Quick answer: An electric field E = F/q is the force per unit positive charge, a vector drawn with field lines. This AP Physics 2 lab on electric field gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How does electric potential change between two parallel electrodes, and what does that tell us about the field? You measure the potential at several points on conducting paper, plot a graph, and use the gradient to find the electric field strength. You then compare it with V/d.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 10
Aim
To map the potential between two parallel electrodes on conducting paper and use it to find the electric field.
Equipment
- Sheet of conducting (carbon-coated) paper with two parallel bar electrodes 10.0 cm apart
- 6.0 V DC supply or battery pack
- Digital voltmeter and a probe
- Ruler and cork board with drawing pins
- Connecting leads
Safety
The voltage is low and safe, but the paper and leads can warm up if a short circuit forms. Switch off between readings, never connect the electrodes with a lead, and keep drinks away from the equipment.
Method
- Pin the paper to the board and connect one electrode to the positive terminal and the other to the negative terminal of the 6.0 V supply.
- Connect the voltmeter negative lead to the negative electrode and use the positive lead as a probe.
- Touch the probe at x = 0, 2, 4, 6, 8 and 10 cm from the negative electrode along the centre line and record the potential.
- Find three points with the same reading and mark them to draw one equipotential line.
- Sketch the field direction perpendicular to your equipotentials.
Results
Sample readings from Mei’s group in Singapore are shown.
| x from negative electrode (cm) | 0 | 2.0 | 4.0 | 6.0 | 8.0 | 10.0 |
|---|---|---|---|---|---|---|
| Potential V (V) | 0.00 | 1.21 | 2.42 | 3.58 | 4.80 | 6.00 |
Analysis questions
Question 1 Plot and describe
Plot V (vertical) against x (horizontal) and describe the line.Question 2 Gradient and field
Calculate the gradient from the first and last points and use it to find the field strength in V/m.Question 3 Compare with V/d
Calculate V/d for the whole paper and find the percentage difference from your gradient.Question 4 Energy of a charge
A 1.0 μC charge is moved across the paper from the negative to the positive electrode. Calculate the work done.Question 5 Equipotentials and field lines
Describe the shape of the equipotential lines and how field lines relate to them.Question 6 Voltmeter resistance
Explain why a voltmeter with a very high resistance is used.Question 7 Edge effects
Near the edges of the paper the equipotential lines curve. Explain why and say how you would reduce the effect.
