Electric circuits (3) Flashcards
What is the definition of electric current?
The rate of flow of charge.
What is the unit of electric current?
Amperes (A) or amps.
What is electric charge?
A property of some particles, such as protons (positive) and electrons (negative).
What is the unit of charge?
Coulombs (C).
How many electrons make up 1 Coulomb of charge?
6.25e18 electrons.
What happens when two oppositely charged conductors are connected by a wire?
Charge flows between them, creating a current.
In electrical wires, what are the charge carriers?
Electrons
In which direction do electrons flow in a circuit?
From the negative terminal to the positive terminal of a cell.
What is conventional current?
The flow of positive charge from the positive terminal to the negative terminal (opposite to electron flow).
What instrument is used to measure electric current?
An ammeter
How should an ammeter be connected in a circuit?
In series with the part of the circuit where the current is measured.
What is potential difference
Energy per coulomb of charge transferred between two points in a circuit
Why must an ammeter be connected in series?
Because current is the same in all components connected in series.
What is potential difference (voltage)?
The work done per unit charge.
How does a cell create potential difference in a circuit?
It makes one end positive and the other negative, setting up a potential difference.
How should a voltmeter be connected in a circuit?
In parallel with the component being measured.
What happens to the potential difference across components in parallel?
It is always the same across each component.
Why is potential difference the same in parallel circuits?
Due to the conservation of energy.
What is resistance?
The opposition to current in a circuit.
How is 1 Ohm (Ω) defined?
1 Volt per Ampere ( 1Ω=1V/A ).
What is Ohm’s Law?
Resistance is the ratio of potential difference to current.
Do all electrical components have resistance?
Yes, even wires have some resistance
What is the role of resistance in a circuit?
It controls the size of the current.
What is Ohm’s law
The current through a component is directly proportional to the potential difference across it, providing the temperature is constant
Variable resistor graph for I against V is…
Straight line increasing
How to tell if a component obeys Ohm’s law
its graph of current against potential difference is a straight line through the origin
Does a resistor obey Ohm’s law
yes
Does a filament lamp obey Ohm’s law
No
What does the electric current rule state?
The algebraic sum of the currents entering and leaving a junction is equal to zero.
What fundamental principle does the current rule follow?
Conservation of charge—charge cannot be created or destroyed at a junction.
What happens to the total current when a circuit splits into branches?
The total current before the junction is equal to the sum of the currents after it splits.
What happens to current in a series circuit?
The current remains the same at all points in a series circuit.
If three identical bulbs are connected in series, how does the current compare through each bulb?
The current is the same through each bulb.
What happens to current at a junction in a parallel circuit?
The current splits at the junction, with each branch having a different current depending on resistance.
How does the current behave in a parallel circuit?
The sum of the currents entering a junction equals the sum of the currents leaving.
If the main circuit has a current of 6A and splits into two branches, carrying 4A and 2A, what is the sum of the branch currents?
6A = 4A + 2A (verifies conservation of charge).
What does the Voltage Rule state?
The sum of the electromotive forces (e.m.f.s) in a closed circuit loop is equal to the sum of the potential differences in that loop.
What fundamental principle does the voltage rule follow?
Conservation of energy—energy supplied by the power source is fully transferred to components in the circuit.
How is voltage distributed in a series circuit?
The voltage is split across all components in proportion to their resistance.
What is the relationship between total voltage and component voltages in a series circuit?
Total voltage = Sum of voltages across all components.
How is voltage distributed in a parallel circuit?
The voltage across each branch of a parallel circuit is the same as the supply voltage.
Why are parallel circuits used in home wiring?
Because each branch receives the same voltage, and if one component fails, others keep working.
What is the equation for total resistance in a series circuit?
R (total) = R1 + R2 + R3 + ….
Why does total resistance increase in a series circuit?
Because current has to pass through each resistor, adding more resistance at each stage.
What is the equation for total resistance in a parallel circuit?
1/R(total)=1/R1 + 1/R2 + 1/R3 +…
Why does total resistance decrease in a parallel circuit?
Because adding more resistors creates more paths for current to flow, reducing overall resistance.
How does resistance in a parallel circuit compare to the smallest individual resistor?
The total resistance is always less than the smallest individual resistor.
If two resistors of equal value are in parallel, how does their combined resistance compare?
The total resistance is half of an individual resistor’s resistance.
Why is the reciprocal method used to calculate parallel resistance?
Because current splits at junctions, so the total resistance must account for multiple current paths.
What is the definition of electrical power?
Electrical power is the rate at which work is done or energy is transferred in a circuit.
Formula for electrical power
P=IV
How can power be expressed using resistance?
P=IIR
What happens to power if the current doubles in the equation
P=IIR
The power quadruples because it depends on the square of current.
How does increasing resistance affect power if voltage is constant?
Higher resistance leads to lower power output, because current decreases.
How is power affected in a circuit with two resistors in series?
The total resistance increases, leading to lower current and lower power.
How do you decide which power equation to use in a problem?
- If given voltage and current, use P=IV
- If given current and resistance, use P=IIR
- If given voltage and resistance, use P=V*V/R
What is the unit of electrical power, and how is it defined?
Watt (W), where 1 W = 1 J/s, meaning 1 joule of energy is transferred per second.
What happens to current when potential difference increases across a component?
The current increases, following Ohm’s Law for some components.
What does the I-V graph of an ohmic conductor look like?
A straight line through the origin, meaning current is directly proportional to voltage.
Why does an ohmic conductor produce a straight-line I-V graph?
Because its resistance remains constant, so V and I increase proportionally.
How do different components behave on an I-V graph?
- Ohmic conductor: Straight line
2.Diode: Sharp increase in one direction
3.Filament lamp: ‘S’ shaped curve
What is the function of a diode in a circuit?
It allows current to flow in only one direction.
What does the I-V graph of a diode look like?
A horizontal section (no current) in reverse, followed by a sharp increase in forward bias.
What is the threshold voltage of a diode?
Around 0.6V, which is the minimum voltage needed for it to conduct.
What happens to current when a diode is in reverse bias?
No current flows, as shown by the flat portion of the graph.
Why does a diode not obey Ohm’s Law?
Its resistance is not constant—it changes with voltage.
What shape is the I-V graph for a filament lamp?
An ‘S’ shaped curve, where the slope decreases as voltage increases.
Why does the resistance of a filament lamp increase with voltage?
- Higher current-Higher temp
- High temp-increased resistance
- Increased resistance slows down current growth
When does a filament lamp obey Ohm’s Law?
Only at low voltages, before significant heating occurs.
How does the I-V graph of a thermistor behave?
It curves upwards, showing increasing current at a faster rate as resistance decreases.
Why does resistance decrease as current increases in a thermistor? (3)
- More current-higher temp
- Higher temp-Lower resistance
- Lower resistance-more current can flow
Do all materials have resistance to the flow of charge?
Yes, all materials have some resistance to the flow of charge
What happens when free electrons move through a metal wire?
They collide with ions that get in their way.
What is the result of electrons colliding with ions in a metal wire?
They transfer some or all of their kinetic energy, causing electrical heating.
What causes electrical resistance in a wire?
The ions in the wire resist the flow of charge, which causes resistance.
How does the length of a wire affect its resistance?
The longer the wire, the greater its resistance.
How does the thickness of a wire affect its resistance?
The thicker the wire, the smaller its resistance.
What is resistivity?
A property that describes how much a material opposes the flow of electric current.
What factors influence the resistivity of a material?
It is a property of the material and depends on temperature.
What are the units of resistivity?
Resistivity is measured in ohm meters (Ω m).
What causes current in a conductor?
The movement of charge carriers.
Can charge carriers be both positive and negative?
Yes, but current is always taken to be in the same direction.
What is drift velocity?
The average velocity of charge carriers traveling through a conductor.
What is the charge carrier in most conductors?
Free electrons.
What is the transport equation for current?
I=nqvA
Why do metals conduct electricity well?
They have a high value of n, meaning many free electrons per unit volume.
How does a negative drift velocity (v) affect current direction?
It indicates current is in the opposite direction to the charge carriers.
How does n affect drift velocity (v)
v is inversely proportional to n, a higher n slows down individual charge carries
Why does increasing the number of charge carriers decrease resistivity?
More charge carriers allow current to flow more easily, reducing resistivity.
Why do insulators prevent current flow?
Insulators have very few charge carriers, giving them high resistivity. A perfect insulator (n=0) would have zero current regardless of voltage.
How do semiconductors compare to metals in terms of charge carriers?
Semiconductors have fewer free electrons than metals, meaning they have higher resistivity.
How does temperature affect a semiconductor’s resistivity?
Higher temperatures increase the number of free electrons, decreasing resistivity. Silicon is an example of a semiconductor.
How is potential difference across a component defined?
Energy transferred per unit charge.
In what units is potential difference measured?
Volts (V), which are equivalent to Joules per Coulomb (J C⁻¹).
How is potential difference shared in a series circuit?
It is divided among all the components in the circuit.
What is the equation for resistivity?
R=pl/A
What do R and p represent in the resistivity equation
R=resistance
p=resistivity
How does the length of a conductor affect its resistance?
As length increases, resistance also increases (if temperature is constant).
How does resistance affect potential difference in a circuit?
As resistance increases, potential difference must also increase. (as long as current remains constant)
Ratio of voltages to resistances across resistors
V1/V2=R1/R2
What is the electrical voltages rule?
The sum of the e.m.f.s in a closed circuit loop is equal to the sum of the potential differences around that loop.
What happens when two resistors are connected in series?
The potential difference across the power source is divided across the two resistors.
What are the three main purposes of potential dividers?
- To provide a variable potential difference.
- To enable a specific potential difference to be chosen.
- To split the potential difference of a power source between components.
What determines the potential difference across each resistor?
The resistance of each resistor.
What is a potential divider?
A circuit that produces an output voltage as a fraction of the input voltage.
Which resistor in a potential divider will have the greatest potential difference across it?
The resistor with the largest resistance.
How do potential dividers work?
They use two resistors in series to split the voltage of the supply in a chosen ratio.
Where are potential dividers commonly used?
In volume controls and sensory circuits using LDRs and thermistors.
What happens if the resistance of one resistor in a potential divider increases?
It receives a greater share of the potential difference, while the other resistor receives a smaller share.
What is a potentiometer?
A single component that acts as a potential divider.
How does a potentiometer work?
It consists of a coil of wire with a sliding contact that adjusts the output voltage.
What happens when the slider moves along the potentiometer?
It separates the potentiometer into two parts with different resistances, changing the output voltage.
What are the maximum and minimum voltage conditions for a 3 Ω potentiometer?
Maximum voltage at 3 Ω, minimum voltage at 0 Ω.
What are sensory resistors, and how are they used?
Thermistors and LDRs are used in potential dividers to vary output voltage.
How can a potential divider circuit control external components?
It can switch components on or off based on resistance changes.
How is electromotive force (e.m.f.) defined?
The amount of chemical energy converted to electrical energy per unit charge when charge passes through a power supply.
In what unit is e.m.f. measured?
Volts (V), which is equivalent to Joules per Coulomb (J C⁻¹).
Is e.m.f. actually a force?
No, it is a measure of energy transferred per coulomb of charge.
When is e.m.f. equal to the potential difference across a cell?
When no current is flowing.
Why must a high-resistance voltmeter be used to measure e.m.f.?
To ensure that no significant current flows, allowing an accurate reading of the open-circuit voltage.
What is internal resistance in a power supply?
The resistance between the terminals of a power supply, denoted as r.
What effect does internal resistance have on electrical energy?
It causes some electrical energy to be converted into heat energy, making the power supply heat up
What are ‘lost volts’?
The voltage lost due to internal resistance in a power supply.
How does internal resistance affect lost volts?
A higher internal resistance results in a greater amount of lost volts.
How does internal resistance affect energy loss?
It reduces the available voltage by causing energy loss in the power supply.
What is terminal potential difference (p.d.)?
The potential difference across the terminals of a cell.
How does internal resistance affect terminal p.d.?
If a cell has internal resistance, the terminal p.d. is always lower than the e.m.f.
When would the terminal p.d. be equal to the e.m.f.?
If there was no internal resistance.
What is the terminal p.d. when a load resistor R is connected?
It is also the potential difference across the load resistor.
How are lost volts calculated?
Vr=I×r, where r is the internal resistance and I is the current.
How is e.m.f. related to terminal p.d. and lost volts?
emf= terminal pd + lost volts
What happens to electrical energy when charge passes through a resistor?
It is converted to heat, creating a potential difference across the resistor.
What is the difference between potential difference and e.m.f.?
Potential difference describes energy loss from charges, while e.m.f. describes energy transfer from the power supply to the charges.
Why do all materials have some resistance to the flow of charge?
Free electrons collide with ions in the material, impeding their flow.
What happens when free electrons collide with ions in a metal wire?
They transfer some or all of their kinetic energy, causing electrical heating.
How does increasing temperature affect ion vibrations?
The ions vibrate more, increasing the likelihood of collisions with conduction electrons.
Why does resistance increase as temperature rises?
More frequent electron-ion collisions impede the flow of charge.
How does temperature affect resistance at small increases?
The increase in resistance is linear with temperature.
What effect does a higher current have on temperature?
It increases temperature due to more frequent electron-ion collisions.
How does temperature affect resistance in metallic conductors?
Higher temperature increases resistance and resistivity, and lower temperature decreases them.
What happens in a filament lamp as current increases?
More electron-ion collisions raise the filament’s temperature.
How does resistance affect current as temperature rises?
Resistance opposes the current, causing it to increase at a slower rate.
What happens when light’s absorbed by a material
More electrons are available for conduction
For an LDR, what happens when light intensity increases
Resistance decreases
What does a resistance/light intensity graph look like for an LDR
Negative exponential
