ELECTRICITY A Flashcards
I, V, R.
Define Current, Voltage and Resistance in words.
Now define 1amp, 1volt, and 1ohm.
Definitions of current, voltage and resistance
- Current is the rate of flow of charge
- Voltage is the work done per unit charge
- Resistance is the ratio of the voltage across a component to the current through it
Definitions of 1A, 1V, and 1ohm.
1A: when 1 coulomb of charge passes a point per second.
1V: the electrical potential difference between two points when 1 Joule of work is required to move 1 C of charge between those two points.
1ohm: the resistance of a component when 1A of current flows through it when 1V of p.d is applied across it.
State KCL and KVL.
- KCL - sum of currents in = sum of currents ou
- KVL - the sum of the voltage changes across any closed loop = 0
I-V graphs of an ohmic conductor, a filament lamp, an NTC thermistor, a diode
Ohmic conductor - Voltage across componenent is directly proportional to the current flowing through, straight line through origin
Filament lamp - as voltage increases, current increases, so temp increases, so the energy of the metal ions increases, so they vibrate with greater amplitude, so more frequent collisions between e- and metal ions, so greater resistance. Since I/V decreases, graph curves down.
NTC thermistor - as voltage increases, current increases, so temp increases, so more e-s released in to the metal lattice, so more charge carriers to carry a greater current for the same voltage, so I/V increases and the graph curves up
Diode - in the negative p.d direction, reverse bias, so high resistance and little current flows.
In the positive p.d direction, forward bias, below threshold voltage, high resistance and little current, above threshold voltage, low resistance and high current.
Resistivity.
Define resistivity.
Give it’s equation. V constant, relationship between R and L.
How does one experimentally determine resistivity?
Resistivity is defined as the resistance of a uniform wire of cross-sectional area 1m^2 with a length of 1m, for a specific material.
rho = RA/L
rearrange to (rho/A) * L = R
Plot R against L, gradient is rho/A.
Superconductors.
What is a superconductor?
Give 3 applications of a superconductor and explain each one.
What are the limitations of superconductors?
A superconductor is a material which has 0 resistivity at or below the critical temperature.
3 applications:
-since superconductors can carry a huge current without overheating, they can act as very strong electromagnets and produce strong magnetic fields, so
Maglev trains and MRI machines.
-since superconductors have no resistance at or below Tc, no energy losses due to Joule heating
Superconducting transmission wires
Limitations:
-rare earth minerals
-cooling is expensive/impractical
Potential dividers.
Define what a potential divider is.
What are 2 real-life uses of potential dividers?
A potential divider is a set of resistors in series with a p.d across them, to divide a larger input voltage into a set of smaller output voltages.
2 uses are - thermostats, and automatic night lights.
Potentiometers.
What is a potentiometer?
Give a use.
A potentiometer is a long coil of wire with a moveable connection, to give 2 resistors with varying resistance.
Used for determining I-V characteristics of components.
Internal resistance.
Define what internal resistance is.
How can one experimentally determine internal resistance?
Internal resistance is the resistance of a cell due to e-s colliding with metal ions and losing energy.
V = E-Ir.
plot V against I.
Power.
Define power.
The work done per unit time.
E.M.F.
Define what emf is.
How does one experimentally determine the emf of a cell?
In practice, why is the reading from the method you describe always less than the true emf?
Definition of EMF:
The work done per coulomb of charge flowing through the cell
Converting chemical potential energy into electrical energy
Experimental determination:
Connect an ideal voltmeter across the terminals of the cell without any additional connections.
In practice, voltmeters have a finite resistance so some current flows through the cell and there are lost volts.
State Ohm’s law.
For a conductor at constant temperature, the voltage across the component is d.p to the current flowing through it.
Terminal p.d
Define terminal p.d.
The voltage across the terminals of the cell
Ideal ammeters and ideal voltmeters.
Explain the resistances of both of these.
Ideal A has no resistance, so it does not change the total resistance of the circuit, and the current does not change
Ideal V has inf resistance, so it does not affect the current between the two points and so does not affect the voltage change between them as V = IR.
