Electric Circuits Flashcards
Define current
Rate of flow of charge
What does the gradient on a charge-time graph tell us?
Current
I = Q/t
How to you work out charge from a current-time graph?
The area under the graph
Q= It
An ammeter must…
have a low resistance and must be placed in series
A voltmeter must…
have infinitely high resistance and must be placed in parallel
Another word for voltage is…
EMF/potential difference
Potential difference is…
A measure of the amount of energy or unit of charge transferred between 2 points in a circuit
What is Kirchoff’s first law?
The total current flowing into a point is equal to the current flowing out of a point.
Conservation of Charge
What is Kirchoff’s second law?
Around any closed loop in a circuit, the sum of the p.d across all components is the p.d of the supply.
Conservation of Energy
Define power
The rate of work done
1 watt of power means…
1 joule of energy is used every second
State Ohm’s Law
V = IR
at a constant temperature
In a series circuit the current is…
the same across all components
In a parallel circuit the current is…
Total current = the sum of the current in all branches
Potential difference in a series circuit…
total p.d is split up across all components, in the ratio of their resistance
Potential difference in a parallel circuit is…
the same across each brach
p.d across each branch = p.d of source
Resistance in a series circuit…
total resistance = the sum of the resistance in all the components
Resistance in a parallel circuit…
Reciprocal of Resistance = the same of all the reciprocals of the resistance of all the components
1/RT = 1/R1 + 1/R2 + 1/R3 …
Is a resistor a conductor?
Yes - it is a conductor so current can still flow around a circuit however resists let less current flow
What effects the resistance of a resistor?
CSA of the wire
Length of the wire
Material of the wire (resistivity)
Temperature of the wire
What is resistance proportional to?
Length
What is resistance inversely proportional to?
CSA
What is the Resistivity of a material?
The resistance of a 1m^2 long sample (the resistance per unit cube)
It is a property of a material
The resistivity is constant if…
the temperature is constant
What is the conductivity?
1/resistivity
On a resistance-length graph, what pattern does the graph follow?
positive straight gradient
On a resistance-length graph, what does the gradient tell you?
resistivity/area
What does a resistance-area graph look like?
A curve getting less steep as it goes down (negative gradient)
What does a resistance-1/CSA graph look like?
Positive straight gradient
What does the gradient of a resistance-1/CSA graph mean?
It is the resistivity multiplied by the length
Prove:
1/RT = 1/R1+ 1/R2 + 1/R3 in parallel
iTotal = i1 + i2 + i3
i = V/R hence:
v/RT = v/R1 + v/R2 + v/R3
v is constant and therefore:
1/RT = 1/R1 + 1/R2+ 1/R3
What is a potentiometer?
A device that provides a p.d ranging from 0v and Vs (supply voltage)
Length needed =
length of wire x (voltage out/total voltage)
What do the letters stand for in I = nAvq
I = Current (A)
v = Drift velocity (ms-1)
A = CSA of conductor (m2)
n = Charge density (no. of electrons per m3)
q = charge on each charge carrier
What is the charge density?
the number of charge carriers per m3
A poor conductor has ___ charge carriers
few
A good conductor has ___ charge carriers
many
4V is equivalent to 4J per…
coulomb of charge
5ohm resistance means that 5v is required for…
1A of current
Vout/Vsupply =
R2 / (R1+R2)
