Electrical quantities: voltage, current and resistance
When you plug in your phone charger at home in Galway, something invisible travels along the lead and reaches the battery. You can't see it, but you can measure it with three numbers: how hard it "pushes", how much of it passes through and how much it resists passing. These are the three quantities of electricity.
Voltage: the push from the cell
If you connect a small bulb to a $1.5\ \text{V}$ button cell it barely glows, but if you connect it to a $4.5\ \text{V}$ battery (three $1.5\ \text{V}$ cells in series) it shines brightly: what property of the battery makes the charges move with more energy?
Think of a raised water tank connected to a pipe. The higher the tank is, the more push the water has to come out. In electricity it is the same: the more voltage, the more push the charges have.
The human body also has a resistance. The current through it depends on the p.d. across it and on its resistance: for the same resistance, a much higher voltage drives a much larger current, and a large current through the body is dangerous. That is why $230\ \text{V}$ is hazardous and a $3\ \text{V}$ button cell is not.
Cells in series
When cells are connected one after another (in series), their voltages add up, like several pumps in the same pipe each giving the water an extra push.
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 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.
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…
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?
Match each electrical quantity to its unit and write out the three complete pairs: a) Voltage. b) Current. c) Resistance. Units: ohm ($\Omega$), volt ($\text{V}$) and…
Match each electrical quantity to its unit and write out the three complete pairs:
- a) Voltage.
- b) Current.
- c) Resistance.
Units: ohm ($\Omega$), volt ($\text{V}$) and ampere ($\text{A}$).
Mains voltage at home is $230\ \text{V}$ and that of a button cell battery is $3\ \text{V}$. a) Which quantity are we comparing? b) Explain, using the idea of "push" on…
Mains voltage at home is $230\ \text{V}$ and that of a button cell battery is $3\ \text{V}$.
- a) Which quantity are we comparing?
- b) Explain, using the idea of "push" on charges, why the mains electricity grid is dangerous while a button cell battery is not.
- 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.
Current: how much flows
Voltage, current and resistance are linked: for a given voltage, more resistance means less current, and more voltage means more current. You will meet the exact relationship, Ohm's law $V = IR$, in the next section. For example, a $6\ \text{V}$ supply across $3\ \Omega$ gives $2\ \text{A}$; across $6\ \Omega$ it gives only $1\ \text{A}$.
Going back to the water tank: current is like the flow rate, the amount of water that comes out of the pipe every second. If you open the tap fully, the flow rate is greater.
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?
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?
The manual of a toy drone states that its motors draw $500\ \text{mA}$. a) Express this current in amperes. b) Does it draw more or less current than a bulb through…
The manual of a toy drone states that its motors draw $500\ \text{mA}$.
- a) Express this current in amperes.
- b) Does it draw more or less current than a bulb through which $0.6\ \text{A}$ passes?
A portable fan has four $1.5\ \text{V}$ batteries connected one after another, as shown in the figure, and the combination supplies $6\ \text{V}$. a) Which quantity adds…
A portable fan has four $1.5\ \text{V}$ batteries connected one after another, as shown in the figure, and the combination supplies $6\ \text{V}$.
- a) Which quantity adds up when connecting the batteries like this?
- b) Explain this result by imagining several water pumps placed one after another in the same pipe.
- c) What voltage would the combination give if we removed one of the batteries?
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…
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:
- a) The height of the tank.
- b) The flow rate of water circulating through the pipe.
- c) The narrowness of the pipe.
Resistance: what slows the flow
If you connect a copper wire to a cell, the current flows easily; but if you put a thin graphite pencil lead in its place, the bulb connected barely glows: what property of the material makes it harder for the charges to pass? (Think of the lead in a pencil in your copy.)
In the water pipe, resistance is like its narrowness: a narrow pipe lets less water through than a wide one, even if the tank is just as high.
Resistances are often given with prefixes: $1\ \text{k}\Omega = 1000\ \Omega$, so $2.5\ \text{k}\Omega = 2500\ \Omega$ and $2.5 \times 10^{3}\ \Omega$ is the same value.
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?
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?
In the laboratory, there are three resistors labelled $2\ \text{k}\Omega$, $2200\ \Omega$ and $2.5\times10^{3}\ \Omega$. a) Express all three values in ohms. b) Order…
In the laboratory, there are three resistors labelled $2\ \text{k}\Omega$, $2200\ \Omega$ and $2.5\times10^{3}\ \Omega$.
- a) Express all three values in ohms.
- b) Order them from lowest to highest resistance.
Water flows easily through a wide, short pipe; through a narrow, long pipe it flows with much more difficulty. a) Which of the two offers more resistance to the flow of…
Water flows easily through a wide, short pipe; through a narrow, long pipe it flows with much more difficulty.
- a) Which of the two offers more resistance to the flow of water?
- 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.
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…
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.
- a) Which of the two materials has higher resistance? How do you know?
- b) Would you say that graphite is a conductor or an insulator? Justify your answer based on what was observed.
- c) Suggest a reason why wire sheaths are made of plastic and not graphite.
The multimeter: measuring the three quantities
-
To measure voltage, you touch the two ends of the component with the probes, without cutting the wire.
-
To measure current, you have to cut the wire and insert the device into the gap, so that all the current passes through it.
-
To measure resistance (ohmmeter setting), you measure the component disconnected from the circuit. A reading of $10\ \Omega$ means the component opposes the current with a resistance of ten ohms.
An ammeter is connected in series and has a very low internal resistance, so it does not reduce the current it measures. A voltmeter is connected in parallel and has a very high internal resistance, so almost no current is diverted through it. In a practical, you measure the current with an ammeter in series and the p.d. with a voltmeter in parallel, change the supply or a variable resistor to get several pairs of readings, and then work out $R = V/I$ for each pair. In the lab you will also meet the circuit symbols for a switch, bulb, resistor, variable resistor, diode, LED, LDR, thermistor and transistor.
In the laboratory, we use a device that can measure voltage, current, and resistance depending on the position of its dial. a) What is it called? b) Which unit would you…
In the laboratory, we use a device that can measure voltage, current, and resistance depending on the position of its dial.
- a) What is it called?
- b) Which unit would you select with the dial to test a toy's $4.5\ \text{V}$ battery?
We want to measure the resistance of a bulb disconnected from the circuit using a multimeter. a) Which unit do we select with the device's dial? b) If the multimeter…
We want to measure the resistance of a bulb disconnected from the circuit using a multimeter.
- a) Which unit do we select with the device's dial?
- 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…
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.
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.…
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.
- a) Reason whether its internal resistance should be very high or very low, considering what would happen to the current we want to measure.
- 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.
These sentences from an exam contain errors in units. Correct each one and justify the correction: a) "The battery has a current of $1.5\ \text{V}$." b) "A voltage of…
These sentences from an exam contain errors in units. Correct each one and justify the correction:
- a) "The battery has a current of $1.5\ \text{V}$."
- b) "A voltage of $0.3\ \text{A}$ passes through the bulb."
- c) "The resistance of this wire is $10\ \text{V}$."
Back to the beginning: In the phone charger, the transformer provides about $5\ \text{V}$ of push (voltage), about $2\ \text{A}$ of current flows towards the battery, and the lead has a very low resistance so that it does not heat up or waste energy.