CURRENT ELECTRICTY Flashcards
Mr Prestwich's lessons
In a series circuit the current…
…is the same at all points.
In a parallel circuit the current in less in…
…branches with more resistance.
The sum of all currents in each branch is…
…the total current entering or leaving the supply.
In a parallel circuit the total p.d. in each branch…
…is the same.
1/ RT = 1/ R1 + 1/R2 +1/ R3. This rule is for what type of circuit?
Parallel.
Resistance in a series circuit is calculated by?
Adding all the individual resistances.
Potential difference is shared between components, with components with bigger resistances taking more p.d. in what type of circuit?
Series.
Charge (Q) =
(change in) Current (I) x (change in) Time (t)
What is Ohm’s law?
Ohm’s Law states that the current through a conductor is directly proportional to the potential difference.
If something doesn’t apply Ohm’s Law what does it mean? Give and example?
It means that current and potential difference is not directly proportional to one another. Examples: Diode, LED, Filament lamp.
What is a coulomb?
A coulomb is the charge transported by a steady current of one amp in one second.
What is an amp?
An amp is what current is measured in.
What is potential difference? What is it measured in?
Potential difference is the work done per unit charge. It is measured in Volts (p.d. is also known as Voltage).
What is current?
The rate of charge passing a point in a circuit.
What are the 4 things does resistivity depend on?
Material, length, cross sectional area and temperature.
What does the characteristic curve of a Filament lamp look like?
It is an S shaped curve, as current increases voltage increases exponentially. (S curves towards the voltage axis. This means you can draw it two different ways depending whether p.d. is on the x or y axis.)
Resistivity is a constant for a material. What is the definition of Resistivity?
The resistivity of a material is the resistance of a sample which is one metre long and has a cross-sectional area of 1m^2.
What is the equation you can find Resistivity from?
Resistance (R) = Resistivity(roh/p ) x (Length (L) /Cross-sectional Area (A))
What are the units for all the components of the Resistivity equation?
R = Ohms
Resistivity = Ohm Metres
Length = Metres
Cross-sectional Area = Metres^2
Describe a conductor? What does a temperature increase do? (2 marks)
- Has delocalised electrons, so charge can be passed.
- Increase in Kinetic Energy, increase in movement of electrons, more resistance.
Describe an insulator? What does a temperature increase do? (3 marks)
- No free electrons (very high resistance)
- Doesn’t generally change arrangement, but very high temperature can cause an electron to escape.
Describe the effect of temperature on a semi-conductor?
- At low temps act like insulator.
- As temperature increases, resistance decreases.
- Resistivity decreases as temp increases.
Describe a super conductor?
- Have very low resistance.
- Have to be super cooled.
- Used in MRI scanners.
How do you work out cross-sectional area? (5 steps)
- Measure diameter
- Halve it to get radius
- Convert into metres
- Calculate using: A= pi x r^2
- UNITS = m^2
Power (Watts) = (3 answers)
- Current (I) x Potential difference (V)
- Current^2 (I) x Resistance (Ohms)
- Potential difference^2 (V) / Resistance (Ohms)
Efficiency (%) =
(Useful Energy (J) / Total energy (J) ) x 100
Energy (J) = (3 answers)
- Power (W) x Time (secs)
- Potential difference (V) x Current (I) x Time (secs)
- Charge (Q) x Voltage (V)
Potential difference (V) =
Current (I) x Resistance (Ohms)
V out (V) =
(V in (V) x Resistance 2 (Ohms) ) / (Resistance 1 + Resistance 1 (both is Ohms) )
What is a potential divider?
It has 2 or more resistors in series and a source with a fixed p.d.
What does a potential divider do?
It supplies a set p.d. which you can vary depending how much p.d. you want. You have a constant input and get a variable output.
What special thing can potential dividers also do? (Hint: thermistor)
Can provide a p.d. which varies with physical condition such as pressure or temperature.
A potential divider can be used as a “potentiometer”. What does this do?
Controls voltage by dividing the resistance along the coil. Has to have to have 3 points of contact.
Define: Electromotive Force (Emf: shown by a Greek Epsilon)
The Emf of a source is the ELECTRICAL POTENTIAL energy transferred FROM other forms of energy to other forms PER COULOMB OF CHARGE.
What is a simple description of Emf, with UNITS?
Potential energy per unit charge, measured in Volts.
What is the internal resistance?
The resistance in the actual cell.
Why do dry batteries have a high internal resistance?
So that they can get hot, as a high internal resistance causes a lot of energy to be dissipated over it.
Emf (V) = (2 answers)
- Terminal p.d. (V) + Volts lost across Internal Resistance (V).
- (Current (I) x Resistance of circuit (Ohms) ) + (Current (I) x Internal resistance (Ohms) )
How does the Emf equations apply to a graph of y = mx + c?
y = V
x = I
m = -r (internal resistance)
+ c = Epsilon
You can measure the Emf across the terminals of the cell when…
…There is no current so no energy dissipated across the internal resistance so full emf value found.
What does a oscilloscope plot?
Voltage against time.
What is direct current? What does it look like on a oscilloscope?
It is where the current only flows in one direction.
A straight line of constant voltage.
What is alternating current? What does it look like on a oscilloscope?
It is where the current is currently changing direction.
It is an oscillating sinusoidal wave.
What is d.c. and a.c. current used for?
Direct current comes from batteries.
Alternating current is like mains current in the UK.
Frequency (Hz) =
1 / Time period of one wave (secs)
What is the V rms?
The root mean squared voltage, meaning the the mean a.c. voltage squared and rooted.
Which is higher V rms or V o (Peak voltage)?
V o because V rms is the mean of the peak voltages.
V rms (V) =
V o (V) / root2
I o (A) / root2 =
I rms (A) (root mean squared current).
What is the x axis and y axis of an oscilloscope?
x axis: time base (seconds or milliseconds per division)
y axis: Y-gain / volts per divison
How does an oscilloscope work?
- Beam of electrons fired at screen
- Y plates (left and right) deflect the beam up and down.
- X plates (above and below) deflect the beam side to side.
What does a spot on a oscilloscope show? (2 marks)
Time base off, no voltage.
What does a dot raised up/down from the centre on a oscilloscope show? (2 marks)
Time base off, d.c. input.
What does a straight horizontal line anywhere on the oscilloscope screen show? (2 marks)
Time base on, d.c. input.
What does a straight vertical line on the centre of the oscilloscope screen show? (2 marks)
Time base off, a.c. input.
What does a sinusoidal wave on the oscilloscope screen show? (2 marks)
Time base on, a.c. input.
Time period (secs) =
Length on one wavelength (in divisions) x Time base (sec per division).
V pp =
The peak to peak distance, measured in the time base.
