9,10. Energy, Power, Resistance, Circuits (4.2-4.3) Flashcards
Can you draw all the circuit symbols?
https://www.savemyexams.com/a-level/physics/ocr/17/revision-notes/4-electrons-waves-and-photons/4-1-charge-and-current/4-1-6-circuit-symbols-and-diagrams/
What are the definitions of potential difference and electromotive force in terms of work done?
- p.d is work done by the charge carriers
- e.m.f is work done on the charge carriers
What are the definitions of p.d and e.m.f in terms of energy stores?
- p.d is the energy transfer from electrical energy to other forms per 1 coulomb of charge
- e.m.f is the energy transfer from other forms to electrical energy per 1 coulomb of charge
What resistance should the ideal voltmeter have?
Infinite resistance so no current will pass through it (all current will go through the component we want to measure)
Equation for energy transfer:
a) in terms of p.d
b) in terms of e.m.f
a) W = VQ
b) W = εQ
What is the equation for energy transfer for electrons and other charged particles (electron gun stuff)
ke = 1/2 x mv^2
eV = 1/2 x mv^2
Thermionic emission
The emission of electrons from a heated source
What is the equation for resistivity?
R = ρL/A
What is resistance a measure of?
How difficult it is to push electrons through a component of a circuit (not completely accurate but you get the point)
What is an ohmic conducted?
A conductor where current is directly proportional to p.d
What is a non-ohmic conductor?
A conductor where current is not directly proportional to p,d
What is ohm’s law?
For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to the p.d.
Draw IV characteristics of:
a) filament lamp
b) diode
c) thermistor
d) light emitting diode (LED)
https://www.savemyexams.com/a-level/physics/ocr/17/revision-notes/4-electrons-waves-and-photons/4-2-resistance-and-resistivity/4-2-2-ohms-law-and-i-v-characteristics/
How does resistivity change as temp increases
a) for metals
b) for semiconductors
a) resistivity increases
b) resistivity decreases
is the temperature coefficient positive or negative
a) for metals
b) for semiconductors
a) positive
b) negative
What is the equation for energy in terms of p.d, current and time?
W = VIt
What is the kilowatt hour a unit of?
Energy
What is Kirchhoff’s first law?
total current in = total current out
ΣI in = ΣI out
What is Kirchhoff’s second law?
If a battery has negligible internal resistance, the e.m.f into the circuit is equal to the p.d of the circuit
Σ ε = ΣV
Relationships of I, V and R in a Series Circuit:
V (total) = V(1) + V(2) + …
I (total) = I(1) = I(2) = …
R (total) = R(1) + R(2) + …
Relationships of I,V and R in Parallel Circuits.
V (total) = V(1) = V(2) = …
I (total) = I(1) + I(2) + …
1/R (total) = 1/R(1) + 1/R(2) + …
What happens to the total resistance of a parallel circuit if you add more resistors in parallel
It decreases.
What is the equation of e.m.f
a) in relation to p.d, current, and internal resistance
b) in terms of current, resistor circuit resistance, and internal resistance
c) terminal p.d and lost volts
a) ε = V + Ir
b) ε = IR + Ir
c) ε = terminal p.d + lost volts
What is the relationship between V1, V2, R1, R2 in a potential divider circuit?
V1/V2 = R1/R2
Equation for Vout in potential divider circuit:
Vout = (R2/(R1+R2))*Vin
Explain what happens to Vout of a potential divider circuit when you add a load (another resistor) in parallel to R2. (3)
- due to 1/Rt = 1/R2 + 1/Rload
- so as you add a load, the total resistance decreases
- thus the voltage decreases with resistance in direct proportion/V=IR
To maintain a high Vout in a potential divider circuit, how large do you want Rload to be?
Very high
What should the resistance be of an ideal voltmeter?
Infinite (so “no” current goes through it)
What are the advantages of a potentiometer compared with a potential divider? (3)
any three from:
- sliding contact/continuous change in resistance (as compared to discrete changes in resistance for potential divider)
- cheaper
- simpler
- more compact
What is the purpose of adding a variable resistor to a potential divider circuit? (1)
So Vout can vary at a specific temperature
