Electricity Flashcards
What is current?
It is defined as the number of coulombs transferred per second (rate of flow of charge).
It is the flow of electrons.
What is potential difference?
It is defined as the energy transferred per coulomb
What is resistance?
This describes how difficult electrons find it to move through a wire/component
What is power?
It is the rate at which electrical energy is supplied to a circuit.
What is the unit for charge?
Coulombs(C)
What is the unit for current?
Amps(A)
What is the unit for resistance?
Ohms
What is the unit for work done?
Joules
What is the unit for power?
Watts/joules
What is the equation that links charge, current and time?
Charge = current*time
Q=IT
What is the equation that links P.D, current and resistance?
P.D = current*resistance
V=IR
What is the equation that links P.D, work done and charge?
P.D = work done / charge
V=J/Q
What is the equation that links power, current and P.D?
Power = current*P.D
P=IV
What is the equation that links power, P.D and resistance?
Power = P.D^2/resistance
P=V^2/R
What is the equation that links power, current and resistance?
Power = current^2 * resistance P= I^2*R
What is Ohm’s law?
The current through an ohmic conductor is directly proportional to the voltage provided the physical conditions such as temperature remain the same
What factors can affect resistance?
Temperature
Light intensity
Type of wire
Length of wire
How does the I/V graph for an ohmic conductor look like?
Straight line through the origin
How does the I/V graph for a filament lamp look like?
It is a curve
Begins steep and becomes shallower as the current increases
Why does a filament lamp I/V graph become shallower as the current and voltage increase?
The temperature and resistance increase as the current flows through the lamp
Why does the resistance of a metal increase as the temperature increases?
The atoms in the metal gain energy from the rising temperature
The atoms vibrate faster
This makes it harder for electrons to move past the metal atoms
What are semiconductors?
Semiconductors are conductors that do not conduct electricity as well as metals as they have fewer charge carriers
What happens when energy is supplied to a semiconductor?
More charge carriers are released
This makes them excellent sensors for detecting changes in their environment
Give two examples of semiconductors
- Thermistors
- Diodes
What is a thermistor?
It is a resistor
The resistance depends on temperature
What happens when the temperature increases in a thermistor?
As the temperature increases, the resistance decreases
Why does the resistance decrease as the temperature increases in a thermistor?
As the temperature increases, more electrons have enough energy to escape from their atoms
This means that there are more charge carriers available
In which direction does the current flow in a diode?
The current flows in the direction that the triangle in the symbol points
What is the threshold voltage for most diodes in the forward bias?
O.6V
What happens to the current in a diode after the threshold voltage?
The current rapidly increases
What are the three factors that affect resistance of a material?
- Length
- Area
- Resistivity
How does length affect resistance?
The longer the wire, the further the current has to travel and the more difficult it is to flow a current through the wire
How does area affect resistance?
The wider the area, the easier it is for current to pass through
How does resistivity affect resistance?
The lower the resistivity, the more current can pass through and the lower the resistance
What is the definition of resistivity?
The resistivity of a material is defined as the resistance of a 1m length with a 1m^2 cross-sectional area.
Measured in ohms-meters
What is the equation that is used to find resistivity?
Resistivity= (resistance*area)/length
Why are metals good conductors?
They have low resistivity
What happens when there is low resistivity?
There is a high current flow
What are superconductors?
Conductors which have zero conductivity at and below the critical temperature
How are superconductors made?
Superconductors are cooled to their critical temperature
How can the critical temperature be increased from -273 to -123?
Using new materials called cuprates(copper oxide)
Why is no energy lost as heat in superconductors?
There is no resistance and therefore no potential difference
What are the uses for superconductors?
1-power cables that transmit electricity without any loss of power
2-high power electromagnets(MRI scanners, Maglev trains)
3-electronic circuits
How do you find the resistivity of a wire?
- Use a micrometre to measure the diameter of the wire
- Use this length to find the radius and then calculate the cross-sectional area
- Clamp the test wire to a ruler with the circuit attached to the wire where the ruler reads zero
- Attach the flying lead to the test wire
- Record the length of the test wire connected to the circuit, the voltmeter reading and ammeter reading
- Calculate the resistance using the voltmeter and ammeter readings
How can you increase the accuracy of the diameter measured?
Measuring the diameter at three different points along the wire and calculate an average
When conducting the experiment, why must the temperature be kept constant?
The resistivity of a material depends on its temperature
How can you try to keep the temperature of the wire constant?
Having only small currents flow through the wire or else the current will cause an increase in temperature leading to invalid results and random errors
What happens to the current in a series circuit?
The current is the same throughout the circuit
What happens to the voltage in a series circuit?
It is shared between all the components attached
What happens to the current in a parallel circuit?
The total current is the sum of the currents throughout each branch
What happens to the voltage in a parallel circuit?
The voltage supplied is the same throughout each branch
How do you calculate the total resistance in series?
Total R = R1+R2…
How do you calculate the total resistance in parallel?
1/total R=1/R1 + 1/R2…..
What is EMF?
Electromotive force
the inherent voltage of the cell
How do you calculate EMF?
E= I(R+r)
What does ‘r’ mean?
internal resistance
What does ‘R’ mean?
external resistance
Why is the voltage not always equal to the EMF?
this is because of the internal resistance in the cell
How do you calculate the voltage in a circuit?
V=EMF-P.D(lost in the source)
what happens when the circuit is open?
There is no current flow which means that there is no P.D
Therefore the voltage = EMF
What happens when the P.D increases?
the current decreases and the resistance increases
What is internal resistance?
The opposition to the flow of charge through the source
What happens to the electrical energy?
it is dissipated inside when the charge flows
How do you calculate the EMF using electrical energy?
EMF = electrical energy/charge
How do you calculate the lost P.D?
Lost P.D = EMF - P.D(throughout the circuit)
How do you calculate the power supplied by the cell?
IE=I^2R + I^2r
Power = power to R + power wasted in the cell
When is max power delivered?
Max power is delivered when the resistance = internal resistance
How do you calculate the EMF in multiple cells in series?
total EMF = EMF1+ EMF2…..
How do you calculate the EMF in multiple cells in parallel?
total EMF = EMF1 = EMF2=EMF3…..
How do you find the emf and internal resistance using a graph?
EMF = y-intercept
internal resistance = gradient(-r)
y=mx+c
V= -rI+EMF
How do you calculate the total energy in a circuit?
Energy = VIt Energy = P.D *current*time(secs)
What is a potential divider?
A circuit with a voltage source and resistors in series
How do you calculate the voltage out of the circuit?
Vout = (Vin * R2)/R1+R2
What is 1 Coulomb(C)?
It is defined as the amount of charge that passes in 1 second if the current is 1 ampere(A)
What is a volt?
The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component
What is an ohm?
A component has a resistance of 1ohm if a potential difference of 1V makes a current of 1A flow through
What is resistivity of a material?
The resistivity o a material is defined as the material of a 1m length with a 1m^2 cross-sectional area
