Table of Contents
Question 1 #
(a) (i) State what is meant by the direction of an electric field.
[1]
The direction of an electric field is the direction a positive charge would move if you put it in the field.
(ii) Fig. 9.1 shows a pair of oppositely-charged horizontal metal plates with the top plate positive.
The electric field between the plates in Fig. 9.1 is uniform.
Draw lines on Fig. 9.1 to represent this uniform field. Add arrows to these lines to show the direction of the field. [3]
The electric field between the plates in Fig. 9.1 is uniform.
Draw lines on Fig. 9.1 to represent this uniform field. Add arrows to these lines to show the direction of the field. [3]
Draw straight lines going up and down between the two plates. Space them evenly. Put arrows on the lines pointing down (from the positive plate to the negative plate).
(b) (i) Fig. 9.2 shows a very small negatively-charged oil drop in the air between a pair of oppositely charged horizontal metal plates. The oil drop does not move up or down.
Suggest, in terms of forces, why the oil drop does not move up or down. [2]
Suggest, in terms of forces, why the oil drop does not move up or down. [2]
There are two forces on the oil drop. The weight pulls it down. The electric force pushes it up. These two forces are equal in size but opposite. They balance each other. So the oil drop stays still.
(ii) Without losing any of its charge, the oil drop begins to evaporate.
State and explain what happens to the oil drop. [2]
State and explain what happens to the oil drop. [2]
The oil drop moves up. When the oil evaporates, the drop gets lighter. So its weight becomes smaller. The electric force stays the same because the charge does not change. Now the electric force is bigger than the weight. So there is a force pushing up, and the drop moves up.
Question 2 #
A plastic rod is rubbed with a cloth and becomes positively charged. After charging, the rod is held close to the suspended table-tennis ball shown in Fig. 9.1. The table-tennis ball is covered with metal paint and is uncharged.
(a) Describe what happens to the charges in the metal paint on the ball as the positively charged rod is brought close to the ball. [1]
(a) Describe what happens to the charges in the metal paint on the ball as the positively charged rod is brought close to the ball. [1]
The negative charges in the metal paint move to the side of the ball closest to the rod. The positive charges move to the far side of the ball.
(b) The ball is attracted towards the charged rod.
Explain why this happens, given that the ball is uncharged. [2]
Explain why this happens, given that the ball is uncharged. [2]
The negative charges on the ball are closer to the positive rod. The positive charges are further away. The pull on the negative charges is stronger than the push on the positive charges. So overall, the ball is pulled towards the rod.
(c) State the unit in which electric charge is measured.
[1]
Coulomb (C)
Question 3 #
A digital watch is powered by a 1.3V cell. The cell supplies a current of 4.1 × 10-5 A (0.000041 A) for 1.6 × 107 s.
Calculate
(a) the charge that passes through the cell in this time, [2]
Calculate
(a) the charge that passes through the cell in this time, [2]
Formula:
$Q = I \times t$
Given:
– Current (I) = 4.1 × 10-5 A
– Time (t) = 1.6 × 107 s
Calculation:
$Q = 4.1 \times 10^{-5} \times 1.6 \times 10^{7}$
$Q = 656$ C
(b) the resistance of the electrical circuit in the watch,
[2]
Formula:
$R = \frac{V}{I}$
Given:
– Voltage (V) = 1.3 V
– Current (I) = 4.1 × 10-5 A
Calculation:
$R = \frac{1.3}{4.1 \times 10^{-5}}$
$R = 31707$ Ω
(c) the output power of the cell.
[2]
Formula:
$P = V \times I$
Given:
– Voltage (V) = 1.3 V
– Current (I) = 4.1 × 10-5 A
Calculation:
$P = 1.3 \times 4.1 \times 10^{-5}$
$P = 0.0000533$ W
Question d #
A smaller metal sphere, also mounted on an insulating plastic stand, is uncharged.
This smaller sphere is moved close to the positively charged sphere. Fig. 8.1 shows the two spheres.
(i) On Fig. 8.1, draw the distribution of charge on the smaller sphere. [2]
This smaller sphere is moved close to the positively charged sphere. Fig. 8.1 shows the two spheres.
(i) On Fig. 8.1, draw the distribution of charge on the smaller sphere. [2]
On the smaller sphere, draw negative signs (-) on the side closest to the positive sphere. Draw positive signs (+) on the far side. Show more negative charges on the left side (nearest to the positive sphere) and more positive charges on the right side (furthest away).
