Topic 3 - Electric Circuits Flashcards
Define electric current
‘The rate of flow of charged particles’
(Spec point 31)
Give the equation for current
I = delta Q/delta t
A)What is the charge of elementary charge
B) what is its unit
1.6 x 10^-19
Coulombs
What is the charge of an electron in terms of elementary charge?
-e
What is the symbol for elementary charge?
e
What is the charge of a proton?
e
What is 1 mole of something?
6.02 x 10 ^23 of that thing
What is avogadros constant (give the number)
6.02 x 10^23
Current is the same/different across a series circuit
Same
Electrons flow -/+ to -/+
Negative to positive always
Conventional current flows from the -/+ terminal to the -/+ terminal and why?
Positive to negative
Current is defined as flow of positive charge (you can see this from the equation) and as electrons carry negative charge, the direction of current flow is the opposite to the direction of the flow of electrons
True or false: only electrons can cause current to flow
False - protons and other ions can as well
What is the equation for potential difference
V = W/Q
Potential difference = work done/charge
What is EMF?
Electromotive force - the work done per unit charge by the power supply or cell converting energy into electrical energy of the charges
Define potential difference
The ENERGY TRANSFERRED between two points in a circuit PER UNIT CHARGE
(Things in capitals are important marking points)
Net electric charge flow is the sum of what
Negative and positive charge flow - do not take the magnitude of the negative flow, as the negative cancels out the positive as net charge flow measures the total imbalance in charge, not the movement of electrons
Explain why a proton in a space between a positively charged surface and a negatively charged one (where it is further from the negative surface than the positive), accelerates towards the negatively charged surface. Explain the energy transfers in this process.
- The proton has a high electrical potential energy at first when it is far from the negative surface and There is a downward force on the proton
a. Because like charges repel and opposite charges attract, so the charge is repelled from the top and attracted to the bottom- The speed of the proton increases as it accelerates and so the kinetic energy of the proton increases
The electrical potential energy decreases and it reaches the negative surface
- The speed of the proton increases as it accelerates and so the kinetic energy of the proton increases
A proton has a higher electrical potential energy far/close to a negatively charged surface
Far - as there is a large potential for work to be done
True or false: potential is the same as electrical potential energy
False
Positive charges move from a high/low potential to a high/low potential
High to low
As think of all positive things as a high potential as they have a high potential to move to the negative thing as their point charge is pointing outwards. They all want to go to the negative (low potential)
Potential is defined between two points/at a point
At a point
What are the units for potential
Volts
What is the mass of an electron
9.1 x 10 ^-31 kg
What is the radius and what is the nucleus of an atom?
Radius = 0.1nm = 1 x 10^-10
Nucleus = 10fm = 1 x 10^-14
[l] = ?
Metres - m
(The question means what is the unit of length)
[ T ] = ?
Kelvin - K
Symbol for time
t
Moles = [?]
n
What is the symbol for the amount of a substance?
n
How many degrees Kelvin is 100 degrees Celsius?
373
How many degrees Kelvin is 0 degrees Celsius?
273
What is 0 degrees Kelvin in Celsius?
-273
What happens at O degrees Kelvin
All particles stop moving, they have 0 kinetic energy
Give the 7 base units and their symbols
Length - l -metres - m
Time - t - seconds - s
Mass - m - kg
Temperature - T - Kelvin - K
Current - I - Amperes - A
Amount of a substance - n - moles - mol
Luminous intensity - Lv - Candela - Cd
How many derived units are there?
Anything that isn’t a base unit is a derived one - and you can always write a derived unit in terms of base units
What is Joules expressed in base units using the equation for kinetic energy?
Kg (m/s)^2
REMEMBER THE BRACKETS - kgm^2/s^2
Describe in words what V = IR means
Potential difference across the resistor = current through the resistor x the resistance of the resistor
What is kirchoff’s current law?
Sum of currents into a junction equals sum of currents out
Explain the derivation of kirchoff’s current law using the conservation of charge
- net charge in a closed system doesn’t change
- therefore sum of charge into a junction = sum of charge out of a junction
- Divide the charge on each side by time (ensure you still use sigma to denote sum of in this line of working!)
- therefore sum of current in = sum of current out
Is current vector or scalar and why?
Scalar - as it can only be positive or negative, so it is one dimensional
It is also defined as change in current over change in time, and both change in current and change in time are scalar quantities, and you can’t get a vector quantity from a scalar one
What is kirchoff’s voltage law?
The sum of the potential difference is equal to the sum of the EMFs around a closed loop in the circuit
How is kirchoff’s voltage law derived?
The law of the conservation of energy states energy cannot be created or destroyed
Sum of Energy released by EMFs = sum of energy dissipated by components
Sum of VQ = sum of VQ
Therefore sum of potential difference released by EMF = sum of potential difference across component
What is the symbol for femto and what order of magnitude is it?
f - 10^-15
What is the symbol for femto and what order of magnitude is it?
f - 10^-15
If you move a charge across a voltage of 1V then the charge gains how much energy
An energy whcih is equal to its charge
(If you moved it across more than 1V then it gains energy that is its charge x the voltage )
Explain why potential difference across branches of a parallel circuit (with the same component on each) is the same
- Kirchhoff’s voltage law says voltage in a closed loop is equal to voltage of EMF in that loop
- therefore applying this law to the two loops of the two branches shows that the voltage on one branch = voltage on other branch = voltage of EMF
Explain why potential difference splits across components connected in series
- kirchoff’s voltage law states that the sum of the voltages = the sum of EMF in a closed loop
- therefore the voltage of the EMF is split across the components connected in series, as they only form one closed loop
Describe how current splits across branches in parallel
It splits according to the ratio of the resistances, favouring lower resistances (more current flows in the lower resistance branch)
How do you find total resistance in series?
The sum of the resistances
What are the only 3 laws you need to know about circuits in a level physics?
Kirchoffs voltage law, his current law, and v = IR
Derive the equation for total resistance in a series circuit
NOTE DO NOT USE KVL OR KCL IN THIS QUESTION
KVL says total voltage = sum of voltages
As V = IR, IR total = sum of IR for each component
KCL says I total = sum of currents
Therefore we can cancel the total current to get
R total = sum of individual resistances
Derive the equation for total resistance in a parallel circuit
Total current = sum of currents due to KCL
As V=IR, total current = V/R = the sum of V/R for each component
Due to KVL, voltage of EMF is the same as voltage of each component, so that can be cancelled
Therefore 1/Rtotal = sum of 1/R for each component
What is the key part of KVL which you need to pay attention to to get questions about KVL correct?
Sum of voltage = sum of EMF IN A CLOSED LOOP (a closed loop means a loop with no breaks (just a loop in series which links the EMF to the component(s))
Are Resistor 1 and resistor 3 connected in series or parallel and why?
They are not connected in series, and they are also not connected in parallel!!
They can’t be in series as they have different currents, but they can’t be in parallel because they are not