Current, voltage, resistance, Ohm's law, circuits and electrical energy.
Current, voltage and resistance with a simple circuit. Video coming soon.
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Electric current I is the rate of flow of charge: I = Q ÷ t, in ampere (1 A = 1 C/s). Potential difference V (volt) is the work done per unit charge between two points; it drives the current. Ohm's law: at constant temperature, the current through a conductor is proportional to the potential difference across it, V = IR, where R is the resistance in ohm (Ω). Resistance rises with length and falls with cross-section area.
120 C of charge flows in 60 s. Find the current.
I = 120 ÷ 60 = 2 A.
A 6 Ω resistor carries 2 A. Find the potential difference.
V = IR = 2 × 6 = 12 V.
Why does a thin wire have more resistance than a thick one of equal length?
Electrons find less cross-section area to move through, so more collisions.
In series, the same current flows through every resistor and R = R₁ + R₂ + …; if one breaks, the circuit breaks. In parallel, each resistor has the same voltage and 1/R = 1/R₁ + 1/R₂ + …; the total is less than the smallest resistor, and branch currents add up to the total. Household appliances are connected in parallel so each gets full voltage and can be switched separately.
Find the total resistance of 2 Ω and 4 Ω in series.
R = 2 + 4 = 6 Ω.
Find the total resistance of 6 Ω and 3 Ω in parallel.
R = 6 × 3 ÷ (6 + 3) = 2 Ω.
A 12 V supply is connected across that parallel pair. Find the total current and each branch current.
Total: 12 ÷ 2 = 6 A. Branches: 12 ÷ 6 = 2 A and 12 ÷ 3 = 4 A, which add to 6 A.
Electric power P = VI = I²R = V²÷R, in watt. Electrical energy = power × time. The commercial unit is the kilowatt-hour (kWh, the "unit" on bills): 1 kWh = 1000 W for 1 h = 3.6 × 10⁶ J. A fuse is a thin wire that melts when too much current flows, protecting the appliance, and is fitted on the live wire. Earthing the metal body prevents shocks.
Find the power when 2 A flows through a 12 V supply.
P = 12 × 2 = 24 W.
A 100 W bulb is used for 5 h. Find the energy used in kWh.
100 × 5 ÷ 1000 = 0.5 kWh.
Why is a fuse fitted in the live wire?
If a fault occurs the fuse melts and cuts the current before the appliance gets live.
1. Define electric current and its unit.
2. Find the current when 300 C passes in 100 s.
3. State Ohm's law.
4. Find V when I = 3 A and R = 5 Ω.
5. Find the resistance of 5 Ω and 10 Ω in series.
6. Find the resistance of two 4 Ω in parallel.
7. Why are household appliances connected in parallel?
8. Define 1 kWh.
9. Find the energy in kWh used by a 2 kW heater in 3 h.
10. What does a fuse do?