Electricity Flashcards
What are lost volts?
The potential difference required to overcome the internal resistance of the power supply. This leaves the rest of the circuit with less pd.
What is the formula for resistance in a series circuit?
R_t = R_1 + R_2 + … + R_n
What is the formula for resistance in a parallel circuit?
1/R_t = 1/R_1 + 1/R_2 + … + 1/R_n
What is Kirchhoff’s 1st rule?
The current going into a junction must equal the current coming out.
What is the value of turn on voltage?
Approximately 0.6V
How does current differ across a series circuit?
It doesn’t, in a series circuit current is always the same.
The pd across all components in a circuit is equal to what?
The total pd available, e.g. the pd provided by the battery.
What is pd proportional to?
Energy
What is terminal pd?
The max pd available in the circuit. The pd across the battery.
How does pd differ across any circuit?
It doesn’t, pd is always the same anywhere in the circuit.
What happens to the resistance in a thermistor as the temperature increases?
The resistance decreases.
What is potential difference?
Work done (energy converted) per unit charge moved.
What is current?
The rate of flow of charge.
What is threshold voltage (turn on voltage)?
The potential difference required for a diode to conduct. This is about 0.6V.
What is power?
The rate of transfer of energy. P = E/t
What is the equation for energy?
E = VIt
What is the equation for emf including lost volts?
ε = V + v where ε is emf, V is potential difference and v is lost volts.
What is the total emf of cells in series?
ε_t = ε_1 + ε_2 +…
What is the total emf of cells in parallel?
ε_t = ε_1 = ε_2 = …
If you have a circuit with a battery and two resistors in series, one with a resistance of 2 Ω and one with 3 Ω, what is the pd across each resistor?
2/5 of the total pd across the 2 Ω resistor and 3/5 across the 3 Ω resistor.
State the steps in the method of an experiment to measure the resistivity of constantan wire.
Measure the thickness of the constantan wire using the micrometer in at least three places and find the mean diameter in metres. Attach crocodile clips to the wire so that the distance apart is 0.1m and connect them in series to a power supply and and ammeter with a voltmeter across the constantan wire. Adjust the pd so that the current is 0.5A. Work out the resistance using V/I. Repeat for lengths increasing by 0.1m up to 0.8m. Repeat the experiment and find the mean value of R for each value of L. Work out the cross-sectional area of the wire using (πd^4) / 4. Plot a graph of R against L and find the gradient. Work out the resistivity using gradient x A in Ωm.
State two physical features which the constantan wire should have before starting the experiment to find its resistivity.
It should be free of kinks and have a thickness of around 0.25mm so that it can be measured accurately with a micrometer.
How should the constantan wire be held when measuring its length?
As straight as possible.
What is the resistance of 10cm of constantan wire and why is this useful?
1Ω which will allow for a reasonably accurate determination of its resistance.
What will make measuring the length of the constantan wire easier?
Mounting 2x4mm terminals at both ends of a meter ruler.
In the experiment to measure the resistivity of constantan wire, why should the length be increased with each measurement rather than decreased from 0.8m?
To minimise the effect of temperature due to current.
How do you measure the emf of a battery?
Put the voltmeter across the battery with no load.
