Components in Series & Parallel Circuits Flashcards

All 29 cards in this deck

What is the formula for the total resistance of resistors in series?

e.g. a $3\,\Omega$ and a $5\,\Omega$ resistor in series

$R_{total} = R_1 + R_2$

e.g. $3 + 5 = 8\,\Omega$

Why does adding a second resistor in series increase the net resistance?

The charge has to pass through both resistors, so the oppositions to current add together.

Why does connecting a second resistor in parallel decrease the net resistance?

It gives the current an extra path, so the total current from the supply increases for the same potential difference.

What is the net resistance of two identical resistors connected in parallel?

Half the resistance of one of them.

e.g. two $10\,\Omega$ resistors in parallel give $5\,\Omega$.

True or false? Two $10\,\Omega$ resistors connected in parallel have a net resistance of $20\,\Omega$.

False. The net resistance of resistors in parallel is always less than the smallest single resistance — here $5\,\Omega$.

What are the steps to find the current and the potential difference across each resistor in a series circuit?

e.g. a $6\,\mathrm{V}$ supply with a $2\,\Omega$ and a $4\,\Omega$ resistor in series

1. Add the resistances: $2 + 4 = 6\,\Omega$
2. Current $I = V \div R = 6 \div 6 = 1\,\mathrm{A}$ (same everywhere)
3. $V = I \times R$ for each: $2\,\mathrm{V}$ and $4\,\mathrm{V}$

How does the current vary around a series circuit?

It is the same at every point in the circuit.

How is the supply potential difference shared in a series circuit?

It is shared between the components — the p.d.s across the components add up to the supply p.d.

What is the potential difference across each branch of a parallel circuit?

The same as the potential difference of the supply.

How does the current from the supply relate to the currents in the branches of a parallel circuit?

The supply current equals the sum of the currents in the separate branches.

How is the supply potential difference shared between identical components in series?

e.g. four identical lamps on a $12\,\mathrm{V}$ supply

Equally between them.

e.g. $12 \div 4 = 3\,\mathrm{V}$ across each lamp.

True or false? Current is used up as it passes through the components of a series circuit.

False. The current is the same before and after every component; it is energy, not charge, that is transferred.

What is the shape of the current–potential difference graph for a fixed resistor at constant temperature?

A straight line through the origin — the resistance stays constant.

What is the shape of the current–potential difference graph for a filament lamp?

An S-shaped curve through the origin that gets shallower as current increases — resistance increases as the filament heats up.

What is the shape of the current–potential difference graph for a diode?

Almost no current until a small forward p.d., then current rises steeply; in reverse the resistance is very high and current is about zero.

How does the resistance of a light-dependent resistor (LDR) vary with light intensity?

Resistance decreases as light intensity increases (and is very high in the dark).

How does the resistance of an NTC thermistor vary with temperature?

Resistance decreases as temperature increases.

True or false? A filament lamp has a constant resistance whatever the current through it.

False. Its resistance increases as the current heats the filament, so its I–V graph is a curve.

How is an ammeter connected in a circuit used to test a component?

In series with the component, so it measures the current through it.

True or false? A voltmeter is connected in parallel with the component whose potential difference is being measured.

True. It measures the p.d. across the component, so it goes across it.

Which component is added to a test circuit to give a continuously variable potential difference across the component being tested, and how is it connected?

A variable resistor (rheostat), connected in series with the component.

What are the steps to investigate how the resistance of a thermistor varies with temperature?

e.g. using a water bath from $20\,^{\circ}\mathrm{C}$ to $80\,^{\circ}\mathrm{C}$

1. Connect the thermistor in series with a cell and ammeter, with a voltmeter across it.
2. Place it in the water bath and record $V$, $I$ and temperature every $10\,^{\circ}\mathrm{C}$.
3. Calculate $R = V \div I$ at each temperature and plot resistance against temperature.

In a test circuit, how do you show that a diode conducts in one direction only?

Reverse the diode (or the supply connections) and take readings again — the current is about zero in reverse.

What are the steps of the core practical to investigate p.d., current and resistance for a resistor?

e.g. a fixed $10\,\Omega$ resistor

1. Connect the resistor in series with a cell, ammeter and variable resistor, with a voltmeter across the resistor.
2. Adjust the variable resistor and record about six pairs of current and p.d. readings.
3. Plot current against p.d. and/or calculate $R = V \div I$.

Why should the circuit be switched off between readings in the core practical on current and potential difference?

To stop the component heating up, which would change its resistance.

What are the steps to test that current is the same everywhere in a series circuit?

e.g. two filament lamps in series with a cell

1. Connect the two lamps in series with the cell and an ammeter.
2. Move the ammeter to a different point in the circuit and read the current again.
3. Compare the readings — they should be equal.

When two identical lamps are connected to the same supply, are they brighter in series or in parallel?

In parallel — each lamp has the full supply p.d., whereas in series they share it.

Give one way to improve the accuracy of the resistance values found in a circuit investigation.

Repeat the readings at each setting and take a mean.

True or false? Reversing the connections to a fixed resistor reverses the current but gives readings of the same size.

True. A fixed resistor has the same resistance in both directions, so its I–V graph is symmetrical through the origin.