# Mahi Tuatahi

- Draw a series circuit with 6V power supply, a bulb and a resistor. Indicate the positive and negative terminals on the power supply, and indicate the direction of conventional current and the actual movement of electrons.
- What is the definition of current?
- Give an equation that relates to the definition.
- A circuit draws 0.4A and in total 2.5C goes past a certain point. How long did was the circuit on for?

## Circuit Diagram Symbols

You can find more symbols in your textbook (197-198) but here is the most basic few.

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Yesterday we talked about current and how it is the rate of transfer of charge per unit time.

\begin{aligned} & I = \frac{q}{t} \end{aligned}

## Current Question

A current of $1.2A$ flows for $0.25s$. How much charge was transferred in that time?

### Answer

\begin{aligned}
& I = \frac{q}{t} \

& q = I \times t \

& q = 1.2 \times 0.25 \

& q = 0.3 Coulombs (C)
\end{aligned}

# What is a circuit?

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## Answer

A circuit is a way to deliver energy to different components!

Open this link and build a circuit with a power supply, light bulb, resistor and switch in series. What happens when you take out the resistor?

PhET DC Circuit Construction Simulation

# Voltage

- The
**charge carriers**flowing around a circuit are**energy carriers** - The
**charge carriers**carry**electrical energy**which comes from the power supply **Voltage is the amount of energy in one coulomb of charge**

\begin{aligned}
& V = \frac{E_{p}}{q} \

& V = \text{ voltage measured in…} \

& E_{p} = \text{ electrical energy measured in… } \

& q = \text{ charge measured in…}
\end{aligned}

## Voltage Question 1

A system has electrical potential energy of $25J$ and has $0.5C$ of charge. What is the voltage of the system?

### Answer

\begin{aligned}
& V = \frac{E_{p}}{q} \

& V = \frac{25}{0.5} \

& V = 12.5V \

\end{aligned}

## Voltage Question 2

A system has a voltage of $12V$ and has $1.75C$ of charge inside it. What potential energy does the system have?

### Answer

\begin{aligned}
& V = \frac{E_{p}}{q} \

& E_{p} = V \times q \

& E_{p} = 12 \times 1.75 \

& E_{p} = 21J \

\end{aligned}

# Power

- The amount of energy transferred/transformed per second
- E.g. A 100W ($100 J/s, 100JS^{-1}$) light bulb transforms 100J of electrical energy per second into light and heat energy.

\begin{aligned}
& P = IV \

& P = \text{ power measured in…} \

& I = \text{ current measured in…} \

& V = \text{ voltage measured in…}
\end{aligned}

## Power Question 1

A system has a current of $0.5A$ and voltage, $6V$. What is the power consumed by the system?

### Answer

\begin{aligned}
& P = IV \

& P = 0.5 \times 6 \

& 3W = 3J/s = 3Js^{-1} \

\end{aligned}

## Power Question 2

A washing machine has a power rating of 65W and the house is using a 12V system. What current is the washing machine drawing?

### Answer

\begin{aligned}
& P = IV \

& I = \frac{P}{V} \

& I = \frac{65}{12} \

& I = 5.42A \

\end{aligned}

# Voltage & Power Summary

By substituting these equations into each other we can begin to see how intertwined all of these concepts are.

\begin{aligned}
& I = \frac{q}{t} \

& V = \frac{E_{p}}{q} \

& \

& P = I \times V = \frac{q}{t} \times \frac{E_{p}}{q} = \frac{E_{p}}{t}
\end{aligned}