Electricity Flashcards
What is alternating current?
An alternating current (a.c.) changes its direction and instantaneous value with time.
What is direct current?
In direct current (d.c.), the electrons flows in one direction only. Its value is constant.
In an oscilloscope what does the “time base” control?
time base” controls how much time each division on the x-axis is worth.
In an oscilloscope what does the “Y- Gain” control?
“Y Gain” controls the voltage each ‘division’ on the yaxis is worth.
What does the a.c current trace look like?
What does A.C look like on an Ossocilsope?
What does D.C look like on an Ossocilsope?
Solid line———————————
How do you calculate the peak voltage?
Peak voltage = Number of divisions x volts per division
Number of divisions x time per division = what
PERIOD (T) of the wave (length of one wavelength across)
What is the equation for frequency?
frequency (f) = 1/T
What happens when the timebase of the ossolociope is turned off?
The a.c. the signal will not spread along the xaxis. The voltage variation will continue to oscillate up and down meaning a straight vertical line will be displayed on the screen.
Aa.c. voltages are stated in terms of their r.m.s. (root mean square) values what are R.M S Values?
These are a kind of ‘average’ of the alternating a.c. supply.
Where is the peak voltage of the wave found?
How do you calculate the RMS value?
VRMS = VPEAK / Square root 2
What is the equation to find out V peak
Vpeak = √2 Vrms
Also, since V = IR where R is a constant, this extends to current too. What is the I peak involving Current
Ipeak = √2 Irms
The r.m.s. value of an a.c. supply is equal to what?
a d.c. supply value which gives the same power output.
What is the voltage of a mains supply in UK?
The voltage of 230 volts
Definition of charge?
Current is defined as the flow of electric charge per second.
The equation to find out the charge?
I= Q/t
Current (Amperes) = Charge(Coulombs)/ Time(Seconds)
That means 1 coulomb is equal to 6.25x1018 electrons.
True or False?
True
What is the definition of voltage (potential difference)?
‘Potential Difference’ as the number of joules of work energy needed to drive each coulomb of charge from one point in the circuit to another.
1 volt is equal to
1 joule for every coulomb of charge.
This gives us the relationship:
V = Ew/Q
or Ew = QV
Ohm’s Law
If you increase the voltage (potential difference) across a component in a circuit with resistance R, the current through it will do what?
What does the gradient of the graph equal?
What equation do you get out of it? (v=Something)
increase in direct proportion
Resistance
V=IR
OHM’s Law
If the resistance is not constant V and I are no longer what?
What does the graph look like?
directly proportional the line of best fit will not longer be straight
What does power mean?
Energy transformed per second.
What are the 4 power equations?
P = E/t
P = IV
P = I^2 R
P = V2/R
Current Series equation
IS = I1 = I2 = I3
Current Parallel equation?
Ip = I1 + I2 + I3
Voltage series equation
VS = V1 + V2 + V3
Voltage parallel equation?
VS = V1 = V2 = V3
Resistance series equation?
Rs=R1+R2+R3
Resistance in parallel circuit?
1/Rp=1/R1+1/R2+1/R3
Potential dividers what do they do?
Potential (Voltage) dividers split a supply voltage between two or more resistors in the same proportion as the resistors to the total resistance in the circuit.
What are the two rules for potential dividers?
V1/V2 = R1/R2
V1 =( R1 / (R1 + R2) )Vs
V1 will decrease (because R1 decreases)
V2 will increase (Because V1 + V2 must equal 6V)
In Thermisters with potential dividers? Temperature increases as what decreases.
TURD
Temperature
UP
Resistance
DOWN
In LDR’s with potential dividers? Temperature increases as what decreases.
TURD
Temperature
UP
Resistance
DOWN
What is this called
Wheatstone bridge
What do voltmeters measure?
Voltmeters measure ‘potential difference’ i.e. the difference in the potential at these two points.
What is E.M. F?
The electromotive force (e.m.f.) of a source is the electrical energy supplied to each coulomb of charge which passes through the source.
What is meant by an E. M.f of 1.5 volts
1.5 J of energy is given for each column of charge (the passes through the supply)
What would an ideal supply be like?
An Ideal Supply would be one in which there is no internal resistance.That’s means the voltage available to the circuit is equal to the emf. In reality this doesn’t happen!
As the battery has a resistance of its own, we say that it has a what?
internal resistance, r.
What are the “LOST VOLTS”
‘Lost volts’ is the energy (per coloumb) that will be used up in overcoming the internal resistance of the supply.
The energy (per coulomb) left for the circuit is called the terminal potential difference or TPD. What is the tpd?
The TPD is the voltage obtained from a source of electrical energy when a current is being drawn from it.
E.m.f = t.p.d + what?
E.m.f = t.p.d + lost volts
From OHM’s Laws what is the equation to find out lost volts and t.p.d
Lost volts = Ir
t.p.d. = IR
What are the 3 equations to figure out E.M.F?
E.m.f = IR + Ir
E.m.f = I(R + r)
E.m.f= t.p.d+ lost volts
Describe how you would carry out the experiment determining the emf and external resistance
1) measure E.M.F of battery
2) set up a circuit with variable resistor, ammeter and voltmeter across the battery
3) Set up variable resistor to measure value
4) gradually lower resistance taking readings of voltage
5) plot readings on graph
Where can r be found on a graph
-gradient of the line
EMF can be found how on a graph?
y intercept
Why does Vtpd decrease when current increases?
The current increase, the lost volts increases (Lost volts = IR)
This means more energy is lost (per coloumb) of elections through the supply. As more voltage is lost across the supply there will be less for the external circuit therefore the TPD will decrease
Describe an open circuit? (I= O)
As there is no current, the lost volts will be 0.
Therefore the Emf = tpd.
You can use an open circuit to measure the Emf. This can be done by connecting a voltmeter or oscilloscope across the supply when there is no current flowing. The reading on the voltmeter will be the emf.
In exam questions if the switch is open (off) or the current is zero the voltmeter reading across the supply will be what?
the emf.
Describe an underload circuit?
This is when the circuit is working normally. There will be ‘lost volts’ due to the internal resistance. In these cases we need to use our internal resistance equations.
Emf = V + Ir
Emf= IR+Ir
In exam questions if the switch is closed (on) the voltmeter reading across the supply will be what?
the tpd.
Describe a short circuit?
There is no external resistance.All of the energy from the battery is lost because of internal resistance. By substituting R = 0 in the above equation we get:
E = I(0 + r)
E = Ir
The maximum current is the shortcircuit current. This would normally be large enough to do what?
blow a fuse or circuit breaker.
When we increase the current the lost volts will increase what will decrease?
The t.p.d
What is a capacitor?
A capacitor is a component that stores an electrical charge. They consist of two conducting layers separated by an insulator. The simplest type is two metal plates with an air gap between them.
The amount of charge (in Coulombs) that a capacitor can store depends on its what?
capacitance (measured in Farads although usually in milliFarads, microFarads, or nanoFarads)
Q and V are directly proportional. The gradient of this graph is constant. This constant is what?
the capacitance of the capacitor.
One Farad of Capacitance means what?
that the capacitor can store one coulomb of charge per volt
What is the equation linking capacitance, charge and voltage?
Capacitance = Charge / Voltage
C= Q / V
What is meant by a capacitance of 1000mf?
1000mc of charge are stored across a capacitor per volt.
What does the charging voltage graph look like?
What does the charging current graph look like?
What does the discharging voltage graph look like?
What does the discharging current graph look like?
What does the p.d across the resistor versus time look like?
The time taken for a capacitor to charge is controlled by what?
- The resistance of the resistor R (because it controls the size of the current, i.e. the charge flow rate)
- The capacitance of the capacitor (since a larger capacitor will take longer to fill and empty).
The voltage against time graph for a small capacitance/ resistance
The effect of changing capacitance on current. Graph large capacitor/ small capacitor.
The effect of changing resistance on current. Graph a large resistor / small resistor
since the area under the I/t graph is equal to charge, for a given capacitor the area under the graphs must be equal. True or False
True
By replacing the fixed resistor with a variable resistor, we could decrease the value of the resistance as the capacitor charges what does this do?
, maintaining the current at a constant value (can be hard to do in real life!)
decreasing the value of the resistance as the capacitor charges, maintaining the current at a constant value- how is this useful?
because it means that we can use the relationship between charge, current and time to calculate the charge stored in a capacitor (just as long as I is constant!)
What is the equation linking charge stored, constant charging current and time to charge?
Q= I x T
charge stored (C), constant charging current (A) and time to charge (S)
In the charge stored vs voltage graph what Is the area underneath the graph equal to?
Area under this graph = Energy stored in the capacitor:
What are the 3 equations for capacitance?
E =½ Q V
E= 1/2 CV^2
E= 1/2 (Q^2) / C
