Unit 2.3 - DC Circuits Flashcards
What is current?
A measure of the flow of charge per second
How does the number of charges flowing into a wire compare to the charges coming out?
Equal
What is equal in terms of charges in a wire?
The number of charges flowing into a wire = the number of charges flowing out
What do charges all do which leads to the current into a wire being the same as the current out?
They all drift at the same velocity
What happens to the sum of current flowing into a junction?
It’s the same as the current flowing out
What is the same flowing in and out of a circuit?
The number of charges
So, the current
Can charges gather in one spot?
No, charge does not accumulate
What’s the principle that states that the number of charges flowing into a wire = the number of charges flowing out?
Conservation of charge
Which energy do charges carry?
The energy which is gained in the cell/battery and lost in circuit components
Where is energy gained in a ciruit?
In the cell/battery
Where is energy lost in a circuit?
In circuit components
Explain the principle of conservation of energy in a circuit
We cannot lose more energy than gained in the battery, so the total energy gained in the battery is equal to the sum of energy lost in the circuit
Principle of conservation of energy in a circuit in electrical terms
The sum of the p.d.s in a circuit is equal to the p.d across the battery
What would count as a circuit component?
A bulb, for example
What happens if the current in a circuit splits into more than 1 wire in terms of potential difference?
The potential difference in each branch will be the same (potential is a measure of energy)
Why does the fact that the potential difference in each branch of a parallel circuit is the same not mean that we’ve got energy “for free”?
The current is spit between the branches
Do more charges pass per second is a series or parallel circuit?
Not as many charges pass per second in a parallel circuit as would in a series circuit
Are ammeters components that cause energy loss in circuits? Why?
No - a perfect one does no resistance and no work is done when passing one
Give an example of an addition to a circuit which isn’t a component
Ammeters
What’s equal between resisters in SERIES? Why?
The current through them is equal (conservation of charge)
In what type of circuit is the current through all of the resistors equal?
Series circuit
What is the current equal between all of in a circuit?
Resistors
How do we work out the total resistance in a series circuit?
It’s just the sum of each individual resistance
In what type of circuit is the resistance the sum of each individual resistance?
Series circuits
What’s the calculation that shows that the total resistance in a series circuit is the sum of each individual resistance?
Power = I^2R
Total power dissipated in circuit = Ptotal = I^2Rtotal
Ptotal = I^2R1 + I^2R2 + I^2r3
Dividing by I^2 gives
Rtotal = R1 + R2 + R3
What is equal in a parallel circuit?
The potential difference across resistors (even if they aren’t of the same value)
In what type of circuit is the potential difference the same across the resistors?
Parallel circuits
What’s the potential difference across the circuit the same for when resistors are in parallel?
The resistors
What’s the calculation that shows how to work out the resistance in a parallel circuit?
Power = P = V^2/R
Ptotal = V^2/Rtotal = V^2/R1 + V2^2/R2 + V^2/R3
Divided by V^2 = 1/Rtotal = 1/R1 + 1/R2 + 1/R3
What would you type into the calculator to work out the total resistance of a parallel circuit with the following values for resistance…?
5, 17, 150
(1/5 + 1/17 + 1/150)^-1
Which values do we put into our calculations for calculating total resistance?
Resistance values
What do we do to resistance if they’re in series?
Just add them
e.m.f
Voltage across a battery
What does a variable resistor control in a circuit?
The current
How many terminals in a circuit does a rheostat have?
2
An electrical instrument used to control a current by varying its resistance
Rheostat
Rheostat
An electrical instrument used to control a current by varying its resistance
What does a variable resistor used as a potential divider control?
The potential difference (not the current) supplied to a circuit or component
When does a variable resistor control the potential difference and not the current supplied to a circuit or component?
When used as a potential divider
Potential divider
Resistances connected in series to “divide up” the potential difference (Vtotal) placed across the combination
What are the 2 ways of making a potential divider in a circuit?
Use variable resistors
Use 2 separate resistors
Label the separate resistors and the following on a circuit…
Vin, Vout, “lost” unwanted volts
(See notes)
What is Vout in a circuit and how do we work this out?
The required volts
Vout = R x Vin
———
R total
What does Vout depend on?
The values of R1 an R2
In what circumstance is Vout a bigger fraction of Vin?
If R2 is a bigger fraction of Rtotal
Which is always highest - Vout or Vin?
Always Vin
What’s the advantage of using variable resistors as opposed to separate ones?
Resistors do not come in a continuous range of values (e.g - you can get 22ohms, but not 23ohms) but it’s possible to get any value from zero to the maximum value with variable ones
What usually implies a potential divider circuit?
A circuit drawn to be on its side
How many terminals of the variable resistor are there when used as a potential divider?
3
What does a variable resistor change the value of and how?
Vout by changing the current, which changes the p.d across R2 (hence changing Vout)
Sketch and label a circuit with a variable resistor forming a potential divider
(See notes)
What can a variable resistor have any value between?
Zero and Rtotal
What can Vout have a value of with a variable resistor?
Any value between zero and Vin
How does the sum of the potential differences across a potential divider compare to that of the supply and why?
It’s equal as a potential divider consists of 2 components connected in series
Make it clear which part of a parallel circuit have the same current and which parts don’t
(See notes)
What happens to the total resistance when the resistors are added in series?
Increases
What happens to the total resistance when resistors are in added in parallel?
Decreases
What are potential divider circuits particularly useful for?
Circuits where the power in pit is needed to supply several components with different p.d.s
How can we use a circuit as a sensor?
Use circuit components that have changing resistance depending on physical factors
circuit components that have changing resistance depending on physical factors
Thermistor
LDR (light dependant resistor)
What do thermistors do?
Has a significant change in resistance as the temperature changes
Draw a thermistor symbol
(See notes)
What’s the resistance caused by a thermistor at high temperatures?
Low
What’s the resistance caused by a thermistor at low temperatures?
High
What does the low resistance caused by thermistors in high temperatures lead to?
Current increase
P.d across the second resistor increases (Vout)
What can thermistors be used in?
To trigger bells in fire alarms
LDR
Light dependant resistor
LDR symbol
(See notes)
What does an LDR have?
Has a change in resistance depending on the light striking it
What happens to the resistance of LDR as the light intensity increases?
Decreases
What happens to positive charges in a battery?
“Pumped” from the negative to the positive end
How are the positive charges in a battery concentrated in one end?
The battery must do work against the electrostatic forces - work done transfers energy to the charges
What transfers energy to positive charges in a battery in order to move to the positive side?
Work done by the battery
What’s the ideal energy transfer when transferring charges in a battery to the positive end?
Chemical potential energy —> electrical energy
What is emf in terms of positive charges in a battery?
The energy that the battery gives the charges to concentrate themselves at the positive end
What’s the energy that a battery gives charges to concentrate themselves at the positive end of the battery known as?
Electromotive force (emf)
Emf symbol
ε
Unit of electromotive force
Volt
Why does emf have volt as its unit?
W/Q
Jc-1
Same as a volt
Difference between emf and pd
Emf - involves the transfer of other types of energy into electrical energies
Pd - involved the change from electrical energy into other types
Emf definition
The energy converted from chemical energy to electrical potential energy per unit charge that flows through the cell
What do we use to measure a cell’s emf?
A digital voltmeter
Current in an open circuit
Zero
What is a voltmeter measuring in an open circuit, with a current of zero, and what is this equivalent to?
Open circuit terminal potential difference
The same as the emf of the circuit
What do cell supply as they work?
A current
If a voltmeter is in parallel with a resistor and a battery, what is it measuring?
The potential difference across both the battery and the load
If a voltmeter is connected in parallel in a circuit, is the radio higher to lower than a reading for the battery in an open circuit?
Lower
What does emf tell us about a circuit?
How much energy is given to a circuit
What does pd tell us that emf doesn’t?
How much of the energy given to a circuit doesn’t come out as current
Why does not all energy given to a circuit come out in the current?
Lost voltage due to internal resistance
When are frictional forces caused in a battery?
When a battery does work to concentrate the charges at the positive end of the battery
What causes internal resistance?
When work is done against frictional forces in a battery
Why does doing work against friction in a battery lead to internal resistance?
As work is done, heat is released
(As happens when charges flow through any resistance)
Draw and label a circuit to demonstrate internal resistance
(See notes)
How many components are there that dissipate energy in a circuit with a resistor?
2
The resistor
Internal resistance of the battery
What’s the relationship between emf and pd?
Sum of emf = sum of pd
Compare the current flowing through internal and external resistances
The same
ε in an equation
ε = Ir + IR
In ε = Ir + IR, which are variables and which are constant?
I and V = variables (can measure)
ε and r = constants
Is r constant?
Very nearly
How do we re-arrange the equation ε = Ir + IR to be in y = mx + c form?
V = -Ir + ε
What’s y = mx + c?
Equation for a straight line graph
Symbol for gradient in y = mx + c?
m
Symbol for y-intercept in y = mx + c
c
What is the intercept on the y-axis on a current-voltage graph?
ε
Why is the y - intercept ε on a current-voltage graph?
The potential difference when the current = zero
What’s the gradient on a current-voltage graph?
-internal resistance
-r
Which point of a current-voltage graph shows the maximum current that the battery can supply?
X-intercept
Draw and label a current-voltage graph and label 3 key parts using the equation
V = -Ir + ε
(See notes)
Resistance in an open circuit
Infinite
Resistance in a short circuit
Zero
What does the fact that an open circuit has infinite resistance mean?
Zero current can flow
Resistance of a short circuit
Zero
What does the fact that a short circuit has zero existence mean for it?
There’s zero voltage difference for any current
Describe the resistance of a voltmeter and explain why
More or less infinite
Draws no energy or charge from the circuit in order to measure the actual potential difference across 2 points
Equation for determining internal resistance
V = E - Ir
E in V = E - Ir
Emf
V in V = E - Ir
Terminal pd of a cell
r in V =E - Ir
Internal resistance
I in V = E - Ir
Current
When is the power supply of a source as maximum?
When internal resistance = external resistance
R = r
What does it mean when internal resistance = external resistance?
Maximum power supply
What is brightness directly proportional to?
Power
If the current and resistance for 2 bulbs stays the same, compare their brightnesses and explain why this happens
Brightness is directly proportional to POWER
P = I^2R
so, they have the same brightness
What happens if the resistance decreases in a parallel circuit?
The potential difference across resistors decreases
In which situation would the current coming up to a junction in a parallel circuit split evenly?
If the resistance values at both junctions were even
Describe the difference in terms of energy between emf and potential difference
Emf - energy supplied by the source
Pd - energy used in external circuit
What happens to the emf and internal resistance of a cell if another identical cell were added in series?
Both would double
What is 1 volt equivalent to?
1 joule per coulomb
What does a battery do?
Gives charge electrical potential energy
Describe the voltage in a series circuit
Shared amongst components
Describe the voltage in parallel circuits
All components get the full voltage
Describe the current in a series circuit
The current is constant in all parts
Describe the current in a parallel circuit
The current is split between branches based on resistance
How do we work out the number of electrons passing through a point per minute?
Charge
———— x 60
e
What would happen to the power of a circuit if the internal resistance decreased and why?
Increase
Total resistance of circuit is less for all values of R
Which path does current always go through?
the one of least resistance
Potential difference across parallel circuits
The same (even if they aren’t of the same value of resistance
What does Ir represent in V = E - Ir?
Energy per unit charge lost in source
What do we not write when using a graph to calculate internal resistance with the y-intercept?
The negative sign, even if it’s a negative gradient
Compare conventional current and electron flow
Convention current —> + to -
Electrons —> - to +
