Electricity (AS) Flashcards
What is current?
The rate of flow of charge.
What is potential difference?
Potential difference is the energy transferred per unit charge between two points in a circuit.
W = V x Q
V = W/Q
What is resistance?
How difficult it is for current to flow through an appliance.
A component has a resistance of 1Ω if a potential difference of 1V makes a current of 1A flow through it.
Define a coulomb (C).
A coloumb is the amount of charge that passes in 1 second if the current is 1A.
Q = I x T
Define a volt (V).
The potential difference across a component is 1V when you convert 1 joule of energy moving 1 couloumb of charge through the component.
V = W/Q
1V = 1JC^-1
Which direction does electron flow act in?
Negative to positive.
Direction that electrons actually flow in a circuit.
Which direction does coventional current flow act in?
Positive to negative.
Opposite to actual electron flow.
How do you measure the potential difference across a component?
Voltmeter should be used connected in parallel to the component.
What should an ideal voltmeter have?
Infinite resistance, so no current is drawn from the circuit.
V = I/R
What is an ohmic conductor?
A conductor that obeys Ohm’s Law:
The current through an ohmic conductor is directly proportional to the potential difference across it when held at a constant temperature.
Ohm’s Law:
The ______ through an ohmic conductor is ______ ______ to the ______ ______ across it, when held at a constant ______.
Ohm’s Law:
The (current) through an ohmic conductor is (directly proportional) to the (potential difference) across it, when held at a constant (temperature).
I ∝ V
What does resistance depend on?
Resistivity
Cross sectional area
Length
ρ = RA/L
What is resistivity?
Resistivity is a quantity that is proportional to an object’s resistance and cross-sectional area, and inversely proportional to the object’s length.
ρ = RA/l
Superconductors are materials that have zero ______ at and below a ______ ______.
Superconductors are materials that have zero (resistivity) at and below a (critical temperature).
What are superconductors?
Superconductors are materials that have zero resistivity at and below a critical temperature.
Why are superconductors called superconductors?
Since there is no resistance, no electrical energy is turned into heat and lost, so no energy is wasted.
What are critical temperatures in terms of superconductors?
A temperature at which, if below, a material will have zero resistivity.
They differ for each material, but are currently too low to be useful.
Why would superconductors with high critical temperatures be useful?
It wouldn’t be as difficult or expensive to keep them at their critical temperature. (Not too cold)
Give three uses of superconductors.
- Power cables without losing too much energy as heat.
- Really strong electromagnets that don’t need a constant power source.
- Really fast electronic circuits, because there’s no resistance to slow them down.
What are semiconductors?
Components for which the resistance changes depending on external conditions.
Give two examples of semiconductors.
- Light Dependant Resistors (LDR) - When light intensity increases, their resistance decreases.
- Thermistors - When the temperature increases, their resistance decreases.
What does an I/V graph look like for an ohmic conductor?
A straight line graph through the origin.
What is power?
Power is defined as the rate of energy transfer.
P = W/T
Peas walk on tables
P = VI
(Painful vagina illness)
What is a Watt?
Unit of power.
1 Watt is the power dissipated when 1 joule of energy is transferred each second.
P = W/T
What does an I/V graph look like for a filament lamp?
A curve that starts steep, but gets shallower as voltage rises.
Current flowing through the lamp increases its temperature.
The resistance of a metal increases as the temperature increases.
Why does resistance increase as temperature increases?
Higher temperature causes metal ions to vibrate with more kinetic energy, making it harder for charge carriers to pass through.
What is a NTC thermistor?
NTC stands for ‘Negative Temperature Coefficient’, which means that resistance decreases as temperature increases.
What are diodes?
Components that only let current flow in one direction.
What is the threshold voltage for diodes?
0.6V. They require this voltage before they will conduct.
What is the current and resistance like for diodes in the reverse direction?
In the reverse direction, the resistance of the diode is very high and the current that flows is very tiny.
What kind of component is an LED?
Diode.
What are the voltage and current like in series circuits?
The current is the same in all positions in a series circuit.
The potential difference over the loop is split between all the components, in the ratio of their resistances.
What are the voltage and current like in parallel circuits?
The current is split between each branch, and so is different in different parts of the circuit.
The potential difference over each loop is the same.
What is Kirchhoff’s First Law?
The total current entering a junction = the total current leaving it.
“The total current entering a junction = the total current leaving it.”
Which Law is this?
Kirchhoff’s First Law.
What is Kirchhoff’s Second Law?
The total e.m.f. around a series circuit = the sum of the p.d’s of each component.
ε = ΣIR
“The total e.m.f. around a series circuit = the sum of the p.d’s of each component.”
Which Law is this?
Kirchhoff’s Second Law.
What is e.m.f., or electromotive force?
The e.m.f. is the amount of energy supplied by the source per unit charge.
ε = E/Q
What is internal resistance?
Internal resistance is resistance that occurs in batteries/cells due to the movement of electrons colliding with atoms.
What makes cells and batteries warm up whilst they’re being used?
Internal resistance.
Why is the terminal p.d. (voltage provided by battery) never equal to the e.m.f.?
Because internal resistance makes the e.m.f. smaller.
What is the terminal p.d. equal to?
V = ε - v V = ε - Ir
Where: V = terminal p.d. ε = e.m.f. v = lost volts I = current r = internal resistance
Need to learn this
What is e.m.f. equal to?
ε = I ( R + r ) (given in exam) ε = V + v (not given in exam)
Where ε = e.m.f. I = current R = load resistance r = internal resistance V = terminal p.d. v = lost volts
What is load resistance?
Load resistance is the total resistance of all the components in the external circuit.
Also called ‘external resistance’.
How do you find the total e.m.f. of the cells in a series circuit?
ε(total) = ε(1) + ε(2) + ε(3) + …
Adding their individual e.m.f.s.
How do you find the total e.m.f. of the cells in a parallel circuit?
ε(total) = ε(1) = ε(2) = ε(3) = …
Same size as the e.m.f. of each of the individual cells.
What is a potential divider?
A potential divider is a circuit with a voltage source and two resistors in series.
This results in the potential difference in the circuit being split into a specific ratio.
So, if you had a 2Ω resistor and a 3Ω resistor, you’d get 2/5 of the p.d. across the 2Ω resistor and 3/5 across the 3Ω.
Why are variable resistors used in potential dividers?
Variable resistors are used in potential dividers to change the voltage supplied to something to a more suitable voltage.
However, this method wastes power.
What is a detecting circuit?
Add a semiconductor, such as an LDR to a potential divider as the second resistor. This will change the V(out) across the first resistor dependant on the light intensity.
What is a potentiometer?
A potentiometer has a single variable resistor replacing both resistors of the potential divider.
