Define electric field strength - Leaving Cert Physics - Question 8 - 2015

To induce a negative charge, bring a positively charged rod close to the metal cap. This will cause the electrons in the electroscope to move towards the cap, resulting in a build-up of negative charge on the gold leaves. Next, touch the earth cap briefly, allowing some electrons to flow from the earth into the leaves, then remove the rod and earth cap. The electroscope now holds a negative charge.

Point discharge occurs when the concentration of charge at a pointed conductor becomes high enough to ionize the surrounding air. A diagram should show a pointed object near a larger conducting body. As the distance decreases, strong electric fields form, neutralizing air particles at the point.

To demonstrate point discharge, use a candle flame near a charged pointed conductor. As the flame approaches the conductor, the air near the point ionizes, allowing the charge to escape and leading to a visible discharge, seen as a spark or glowing ionized air around the flame.

Given the dome has a diameter of 40 cm, the radius is:

$d = 40 ext{ cm} = 0.40 ext{ m} \implies r = \frac{0.40}{2} = 0.20 ext{ m}$

The distance from the center of the dome to the point is:

$r' = r + 0.04 ext{ m} = 0.20 + 0.04 = 0.24 ext{ m}$

The electric field strength can be calculated using:

$E = \frac{q}{4\pi\epsilon_0r'^2}$

Where:

• $q = 3.8 \mu C = 3.8 \times 10^{-6} C$
• $\epsilon_0 = 8.85 \times 10^{-12} F/m$

Thus:

$E = \frac{3.8 \times 10^{-6}}{4\pi(8.85 \times 10^{-12})(0.24)^2}\approx 5.9 \times 10^5 N/C$

So the electric field strength is approximately (5.9 \times 10^5 N/C ) away from the center of the dome.