Suggested time to complete: 25 minutes

1. Define electric current.
2. Explain the difference between static electricity and current electricity.
3. Give the units in which the following are measured:
   1. current
   2. charge
4. An electric current of $\scriptsize 10\text{ A}$ flows in a circuit for one minute. Calculate the electrical charge which flows past a point in the conductor during this time.
5. Calculate the amount of current passing a point in a conductor when $\scriptsize 2\text{ 400 C}$ takes $\scriptsize \text{5}$ minutes to pass this point.
6. $\scriptsize \text{95}$ electrons move past a fixed point in a circuit in one tenth of a second, what is the current in the circuit?
7. How many electrons move through a fixed point in a circuit in $\scriptsize 10\text{ s}$ if the current is $\scriptsize 6\text{ A}$?
8. What is the current, in milliamperes, produced by the solar cells of a pocket calculator through which $\scriptsize 4\text{ C}$ of charge passes in $\scriptsize \text{4 hrs}\text{.}$?
9. A total of $\scriptsize 600\text{ C}$ of charge passes through a flashlight in $\scriptsize 0.5\text{ hrs}\text{.}$ What is the average current?
10. What is the current when a typical charge of $\scriptsize 0.25\text{ C}$ moves from your finger to a metal doorknob in one millisecond?
11. A large lightning bolt had a $\scriptsize 20\text{ 000 A}$ current and moved $\scriptsize 30\text{ C}$ of charge. What was its duration?
12. The $\scriptsize 200\text{ A}$ current through a spark plug moves $\scriptsize 0.003\text{ mC}$ of charge. How long does the spark last?
13. Explain the difference between direct and alternating current.

The full solutions are at the end of the unit.

Exercise 1.1

1.  . 
   

   	Circuit 1	Circuit 2	Circuit 3	Circuit 4	Circuit 5
   $\scriptsize I\text{ }=\text{ }\displaystyle \frac{Q}{{\Delta t}}$	$\scriptsize =\text{ }\displaystyle \frac{1}{{20}}$	$\scriptsize =\text{ }\displaystyle \frac{{0.3}}{2}$	$\scriptsize =\text{ }\displaystyle \frac{{0.9}}{3}$	$\scriptsize =\text{ }\displaystyle \frac{{0.2}}{4}$	$\scriptsize =\text{ }\displaystyle \frac{{24}}{{120}}$
   	$\scriptsize =\text{ }0.05\text{ A}$	$\scriptsize =\text{ 0}\text{.15 A}$	$\scriptsize =\text{ 0}\text{.3 A}$	$\scriptsize =\text{ 0}\text{.05 A}$	$\scriptsize =\text{ 0}\text{.2 A}$

   Notes: In circuits $\scriptsize 2$ and $\scriptsize 4$ the current needs to be converted from mA to A by dividing by $\scriptsize 1000$.
   In circuit $\scriptsize 5$, the time needs to be converted from minutes to seconds by multiplying by $\scriptsize 60$.

2. . 
   

   	Circuit 1	Circuit 2	Circuit 3	Circuit 4	Circuit 5
   $\scriptsize \Delta t\text{ }=\text{ }\displaystyle \frac{Q}{I}$	$\scriptsize =\text{ }\displaystyle \frac{{1.0}}{2}$	$\scriptsize =\text{ }\displaystyle \frac{{0.5}}{{0.5}}$	$\scriptsize =\text{ }\displaystyle \frac{{0.6}}{{0.3}}$	$\scriptsize =\text{ }\displaystyle \frac{{0.2}}{{0.4}}$	$\scriptsize =\text{ }\displaystyle \frac{{1.5}}{{0.02}}$
   	$\scriptsize =\text{ 0}\text{.5 s}$	$\scriptsize =\text{ 1 s}$	$\scriptsize =\text{ 2 s}$	$\scriptsize =\text{ 0}\text{.5 s}$	$\scriptsize =\text{ 75 s}$

   Note: In circuit $\scriptsize 3$ the current must be converted from mA to A by dividing by $\scriptsize 1000$.

3. . 
   

   	Circuit 1	Circuit 2	Circuit 3	Circuit 4	Circuit 5
   $\scriptsize Q\text{ }=\text{ }I\text{ x }\Delta t$	$\scriptsize =\text{ 2 x 20}$	$\scriptsize =\text{ 0}\text{.5 x 2}$	$\scriptsize =0.3\text{ x 0}\text{.3}$	$\scriptsize =\text{ 0}\text{.4 x 0}\text{.4}$	$\scriptsize =\text{ 0}\text{.2 x 120}$
   	$\scriptsize =\text{ 40 C}$	$\scriptsize =\text{ 1 C}$	$\scriptsize =\text{ 0}\text{.9 C}$	$\scriptsize =\text{ 1}\text{.6 C}$	$\scriptsize =\text{ 24 C}$

   Note: In circuit $\scriptsize 3$ the current needs to be converted from mA to A by dividing by $\scriptsize 1000$.
   In circuit $\scriptsize 5$ the time needs to be converted from minutes to seconds by multiplying by $\scriptsize 60$.

Back to Exercise 1.1

Unit 1: Assessment

1. Current is the rate at which charge moves through a fixed point in a circuit OR current is how much charge passes a particular point in a circuit per second.
2. Static electricity occurs when electrons move from one object into another object creating two objects of opposite charge and how these charged objects interact to create a force between them. Current electricity is the continuous flow of electrons in a conductor in a closed circuit because of energy supplied by a power source.
3. .
   1. Amperes
   2. Coulombs
4. Convert minutes to seconds: $\scriptsize 1$ minute = $\scriptsize 60$ seconds
   $\scriptsize \begin{align*}Q&=I\times \Delta t\\&=10\times 60\\&=600 \text{ C}\end{align*}$
5. Convert minutes to seconds: $\scriptsize 5$ minutes = $\scriptsize 300$ seconds
   $\scriptsize \begin{align*}I\text{ } & =\text{ }\displaystyle \frac{Q}{{\Delta t}}\\\text{ } & =\text{ }\displaystyle \frac{{2\text{ 400}}}{{300}}\\\text{ } & =\text{ 8 A}\end{align*}$
6. The charge of $\scriptsize 95$ electrons must be calculated first:
   $\scriptsize 1\text{ C}=6.24\times 10^{18}\text{ electrons. Then the charge on }1\text{ electron}=\displaystyle \frac{1}{6.24}\times 10^{18}=1.6 \times 10^{-19}\text{ C}$
   $\scriptsize \begin{align*} Q_{95} \text{ electrons} = 95 \times 1.6 \times 10^{-19}&= 1.52 \times 10^{-17} \text{ C}\\ I&=\displaystyle \frac{Q}{\Delta t}\\ &=\displaystyle \frac{1.52 \times 10^{-17}}{0.1}\\ &=1.52 \times 10^{-16} \text{ A} \end{align*}$
7. .
   $\scriptsize \begin{align*}Q\text{ } & =\text{ }I\text{ x }\Delta t\\ & = \text{ 6 x 10}\\ & = \text{ 60 C}\end{align*}$
   $\scriptsize \begin{align*}\text{no}\text{. of electrons } & =\text{ }\displaystyle \frac{Q}{{\text{charge on one electron}}}\\\text{ } & =\text{ }\displaystyle \frac{{60}}{{1.6\text{ x 1}{{\text{0}}^{{-19}}}}}\\\text{ } & =\text{ 3}\text{.75 x 1}{{\text{0}}^{{20}}}\text{ electrons}\end{align*}$
8. Convert time in hrs. to seconds. $\scriptsize 4\text{ x 60 x 60 = 14 400 s}$
   $\scriptsize \begin{align*}I\text{ } & =\text{ }\displaystyle \frac{Q}{{\Delta t}}\\\text{ } & =\text{ }\displaystyle \frac{4}{{14\text{ 400}}}\\\text{ } & =\text{ 2}\text{.78 x 1}{{\text{0}}^{{-4}}}\text{A}\end{align*}$
   Convert to milliamperes: $\scriptsize \text{2}\text{.78 x 1}{{\text{0}}^{{-4}}}\text{ x 1 000 = 0}\text{.28 mA}$
9. Convert time from hours to seconds: $\scriptsize 0.5\text{ x 60 x 60 = 1800 s}$
   $\scriptsize \begin{align*}I\text{ } & =\text{ }\displaystyle \frac{Q}{{\Delta t}}\\\text{ } & =\text{ }\displaystyle \frac{{600}}{{18\text{00}}}\\\text{ } & =\text{ 0}\text{.33 A}\end{align*}$
10. Convert one ms to seconds: $\scriptsize 1\text{ ms = 0}\text{.001 s}$
    $\scriptsize \begin{align*}I\text{ } & =\text{ }\displaystyle \frac{Q}{{\Delta t}}\\\text{ } & =\text{ }\displaystyle \frac{{0.25}}{{0,001}}\\\text{ } & =\text{ 250 A}\end{align*}$
11. .
    $\scriptsize \begin{align*}\vartriangle t\text{ } & =\text{ }\displaystyle \frac{Q}{I}\\\text{ } & =\text{ }\displaystyle \frac{{30\text{ }}}{{20\text{ }000}}\text{ }\\\text{ } & =\text{ 1}\text{.5 x 1}{{\text{0}}^{{-3}}}\text{ s}\end{align*}$
12. Convert mC to C: $\scriptsize 0.003\text{ mC = 0}\text{.000003 C}$
    $\scriptsize \begin{align*}\Delta t\text{ } & =\text{ }\displaystyle \frac{Q}{I}\\\text{ } & =\text{ }\displaystyle \frac{{0.000003}}{{200}}\\\text{ } & =\text{ 1}\text{.5 x 1}{{\text{0}}^{{-8}}}\text{ s}\end{align*}$
13. In direct current, the charges flow in one direction only. In alternating current the flow of charge reverses direction at regular intervals.

Back to Unit 1: Assessment