5. Electricity Flashcards
What is electrical current? What is its units?
The rate of flow of charge. It is measure in Amperes
What is potential difference?
The work done moving a unit charge between 2 points in a circuit
V = W/Q
What is resistance?
How difficult it is for current to flow through a circuit
R= V/I
What is meant by an ohmic conductor?
A conductor that obeys Ohm’s law, meaning the current is directly proportional to the potential difference providing physical conditions such as temperature stay constant
How can you measure the current in a circuit?
By using an ammeter in series with the circuit
How do you measure the potential difference across a component?
By connecting a voltmeter parallel to the component being measured
What does the gradient of a current-potential difference graph represent?
1/R
Why does current increasing on a filament lamp increase resistance?
As current flows through the lamp , electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions
What is a diode?
An appliance that allows current to travel in one direction
Unless stated in the question should you assume voltmeters to have 0 or infinite resistance?
You should assume they have infinite resistance. Current takes the path of least resistance so if the voltmeter is connected in parallel with a component all current should flow through the component and no the voltmeter
Why should you assume that the ammeter has 0 resistance unless stated otherwise?
This assumption means there would be no potential difference across it so no energy lost : ; does not affect circuit
What is a Light Dependent Resister (LDR)?
A semiconductor that is sensitive to light so as light increases resistance decreases
How does a thermistor work?
Similar to LDR but as temperature increases resistance decreases
What is resistivity?
p = RA/L
p = resistivity (ohm m) R = resistance (ohm)
A = cross sectional area (m^2) L = length(m)
Describe an experiment to determine the resistivity of a metal
1) Measure the diameter with a micrometer of the wire and workout the cross sectional area
2) Set up the circuit with an ammeter in series and a voltmeter parallel to the wire
3) Vary the wire length by a set amount each time and record the voltage and current for each length
4) Use R = V/I to work out resistance
5) Plot a graph of resistance against wire length
6) Gradient = resistivity / cross sectional area
7) So resistivity = cross sectional area x gradient
What is a superconductor?
A material with a resistance of zero at or below a critical temperature. The critical temperature is an inherit property of the material
How do you find the total resistance in a series circuit?
Add the individual resistances of the components
How does the current vary between each component in a series circuit?
The current through all of the components is the same so current does not vary
Is the current in parallel components the same?
No , each branch of parallel components can carry different currents through them according to Kirchhoff’s first law
What is Kirchhoff’s first law?
All of the current going into a junction is equal to the total current leaving the junction
What is Kirchhoff’s second law?
For any path (loop) of the circuit, the sum of all the potential differences must be equal to the total emf of the circuit
What happens when two cells connect negative to negative in series?
They cancel out
What is power?
The rate of energy transfer
Give an equation for power in terms of current and voltage
P = VxI
What is the purpose of a potential divider?
1) To provide variable potential difference
2) To provide a constant specific potential difference
What is emf?
Electromotive force: the electrical energy transferred by a power supply per unit charge
What are two applications of superconductors?
1) Power cables which would reduce the energy lost through heat to zero through transmission
2) Strong magnetic fields that do not need a constant power source
How do you calculate the total resistance of resistors in parallel?
1/R total= 1/R1 + 1/R2 + 1/R3+……+ 1/Rn
