Current, Potential Difference and Ohm's Law: Quick Revision

A brief note on Current, Potential Difference and Ohm's Law. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Current is the charge passing each second: $I = \dfrac{Q}{t}$, in amperes. An electron carries $1.6 \times 10^{-19}\ \text{C}$.
• The conventional current goes from $+$ to $-$ outside the cell; the electrons drift the other way.
• Potential difference is the work per coulomb: $V = \dfrac{W}{Q}$, in volts.
• Ohm's law: at a steady temperature, $V = IR$. The V-I graph is a straight line through the origin, with slope $R$.
• An ammeter goes in series (tiny resistance); a voltmeter goes in parallel (huge resistance).

The facts at a glance

A worked example

A current of $0.45\ \text{A}$ flows for $3$ minutes through a $12\ \Omega$ resistor. Find the charge that passes and the potential difference across the resistor.

1. The time. $3\ \text{min} = 180\ \text{s}$.
2. The charge. $Q = 0.45\ \text{A} \times 180\ \text{s} = 81\ \text{C}$.
3. The potential difference. $V = IR = 0.45\ \text{A} \times 12\ \Omega = 5.4\ \text{V}$.

Practice with new numbers

A program to try

The program works out the charge, the potential difference and the energy for one resistor.

Mistakes to avoid

• Leaving the time in minutes in $Q = It$. Use seconds.
• Joining an ammeter in parallel. It would short the circuit.
• Thinking current is used up in a bulb. The same current flows in and out; energy is what is used.
• Applying Ohm's law to a hot filament. Its resistance changes as it heats.

Check yourself

1. A charge of $54\ \text{C}$ passes in $15\ \text{s}$. What is the current?

2. A voltmeter reads $2.2\ \text{V}$ across a wire, and an ammeter $0.08\ \text{A}$ through it. Find its resistance.

3. A $6\ \text{V}$ battery moves $3.5\ \text{C}$. How much energy does it give?

4. Two conductors have straight V-I lines. Which one has the larger resistance?