What Is Electric Current?

Electric current is the rate at which electric charge flows past a point in a circuit.

In a metal wire, the moving charged particles are electrons. When a complete circuit is connected to a source such as a cell or battery, electrons have a net movement through the conducting wires.

We describe this movement of charge using current.

A useful way to think about it is:

more charge passing each second → larger current

less charge passing each second → smaller current

 

 

 

Current does not tell us how much charge exists in the entire circuit. Instead, it tells us how quickly charge passes a particular point.

Current as a Rate of Flow

Imagine standing beside a road and counting cars.

If 5 cars pass you each minute, the traffic flow is relatively small.

If 100 cars pass you each minute, the traffic flow is much larger.

Electric current is similar, except instead of counting cars, we consider electric charge.

The relationship is:

\[ \boxed{I=\frac{Q}{t}} \]

where:

• \(I\) = electric current
• \(Q\) = electric charge
• \(t\) = time

Current therefore tells us how much charge passes a point per unit time.

The Unit of Electric Current

The SI unit of electric current is the ampere.

Its symbol is:

\[ \boxed{A} \]

The ampere is named after French physicist and mathematician André-Marie Ampère, whose work contributed greatly to our understanding of electricity and magnetism.

In equations, electric current is usually represented by the symbol:

\[ \boxed{I} \]

So we might write:

\[ I=2\text{ A} \]

This means that the electric current is 2 amperes.

What Does One Ampere Mean?

Charge is measured in coulombs (C).

One ampere means that one coulomb of charge passes a point every second.

Therefore:

\[ \boxed{1\text{ A}=1\text{ C/s}} \]

This is an extremely important relationship.

For example:

If 5 C of charge passes a point every second:

\[ I=5\text{ A} \]

If only 0.5 C passes every second:

\[ I=0.5\text{ A} \]

 

 

 

Current is therefore fundamentally a measurement of the rate of movement of electric charge.

Calculating Current

The equation for current is:

\[ I=\frac{Q}{t} \]

Worked Example 1

Suppose 20 C of charge passes through a wire in 5 seconds.

\[ I=\frac{Q}{t} \]

Substitute the values:

\[ I=\frac{20}{5} \]\[ \boxed{I=4\text{ A}} \]

The current is 4 A.

This means charge is passing the point at an average rate of 4 coulombs per second.

Rearranging the Current Equation

The equation:

\[ I=\frac{Q}{t} \]

can also be rearranged.

To calculate charge:

\[ \boxed{Q=It} \]

To calculate time:

\[ \boxed{t=\frac{Q}{I}} \]

So we have three useful forms:

\[ \boxed{I=\frac{Q}{t}} \]\[ \boxed{Q=It} \]\[ \boxed{t=\frac{Q}{I}} \]

Worked Example 2: Finding Charge

A current of 3 A flows for 10 seconds.

How much charge passes through the circuit?

Use:

\[ Q=It \]\[ Q=3\times10 \]\[ \boxed{Q=30\text{ C}} \]

Therefore, 30 coulombs of charge pass through the circuit.

Worked Example 3: Finding Time

A total charge of 24 C passes through a component while the current is 6 A.

How long does this take?

Use:

\[ t=\frac{Q}{I} \]\[ t=\frac{24}{6} \]\[ \boxed{t=4\text{ s}} \]

What Is Actually Moving?

In a metallic conductor, such as a copper wire, the moving charge carriers are electrons.

Metals contain large numbers of mobile or delocalized electrons.

When no electric field is present, these electrons already move randomly through the material.

However, random movement does not produce a net current.

When a source such as a battery establishes an electric field through a complete circuit, the electrons acquire an overall drift direction.

 

 

 

It is this net movement of charge that produces electric current.

Electron Flow

Electrons have a negative charge.

In the external metallic part of a simple DC circuit, their net drift is:

\[ \boxed{\text{negative terminal}\rightarrow\text{positive terminal}} \]

This direction is called electron flow.

 

 

 

However, this creates an important complication.

Circuit diagrams usually show current travelling in the opposite direction.

Conventional Current

Before scientists discovered electrons, they had already chosen a direction to represent electric current.

They assumed that positive charge moved from the positive terminal toward the negative terminal.

This direction became known as conventional current.

Therefore:

Conventional current

\[ \boxed{+\rightarrow-} \]

Electron flow in metallic conductors

\[ \boxed{-\rightarrow+} \]

 

 

 

Both directions are useful descriptions, but they refer to different conventions.

In most circuit diagrams and calculations, we use conventional current.

Why Do We Still Use Conventional Current?

It might seem strange to keep using conventional current when we know electrons move in the opposite direction through metal wires.

The reason is largely historical.

Scientists developed many of the rules and symbols of circuit theory before the electron was discovered. By the time scientists understood electron motion, conventional current was already deeply established.

Changing every equation, diagram, textbook, and engineering convention would have created unnecessary confusion.

So conventional current remains the standard.

Remember:

Circuit arrows usually show conventional current.

They do not usually show the direction that electrons drift through a metal wire.

Comparing Electron Flow and Conventional Current

A common mistake is to assume that one direction must be "wrong."

It isn't.

They are simply different ways of describing charge transport.

Measuring Electric Current

Electric current is measured using an instrument called an ammeter.

The circuit symbol for an ammeter is usually shown as:

\[ \boxed{\text{Ⓐ}} \]

An ammeter measures the amount of current flowing through a particular part of a circuit.

 

 

 

To measure current correctly, the ammeter must be placed so that the current being measured passes through the ammeter.

This means the ammeter is connected in series with the component.

Connecting an Ammeter

Suppose we want to measure the current through a lamp.

The circuit might contain:

cell → ammeter → lamp → cell

The ammeter becomes part of the conducting path.

The current flows through:

1. the wire
2. the ammeter
3. the lamp
4. the remaining wire
5. back to the source

 

 

 

Because the charge passes through the ammeter, the instrument can measure the rate at which charge is flowing.

Ammeters Are Connected in Series

This is one of the most important practical rules when measuring current:

\[ \boxed{\text{An ammeter is connected in series.}} \]

Why?

Because the charge flowing through the component must also flow through the ammeter.

If we want to know the current through a lamp, the ammeter must therefore be placed in the same current path as the lamp.

Later, when we study voltage, we will see that voltmeters are connected differently.

Using a Digital Multimeter

In laboratories, current is often measured using a digital multimeter rather than a dedicated ammeter.

A multimeter can usually measure several electrical quantities, including:

• current
• voltage
• resistance

To measure current, the meter must be placed in the correct current-measuring mode and connected in series with the circuit.

 

 

 

The correct terminals and range must also be selected.

Incorrectly connecting a multimeter while it is set to measure current can create a very low-resistance path, so laboratory instructions and meter ratings should always be followed.

Current in a Simple Series Circuit

Consider a simple circuit containing:

• one cell
• one switch
• one lamp

When the switch is open, the conducting path is incomplete.

There is no sustained current around the circuit.

When the switch is closed, a complete conducting path exists.

Current can then flow.

 

 

 

This gives us an important principle:

A sustained current requires a complete conducting path.

Current Is Not "Used Up"

A very common misconception is that a lamp "uses up" current.

It does not.

In a simple series circuit operating steadily, charge does not disappear as it passes through a lamp.

Charge is conserved.

Therefore, the rate of charge flow through one point in a simple series circuit is the same as the rate of charge flow through another point.

Imagine measuring current immediately before and immediately after a lamp.

If:

\[ I_{\text{before}}=0.50\text{ A} \]

then, for a simple steady series circuit:

\[ I_{\text{after}}=0.50\text{ A} \]

The lamp transfers energy, but it does not consume electric charge.

Current and Charge Conservation

Suppose a current of 2 A enters a lamp.

This means:

\[ 2\text{ C} \]

of charge passes into the lamp every second.

If only 1 C per second came out, charge would continuously accumulate inside the lamp.

That does not happen during normal steady operation.

Instead:

\[ 2\text{ C/s enters} \]

and

\[ 2\text{ C/s leaves} \]

Therefore:

\[ I_{\text{in}}=I_{\text{out}} \]

This is another consequence of conservation of charge.

Does a Larger Current Mean Faster Electrons?

This needs careful interpretation.

A larger current means more charge passes a point each second.

That can be related to factors including:

• the number of available charge carriers
• the charge carried by each particle
• their average drift speed
• the cross-sectional area of the conductor

So current is not simply "how fast an electron moves."

Current measures the rate of charge flow.

Large and Small Currents

Not every electrical device operates with the same current.

Very small currents may be measured in:

Milliamperes

\[ 1\text{ mA}=0.001\text{ A} \]

or:

\[ 1000\text{ mA}=1\text{ A} \]

Even smaller currents can be measured in microamperes (μA).

For example:

\[ 1\text{ μA}=0.000001\text{ A} \]

 

 

Being comfortable converting between amperes and milliamperes is useful when reading meters and working with electronic circuits.

Worked Example: Milliamperes

A meter reads:

\[ 250\text{ mA} \]

Convert this to amperes.

Because:

\[ 1000\text{ mA}=1\text{ A} \]

divide by 1000:

\[ 250\div1000=0.250 \]

Therefore:

\[ \boxed{250\text{ mA}=0.250\text{ A}} \]

Worked Example: Comparing Currents

Two circuits are measured.

Circuit A

\[ I=0.8\text{ A} \]

Circuit B

\[ I=1.5\text{ A} \]

Circuit B has the larger current.

This means that charge is passing a point at a greater rate in Circuit B.

It does not mean Circuit B contains "more electricity." Current specifically describes the rate of charge flow.

A Useful Analogy: Water Flow

Electric current is sometimes compared with water flowing through a pipe.

 

 

 

Imagine measuring how much water passes through a pipe every second.

A small flow:

small amount of water per second

A large flow:

large amount of water per second

Similarly:

small current → small amount of charge per second

large current → large amount of charge per second

The analogy is useful for understanding rate of flow, although electric circuits do not behave exactly like water systems.

Real-World Connections

Electric current is important in almost every electrical technology.

Phone Chargers

Chargers are labelled with current ratings, often expressed in amperes.

Household Appliances

Different appliances require different currents depending on their electrical characteristics and operating conditions.

Electronics

Small electronic components may operate with currents measured in milliamperes or even microamperes.

Electric Motors

Electric motors rely on electric current flowing through conductors to produce magnetic effects.

Circuit Protection

Fuses and circuit breakers are designed to respond when current becomes too large for a circuit.

Understanding current is therefore important both for designing circuits and for using electrical systems safely.

Common Mistakes

Mistake 1: Current is the amount of charge

Current is not simply charge.

Current is the rate at which charge flows:

\[ I=\frac{Q}{t} \]

Mistake 2: Current is measured in coulombs

Charge is measured in coulombs (C).

Current is measured in amperes (A).

Mistake 3: Electrons move in the direction of conventional current

In metallic conductors, they move in opposite directions.

Electron flow: − → +

Conventional current: + → −

Mistake 4: An ammeter is connected across a component

An ammeter is normally connected in series so that the current being measured passes through it.

Mistake 5: Components use up current

Components can transfer electrical energy into other forms, but electric charge is conserved.

Current is not gradually "used up" as it travels around a simple series circuit.

Mistake 6: A larger current simply means individual electrons are moving extremely quickly around the circuit

Current measures charge passing per second. Electron drift speed is a related but different concept.

Check Your Understanding

1. Define electric current.

2. What is the SI unit of electric current?

3. What symbol is used for the ampere?

4. What quantity is measured in coulombs?

5. Explain what a current of 1 A means.

6. Calculate the current if 12 C of charge passes a point in 3 seconds.

7. A current of 4 A flows for 5 seconds. Calculate the charge transferred.

8. 30 C of charge passes a point while the current is 5 A. Calculate the time.

9. State the direction of conventional current through the external circuit of a simple cell.

10. State the direction of electron flow through the metallic wires.

11. Explain why conventional current and electron flow point in opposite directions.

12. What instrument is used to measure electric current?

13. How should an ammeter be connected in a circuit?

14. Convert 500 mA into amperes.

15. Convert 0.075 A into milliamperes.

16. Explain why saying that a lamp "uses up current" is incorrect.

17. Circuit A carries 0.4 A and Circuit B carries 1.2 A. What does this tell you about the rates of charge flow?

18. Challenge: A current of 2.5 A flows through a wire for 3 minutes. Calculate the total charge that passes through the wire.

Key Terms

• Electric current – the rate of flow of electric charge.
• Charge – an electrical property of matter measured in coulombs.
• Coulomb (C) – the SI unit of electric charge.
• Ampere (A) – the SI unit of electric current.
• Ammeter – an instrument used to measure electric current.
• Electron flow – the net movement of electrons through a conductor.
• Conventional current – the standard current direction from positive toward negative in an external circuit.
• Drift – the overall directional movement of charge carriers in response to an electric field.
• Series connection – a connection in which components lie along the same current path.
• Milliampere (mA) – one thousandth of an ampere.
• Complete circuit – a continuous conducting path through which sustained current can flow.

Key Takeaways

• Electric current is the rate of flow of electric charge.
• Current is represented by the symbol \(I\).
• The SI unit of current is the ampere (A).
• One ampere means one coulomb of charge passes a point each second:

\[ \boxed{1\text{ A}=1\text{ C/s}} \]

• Current can be calculated using:

\[ \boxed{I=\frac{Q}{t}} \]

• Charge can be calculated using:

\[ \boxed{Q=It} \]

• In metallic conductors, mobile electrons carry charge.
• Electron flow through the external circuit is from negative to positive.
• Conventional current is defined in the opposite direction, from positive to negative.
• Current is measured using an ammeter.
• An ammeter is connected in series with the part of the circuit being measured.
• Current is not "used up" by a circuit component; electric charge is conserved.
• A larger current means more charge passes a point each second.

What Is Voltage?

Voltage is another name for electrical potential difference.

It describes the difference in electrical potential energy between two points in a circuit.

A useful definition is:

Voltage is the energy transferred per unit charge.

The equation is:

\[ \boxed{V=\frac{E}{Q}} \]

where:

• \(V\) = voltage or potential difference
• \(E\) = energy transferred
• \(Q\) = electric charge

The SI unit of voltage is the volt (V).

One volt means that 1 joule of energy is transferred for every 1 coulomb of charge:

\[ \boxed{1\text{ V}=1\text{ J/C}} \]

 

 

 

Understanding Potential Difference

The term potential difference is important because voltage is always measured between two points.

Imagine two locations in a circuit.

If a charge has more electrical potential energy at one point than another, there is a potential difference between those points.

When charge moves between them, energy can be transferred.

This is why voltage is closely connected to what circuit components do.

For example:

• a battery increases the electrical potential energy of charge
• a lamp transfers electrical energy into light and thermal energy
• a motor transfers electrical energy into kinetic energy

Voltage therefore tells us something about the energy transferred by each coulomb of charge.

A Simple Analogy: Height

A useful comparison is gravitational potential energy.

Imagine water stored at the top of a hill.

The water has gravitational potential energy because of its height.

As the water moves downhill, its potential energy can be transferred into other forms.

Similarly, a battery creates a difference in electrical potential between its terminals.

 

 

 

This analogy is useful because:

greater height difference → greater gravitational potential difference

and similarly:

greater voltage → greater electrical potential difference

However, voltage is not literally height or water pressure. It is an electrical quantity measured in volts.

How Batteries Produce Voltage

A battery produces voltage using chemical reactions.

Inside a battery, chemical processes separate electric charge and maintain a difference in electric potential between the terminals.

One terminal becomes relatively:

negative

while the other becomes relatively:

positive

This separation creates an electric potential difference.

 

 

 

When the battery is connected into a complete circuit, the electric field established around the circuit causes charge carriers in the conductors to have a net movement.

The battery continuously converts:

chemical energy → electrical energy

as long as the relevant chemical reactions can continue.

Cells and Batteries

A single electrochemical source is often called a cell.

A battery technically consists of two or more cells connected together, although in everyday language we commonly call individual cells "batteries."

A typical cell might provide:

\[ 1.5\text{ V} \]

This means approximately:

\[ 1.5\text{ J} \]

of energy is transferred per coulomb of charge.

So:

\[ 1.5\text{ V}=1.5\text{ J/C} \]

Worked Example: Energy per Charge

A battery provides 12 J of electrical energy to 3 C of charge.

Calculate the voltage.

Use:

\[ V=\frac{E}{Q} \]

Substitute:

\[ V=\frac{12}{3} \]\[ \boxed{V=4\text{ V}} \]

The battery provides 4 joules of energy per coulomb of charge.

Rearranging the Voltage Equation

Starting with:

\[ V=\frac{E}{Q} \]

we can calculate energy using:

\[ \boxed{E=VQ} \]

and charge using:

\[ \boxed{Q=\frac{E}{V}} \]

So the three useful forms are:

\[ V=\frac{E}{Q} \]\[ E=VQ \]\[ Q=\frac{E}{V} \]

Worked Example: Calculating Energy

A charge of 5 C moves through a potential difference of 6 V.

How much energy is transferred?

Use:

\[ E=VQ \]\[ E=6\times5 \]\[ \boxed{E=30\text{ J}} \]

Therefore, 30 J of energy is transferred.

Voltage and Current Are Not the Same

Voltage and current are closely related, but they describe different things.

Voltage

Voltage describes the energy transferred per unit charge.

Unit:

\[ \text{volt (V)} \]

Current

Current describes the rate at which charge flows.

Unit:

\[ \text{ampere (A)} \]

 

 

 

A useful comparison is:

Voltage → energy available per charge

Current → amount of charge flowing per second

These quantities should not be confused.

Voltage Does Not "Flow"

Students sometimes say that "voltage flows through the wire."

That is incorrect.

Charge flows.

The rate of charge flow is current.

Voltage is a difference in electric potential between two points.

So:

Current flows through a component.

Voltage exists across a component.

This distinction becomes very important when connecting meters.

How Voltage Affects Current

In many circuits, increasing voltage causes the current to increase.

Why?

A larger potential difference creates a stronger electric field in the conducting circuit, producing a greater net movement of charge carriers.

For a simple resistor at approximately constant temperature:

higher voltage → higher current

lower voltage → lower current

This relationship is described by Ohm's law.

For example, if a resistor has a resistance of \(6\,\Omega\):

At:

\[ V=6\text{ V} \]

the current is:

\[ I=\frac{6}{6}=1\text{ A} \]

At:

\[ V=12\text{ V} \]

the current becomes:

\[ I=\frac{12}{6}=2\text{ A} \]

Doubling the voltage doubled the current because the resistance remained constant.

Voltage, Current, and Resistance

The effect of voltage on current also depends on resistance.

Resistance tells us how strongly a component opposes the flow of electric charge.

For a given resistance:

greater voltage generally produces greater current.

However, two circuits with the same voltage may have different currents if their resistances are different.

Example

Circuit A:

\[ V=6\text{ V} \]\[ R=3\,\Omega \]\[ I=2\text{ A} \]

Circuit B:

\[ V=6\text{ V} \]\[ R=12\,\Omega \]\[ I=0.5\text{ A} \]

Both circuits have the same voltage, but Circuit A has the larger current because it has less resistance.

Predicting the Effect of Changing Voltage

Suppose a lamp is connected to a variable power supply.

If we gradually increase the voltage, we would generally expect the current through the lamp to increase.

 

 

 

However, real components do not always behave perfectly.

For example, as a filament lamp gets hotter, its resistance increases.

This means that doubling the voltage may not exactly double the current.

Later, this can be studied using current–voltage graphs.

Measuring Voltage

Voltage is measured using an instrument called a voltmeter.

The circuit symbol for a voltmeter is:

\[ \boxed{\text{Ⓥ}} \]

A voltmeter measures the potential difference between two points.

 

 

 

Because voltage is measured between two points, a voltmeter is connected across the component.

This is called a parallel connection.

Connecting a Voltmeter

Suppose we want to measure the voltage across a lamp.

The voltmeter must be connected with one lead on one side of the lamp and the other lead on the other side.

The voltmeter is therefore connected:

\[ \boxed{\text{in parallel with the lamp}} \]

The current does not need to pass through the voltmeter in the same way it passes through an ammeter.

Instead, the voltmeter compares the electrical potential at two points.

 

 

 

Ammeter vs. Voltmeter

It is important to distinguish the two meters.

A good memory rule is:

Ammeter → Along the circuit

Voltmeter → across the component

Why Is a Voltmeter Connected in Parallel?

Voltage is a comparison between two points.

Imagine a lamp.

To find the energy transferred per unit charge by the lamp, we need to compare:

electrical potential before the lamp

with:

electrical potential after the lamp

Therefore, the voltmeter must connect to both sides of the component.

This creates a parallel connection.

Using a Digital Multimeter

A digital multimeter can usually measure:

• voltage
• current
• resistance

When measuring voltage:

1. select the appropriate voltage setting
2. connect the leads to the correct meter terminals
3. place the probes across the two points being compared
4. read the voltage from the display

 

 

 

For a DC circuit, the polarity of the leads may affect whether the reading appears positive or negative.

Measuring the Voltage of a Cell

A voltmeter can be connected directly across the terminals of a cell.

If the meter reads:

\[ 1.5\text{ V} \]

the potential difference between the terminals is approximately 1.5 volts.

This means that the source can transfer approximately:

\[ 1.5\text{ J} \]

of electrical energy for every coulomb of charge passing through the source.

Voltage Across Circuit Components

Components transfer electrical energy into other forms.

For example, a lamp converts electrical energy mainly into:

• light energy
• thermal energy

If the voltage across a lamp is:

\[ 6\text{ V} \]

then each coulomb of charge transfers approximately:

\[ 6\text{ J} \]

of electrical energy as it passes through the lamp.

This connection between voltage and energy transfer is one of the most important concepts in circuit physics.

Batteries in Series

Cells can be connected so that their voltages combine.

For example, two 1.5 V cells connected appropriately in series can provide approximately:

\[ 1.5+1.5=3.0\text{ V} \]

Three such cells can provide:

\[ 1.5+1.5+1.5=4.5\text{ V} \]

 

 

 

A larger total voltage can produce a larger current if the resistance of the circuit remains unchanged.

Worked Example: Comparing Batteries

Battery A provides:

\[ 3\text{ V} \]

Battery B provides:

\[ 9\text{ V} \]

If each sends 2 C of charge through a component, how much energy can each transfer?

Battery A

\[ E=VQ \]\[ E=3\times2 \]\[ E=6\text{ J} \]

Battery B

\[ E=9\times2 \]\[ E=18\text{ J} \]

Battery B transfers more energy to each amount of charge because it has the greater voltage.

Real-World Voltage Values

Different electrical systems operate at very different voltages.

Examples include:

• individual cells: often around a few volts
• USB-powered electronics: low-voltage DC systems
• vehicle electrical systems: typically higher than individual cells
• household electrical supplies: much higher voltage
• power transmission systems: extremely high voltage

The voltage chosen depends on the purpose of the electrical system.

Higher voltage can allow more energy to be transferred per coulomb of charge, but electrical safety becomes increasingly important as voltage rises.

Voltage and Electrical Safety

Voltage is one factor that affects electrical hazards.

A sufficiently large potential difference can drive harmful current through the human body.

The actual current depends on factors including:

• voltage
• resistance of the body
• skin condition
• path through the body
• duration of contact

For this reason, electrical systems use insulation, grounding, protective devices, and safe operating procedures.

Common Mistakes

Mistake 1: Voltage and current are the same thing

They are different quantities.

Voltage: energy transferred per unit charge.

Current: rate of flow of charge.

Mistake 2: Voltage flows through a wire

Voltage does not flow.

Charge flows, producing current.

Voltage is a potential difference between two points.

Mistake 3: A voltmeter is connected in series

A voltmeter is connected in parallel across the component.

Mistake 4: A battery contains voltage like a container contains water

A battery maintains a potential difference through chemical processes.

Voltage describes a difference between the terminals, not a substance stored inside the battery.

Mistake 5: Higher voltage always means exactly proportional current

For a fixed resistance, increasing voltage increases current proportionally.

However, the resistance of some components can change with temperature or operating conditions.

Mistake 6: Voltage measures the total amount of energy

Voltage measures energy transferred per unit charge, not simply total energy.

Check Your Understanding

1. Define voltage.

2. What is another name for voltage?

3. State the SI unit of voltage.

4. What does a potential difference of 1 V mean?

5. Write the equation relating voltage, energy, and charge.

6. A battery transfers 18 J of energy to 3 C of charge. Calculate the voltage.

7. A charge of 4 C moves through a potential difference of 5 V. Calculate the energy transferred.

8. Explain how a battery creates a potential difference.

9. State one important difference between voltage and current.

10. Which quantity is measured in amperes?

11. Which quantity is measured in volts?

12. Predict what normally happens to current if voltage increases while resistance remains constant.

13. What instrument is used to measure voltage?

14. How should a voltmeter be connected to a lamp?

15. Explain why a voltmeter must be connected across a component rather than simply placed somewhere in series.

16. Two 1.5 V cells are connected appropriately in series. What total voltage do they provide?

17. Explain why saying "voltage flows through the circuit" is scientifically incorrect.

18. Challenge: A resistor has a resistance of \(8\,\Omega\). Calculate the current when the voltage is 4 V and again when the voltage is 12 V. Describe the relationship you observe.

Key Terms

• Voltage – electrical potential difference; the energy transferred per unit charge.
• Potential difference – the difference in electrical potential between two points.
• Volt (V) – the SI unit of voltage.
• Charge (Q) – an electrical quantity measured in coulombs.
• Coulomb (C) – the SI unit of electric charge.
• Electrical energy – energy associated with electric charges and electric circuits.
• Cell – an electrochemical source of electrical potential difference.
• Battery – a source consisting of one or more electrochemical cells.
• Current – the rate of flow of electric charge.
• Resistance – opposition to the movement of electric charge.
• Voltmeter – an instrument used to measure potential difference.
• Parallel connection – a connection across the same two points of a circuit.

Key Takeaways

• Voltage is electrical potential difference.
• Voltage tells us how much energy is transferred per coulomb of charge.
• Voltage is measured in volts (V).
• One volt equals:

\[ \boxed{1\text{ V}=1\text{ J/C}} \]

• Voltage can be calculated using:

\[ \boxed{V=\frac{E}{Q}} \]

• Batteries create voltage through chemical reactions that separate charge and maintain a potential difference.
• Current and voltage are different: current is the rate of charge flow, while voltage is energy transferred per unit charge.
• For a fixed resistance, increasing voltage causes the current to increase.
• Voltage is measured using a voltmeter.
• A voltmeter is connected in parallel across a component.
• Voltage exists between two points; it does not "flow" through the circuit.
• Circuit components transfer electrical energy, and the voltage across them indicates the energy transferred per coulomb of charge.