5: Electricity Flashcards
What is terminal p.d?
The voltage directly across a power supply (with internal resistance)
What are the equations for emf?
emf=current multiplied by the total resistance
(ε=I(R+r))
emf=energy supplied/charge
Where:
ε=emf
I=current
R=circuit resistance
r=internal resistance
What are two applications of superconductors?
-Used in power cables to reduce energy loss via heating to zero
-Used in strong magnets
What is the emf?
-The electrical energy transferred by a power supply per unit charge
-The work done required to push a charge around a circuit
What is Kirchhoff’s first law?
-The current into a junction must equal the current leaving that junction
What is Kirchhoff’s second law?
-The sum of all voltages in a loop equals zero
What is a superconductor?
-A material which has zero resistance at or below a critical temperature
How does resistance change for a Light Dependent Resistor?
-As light intensity increase the resistance decreases
What is a potential divider?
-A circuit that turns a changing resistance into a change potential difference
How is charge conserved in a circuit?
-The current is equal at each point in a series circuit
How is energy conserved in a circuit?
-Total voltage input equals the sum of voltages across components
How do voltages sum when cells are connected in series?
-The voltages add together
How do voltages sum when cells are connected in parallel?
-The overall voltage remains the same
How do currents sum when cells are connected in series?
-There is no change in overall current
How do currents sum when cells are connected in parallel?
-The current will increase as the total resistance decreases
How do resistances sum when cells are connected in series?
-The resistances add together
How do resistances sum when cells are connected in parallel?
-Combining cells in parallel will decrease the overall resistance
Why does increasing current increase resistance for a filament lamp?
-As current flows electrical energy is converted to thermal energy
-This causes the metal ions to vibrate more
-Which in turn further impedes the movement of electrons thus making it harder of current to flow
How would you determine the resistivity of a metal (experiment)?
-Measure the diameter with a micrometer in several places and calculate a mean
-Calculate the cross-sectional area by π*(d^2)/4
-Vary the wire length and record voltage with a voltmeter and current with an ammeter
-Repeat this for multiple lengths of wire and calculate the resistance for each length
-Repeat the experiment twice more to calculate mean resistances
-Plot a graph of resistance against length
-The resistivity is the gradient multiplied by the area
What are the power equations?
P=V.I
P=(V^2)/R
P=(I^2)*R
W=P.T
Where:
W=work done
P=power
V=voltage
I=current
R=resistance
How do resistors sum in a series cirucit?
-RT=R1+R2…
How do resistors sum in a parallel circuit?
-1/RT=1/R1+1/R2…
How does resistance change for a thermistor?
-As temperature increase the resistance decreases
How are thermistors used in temperature sensors?
-As temperature of thermistor increases the resistance will decrease
-This will decrease the total resistance and increase the current
-This will increase the voltage across the fixed resistor and decrease the voltage across the thermistor
-This allows you to measure a voltage for the thermistor and when the voltage is too low you will know the temperature is too high
How do you calculate resistivity?
-Resistivity equals (Resistance*Area)/Length
How much resistance does an ideal voltmeter have?
-Infinite resistance
How much resistance does an ideal ammeter have?
-Zero resistance
What is ohm’s law
-For a constant temperature the current is directly proportional to the potential difference
What is resistance?
-A measure of how difficult it is for current to flow through a component
-R=V/I
Where:
V=voltage
I=current
R=resistance
-Measure in ohms
What is potential difference?
-The work done per unit charge
-V=W/Q
Where:
V=voltage
W=work done
Q=charge
-Measure in volts
What is current?
-The rate of flow of charge
-I=ΔQ/Δt
Where:
I=current
Q=charge
t=time
-Measured in amps
Information that can’t be put on flashcards
-Graph for filament lamp
-Graph for semiconductor diode
-Graph for Ohmic conductor
-Circuits
