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### College Physics (Urone)

Book edition 1st Edition
Author(s) Paul Peter Urone
Pages 1272 pages
ISBN 9781938168000

# The motor in a toy car operates on $$6.00V$$, developing a $$4.50V$$ back emf at normal speed. If it draws $$3.00A$$ at normal speed, what current does it draw when starting?

The current it draws when starting is $$12\;A$$.

See the step by step solution

## Step 1: Definition of current and resistance

Current is the term used to describe the speed at which charge moves. Resistance is the propensity of a substance to oppose the flow of charge.

## Step 2: Given information and Formula to be used

$$\begin{array}{c}{I_{back{\rm{ }}}} = 3A\\V = 6V\\Backemf = 4.50\;V\end{array}$$

The current in a circuit,

$$I = \frac{V}{R},$$

where $$V$$ is the potential and $$R$$ is the resistance.

## Step 3: Calculating the current

First we need find the resistance value,

$$\begin{array}{c}{I_{back{\rm{ }}}} = \frac{{V - {\rm{ }}Back{\rm{ }}emf{\rm{ }}}}{R}\\3 = \frac{{6 - 4.5}}{R}\\R = 0.5\Omega \end{array}$$

Now finding current it draws when starting,

$$\begin{array}{c}I = \frac{V}{R}\\{I_{start{\rm{ }}}} = \frac{V}{R}\\{I_{start{\rm{ }}}} = \frac{6}{{0.5}}\\{I_{start{\rm{ }}}} = 12\;A\end{array}$$

Therefore, current value is $$12\;A$$.