Section 5 - Electricity Flashcards
What is needed for current to flow?
Mobile charge carriers
What way does convetional current flow?
From positive to negative
Define current. Equation and words
- I = ΔQ/Δt
- Current is the rate of change of charge
What is an insulator?
Every electron is attatched to an atom
What is a conductor?
There are de-localised electrons
What is a semi-conductor?
The number fo charge carriers changes with a physical conditon
Define voltage. Equation and words
- V = W/Q
- Volatge is the workdone per unit charge
Define EMF
- Electromotive force
- The electrical energy produced per unit charge passing through a source
What are the 3 power eqautions?
- P = IV
- P = I²R
- P = V²/R
What is the definiton of resistance? Equation and words.
- R = V/I
- Resistance is the ratio of Voltage across a compnent to the current through it.
How much resistance should an ideal Ammeter have and why?
- It should have no or very low resistance
- Because they are always connected in series and they should not affect the overall ciruit resistance
How much resistance should an ideal Voltmeter have and why?
- It should have infinite resistance
- Because if current flows through the voltmeter the voltage will be shared across the component and the voltmeter.
What is ohms law?
PD across and ohmic conductor is proportional to current through it provied the physical conditons don’t change
Define resistivity
A property of a material which gives a reference to the amount of resistance in a material
What is the resistivity equation?
R = ρ . L/A
What happens to resistance as the length of a material increases and why?
- It goes up.
- Because as length increases the amount of collisions wiht other electrons increases.
What happens to resistance as area increases?
It decreases becuase there is more spaces for more free electrons to flow without colliding
What is a super conductor?
A material that has no resistance when cooled below the critical temperature
What are some uses of superconductors?
- Mag-lev trains
- MRI scaners
- Particle accelerators
- Strong electromagnets
Draw a resistivity(Ω) vs temperature(K) graph for a superconductor
Check notes for graph
Draw the I / V graph for a wire (ohmic conductor)
Linear straight through origin
Draw the I / V graph for a fillament lamp
s-shape
Draw the I / V graph for a high temperature and low temperature graph on the same axes
check notes
Draw the I / V graph for a diode
Define threshold voltage
The minimum volatage needed to allow current to flow through a diode
Draw the graph for an ntc
Check notes
What happens to resistance in a PTC as temperature increases?
Resistance increases
What happens to resistance in a NTC as temperature increases?
Resistance decreases
What is Kirchoff’s first law?
the total current flowing into a junction is equal to the current
flowing out of that junction
What is Kirchoff’s second law?
the sum of all the voltages in a series circuit is equal to the
battery voltage
Formula for resistors in series?
R₁+R₂+R₃+…
Formula for resistors in parallel?
1/R₁ + 1/R₂ + 1/R₃ + …
What is the total current equal to in a parallel circuit?
The sum of the current in all the branches
True or false the PD across in each branch in the same as the supply voltage?
True
Formula for voltages in series
V₁+V₂+V₃ + …
Formula for indentical voltages in parallel
V₁=V₂=V₃ = …
For a series circuit the current is?
The same everywhere in the circuit
For a series circuit the total volatage is equal to?
The supply voltage
What is the rate of heat transfer equation?
I²R
Define internal resistance
The resistance to the flow of charge within a source
What is the emf formula?
- ε = I(R+r)
- ε = V(load) + V(int-resistance)
What is the formual for power supplied by the cell?
I.ε =I²R + I²r
What is terminal PD?
The voltage across the terminals of the battery
What is the y-int and the gradient on an terminal pd vs I graph?
- The y-int is the emf
- The gradient is -r
How is Y = mX + c realted to the emf formula?
V = -r.I + ε
Y = mX+c
What is a potential divider?
A circuit with two or more resistors in parallel with a source of fixed PD
What is the purpose of a potential divider?
- To supply a fixed voltage between 0v and the supply voltage
- To supply a variable PD
- To supply a varibale PD that changes with physical conditions
What is the potential divider formula?
V(out) = (V(supply) R₁ )
——————-
R₁+R₂
What is an ohmic conductor?
A component with constant resistance
The net motion of electrons are the cause of electric currents in metallic wires. What particles with charge that is greater than ±1e can also cause an electric current?
Ions
How to find short circuit current on an internal resistance graph?
x intercept
Typical voltage accross a led that is conducting?
1.8v
