Unit 19 Current of Electricity & Unit 20 DC Circuits
In an LDR, what lowers the resistance?
A greater light intensity
In I-V graph, what does a low gradient show?
A high resistance
What causes free electrons to drift within a circuit?
A potential difference being applied to a circuit, creating an electric field which exerts a force on the free electrons, causing them to 'drift' in the direction of the force
Describe the I-V characteristic of a filament lamp.
As the p.d across the filament lamp gets larger, the filament lamp gets hotter and its resistance increases
Why are metals good at conducting electricity?
As they have a sea of delocalised electrons that are not bound to a specific atom. These electrons can flow freely through the metal lattice carrying charge.
Where must be the ammeter be placed in a circuit?
Must be placed in series to measure the current flowing in a circuit
How do thermistors work?
Warming the thermistors gives more electrons enough energy to escape from their atoms. This means that there are more charge carriers available, so the resistance is lower.
What is power measured in?
Watts (W)
Is the current the same everywhere around a circuit connected in series?
Yes, since there is only one path for current to flow
I = ?
nqvA
What is potential difference measured in?
volts
R = ? (using resistivity)
ρ x l/A
What is 1 watt equivalent to?
1 joule transferred per second
What is the charge of one electron?
1.6 x 10^-19 C
Describe the I-V characteristic of an ohmic conductor.
Doubling the current, doubles the potential difference
What is ohm's law?
For metals at a constant temperature, the current in the metal is proportional to the potential difference across it. In other words, the ratio V/I is constant
Ideal voltmeters have
Infinite resistance
On a graph plotted as I against V, how can we find the resistance?
Inverse the gradient
Where must be the voltmeter be placed in a circuit?
It must be placed in parallel to measure the difference in electrical potential between two points.
When a diode is reverse biased, what can we say about its resistance?
It's infinite (very, very large)
What does LDR stand for?
Light Dependent Resistor
What is the unit of resistivity?
Ohm metres (Ωm)
Give the circuit symbol of a diode and state it's function
Only allows current to flow in one direction
Using P = VI and V=IR, what are the another equations can you get from power?
P = V^2 / R P = I^2 x R
How does conventional current flow?
Positive to negative
Total energy transferred = ?
Power x time (V x I x t)
ρ = ?
RA/l
How do electrons flow in a wire?
Randomly in all directions (but overall drifting in one direction).
What is resistance and how is it defined?
Ratio of potential difference across component in a circuit to the current. It is defined by the equation : R=V/I
What happens to the resistance in a thermistor as the temperature increases?
Resistance decreases.
What happens to resistance in a metal as temperature increases?
Resistance increases.
If a conductor obeys ohms law, what can be deduced about its resistance?
Resistance is constant
What is one Coulomb?
The amount of charge that passes in 1 second when the current is 1 ampere.
What is "q" in the drift velocity equation?
The charge in Coulomb carried by each charge carrier
What is forward bias?
The direction in which the current is allowed to flow in a diode.
How do LDRs work?
The light provides the energy that release the electrons from their atoms. More charge carriers = a lower resistance.
Give the circuit symbol of a cell and state which side is positive
The longer line is the positive side
What is "n" in the drift velocity equation?
The number of charge carriers per unit volume
What is current?
The rate of flow of charge
What is power?
The rate of transfer of energy
In I-V graph, what does a curve show?
The resistance is changing
Define resistivity
The resistance of a 1 m length component with a 1 m^2 cross-sectional area. It is a property of a material.
In the equation W=QV what does W represent?
The work done on the particle when it is accelerated by a potential difference
Define potential difference
The work done per unit charge
What are diodes?
They are resistors that only let current flow in one direction (when connected in the forward bias, they give a low resistance. When connected in the reverse bias, they give a high resistance)
