Current, Potential Difference & Resistance Flashcards

All 51 cards in this deck

What are the circuit symbols for a cell and for a battery?

Cell: one long thin line and one short thick line. Battery: two or more cells drawn in series.

What is the circuit symbol for a thermistor?

A resistor rectangle with a diagonal line across it that ends in a short horizontal bar.

What is the circuit symbol for an LDR?

A resistor rectangle inside a circle with two arrows pointing towards it.

How does the circuit symbol for an LED differ from the symbol for a diode?

An LED is the diode symbol (triangle pointing at a bar) with two arrows pointing away from it.

True or false? An ammeter is connected in parallel with the component whose current is being measured.

False. An ammeter is connected in series with the component; a voltmeter goes in parallel.

What are the steps to complete a circuit diagram so that the current through and pd across a lamp can be measured?
e.g. a cell and lamp already drawn in a single loop

1. Draw an ammeter in the loop, in series with the lamp.
2. Draw a voltmeter in parallel, connected across the lamp.
3. Use the correct symbols: A in a circle, V in a circle.

What is electric current?

A flow of electrical charge; its size is the rate of flow of electrical charge.

What must a circuit include for electrical charge to flow?

A source of potential difference, and the circuit must be closed.

What is the equation linking charge flow, current and time?

charge flow = current × time, or $Q = It$

What units are used for charge flow, current and time in $Q = It$?

Charge in coulombs (C), current in amperes/amps (A), time in seconds (s).

What are the steps to calculate charge flow when the current is given in milliamps?
e.g. a current of 50 mA for 20 s

1. Convert current to amps: $50\text{ mA} = 0.050$ A
2. Write the equation: $Q = It$
3. Substitute: $Q = 0.050 \times 20 = 1.0$ C

True or false? In a single closed loop the current is smaller after it has passed through a lamp.

False. The current has the same value at every point in a single closed loop.

What is the equation linking potential difference, current and resistance?

potential difference = current × resistance, or $V = IR$

What is the unit of resistance, and its symbol?

The ohm, symbol $\Omega$.

How does the current through a component change if its resistance is greater, for the same potential difference?

The current is smaller.

What are the steps to calculate the current through a component from its potential difference and resistance?
e.g. $V = 12$ V across a $4\ \Omega$ resistor

1. Write the equation: $V = IR$
2. Rearrange: $I = V/R$
3. Substitute: $I = 12/4 = 3$ A

Describe the circuit used to measure the resistance of a component.

The component in series with a cell and an ammeter, with a voltmeter connected in parallel across the component; resistance is found from $R = V/I$.

True or false? 'Voltage' is an acceptable alternative term for potential difference in AQA answers.

True. Credit is given for correct use of either potential difference or voltage.

What are the steps to investigate how the resistance of a wire depends on its length?
e.g. lengths from 10 cm to 100 cm

1. Connect the test wire in series with a cell, switch and ammeter, with a voltmeter across the wire.
2. For each length, close the switch briefly, record $V$ and $I$, and calculate $R = V/I$.
3. Plot resistance against length — a straight line through the origin.

How does the resistance of a wire depend on its length at constant temperature?

Resistance is directly proportional to length — a straight line through the origin on a resistance–length graph.

Why is the switch only closed briefly for each reading in the resistance-of-a-wire practical?

To stop the wire heating up, so the temperature stays constant.

Which variables must be controlled when investigating how a wire's resistance depends on its length?

The material and thickness (cross-sectional area) of the wire, the supply pd, and the temperature of the wire.

What is a zero error on an ammeter?

The meter reads a value other than zero when there is no current, so every reading is out by that amount.

True or false? A 1.5 V cell is an electrical hazard to the student in the resistance-of-a-wire practical.

False. The potential difference is very low, so there is no risk of electric shock.

What is an ohmic conductor?

A component whose current is directly proportional to the pd across it at constant temperature, so its resistance stays constant.

What happens to the resistance of a filament lamp as the temperature of the filament increases?

The resistance increases.

Why does the resistance of a filament lamp increase as the current through it increases?

A larger current heats the filament, and resistance increases with temperature.

What is the key property of a diode?

Current flows through it in one direction only; it has a very high resistance in the reverse direction.

True or false? The resistance of every resistor stays constant as the current changes.

False. For some (ohmic resistors at constant temperature) $R$ is constant, but for lamps, diodes, thermistors and LDRs it changes.

Name four components whose resistance is not constant.

Filament lamp, diode, thermistor and LDR.

What does a straight line through the origin on an I–V graph tell you about a component?

It is linear (ohmic): current is directly proportional to pd, so resistance is constant.

What does a curved I–V graph tell you about a component?

It is non-linear (non-ohmic): its resistance changes as the current changes.

What is the shape of the I–V graph for a filament lamp?

An S-shaped curve through the origin that gets less steep as the pd increases, because resistance rises with temperature.

What is the shape of the I–V graph for a diode?

Almost zero current in the reverse direction, and current rising steeply only after a small forward pd.

What are the steps to determine the resistance of a component at one point on its I–V graph?
e.g. at a pd of $1.0$ V the graph gives $I = 0.08$ A

1. Read the current at that pd: $I = 0.08$ A
2. Rearrange $V = IR$ to $R = V/I$
3. Substitute: $R = 1.0/0.08 = 12.5\ \Omega$

True or false? The resistance of a component at a point on its I–V graph is equal to the gradient of the graph at that point.

False. Resistance is $V \div I$ at that point; for a straight line through the origin the gradient is $1/R$.

How does the resistance of a thermistor change as its temperature increases?

The resistance decreases.

Give one application of a thermistor in a circuit.

A thermostat — it senses temperature and switches a heater on or off (also used in fire alarms, ovens, fridges).

In a thermostat circuit, what happens to the current through the thermistor as the room gets colder?

The thermistor's resistance increases, so the current decreases — this is used to switch the heater on.

True or false? A thermistor is an ohmic conductor.

False. Its resistance is not constant — it changes with temperature (and so with current).

What does LDR stand for?

Light-dependent resistor.

How does the resistance of an LDR change as light intensity increases?

The resistance decreases.

Give one application of an LDR in a circuit.

Switching lights on automatically when it gets dark, e.g. street lights or a night-light.

True or false? In darkness an LDR has a low resistance.

False. In darkness the light intensity is low, so the LDR's resistance is high.

What are the steps to investigate how the resistance of a thermistor varies with temperature?
e.g. water from 20 °C to 80 °C

1. Put the thermistor and a thermometer in a beaker of water, with an ammeter in series and a voltmeter across the thermistor.
2. At each temperature (e.g. every 10 °C) record $V$ and $I$ and calculate $R = V/I$.
3. Plot resistance against temperature — the curve falls as temperature rises.

Why is the water stirred when investigating a thermistor's resistance in a water bath?

So the temperature is the same throughout, and the thermometer reading matches the thermistor's temperature.

True or false? Repeating the readings at each temperature or light level lets you check the repeatability of the results.

True. Repeats also let you spot and ignore anomalous results before taking a mean.

What is the purpose of the variable resistor in an I–V characteristics circuit?

To change the current through, and the potential difference across, the component being tested.

What is meant by the resolution of a measuring instrument?

The smallest change in the quantity that the instrument can detect, e.g. an ammeter with 0.2 A scale divisions.

Give one advantage, other than higher resolution, of using a digital ammeter instead of a moving coil ammeter.

No parallax error, so less chance of misreading and a reading closer to the true value.

True or false? If a resistor heats up during the I–V practical, its I–V graph will still be a straight line.

False. Its resistance increases, so current and pd are no longer directly proportional and the graph curves.