Electricity Flashcards
What is electric current?
What is it units?
The rate of flow of charge. Measured in amperes
What is potential difference?
The work done per unit charge between 2 points in a circuit
What is resistance?
Opposition to current, for a given potential difference.
How difficult it is for current to flow through a circuit
What is meant by an ohmic conductor
A conductor which obeys ohm law
Meaning that current is directly proportional to potential difference provided the physical conditions remain constant (temperature).
Draw the iv graph for an ohmic conductor.
Explain it
Draw the iv graph for a filament bulb
Explain it.
Due to increasing resistance from higher temperatures causing metal ions to vibrate more, causing a greater number to collide with the flow of electrons
Why does the current increasing on a a filament lamp cause an increase in resistance
As current flows through the lamp
Electrical energy is converted to heat energy
Metal ions therefore vibrate with increased amplitudes
Which impedes the movement of electrons through the lamp as they have a higher chance of collision with these metal ions, showing an increase in resistance
What is a diode
An electrical appliance that only allows current in a specific direction
Draw the IV graph of a diode
Why are voltmeters assumed to have infinite resistance
Current takes the path of least resistance
When applied in parallel to the original circuit no current will flow through the voltmeters branch
So conditions are exactly the same before the addition of the voltmeter
Why must you assume the ammeter has zero resistance
So no power or potential difference is disapated in the ammeter
So that conditions for the rest of the components are exactly the same as before
What is a LDR?
A semi conductor that varies its resistance depending on the intensity of light incident
As light intensity increases its resistance decrease
How does a thermistor work
What is the difference between a NCR and a PCR resistors
A semi conductor that is sensitive to temperature
NCR’s resistance decreases as temperature increases
PCR’s resistance increases as temperature increases
What is resistivity
Resistance of a conductor of a cross sectional area per unit length.
The resistance of a 1m cylinder with a cross sectional area of 1M^2
Describe an experiment to determine the resistivity of a material
1) Measure the diameter of the wire with a vernier calliper at multiple places along the wire and take an average
2) Set up the circuit shown
3) Vary the wire length and record the voltage and current for each length
4) Use R = V/I to work out the resistance
5)Plot a graph of resistance against wire length
6) Gradient = Resistivity/ Cross sectional area
7)Resistivity = Gradient * cross sectional area
What is a superconductor
A material that has a resistivity of zero at or below a critical temperature
How do you find the total resistance of a series circuit
R1+R2+R3…… = RT
If 6 cells have a voltage of 5v, arranged in parallel
What is the voltage in the circuit
5 volts
How does the current vary between each component in a series circuit
The current through all of the components is the same
What is Kirchoffs first law
The sum of the currents entering a junction is the same as the sum of the currents exiting a junction
So charge is conserved
What is Kirchoffs 2nd law
The sum of the EMF’s in a closed loop is equal to the sum of the potential differences.
So energy is conserved
What is power?
The rate of which work is done.
What is the purpose of a potential divider
To provide a constant specific potential difference to an external circuit
What is the equation for a potential divider?
Where R2 is the resistor that you want to measure the resistance of.
What is emf?
Electromotive force
The energy transferred from other forms to electrical potential per unit charge
V = -1 * internal resistance * current + emf
The temperature is increased in the following circuit
Explain what happens to the output potential difference
Resistance of thermistor decreases
Output PD decreases, as the resistance of the parallel combination decreases.
Resulting in a less proportion of the potential difference across it.
Explain why the reading on the voltmeter decreases as the brightness of the lamp increases
For the brightness of the lamp to increase, currents must increase
Higher current means that the variable resistor decreases in resistance
This means that the ratio of internal to external resistance increases
A higher proportion of emf is dissipated in the internal resistance of the cell / increase in lost volts
So PD across external circuit decreases
Explain the effect on the brightness of the lamp if another cell is connected in parallel to the other cell
Emf of circuit is identical
Parallel combination of the Cells Internal resistances means a lower total internal resistance
Effective internal resistance is a smaller proportion of the total resistance.
Higher proportion of the potential difference is dissipated across the external circuit.
