Section 6 - Electricity Flashcards
Define current
The rate of flow of charge
Which direction does conventional current flow?
positive to negative
Define a coulomb
The amount of charge that passes in 1 second when the current = 1A
Define potential difference
Work done per unit charge moved
What is the potential difference across components connected in parallel
The same
Define a volt
When you convert 1J of energy moving 1 coulomb of charge through the component
Define Ohm’s law
The current across an ohmic conductor is directly proportional to the potential difference when the physical conditions are kept constant
What do I/V graphs show?
How resistance varies
What does the I/V graph for an ohmic conductor look like
Straight line through the origin
I/V graph: What would a low resistance and high resistance look like on the graph?
Straight steep line for low resistance
Straight shallow line for high resistance
What is the I/V graph for a filament lamp?
A curve that starts steep but gets shallower, and passes through the origin
What is a filament lamp?
A coiled up length of metal wire
When does the resistance of a metal increase?
When the temperature increases
Why aren’t semi conductors as good as conducting as metals?
Far fewer charge carriers
What happens when you supply energy to a semi conductor?
Charge carriers are released
Why do semiconductors make good sensors?
Because when energy is supplied, more charge carriers can be released
What happens as temperature increases in a thermistor?
Resistance decreases
What does the resistance/temperature graph look like for a thermistor?
Top left to bottom right curve
What does the I/V graph look like for a thermistor?
bottom left upward curve to origin, top right downward curve from origin
What happens as the voltage increases in a thermistor?
Current increases
What happens to a thermistor as current increases?
It heats up, and resistance decreases -> graph curves upwards as more current can flow
Why does heating up a thermistor reduce resistance?
Gives electrons enough energy to escape from atoms -> more charge carriers
What do diodes do?
Only let current flow in one direction
What is forward bias on a diode?
The direction in which the current is allowed to flow
What is the threshold voltage on most diodes?
0.6V in the forward direction
Explain what happens when the diode is in reverse direction
Resistance is very high, so a tiny amount of current can flow
What things determine resistance?
Length, area, resistivity
How does length effect resistance?
Longer the wire, more difficult it is for current to flow
How does area effect resistance?
Wider the wire, easier it is for current to flow
How does resistivity effect resistance?
Depends on material and structure. Generally depends on temperature and light intensity
Define resistivity
The resistance of a 1m length with a 1m^2 area
What are superconductors?
Materials with 0 resistance
How do you lower the resistivity of many materials?
Cooling them down
How do you make a material a superconductor?
Cool them down below the transition temperature and their resistivity disappears entirely. ONLY WORKS WITH CERTAIN MATERIALS e.g mercury
What happens when a material has no resistance?
No electrical energy is turned into heat, so you can use a magnetic field to flow current
What’s the down side of superconductors?
Most normal conductors have transition temperatures at 10K (-263C) - difficult to do and expensive
What are applications of superconductors?
Power cables, strong electromagnets - Maglev trains, electric circuits work really fast
Define power
The rate of transfer of energy
How can you calculate the energy if you know the power and the time?
E=Pt
How can you calculate the energy if you know the power?
E=IVt
Where does resistance come from?
Electrons colliding with atoms and losing energy
What type of energy makes electrons move?
Chemical energy
Why must batteries have resistance?
Otherwise there wouldn’t be any atoms for electrons to collide with
What is the resistance inside the battery called?
Internal resistance
Define e.m.f
The amount of electrical energy the battery produces for each coulomb of charge
What is the potential difference across the load resistance?
Energy transferred when 1 coulomb of charge flows through the load resistance
What are lost volts?
Energy wasted per coulomb overcoming the internal resistance
How do you calculate the e.m.f of cells in a series circuit?
Etotal = e1 + e2 + e3
How do you calculate the e.m.f of cells in a parallel circuit?
Etotal = e1 = e2 = e3
What’s the easiest way to measure e.m.f of a power supply?
Connecting a high resistance voltmeter across its terminals
What happens to charge in a circuit?
It is conserved
How do you calculate total I, V and R in a series circuit
I = I1 = I2 = I3, V = V1 + V2 + V3, R = R1 + R2 + R3
How do you calculate total I, V and R in a parallel circuit
I = I1 + I2 + I3, V = V1 + V2 + V3, 1/R = 1/R1 + 1/R2 + 1/R3
`NEED TO DO P86,87
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