Y1 Ch 13 & 14 Current Electricity and Flashcards
Current Definition
The rate of flow of electrons.
Current =
Charge/Time
Direction of current
From positive to negative.
Current - Vector or Scalar
Vector
A gradient of a Q/t graph is
the current.
The area beneath an I/t graph is the
Charge.
Potential Difference Definition
Work done per unit charge between two points.
P.D. =
Work Done/Charge
Electromotive force definition
Energy supplied per unit charge by the supply.
p.d. - Vector or Scalar
Vector.
Resistance definition
The opposition to current flow.
Resistance =
P.d./Current
Superconductor
A material that has 0 resistance below its critical temperature.
Thermistors
A temperature dependent resistor.
As the temperature increases the resistance of a thermistor
decreases
Resistivity equation
R = Resistivity * L/A
Ammeters
placed in series, therefore, have no resistance.
If the resistance of an ammeter increases
The current and p.d. will decrease.
Voltmeter
Placed in series so has infinite resistance
If the resistance of a voltmeter decreases
The current of the component will decrease but the p.d. will stay the same. The overall resistance will decrease therefore total current will also increase.
Cells - with negligible internal resistance
If they are in series add together. If they are in parallel the total emf is equal to the emf of one of the cells if they are identical.
Cells - with internal resistance in series
emf = nemf and R = nr
Cells - with internal resistance in parallel
emf = emf and R = r/n
Power definition
The rate at which work is done or energy is transferred.
Power equation
P = W/T = VQ/T = VIT/T = IV
Current in a series circuit
The same across the circuit
P.d in a series circuit
emf = sum of p.d. across each component - Kirchhoff’s second law.
Resistance in a series circuit
Total resistance = sum of resistances
Current in a parallel circuit
total current = total current across all branches Kirchhoff’s first law.
Potential difference in a parallel circuit
p.d. between 2 points is the same for every loop.
Resistance in a parallel circuit
1/Rt = 1/R1 + 1/R2 + …
Contact resistance
Resistance between connections.
Internal resistance
The resistance inside the power supply
On a V/I Graph the gradient is
Internal resistance
On a V/I graph the y-intercept is
The Emf
Potential divider equation
V1 = Vin * R1/(R1 + R2)
Potential dividers with thermistors when hot
The thermistor will take out a low proportion of p.d.
Potential divider with thermistors when cold
The thermistor will take out a high proportion of p.d.
