.
Equation for resistivity
RA/I
Define temperature coefficient.
Value representing the relationship between the change in resistance and change in temperature of a conductor/semi conductor
Define electrical power
Rate at which energy is converted to or from electrical energy to or from another form
Power equations
Power = work done /time Power = current^2 x resistance Power = voltage^2 / resistance
What is conservation of charge?
the law stating that charge can’t be created nor destroyed, it can only be transferred and the total will always be constant
What is a coulomb?
Unit of charge, the current that pass through a point over a given amount of time. 1 amp for 1 second
Define resistance:
The potential difference across a component per current through it
Kirchoff’s second law
The sum of the EMFs in a closed loop circuit is equal to the sum of the pds, hence energy is conserved.
Define watt
Joule per second
Define electron drift velocity
The average displacement travelled of the electrons along the wire per second
On average, they move slowly in one direction through the metal
Because they collide constantly
