7.3 Electric field: concept of a field revisited  (Page 2/5)

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$E=|\frac{F}{q}|=k|\frac{\text{qQ}}{{\mathrm{qr}}^{2}}|=k\frac{|Q|}{{r}^{2}}.$

Since the test charge cancels, we see that

$E=k\frac{|Q|}{{r}^{2}}.$

The electric field is thus seen to depend only on the charge $Q$ and the distance $r$ ; it is completely independent of the test charge $q$ .

Calculating the electric field of a point charge

Calculate the strength and direction of the electric field $E$ due to a point charge of 2.00 nC (nano-Coulombs) at a distance of 5.00 mm from the charge.

Strategy

We can find the electric field created by a point charge by using the equation $E=\text{kQ}/{r}^{2}$ .

Solution

Here $Q=2\text{.}\text{00}×{\text{10}}^{-9}$ C and $r=5\text{.}\text{00}×{\text{10}}^{-3}$ m. Entering those values into the above equation gives

$\begin{array}{lll}E& =& k\frac{Q}{{r}^{2}}\\ & =& \left(\text{8.99}×{\text{10}}^{9}\phantom{\rule{0.25em}{0ex}}\text{N}\cdot {\text{m}}^{2}{\text{/C}}^{2}\right)×\frac{\left(\text{2.00}×{\text{10}}^{-9}\phantom{\rule{0.25em}{0ex}}\text{C}\right)}{\left(\text{5.00}×{\text{10}}^{-3}\phantom{\rule{0.25em}{0ex}}\text{m}{\right)}^{2}}\\ & =& \text{7.19}×{\text{10}}^{5}\phantom{\rule{0.25em}{0ex}}\text{N/C.}\end{array}$

Discussion

This electric field strength is the same at any point 5.00 mm away from the charge $Q$ that creates the field. It is positive, meaning that it has a direction pointing away from the charge $Q$ .

Calculating the force exerted on a point charge by an electric field

What force does the electric field found in the previous example exert on a point charge of $–0.250\phantom{\rule{0.25em}{0ex}}\mu \text{C}$ ?

Strategy

Since we know the electric field strength and the charge in the field, the force on that charge can be calculated using the definition of electric field $\mathbf{\text{E}}=\mathbf{\text{F}}/q$ rearranged to $\mathbf{\text{F}}=q\mathbf{\text{E}}$ .

Solution

The magnitude of the force on a charge $q=-0\text{.}\text{250}\phantom{\rule{0.25em}{0ex}}\text{μC}$ exerted by a field of strength $E=7\text{.}\text{20}×{\text{10}}^{5}$ N/C is thus,

$\begin{array}{lll}F& =& -\text{qE}\\ & =& \left(\text{0.250}×{\text{10}}^{\text{–6}}\phantom{\rule{0.25em}{0ex}}\text{C}\right)\left(7.20×{\text{10}}^{5}\phantom{\rule{0.25em}{0ex}}\text{N/C}\right)\\ & =& \text{0.180 N.}\end{array}$

Because $q$ is negative, the force is directed opposite to the direction of the field.

Discussion

The force is attractive, as expected for unlike charges. (The field was created by a positive charge and here acts on a negative charge.) The charges in this example are typical of common static electricity, and the modest attractive force obtained is similar to forces experienced in static cling and similar situations.

Section summary

• The electrostatic force field surrounding a charged object extends out into space in all directions.
• The electrostatic force exerted by a point charge on a test charge at a distance $r$ depends on the charge of both charges, as well as the distance between the two.
• The electric field $\mathbf{\text{E}}$ is defined to be
$\mathbf{\text{E}}=\frac{\mathbf{\text{F}}}{q,}$

where $\mathbf{\text{F}}$ is the Coulomb or electrostatic force exerted on a small positive test charge $q$ . $\mathbf{\text{E}}$ has units of N/C.

• The magnitude of the electric field $\mathbf{\text{E}}$ created by a point charge $Q$ is
$\mathbf{\text{E}}=k\frac{|Q|}{{r}^{2}}.$

where $r$ is the distance from $Q$ . The electric field $\mathbf{\text{E}}$ is a vector and fields due to multiple charges add like vectors.

Conceptual questions

Why must the test charge $q$ in the definition of the electric field be vanishingly small?

Are the direction and magnitude of the Coulomb force unique at a given point in space? What about the electric field?

Problem exercises

What is the magnitude and direction of an electric field that exerts a $2\text{.}\text{00}×{\text{10}}^{-5}\phantom{\rule{0.25em}{0ex}}\text{N}$ upward force on a $–1.75\phantom{\rule{0.25em}{0ex}}\mu \text{C}$ charge?

What is the magnitude and direction of the force exerted on a $3.50\phantom{\rule{0.25em}{0ex}}\mu \text{C}$ charge by a 250 N/C electric field that points due east?

$8\text{.}\text{75}×{\text{10}}^{-4}$ N

Calculate the magnitude of the electric field 2.00 m from a point charge of 5.00 mC (such as found on the terminal of a Van de Graaff).

(a) What magnitude point charge creates a 10,000 N/C electric field at a distance of 0.250 m? (b) How large is the field at 10.0 m?

(a) $6\text{.}\text{94}×{\text{10}}^{-8}\phantom{\rule{0.25em}{0ex}}\text{C}$

(b) $6\text{.}\text{25}\phantom{\rule{0.25em}{0ex}}\text{N/C}$

Calculate the initial (from rest) acceleration of a proton in a $5\text{.}\text{00}×{\text{10}}^{6}\phantom{\rule{0.25em}{0ex}}\text{N/C}$ electric field (such as created by a research Van de Graaff). Explicitly show how you follow the steps in the Problem-Solving Strategy for electrostatics.

(a) Find the direction and magnitude of an electric field that exerts a $4\text{.}\text{80}×{\text{10}}^{-\text{17}}\phantom{\rule{0.25em}{0ex}}\text{N}$ westward force on an electron. (b) What magnitude and direction force does this field exert on a proton?

(a) $\text{300}\phantom{\rule{0.25em}{0ex}}\text{N/C}\phantom{\rule{0.25em}{0ex}}\left(\text{east}\right)$

(b) $4\text{.}\text{80}×{\text{10}}^{-\text{17}}\phantom{\rule{0.25em}{0ex}}\text{N}\phantom{\rule{0.25em}{0ex}}\left(\text{east}\right)$

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