Skip to content Skip to navigation Skip to collection information

Connexions

You are here: Home » Content » Siyavula textbooks: Grade 10 Physical Science [CAPS] » Current and measurement

Navigation

Table of Contents

Lenses

What is a lens?

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

This content is ...

Affiliated with (What does "Affiliated with" mean?)

This content is either by members of the organizations listed or about topics related to the organizations listed. Click each link to see a list of all content affiliated with the organization.
  • FETPhysics display tagshide tags

    This module is included inLens: Siyavula: Physics (Gr. 10-12)
    By: Siyavula

    Review Status: In Review

    Click the "FETPhysics" link to see all content affiliated with them.

    Click the tag icon tag icon to display tags associated with this content.

Recently Viewed

This feature requires Javascript to be enabled.

Tags

(What is a tag?)

These tags come from the endorsement, affiliation, and other lenses that include this content.
 

Current and measurement

Module by: Free High School Science Texts Project. E-mail the author

Current

Flow of Charge

We have been talking about moving charge. But how much charge is moving, and how fast is it moving? The concept that represents this information is called current. Current allows us to quantify the movement of charge.

When we talk about current we talk about how much charge moves past a fixed point in circuit in one second. Think of charges being pushed around the circuit by the battery; there are charges in the wires but unless there is a battery they won't move. When one charge moves, the charges next to it also move. They keep their spacing as if you had a tube of marbles like in this picture.

Figure 1
Figure 1 (PG10C9_025.png)

If you push one marble into the tube then one must come out the other side. If you look at any point in the tube and push one marble into the tube, one marble will move past the point you are looking at. This is similar to charges in the wires of a circuit.

If one charge moves then they all move and the same number move at every point in the circuit. This is due to the conservation of charge.

Current

Now that we've thought about moving charges and visualised what is happening we need to get back to quantifying moving charge. We've already said that we call moving charge current but we define it precisely as follows:

Definition 1: Current

Current is the rate of flow of charge, i.e. the rate at which charges move past a fixed point in a circuit. We use the symbol I to show current and it is measured in amperes (A). One ampere is one coulomb of charge moving in one second. The relationship between current, charge and time is given by:

I = Q Δ t I = Q Δ t
(1)

When current flows in a circuit we show this on a diagram by adding arrows. The arrows show the direction of flow in the circuit. By convention we say that charge flows from the positive end (or terminal) of a battery, through the circuit, and back to the negative end (or terminal) of the battery. This is shown in the diagram below. We measure the current with an instrument called an ammeter.

Figure 2
Figure 2 (PG10C9_026.png)

The arrows in this picture show you the direction that charge will flow in the circuit. Note that the arrows point from the positive end of the battery, through the circuit, towards the negative end of the battery.

Note: Interesting Fact :

Benjamin Franklin made a guess about the direction of charge flow when rubbing smooth wax with rough wool. He thought that the charges flowed from the wax to the wool (i.e. from positive to negative) which was opposite to the real direction. Due to this, electrons are said to have a negative charge and so objects which Ben Franklin called "negative" (meaning a shortage of charge) really have an excess of electrons. By the time the true direction of electron flow was discovered, the convention of ´positive¡ and ´negative¡ had already been so well accepted in the scientific world that no effort was made to change it.

Tip:

A battery does not produce the same amount of current no matter what is connected to it. While the voltage produced by a battery is constant, the amount of current supplied depends on what is in the circuit.

Exercises: Current

Exercise 1

Using the relationship between current, charge, and time, calculate the current in a circuit which has 0,8C0,8C of charge passing a point every second.

Solution
  1. Step 1. Write down the relationship between current, charge and time: I=QΔtI=QΔtI = {Q} over {%DELTA t}
  2. Step 2. Figure out what is given and what is being asked for:

    Given: Q=0,8CQ=0,8C

    Δt=1sΔt=1s
    (2)

    Asked for: I

  3. Step 3. We can now substitute the numbers in the equation and solve for current, I. :
    I=0,8C1sx=0,8AI=0,8C1sx=0,8A
    (3)
Exercise 2

How much charge flows per second in a circuit with a current of 1,5A1,5A?

Solution
  1. Step 1. Determine what is being asked for and what is given:

    Asked for: Charge, Q

    Given: Current I=1,5AI=1,5A

    Δt=1sΔt=1s
    (4)

  2. Step 2. Write down the relationship between current and rate of flow of charge: I=QΔtI=QΔtI = {Q} over {%DELTA t}
  3. Step 3. Rearrange the equation to solve for Q:

    Q = I×Δt = 1,5A×1s = 1,5C Q = I×Δt = 1,5A×1s = 1,5C

Exercise 3

If 500×103μC500×103μC of charge flow past a point in a circuit in 1 second, what is the current in the circuit?

Solution
  1. Step 1. Determine what is being asked for and what is given:

    Asked for: current, I

    Given: charge Q=500×103μCQ=500×103μC
     Δt=1sΔt=1s

  2. Step 2. Convert all non-SI units to SI units:

    Q=500×103μCx=500×103×10−6Cx=0,5CQ=500×103μCx=500×103×10−6Cx=0,5C

  3. Step 3. Write down the relationship between current and rate of flow of charge:

    I=QΔtI=QΔt

  4. Step 4. We can now substitute the numbers into the equation and solve for current, I. :

    I=QΔtx=0,5C1sx=0,5AI=QΔtx=0,5C1sx=0,5A

Exercise 4

I measure the current in a circuit to be 500mA500mA. How much charge is flowing per second in the circuit?

Solution
  1. Step 1. Determine what is being asked for and what is given:

    Asked for: Charge, Q

    Given: Current I=500mAI=500mA,
     Δt=1sΔt=1s

  2. Step 2. Convert all non-SI units to SI units:

    500mA=500×10−3A500mA=500×10−3A

  3. Step 3. Write down the relationship between current and rate of flow of charge:

    I=QΔtI=QΔtI = {Q} over {%DELTA t}

  4. Step 4. Rearrange the equation to solve for Q.:

    Q=I×Δtx=500×10−3A×1sx=0,5CQ=I×Δtx=500×10−3A×1sx=0,5C

Measuring voltage and current in circuits

As we have seen in previous sections, an electric circuit is made up of a number of different components such as batteries, resistors and light bulbs. There are devices to measure the properties of these components. These devices are called meters.

For example, one may be interested in measuring the amount of current flowing through a circuit using an ammeter or measuring the voltage provided by a battery using a voltmeter. In this section we will discuss the practical usage of voltmeters and ammeters.

Voltmeter

A voltmeter is an instrument for measuring the voltage between two points in an electric circuit. In analogy with a water circuit, a voltmeter is like a meter designed to measure pressure difference. Since one is interested in measuring the voltage between two points in a circuit, a voltmeter must be connected in parallel with the portion of the circuit on which the measurement is made.

Figure 3: A voltmeter should be connected in parallel in a circuit.
Figure 3 (PG10C9_034.png)

Figure 3 shows a voltmeter connected in parallel with a battery. The positive lead of the voltmeter must be connected closest to the positive end of the battery and the negative lead closest to the negative end of the battery. The voltmeter may also be used to measure the voltage across a resistor or any other component of a circuit that has a voltage drop or potential difference.

Ammeter

An ammeter is an instrument used to measure the flow of electric current in a circuit. Since one is interested in measuring the current flowing through the circuit, the ammeter must be connected in series with the measured circuit component (Figure 4). The positive lead from the ammeter must be connected closest to the positive end of the battery and the negative lead must be connected closest to the negative end of the battery.

Figure 4: An ammeter should be connected in series in a circuit.
Figure 4 (PG10C9_035.png)

Impact of meters on circuits

A good quality meter used correctly will not significantly change the values it is used to measure. This means that an ammeter has very low resistance so as to not slow down the flow of charge. A voltmeter has a very high resistance so that it does not add another parallel pathway to the circuit for the charge to flow along.

Investigation : Using meters

If possible, connect meters in circuits to get used to how to use meters to measure electrical quantities. If the meters have more than one scale, always connect to the largest scale first so that the meter will not be damaged by having to measure values that exceed its limits.

The table below summarises the use of each measuring instrument that we discussed and the way it should be connected to a circuit component.

Table 1
Instrument Measured Quantity Proper Connection
Voltmeter Voltage In Parallel
Ammeter Current In Series

Collection Navigation

Content actions

Download module as:

PDF | More downloads ...

Add:

Collection to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks

Module to:

My Favorites (?)

'My Favorites' is a special kind of lens which you can use to bookmark modules and collections. 'My Favorites' can only be seen by you, and collections saved in 'My Favorites' can remember the last module you were on. You need an account to use 'My Favorites'.

| A lens I own (?)

Definition of a lens

Lenses

A lens is a custom view of the content in the repository. You can think of it as a fancy kind of list that will let you see content through the eyes of organizations and people you trust.

What is in a lens?

Lens makers point to materials (modules and collections), creating a guide that includes their own comments and descriptive tags about the content.

Who can create a lens?

Any individual member, a community, or a respected organization.

What are tags? tag icon

Tags are descriptors added by lens makers to help label content, attaching a vocabulary that is meaningful in the context of the lens.

| External bookmarks