4.3 Electricity:Electrical circuits Flashcards
Kirchhoff’s Second Law
The sum of the e.m.f. around a loop/series circuit is equal to the sum of the p.d. across each component in the same loop/series circuit.
∑ e.m.f = ∑ p.d
This is a consequence of the conservation of energy, as energy cannot be created or destroyed
Series circuit rules?
Whats the proof for the resistance rule?
Current same everywhere
ε = 𝑉1 + 𝑉2 + 𝑉3 + ⋯ + 𝑉n
𝑉Total = 𝑉1 + 𝑉2 + 𝑉3 + ⋯ + 𝑉n
𝑅Total = 𝑅1 + 𝑅2 + 𝑅3 + ⋯ + 𝑅n
PROOF
- 𝑉Total = 𝑉1 + 𝑉2 + 𝑉3 + ⋯ + 𝑉𝑛
- V = IR , so if 𝐼 is constant
- 𝐼𝑅Total = 𝐼𝑅1 + 𝐼𝑅2 + 𝐼𝑅3 + ⋯ + 𝐼𝑅𝑛
- Dividing through by 𝐼 gives 𝑅Total = 𝑅1 + 𝑅2 + 𝑅3 + ⋯ + 𝑅n
Parallel circuit rules?
Whats the proof for the resistance rule?
Current is split at each junction, so 𝐼Total = 𝐼1 + 𝐼2 + 𝐼3 + ⋯ + 𝐼𝑛
The p.d. is the same across each component in a parallel circuit.
This is proven by Kirchhoff’s 2nd law. The sum of e.m.f equals the sum of individual p.d.s in a certain
loop.
1/Rtot=1/R + 1/R…
PROOF
- 𝐼Total = 𝐼1 + 𝐼2…
- V=IR, v is constant
- V/Rtotal=V/R1 + V/R2 + V/R3….
- Divide by V
What type of emf sources have internal resistances?
All sources of e.m.f. have internal resistance.
In a battery, this is due to the chemicals inside of it.
In a power supply it is due to the components and wires inside.
Terminal p.d
The p.d. across the load resistance (R) is the work done when one coulomb of charge flows through the load resistance
Lost Volts
the p.d. across the internal resistance, is equal to the energy wasted per coulomb of charge that overcomes the internal resistance.
How to calculate emf?
ε = I (R + r) (given)
ε = V + Ir (given)
Where R is the load resistance (total external resistance) and r is the internal resistance and Ir is the “lost volts”.
How do you find the internal resistance of an e.m.f. source?
- Set up a circuit with a powerpack, ammeter, variable resistor with voltmeter around it.
- Set power pack to 10V
- Set resistance to low
- Record I and V
- Change R with the variable resistor.
- Record new I and V
- Repeat
- Plot V against I
- Compare to y = mx + c, where y= V , x= I , m = -r and c = e.m.f.
What’s a Potential Divider circuit?
A potential divider circuit is a type of circuit containing two components designed to divide up the p.d. in proportion to the ratio of the resistances of the components.
A potential divider is a circuit that can be adjusted to produce a desired p.d. Vout,
that can be used to power an external circuit
Thermistors in a Potential Divider Circuit
Ciruit diagram:
powersupply (label v in)
Resistor
Thermistor (V out)
Thermistors can be used in thermostats. This is because:
i. In Cold - thermistor Resistance is high, so higher proportion of p.d. will flow through thermistor, so VOut increases. Therefore enough p.d. is supplied to trigger a heater to increase.
ii. In Warm - thermistor Resistance is low, so lower proportion of p.d. will flow through thermistor, so less p.d. will be applied through VOUT, which turns the heater off.
iii. If we change thermistor to a variable resistor, we can control and calibrate the range at which VOUT varies, allowing you to set the temperature at which a heater
operates
Thermistors can be used in fridges
powersupply (label v in)
Thermistor
Resistor (R2) (Vout)
Thermistors can be used in fridges. This is because:
i. In Cold - Resistance of thermistor is high, so a lower proportion of the p.d. will be pushed past R2, meaning that VOUT is low so the fridge does not need to turn
on to lower the heat.
ii. In Warm - Resistance of thermistor is low, so a higher proportion of the p.d. will be pushed past R2, meaning that VOUT is high so that the fridge turns on to lower
the heat.
LDRs in a Potential Divider Circuit
powersupply (label v in)
Resistor (R1)
LDR (R2) (Vout)
LDRs can be used in Roadside lamps because:
i. In Dark - R2 > R1, so Vout is large. This triggers the light to switch on.
ii. In Light - R2 < R1, so Vout is small. This triggers the light to switch off.
Potentiometers in Potential Divider Circuit:
A potentiometer is used in a potential divider circuit because:
i. It is more useful because it consists of a single variable resistor. It still acts as a current limiting resistor, but by moving the sliding contact we can achieve
any value of Vout between 0V up till the input voltage.
ii. This type of circuit is used in a stereo. When you want to change the volume control, the potentiometer can be adjusted to achieve the chosen volume strength:
- Increase in R2 = increase in Vout = increase in volume.
- Decrease in R2 = decrease in Vout = decrease in volume.
Advantages of using data-loggers to monitor physical changes
- eliminate chance of human error
- plots graphs straight away
- very good at processing data
- if a continuous record of temperature or light intensity was needed, then connecting the data logger to the thermistor/LDR because they will produce electrical outputs
Explain what is meant by internal resistance
(some) energy is transferred into thermal energy (driving charge through) the battery. It behaves as if it has an (internal) resistance
