Topic 3 Flashcards
Define electrical current
Electrical current is the rate of flow of charge
State the defining equation for current
I = ΔQ / Δt
What is the unit of current
Ampere (Amps), A
Define voltage
Voltage is the work done per unit charge
State the defining equation for voltage
V = W / Q
What equation links current, voltage and resistance?
Voltage (V) = Current (A) x Resistance (Ω)
V=IR
What is ohm’s law?
If Ohm’s law is obeyed, the current and voltage passing through a component are directly proportional, when at a constant temperature
Describe the distribution of current in a series circuit
In a series circuit, the current is the same at all positions in the circuit
What law is the distribution of current in a series circuit a consequence of?
The law of the conservation of charge. Charge cannot be created or destroyed and so in a closed loop, the flow of charge must be the same throughout
Describe the distribution of current in a parallel circuit
In a parallel circuit, the current is split between the different branches. The current entering each branch must be the same as the current leaving it
Explain how the conservation of charges applies to parallel cicuits
Charge cannot be created or destroyed meaning the total current leaving the source must equal the sum of the currents in all the individual branches
What law is the distribution of potential differences in a circuit a consequence of?
The law of the conservation of energy
Describe the distribution of potential differences in a series circuit
The total potential difference is split across all the components in a series circuit, in the ratio of their resistances.
Describe the distribution of potential differences in a parallel cicuit
The sum of the potential differences in each branch of a parallel circuit, is the same and will be equal to the potential difference of the source
What equation relates power, current and potential difference?
P=IV
How can you calculate the work done over a given time period, by a component with a known potential difference and current?
Combining P = W/t and P=IV gives:
W=IVt
What is the IV characteristic for an ohmic conductor?
Directly proportional
Explain the IV characteristic for a filament bulb
The higher the current, the higher the temperature of the metal filament
The higher the temperature, the higher the KE of the metal ions and so the more they vibrate
This makes it harder for current to flow and so the resistance of the bulb increases
What three physical factors affect the resistance of a wire?
Length
Cross-Sectional Area
Resistivity
How does increasing the length of a conductor affect its resistance?
The longer the conductor is, the higher its resistance, assuming all other factors remain the same
How does increasing the cross-sectional area of a conductor affect its resistance?
The larger the cross-sectional area of a conductor, the lower its resistance, assuming all other factors remain the same
What is resistivity?
A characteristic property of a material that determines its resistance to current flow
Compare the resistance of two conductors of the same dimensions, but with different resistivities
The conductor with the higher resistivity will have the higher resistance
State the equation linking the factors affecting the resistance of a conductor
R = ρL / A
What equation links the number of charge carriers in a metal, along with their drift velocity, to the current flow?
I = nqAv
How does the potential along a uniform current-carrying wire vary as you move along it?
The potential will increase as you move along the wire
What is the electromotive force of a power supply?
The electromotive force is the amount of energy that is transferred to each Coulomb of charge that passes through it
What is the terminal potential difference of a power supply?
The terminal potential difference of a power supply is the actual potential difference that is produced across the terminals of the supply and is provided to the circuit
Why does the electromotive force of a supply differ from its terminal potential difference?
Some of the energy that is transferred to the charges is used to overcome the internal resistance of the power supply
Define internal resistance
The internal resistance of a power supply is the electrical resistance, when a current flows, due to the materials within the supply itself
Describe a method to determine the terminal potential difference of a battery
Attach a voltmeter directly across the terminals of the battery when it is isolated. Since no current flows, no potential is lost due to internal resistance and so the voltmeter will read the value of the terminal p.d.
What is a semiconductor?
A semiconductor is a component that can change its resistance based on external conditions
Give two examples of semiconductors and state what they are sensitive to
Thermistors are sensitive to temperature
LDRS are sensitive to light
Describe how the resistance of a thermistor changes as temperature decreases
As temperature decreases, the resistance of a thermistor will increase
Explain why the resistance of a thermistor will decrease with an increase of temperature
As temperature increases, conduction electrons are liberated, meaning there are more charge carriers and so current can flow more easily
Describe how the resistance of an LDR changes as light intensity increases
As light intensity increases, the resistance of an LDR will decrease
What is a potential divider?
A potential divider is a pair of resistors used to split the potential difference over them into a given ratio
What is the link between the resistances and the potential differences in a potential divider?
The potential difference of the circuit is split in the ratio of the resistances of the resistors
