Current, voltage, and what they measure

Science · Energy · ages 11-12

Name ______________________ Date ____________

What instrument measures potential difference (voltage)?
- Thermometer
- Voltmeter
- Ammeter
What unit is used to measure electric current?
- Amperes
- Volts
- Watts
An ammeter should be connected in series in the circuit loop.
Circle one: True False
Look at the chart. How many amps does Bulb B's ammeter read?
Answer: ______________
Which best describes what current measures?
- How much energy each unit of charge transfers
- How bright a bulb glows
- How much charge passes a point each second
A student wants to measure the potential difference across a bulb. How should the voltmeter be connected?
- Anywhere in the circuit
- In parallel, across the bulb
- In series, inside the loop
Two circuits have the same current of 0.5 A, but Circuit X has a higher voltage than Circuit Y. What does this tell you?
- Circuit X and Circuit Y are identical
- More charge passes a point each second in Circuit X than in Circuit Y
- Each unit of charge in Circuit X transfers more energy
Priya wires an ammeter in parallel across the bulb instead of in series. What is most likely to happen?
- Nothing changes, because an ammeter reads the same in either place
- It gives a wrong reading and the current may become unsafe
- The bulb glows brighter, because the ammeter adds a second path for charge

Answer key

For grown-ups. Fold this page away before handing over the rest.

Voltmeter · A voltmeter measures voltage, the energy transferred per unit of charge.
Amperes · Current is measured in amperes (A), often just called amps.
True · Yes. The ammeter sits inside the loop so all the charge flows through it.
4 · Read the height of Bulb B's bar: it reaches 4, so its ammeter shows 4 A.
How much charge passes a point each second · Current measures the amount of charge passing a point each second. It does not directly measure the speed of individual charge carriers. Energy per unit charge is voltage.
In parallel, across the bulb · A voltmeter reads the difference between two points, so it connects in parallel across the bulb.
Each unit of charge in Circuit X transfers more energy · Equal currents mean equal amounts of charge pass a point each second. The higher voltage in Circuit X means more energy is transferred per unit of charge, not a greater charge flow rate.
It gives a wrong reading and the current may become unsafe · An ammeter lets charge through very easily, so across the bulb it becomes a shortcut around it. The reading is wrong, and the current can grow large enough to be unsafe.