Electricity Flashcards
Define current
- The rate of flow of charged particles
What is the equation for current?
- I = ΔQ / Δt
What is the equation for Voltage?
- V = W / Q
- or
- V = E / Q
Define Resistance
- R = V / I
Define Voltage
- One volt is one Joule per coulomb
- It is what makes the current flow
- V=W/Q
What is Ohms law?
- Special case where current is directly proportional to voltage for constant temperature
What is the the ditribution of current a consequence of?
- Conservation of charge
- Total current into a junction = Total current out
What is the distribution of potential difference a consequence of?
- Conservation of energy
- Total emf from cells = Total p.d in circuit
Derive the equation for combining resistances in parallel
- IT = I1 + I2 (conservation of charge)
- VT / RT = V1 / R1 + V2 / R2
- 1 / RT = 1 / R1 + 1 / R2
Derive the equation for combining resistances in series
- VT = V1 + V2 (Conservation of energy)
- ITRT = I1R1 + I2R2 (I cancels as current is the same in series circuit)
- RT = R1 + R2
What are the seven base units?
- Second
- Metre
- Kilogram
- Amp
- Kelvin
- Mole
- Candela
What does V and I do in series?
- V splits (Conservation of energy)
- I is same (Conservation of charge)
What do V and I do in parallel
- V is same across each branch (conservation of energy)
- I splits between branches (conservation of charge)
What are the equations for power
- P = IV
- P = I2R and P = V2 / R
What is the resistivity equation?
R = pl / A
What is the equation which includes charge carrier density?
- I = nAve or I = nAvq
What is the difference between potential difference and potential
- P.D is difference in potential between two points
- Potential is joules per coulomb at a point
Define emf
- emf is the energy per coulomb converted into electrical energy by the cell
Define internal resistance
- Resistance in battery itself which causes lost volts as heat
Define terminal p.d
- Terminal p.d is the electrical energy converted per coulomb into other forms
Define Resistivity
- A constant for a given material at a given temperature
What does emf stand for?
- Electromotive force
What are Thermistors and LDRs (in general)?
- Semiconducters
- They are resistors whose resistance is based on local conditions
What is the rule for Thermistors for a negative thermal coefficient?
- As temperature increases, resistance decreases
Explain why when temperature increases in Thermistors, the resistance decreases?
- I = nAve
- As temperature increases, drift velocity decreases as there are more collisions between electrons and lattice. Amplitude of oscillations increases.
- But charge carrier density increases as more charge carriers are released hence why resistance decreases
What is a use for Thermistors and LDRs?
- Sensors
What happens if you increase the light intensity at LDR?
- As more photons hit the LDR, more charge carriers are released
- This means charge carrier density increases, current increases and resistance decreases
What is the ideal voltmeter?
- Ideal voltmetere draws no current
- Has ∞ resistance
What is an ideal Ameter?
- Ideal Ameter has no voltage drop
- Resistance of 0
What is the real resistance of an Ameter?
- 0.5 Ohms
What is real resistance of a voltmeter?
- 50 mega Ohms
What happens to the potential as you travel along a wire with no components?
- It decreases linearly
Draw a graph for distance along wire and potential difference?
Learn this diagram on Potential Dividers!
Equations for Energy?
- E = IVt or W = IVt
- E = Pt
Core practical 2: Determine electrical resistivity
- Independant
- Dependant
- Equation
- y = mx + c
- Graph to plot
- Length
- Resistance
- R = pl / A
- y = mx + c
- y axis is R
- x axis is l
- m = resistivity/A
Core practical 3: Determine emf and internal r of cell
- Independant
- Dependant
- Equation
- y = mx + c
- Plot graph
- Current
- Terminal p.d
- E = V + Ir
- y = mx + c
- y axis = V
- x axis = I
- m = r
- y intercept = emf
