In a thunderstorm different parts of a cloud become positively and negatively charged - Leaving Cert Physics - Question 7 - 2019

Electric field strength (E) is defined as the force (F) experienced by a unit positive charge (Q) placed in the field. Mathematically, it is expressed as:

$E = \frac{F}{Q}$

Its unit is newtons per coulomb (N/C).

To charge an insulated spherical conductor positively by induction:

1. Bring a negatively charged rod close to the conductor without touching it.
2. The presence of the negative charge induces a positive charge on the near side of the conductor, while the far side becomes negatively charged.
3. Ground the far side of the conductor to allow the negative charges to flow away.
4. Remove the ground connection and then the negatively charged rod. The conductor now retains a net positive charge.

The electric field lines around a positively charged spherical conductor radiate outward, away from the surface of the sphere. The lines are denser near the surface, indicating a stronger field, and they taper off as they move further away.

To calculate the charge on the conductor, we use the formula for electric field strength:

$E = \frac{Q}{4 \, \pi \, \epsilon_0 \cdot r^2}$

Given:

• Electric field strength, $E = 2.3 \text{ N C}^{-1}$
• Distance from the surface, $r = 0.05 \text{ m}$:

Rearranging gives:

$Q = E \cdot 4 \pi \epsilon_0 \cdot r^2$ Substituting $\,\epsilon_0 = 8.85 \times 10^{-12} \text{ C}^2/\text{N m}^2$,

$Q = 2.3 \cdot 4 \pi (8.85 \times 10^{-12})(0.05^2)$

Calculating this gives:

$Q = 3.1 \times 10^{-12} \text{ C}$

Point discharge occurs when charge accumulates at a pointed electrode. The electric field around the point becomes extremely strong, ionizing the air at that point. The ionized air allows charges to escape into the atmosphere, creating a stream of ions. Consequently, point discharge creates a visible spark or corona effect due to the rapid movement of ions.

Point discharge can be demonstrated in the laboratory using:

1. Charged Points: A pointed conductor charged positively can be used.
2. Candle at Point: Placing a candle near the point while it is charged will show the light flickering due to air ionization.
3. Flame Blown Away from the Point: The flame can be blown away from the point, indicating the discharge occurring away from the point.