Exercises: Electrical quantities: voltage, current and resistance

The TV remote works with two batteries printed with $1,5\ \text{V}$. What electrical quantity does this number indicate? Explain in your own words what it tells us about the "push" the battery provides to the charges in the circuit.

What electrical quantity indicates the amount of charge that passes through a wire every second? In what unit is it measured in the International System of Units?

All components in a circuit oppose the flow of current to a greater or lesser extent. What quantity measures this opposition? In what unit is it expressed?

Match each electrical quantity to its unit and write out the three complete pairs:

1. a) Voltage.
2. b) Current.
3. c) Resistance.

Units: ohm ($\Omega$), volt ($\text{V}$) and ampere ($\text{A}$).

In the water circuit analogy, an electrical circuit is similar to a water circuit with an elevated tank and pipes, as shown in the diagram.

State which electrical quantity corresponds to each feature:

1. a) The height of the tank.
2. b) The flow rate of water circulating through the pipe.
3. c) The narrowness of the pipe.

In the laboratory, we use a device that can measure voltage, current, and resistance depending on the position of its dial.

1. a) What is it called?
2. b) Which unit would you select with the dial to test a toy's $4,5\ \text{V}$ battery?

Complete the sentence with the missing words: "Voltage is measured in $\dots$, current in $\dots$ and resistance in $\dots$". Then write the symbol for each unit.

Order these batteries from greatest to least "push" on the charges: a smoke detector's $9\ \text{V}$ battery, an old torch's $4,5\ \text{V}$ battery, and a wall clock's $1,5\ \text{V}$ battery. What quantity did you compare to order them?

A hose is connected to an elevated water tank, as shown in the diagram.

1. a) If we raise the tank to a higher level, what happens to the flow rate coming out of the hose?
2. b) Apply this answer to a circuit: what electrical quantity is represented by the height of the tank, and what happens to the current if it increases?
3. c) If we now narrow the hose, what happens to the flow rate? What does this correspond to in the circuit?

We want to measure the resistance of a bulb disconnected from the circuit using a multimeter.

1. a) Which unit do we select with the device's dial?
2. b) If the multimeter reads $10\ \Omega$, explain what this value means.

Observe the circuit in the diagram, showing the two possible placements (A and B) of the multimeter.

To measure the voltage across a bulb connected to a battery, where do we place the two probes of the multimeter: touching both terminals of the bulb or disconnecting a wire and inserting the device into the circuit? Explain what the multimeter measures in each placement.

The manual of a toy drone states that its motors draw $500\ \text{mA}$.

1. a) Express this current in amperes.
2. b) Does it draw more or less current than a bulb through which $0,6\ \text{A}$ passes?

In the laboratory, there are three resistors labelled $2\ \text{k}\Omega$, $2200\ \Omega$ and $2,5\cdot10^{3}\ \Omega$.

1. a) Express all three values in ohms.
2. b) Order them from lowest to highest resistance.

We want to measure the current passing through a bulb in a circuit with a battery, as shown in the diagram, which displays two possible placements (A and B) of the meter.

1. a) Where must the ammeter be placed?
2. b) Explain why simply touching the two ends of the bulb with the probes is not correct.

Water flows easily through a wide, short pipe; through a narrow, long pipe it flows with much more difficulty.

1. a) Which of the two offers more resistance to the flow of water?
2. b) Apply this idea to two copper wires, one short and thick and the other long and thin: through which one will the current flow better? Justify your answer.

When measuring a remote control battery that should give $1,5\ \text{V}$ using a multimeter, the device reads $1,2\ \text{V}$.

1. a) Which quantity have we measured and in which unit?
2. b) What do you think has happened to the battery? Would you keep using it in a device that needs a lot of push?

We set up a circuit with a battery, connecting wires and a light bulb.

1. a) Construct the complete hydraulic analogy: state what the battery, the wires, the light bulb and the current represent in a water circuit with a pump.
2. b) Using the analogy, predict what happens to the brightness of the bulb if we switch to a higher-voltage battery.
3. c) And what if we add a second bulb that further narrows the path?

A portable fan has four $1,5\ \text{V}$ batteries connected one after another, as shown in the figure, and the combination supplies $6\ \text{V}$.

1. a) Which quantity adds up when connecting the batteries like this?
2. b) Explain this result by imagining several water pumps placed one after another in the same pipe.
3. c) What voltage would the combination give if we removed one of the batteries?

Observe in the figure how the ammeter and the voltmeter are connected.

The ammeter is connected in series within the circuit so that all the current passes through it.

1. a) Reason whether its internal resistance should be very high or very low, considering what would happen to the current we want to measure.
2. b) The voltmeter, on the other hand, is connected across the two terminals of the component. Reason whether it should have a high or low internal resistance, considering which path the current will prefer to take.

Mains voltage at home is $230\ \text{V}$ and that of a button cell battery is $3\ \text{V}$.

1. a) Which quantity are we comparing?
2. b) Explain, using the idea of "push" on charges, why the mains electricity grid is dangerous while a button cell battery is not.
3. c) The human body offers a certain resistance to the flow of current. Using this idea, reason what will happen if the voltage we touch is much higher.

You have a battery, a light bulb, wires and a multimeter. Describe step by step how you would measure:

1. a) The voltage that the battery applies to the bulb.
2. b) The current passing through it.
3. c) Explain why in one case the device is placed across the ends of the bulb and in the other it is connected in series in the circuit.

In a laboratory setup, Iker replaces the thick wire with a very thin and long copper wire, and observes that the bulb shines less brightly.

1. a) Which property of the new wire has changed compared to the previous one?
2. b) Explain using the hydraulic analogy why the bulb shines less brightly.
3. c) Predict what will have happened to the ammeter reading in the circuit: does it read more, less or the same?

We connect a copper rod and a pencil lead (graphite) of the same size separately to the same battery along with a light bulb: with the copper, the bulb shines clearly brighter.

1. a) Which of the two materials has higher resistance? How do you know?
2. b) Would you say that graphite is a conductor or an insulator? Justify your answer based on what was observed.
3. c) Suggest a reason why wire sheaths are made of plastic and not graphite.

These sentences from an exam contain errors in units. Correct each one and justify the correction:

1. a) "The battery has a current of $1,5\ \text{V}$."
2. b) "A voltage of $0,3\ \text{A}$ passes through the bulb."
3. c) "The resistance of this wire is $10\ \text{V}$."