12th grade electricity and magnetism

Question 1

A 12 Ω resistor and a 4 Ω resistor are connected in parallel. What is their combined resistance, in Ω? (1/R = 1/R₁ + 1/R₂)

Answer Ω

Question 2

A transformer has 2000 turns on the primary coil and 20000 turns on the secondary. The input voltage is 12 V. What is the output voltage, in V? (Vp ÷ Vs = Np ÷ Ns)

Answer V

Question 3

A 100 % efficient transformer has an input of 400 V and 10 A. The output voltage is 115 V. What is the output current, in A? (Vp × Ip = Vs × Is; give 2 decimal places)

Answer A

Question 4

Why does the National Grid transmit electricity at very high voltages?

• A. It makes the cables lighter
• B. High voltage makes electricity travel faster
• C. Homes need very high voltages
• D. A high voltage means a low current, so less energy is lost heating the cables

Question 5

How is Coulomb's law similar to Newton's law of gravitation?

• A. Both are inverse square laws
• B. Both are unaffected by distance
• C. Both depend on mass
• D. Both forces are always attractive

Question 6

A 6 Ω resistor is in series with a parallel pair of 12 Ω and 4 Ω. The supply is 13.5 V. What current flows from the supply, in A?

Answer A

Question 7

One charge is multiplied by 2 and the distance is unchanged. By what factor does the force change?

Answer 

Question 8

Three resistors of 12 Ω, 6 Ω and 1 Ω are connected in series. What is the total resistance, in Ω?

Answer Ω

Question 9

Two small charged spheres carry 3 µC and 3 µC and are 0.3 m apart. What is the size of the force between them, in N? (F = k q₁ q₂ ÷ r², k = 8.99 × 10⁹ N m²/C², 1 µC = 10⁻⁶ C; give 2 decimal places)

Answer N

Question 10

A 20 Ω resistor and a 30 Ω resistor are connected in parallel. What is their combined resistance, in Ω? (1/R = 1/R₁ + 1/R₂)

Answer Ω

Question 11

A transformer has 500 turns on the primary coil and 25 turns on the secondary. The input voltage is 120 V. What is the output voltage, in V? (Vp ÷ Vs = Np ÷ Ns)

Answer V

Question 12

A 100 % efficient transformer has an input of 25000 V and 5 A. The output voltage is 275000 V. What is the output current, in A? (Vp × Ip = Vs × Is; give 2 decimal places)

Answer A

Question 13

Why does the National Grid transmit electricity at very high voltages?

• A. It makes the cables lighter
• B. Homes need very high voltages
• C. A high voltage means a low current, so less energy is lost heating the cables
• D. High voltage makes electricity travel faster

Question 14

Two positive charges are moved closer together. What happens to the force between them?

• A. the attractive force increases
• B. the force becomes zero
• C. the force decreases
• D. the repulsive force increases

Question 15

A 2 Ω resistor is in series with a parallel pair of 10 Ω and 15 Ω. The supply is 8 V. What current flows from the supply, in A?

Answer A

Question 16

Both charges are multiplied by 3 and the distance is unchanged. By what factor does the force change?

Answer