DC Electricity Flashcards
Current
The rate of flow of charge at a point in a circuit
What is used instead of an ammeter for very small currents?
A galvanometer / galvo
What is the resistance of an ideal ammeter?
0 ohms
Direction of flow of current
Positive to negative
Direction of flow of electrons
Negative to positive
Potential difference between two points
The energy converted when a unit charge flows between the points
What is the resistance of an ideal voltmeter?
Infinite resistance
Resistance of a devise
The ratio of the potential difference across its ends to the current flowing through it
What is resistance a measure of?
How difficult it is for current to flow
Ohm’s law
Provided the temperature and other physical factors remain constant, the current through a conductor is directly proportional to the p.d across it.
Power
The rate at which energy is converted
A kilowatt-hour
The energy supplied by 1 kilowatt of power in 1 hour
Kirchoff’s 1st law
At a junction, the total current entering the junction is equal to the total current leaving the junction
Explanation for the shape of a V-I graph
- Obays Ohm’s law at low PD and I
- As PD increases, I increases
- As I increases, more power is dissipated
- As more power is dissipated, the temp increases
- As temp increases, resistance increases
- As resistance increases, there is a smaller increase in I for the same increase in PD
Why does resistance increase at high temperatures?
The metal ions vibrate more vigorously, making it more difficult for electrons to flow through a wire
Kirchhoff’s seccond law
The sum of the emfs around a circuit loop is equal to the sum of the p.ds around the loop
Breakdown voltage of a semiconductor diode?
Around -50V
Minimum voltage required for current to flow in a diode
Around 0.7V
What is the maximum voltage of a diode?
Around 0.7V regardless of current
What is the resistance of an ideal diode when reverse biased?
Infinite resistance
What is a protective resistor used for?
Preventing voltage from rising too high and destroying a diode in the circuit
What determines resistivity?
The material
Units of resistivity
Ωm
A low resistivity means that a material is…
A good conductor
Effect of increasing temperature on resistivity
Resistivity increases
Why does resistivity increase with temperature?
- Metal ions gain KE
- More vibrations
- More collisions with electrons
- Rate of flow of electrons decreases
- Current is lower, so resistance increases
Superconductors
Materials where the resistivity drops to zero at very low temperatures
Critical temperature of supercoductors
The temperature at or below which the resistivity of a material is zero
Uses of superconductors
- Mag-lev trains
- MRI
- Computing
- More efficient power cables
Electromotive force (emf)
The number of joules of electrical energy transferred to each coulomb that passes through it
What causes internal resistance?
The materials inside the power supply
Why is the terminal voltage lower than the emf?
Some of the emf goes across the internal resistance
Effect of increasing current on terminal voltage
- Current increases internal resistance
- A greater proportion of the pd goes across the internal resistance
- Terminal pd is lower
Why would a low internal resistance be used?
This will cause the current produced by the supply to be high
Why would a high internal resistance be used?
In high voltage supplies, this is used to keep current low (prevents electrocution)
Short circuit
When something connects the terminals of the supply, causing load resistance to be 0
Total emf in series
Sum of the emfs
Total internal resistance in series
Sum of the individual resistances
Total emf in parallel (identical cells)
Emf of a single cell
Total internal resistance in parallel (identical cells)
Combined resistance of all resistances in parrallel
Why would cells be used in parrallel?
- Batteries last longer (produce 1/n of the current)
- Lower internal resistance reduces heating
- Total emf is not effected if a cell brakes
Potential divider
A devise that shares the supply voltage between two or more resistors in series
Potentiometer
A device that controls voltage across a component
What is an ntc thermistor?
A negative temperature coefficient thermistor
Effect on resistance of an ntc thermistor when temperature increases
Resistance decreases
How can temperature of a thermistor be controlled during an experiment?
Using a water bath (ice to reach colder temperatures)
LDR
Light dependant resistor - has a high resistance which decreases greatly as light intensity increases
