Electricity Flashcards
Define current (I)
The rate of flow of charge
How do you work out the number of electrons carrying a charge (eg 10C)?
Divide charge by the charge of each electron (1.6x10-19)
What is the difference between conventional current and electron flow?
Conventional current flows from the +ve terminal to the -ve terminal
Electron flow shows the direction the electrons flow, from -ve to +ve
How is the current in a circuit related to potential difference and resistance?
Increasing potential difference increases the current
Increasing resistance decreases the current
What is Ohm’s law?
The current flowing through a metallic conductor is proportional to the potential difference applied across it at constant temperature
When does Ohm’s law apply?
When the component has a fixed resistance (eg a fixed resistor at a constant temperature, or a filament at a low current)
Define potential difference
The work done (energy transferred) by each coulomb of charge moving between two points
(Eg a 12V battery adds 12J of energy to each coulomb of charge passing through)
How does a circuit ‘short circuit’?
If there is an available path with 0 resistance
Current → ∞
And the circuit heats up
What is the I-V graph for a fixed resistor?
What is the I-V graph for a filament bulb?
What is the graph for a semiconductor diode?
What’s wrong with this?
Resistance is not calculated using the gradient (of a tangent) of an I-V graph!!!
Instead just use the voltage and current at that point
Explain the shape of the I-V graph for a filament
As current increases, temperature of filament increases
This increases lattice ion vibrations.
Which increases the number of collisions per second with electrons.
So resistance increases.
How does the I-V graph for a fixed resistor prove it is ohmic?
The straight line passing through the origin
proves that current ∝ voltage
Explain the shape of the semiconductor diode (in positive bias)
- As the potential difference increases weakly bound electrons in the conductor gain energy
- After the threshold pd, some electrons become free to carry a current
- The lattice vibrations still increase but this is less significant
What happens if a semiconductor diode is connected in reverse bias?
No current flows until the breakdown voltage is reached (~50V)
The diode breaks and all current flows through
What is the difference between a series and a parallel circuit?
Parallel circuits have junctions (3 or more wires connect)
Why doesn’t adding voltmeters in parallel affect the circuit? (it is still series)
Voltmeters have ~ ∞ R so no current flows through
What are the p.d and current rules for a series circuit?
P.D is shared across the components (by resistance)
Current is constant throughout
What are the p.d and current rules for a series circuit?
P.D is shared across the components (by resistance)
Current is constant throughout
What are the p.d and current rules for a parallel circuit?
P.D is same for parallel branches
Current separates at junctions (according to branch resistance)
What is Kirchoff’s 1st Law?
At any junction in a circuit the sum of the current flowing into the junction is equal to the sum of the current flowing away from it.
What is Kirchoff’s 2nd Law?
In any complete “loop” of a circuit the sum of p.d’s equals the source p.d.
How do you combine series resistors in the same branch? (no junction between them)
Add up their resistances
How do you combine resistors in parallel branches? (one junction between them)
Use the following equation…
What is the advantage of placing resistors in parallel arrangements?
The total resistance is always less than the smallest resistance
Will the current split equally?
No, because the resistance of each branch is different
Will each component receive the same voltage?
No, because the resistance of the components are different
Why would you place batteries in parallel?
- The power delivered is the same
- But they take longer to run flatter
What is a potential divider circuit?
A circuit with 2 or more resistors connected in series with a power supply. (usually one is a thermistor or LDR)
How does resistance change for an NTC Thermistor?
As temperature increases, resistance decreases
How does resistance change for a Light Dependent Resistor (LDR)?
As light intensity increases, resistance decreases
What is the advantage of setting up a rheostat as a variable resistor?
- Simpler circuit
- Current constant throughout
- But cannot get 0V across bulb
What is the advantage of setting up a rheostat as a potential divider?
- Bulb can receive full range of voltage 0V → Vsource
- Current through bulb can be reduced to 0A
- But maximum current is lower
How does changing the dimensions of a piece of metal affect its resistance?
- Increased length → increased resistance
- Increases cross sectional area → decreased resistance
- Increased resistivity (using different material) → increased resistance
How do you calculate the cross sectional area of a wire?
Assume it to be a cylinder (unless told otherwise)
A=∏r2
Why do metals with a greater cross sectional area have a lower resistance?
There are more paths for the electrons to propagate
How do you calculate the potential difference across branches?
- Work out the P.D of each component
- Make a loop connecting the branches
- Subtract the PDs of one branch from the other
What is a superconductor?
A material with 0 resistance at and below the critical temperature
Why does a material become superconducting at and below its critical temperature?
- The lattice ion vibrations reduce to 0
- So electrons can pass through without collision
What is the advantage of superconductors and name a use?
- Transmit large currents with 0 resistance
- So negligible thermal energy losses
- Used to create high power magnets → MRI machines
- High processing power circuits → Supercomputers
Define emf of a power source
Work done per unit charge on the whole circuit by the power source
Define terminal potential difference of a circuit
The potential difference across the terminals when a current is flowing through
What is the lost voltage in a circuit?
The potential difference used up pushing a current through the battery (vlost = emf - TPD)
How should you work with a circuit involving internal resistance?
Treat the internal resistance as another resistor in series with the components
Then solve as a regular circuit (using ohm’s law, kirchoff’s laws, P=IV etc)
When reading an electricity question what should you first be looking out for?
The phrase ‘negligible internal resistance’
What should you remember about an ideal voltmeter?
- Resistance of voltmeter effectively infinite
- No current goes through the voltmeter
- Has no effect on circuit
What should you remember about a non-ideal voltmeter?
- Has a large resistance (but not infinite)
- Small current passes through
- Treat as a resistor in parallel
What should you remember about an ideal ammeter?
- Resistance of ammeter effectively 0
- No potential drop across ammeter
- Has no effect on circuit
What should you remember about a non-ideal ammeter?
- Has a small resistance (but not zero)
- Small potential drop across
- Treat as a resistor in series
What is a kilowatt hour?
How many hours a power source could deliver a power of 1kW for
(Measure of total energy stored in power source)
What is an Amp-hour?
How many hours a power source could deliver a current of 1A for
(Measure of total charge stored in power source)
