Electricity Flashcards
Define current
It is the rate of flow of charge
What is Kirchhoff’s first law
That the sum of the currents entering in a junction is equal to the sum leaving the junction
What is the distinction between conductors, semi conductors and insulators in terms of n
Conductors have the greatest number density followed by semi conductors, then insulators.
Define emf
Work done on charge carriers per unit charge
Define p.d.s
Is energy transferred from electrical energy to other forms per unit charge
What is Kirchhoff’s second law
Sum of emf = sum of p.d.s around a closed loop
What are the energy transfer equations
W=VQ W=ξQ eV=0.5mv²
What is ohms law
Current is directly proportional to p.d.s when there is a constant temperature
What is the I-V characteristics of a resistor
Resistance is constant
What is the I-V characteristic of a filament lamp
As current increases so does temperature, increasing resistance
What is the I-V characteristic of a diode
Current only flows in one direction. Resistance decreases when currents increases.
What is the I-V characteristics of a thermistor
As temperature increases resistance decreases
What happens to resistance when length increases
Length is directly proportional to resistance. If length doubles so will resistance.
What happens to resistance when area increases
Area is inversely proportional to resistance. If area doubles resistance will half.
How does resistance vary in an LDR
As light intensity increases resistance decreases
Equation for resistivity
ρ=RA/l measured in ohm meters
To find resistivity of a metal
- Find the area of the wire using a micrometer to find the diameter.
- Create a circuit with the test wire being attached to a ruler.
- Using crocodile clips vary the length of the wire recording current and voltage. Get repeats and do this for different lengths.
- Then work out resistance and plot a resistance against length graph
- The gradient will be ρ/a.
How does temperature effect resistivity
As temperature increases resistivity increases
Equations for electrical power
P=VI
P=I²R
P=V²/R
Energy transfer equation
W=VIt
What is a kilowatt-hour
Is a unit of energy calculating cost of energy
How calculate resistance in series
R=R1 + R2 + …
How to calculate resistance in parallel
1/R= 1/R1 + 1/R2 + …
What is internal resistance
It is resistance inside the battery from electrons colliding with atoms.
What is lost volts
It is the difference between between emf and terminal p.d.s
Equations for emf
ε=I(R+r)
ε=V+Ir
How to determine internal resistance
- Vary the circuit by changing the value of the variable resistor and measure p.d. And current at different values.
- Plot a V against I graph
- rearrange to give V=-rI+ε
- emf is the y intercept and -r is the gradient
What are potential divider equations
Vout=(R2/R1+R2)xVin
V1/V2=R1/R2
What is a potentiometer
A potentiometer has a variable resistor to vary the voltage using a slider or knob. This adjusts the relative sizes of R1 and R2.
How to investigate potential divider
You can make a potential divider using an LDR or a thermistor.
Set up equipment with a fixed resistor and a variable resistor such as an LDR. You should measure Vout across the LDR as you change light intensity and plot a graph.
