Electricity Flashcards
What is a coulomb?
One coulomb is the amount of charge which flows past a point when a current of one amp flows for one second
What is Kirchhoff’s 1st law?
It states that the sum of the currents flowing into any point in on a circuit is equal to the sum of the currents flowing out of that point
What is a volt?
One volt is one joule per coulomb
What is electromotive force (emf)?
The energy transferred per unit charge when one other type of energy is converted into electrical energy
What is potential difference?
The electrical energy transferred per unit charge when electrical energy is converted into another form of energy
What is resistance?
The resistance of a component is the ratio of potential difference across the component to the current flowing through it
What is resistance caused by?
Resistance is caused by collisions between free electrons and the metal ions
What factors cause resistance to increase?
If the length increases, temperature increases, or cross sectional area decreases
What is Ohm’s law?
For a conductor at constant temperature, the current in the conductor is proportional to the potential difference across it
Why does an increase in length of a wire cause increased resistance?
It increases the chance of collisions
Why does an increase in temperature cause increased resistance?
Higher temperature means the ions move more, increasing the chance of collisions
Why does increased cross-sectional area cause decreased resistance?
There are a greater number of possible routes for electrons to take and so there is a lower chance of collisions (cross sectional area is inversely proportional to resistance)
What is conventional current?
Flow of charge, opposite to electron flow
What is current?
Electric current is the flow of charged particles per unit time
What are the benefits of diodes?
- Efficient, they operate on low pds
- They switch on instantly
- They are robust and have a long working life
What are the uses of diodes?
- Converting AC to DC
- Generating radio frequency oscillations
- Electronically tuning radio and TV receivers
- Producing light (LEDs)
What is resistivity?
The resistance per unit length of a piece of material of cross-sectional area 1m^2
The units are Ωm
What is the difference between resistance and resistivity?
Resistance is the ratio of current and voltage, and resistivity is the resistance per unit volume of a material
What are each of the terms in I = nqvA?
n = number density (m^-3) (number of charge carriers per m^3) v = drift velocity (ms^-1) A = cross sectional area (m^2) q = electric charge (C)
What are semi-conductors?
Materials that sometimes conduct, depending on the environment
What materials are semiconductors often made of?
Silicon
How do semiconductors work?
At higher temperatures, extra energy moves electrons to the “conduction band” (from the valence band, more energy in the system allows electrons to move out of the orbital and become delocalised, which only happens in semiconductors, not metals), meaning there are more conduction electrons, so resistance decreases
What components can be made from semiconductors?
- Thermistors (resistance decreases as temperature increases)
- Light dependent resistors (resistance decreases as light intensity increases)
What is meant by superconductivity?
If we cool a material to less than a critical temperature (-243°C), the resistance drops to zero. This is useful when a large current is needed
How is the voltage in series relationship derived?
Etotal = E1 + E2 + E3 (conservation of energy) VQtotal = VQ1 + VQ2 + VQ3 Vtotal = V1 + V2 + V3 (Q is the same throughout)
How is the relationship for resistors in series derived?
Vtotal = V1 + V2 + V3 (voltage in series relationship) IRtotal = IR1 + IR2 + IR3 Rtotal = R1 + R2 + R3
How is the relationship for resistors in parallel derived?
Qtotal = Q1 + Q2 + Q3 (conservation of charge) Ittotal = It1 + It2 + It3 (cancel t) Itotal = I1 + I2 + I3 (V/R)total = (V/R)1 + (V/R)2 + (V/R)3 (1/R)total = 1/R1 + 1/R2 + 1/R3 (voltage is the same in each loop)
What are the best ways to measure the voltage and current characteristics of a component?
It is best done using a variable power supply, or it can be done using a fixed power supply and variable resistor
To investigate the voltage and current characteristics of a component, is it better to use a circuit with all components in series, or one with the power supply and variable resistor in one loop, and fixed resistor, ammeter and voltmeter in the other?
The second, with the parallel circuit. This allows the full range from 0-6V, whereas, in series, the range is much more limited
What is internal resistance?
The resistance in the power supply that comes as a result of current flowing through chemicals and electrodes in the power supply.
What are the lost volts?
The voltage which is needed to push current through the internal resistance
What is emf?
The maximum theoretical voltage that the power supply can deliver (which cannot be measured directly).
The actual voltage available is the terminal/circuit pd
Potential dividers
- A potential divider allows the voltage to be shared between two components
- It is usually constructed of 2 resistors in series
- The same current passes through both resistors, so the pd across each resistor is proportional to its resistance, expressed by V1 = (R1/ (R1 +R2)) x Vsupply
N-type thermistors
- Resistance of an n-type thermistor decreases as temperature increases
- Made from a semiconductor (e.g. sintered metal oxide)
- Raising the temperature increases the number of electrons able to move about. The more charge carriers available, the more current
What is a use for thermistors?
In an electrical circuit to monitor ambient temperature and control heating/air conditioning system
Light dependent resistors
- The resistance of an LDR decreases exponentially as light intensity increases
- Made from a semiconductor
What is a use for an LDR?
In an electrical circuit to monitor ambient light conditions, so can control lighting systems like street lights
How can an LDR be used in nightlights?
- Lightbulb is connected across the LDR (in parallel)
- As light intensity drops, resistance of LDR increases, so voltage share across the LDR increases
- Therefore the voltage in the bulb increases because it is connected in parallel
- So when light intensity drops to a certain level, there will be a high enough voltage and the bulb will turn on
Method to investigate efficiency of an electric motor
- Set up a circuit with a battery, ammeter connected in series and voltmeter connected in parallel across the motor
- Secure the motor and pulley apparatus
- Measure the height through which the mass will be lifted (top of the hanger to the cut-off switch)
- Turn on the motor and time how long it takes to raise the mass, recording time, current and voltage
- Calculate energy in, energy out and then efficiency
- Repeat for more masses
How does a fuse work?
When the current in a circuit gets too high, it will be above the fuse rating of the fuse fitted in a circuit. This means that the small wire in the fuse will melt if the current is high, breaking the circuit and stopping current flow. This stops the circuit being damaged or catching fire, as well as stopping the danger of electrocution
Explanation of the graph relationship of voltage and current when investigating the internal resistance of a cell (voltmeter across the power supply, variable resistor, ammeter in series)
- As circuit resistance is reduced, the current increases (for the same emf)
- This means that the lost volts increases
- Therefore less useful voltage is available in the circuit
- So the the terminal pd decreases
