Electricity

Electric current

100

A phone transfers 240 C in 2 minutes. Find the current I.

I=2 A

200

A power bank gives 1.5 A for 8 minutes. Find the charge Q, then say what this equals in mAh. (1 Ah = 3600 C)

Q=720 C =200 mAh

300

A smartwatch uses 0.12 A. How much charge flows in 45 minutes?

Q=324 C

400

A device draws 0.8 A from 12 V. Find the charge that flows in 5 minutes, then the energy used in J.

Q=240 C; E=2880 J

500

A gadget transfers 900 C using 18 V. Find the energy in J, then the average power if it happens in 2 minutes.

E=16200 J; P=135 W

Ohm’s law

100

A 9 V battery is connected to 15 Ω. Find I.

I=0.60 A

200

A circuit draws 0.6 A from 12 V. Find R, then say whether 20 Ω would increase or decrease current.

R=20 Ω; 

does not change

300

A bulb is rated 12 V, 24 W. Find its resistance R (assume constant).

R=6 Ω

400

A device must not exceed 1.5 A on 12 V. What is the minimum resistance needed?

Rmin=8 Ω

500

A cable has resistance 0.8 Ω. Current is 5 A. Find the voltage drop on the cable, and the voltage left from a 12 V supply for the device.

V_drop=4 V; V_device=8 V

Complex combinations of resistors.

100

Series: 3 Ω and 7 Ω on 10 V. Find total current.

I=1 A

200

Parallel: 6 Ω and 12 Ω on 12 V. Find R_eq and I_total.

R_eq=4 Ω; I_tot=3 A

300

A circuit has 4 Ω in series with a parallel pair (6 Ω || 3 Ω) on 12 V. Find R_eq and I_total.

R_eq=6 Ω; I_tot=2 A

400

Two branches in parallel: Branch A has 2 Ω + 4 Ω (series), Branch B has 6 Ω. Supply is 12 V. Find I_A, I_B, I_total.

I_A=2 A; I_B=2 A;  I_tot=4 A

500

Three resistors: R₁ = 3 Ω in series with a parallel pair R₂ = 6 Ω and R₃ = 12 Ω. Supply 24 V. Find Req, I_total, and power in R₁.

R_eq=7 Ω; I_tot=3.43 A; P_R1≈35.3 W

Work and power of electric current.

100

A laptop uses 60 W on 20 V. Find current.

I=3 A

200

A heater uses 2 kW for 15 min. Find energy in kWh and J.

E=0.50 kWh; =1.8×106 J

300

A device uses 120 W. Electricity costs 25 KZT/kWh. Find the cost for 8 hours.

E=0.96 kWh; құны =24 KZT

400

A bulb is 230 V, 60 W. Find current and resistance.

I≈0.261 A; R≈882 Ω

500

You have a 12 V battery and want a device to use 36 W. What resistance should it have? Then find current.

R=4 Ω; I=3 A

Joule–Lenz law.

100

A wire has R = 5 Ω, current 2 A for 20 s. Find heat Q.

Q=400 J

200

A resistor 10 Ω releases 2400 J in 2 minutes. Find current.

I≈1.41 A

300

A 12 V supply is connected to 6 Ω for 3 minutes. Find I and heat Q.

I=2 A; Q=4320 J

400

Two resistors in series 2 Ω and 4 Ω on 12 V for 60 s. Find heat in each resistor and total heat.

Q_2Ω=480 J; Q_4Ω=960 J; Q_total=1440 J

500

A fuse wire has 0.4 Ω and melts after absorbing 1200 J in 8 s. Find the current that would melt it (assume all energy is Joule heating).

I≈19.36 A