Electricity · Lesson 1 of 9
Electric Current and Circuit
“Electric current gets charges moving, because even electrons dislike standing in a queue.”
• Explain electric current as the rate at which charge crosses a section of a conductor. • Relate conventional current direction to the motion of electrons. • Use the relationship among current, charge and time with correct units. • Describe a closed electric circuit and identify the role of its main components. • Connect an ammeter correctly and interpret its reading.
A bulb glows only when its two terminals are joined to a cell through an unbroken conducting path. Closing the key does not create electrons inside the wire; the wire already contains mobile electrons. The cell establishes an electric influence around the complete circuit, causing charge to drift in an organised way. This orderly movement is described by electric current.
The rate at which electric charge flows through a cross-section of a conductor.
Imagine an invisible surface cutting across a wire. If a charge Q passes through that surface in time t, the current tells us how rapidly charge is crossing. A greater current may mean more charge passes in the same time, or the same charge passes in less time. Current therefore describes a rate, not an amount stored at one point.
One ampere is the current produced when one coulomb of charge crosses a section each second. Smaller currents are often written in milliamperes and microamperes: 1 mA = 10⁻³ A and 1 μA = 10⁻⁶ A. Converting to amperes before substitution prevents errors.
Charge Carried by Electrons
An electron carries a charge of magnitude 1.6 × 10⁻¹⁹ C. Consequently, one coulomb corresponds to about 6.25 × 10¹⁸ electrons. In a metal, electrons drift from the negative terminal toward the positive terminal. Historically, current direction was chosen before electrons were known, so conventional current is taken in the opposite direction—from the positive terminal toward the negative terminal through the external circuit.
Electrons move slowly by drift, but a circuit responds quickly when the key is closed because the electric influence is established throughout the circuit. Current is also not used up by a bulb; the bulb transfers electrical energy while charge continues around the circuit.
Electric Circuit
A continuous conducting path through which electric current can flow.
A simple circuit contains a source such as a cell, connecting wires, a component such as a bulb, and a key. With the key open, the path is broken and the current is zero. With the key closed, the path is complete. The cell maintains a potential difference that drives charge, while the bulb transforms electrical energy into light and heat.
Measuring Current
An ammeter measures the current through a branch. It must be inserted in series: the circuit is opened at the required point and the meter becomes part of the conducting path. Its positive terminal is connected toward the positive terminal of the source. Connecting an ammeter directly across a cell provides a very low-resistance path and can damage the meter.
A Reliable Solving Method
- Write the given quantities and convert time to seconds and current to amperes.
- Identify whether current, charge or time must be found.
- Start with I = Q/t and rearrange before substituting.
- Attach the correct unit to the result.
- Check that a larger charge or a shorter time gives a larger current.
Problem
A charge of 24 C passes through a wire in 6 s. Find the current.
- 1.Given: Q = 24 C and t = 6 s. Required: I.
- 2.Use I = Q/t because charge and time are known.
- 3.I = 24 C ÷ 6 s = 4 C/s = 4 A.
- 4.The current is 4 A. The result means 4 C crosses the section each second.
Problem
A bulb carries a current of 0.50 A for 10 minutes. Find the charge that flows.
- 1.Given: I = 0.50 A and t = 10 min. Required: Q.
- 2.Convert time: 10 min = 10 × 60 s = 600 s.
- 3.Rearrange I = Q/t to Q = It.
- 4.Q = 0.50 A × 600 s = 300 C.
- 5.The charge transferred is 300 C. Substitution back gives 300/600 = 0.50 A.
Problem
A current of 320 mA flows for 5 minutes. Estimate the number of electrons passing a point.
- 1.Given: I = 320 mA = 0.320 A and t = 5 min = 300 s.
- 2.First find charge: Q = It = 0.320 × 300 = 96 C.
- 3.Each electron has charge magnitude e = 1.6 × 10⁻¹⁹ C, so N = Q/e.
- 4.N = 96 ÷ (1.6 × 10⁻¹⁹) = 60 × 10¹⁹ = 6.0 × 10²⁰ electrons.
- 5.The very large number is reasonable because the charge on one electron is extremely small.
Quiz
What does an electric current of 2 A mean?
In a metal wire, conventional current is directed how relative to electron drift?
Which connection is correct for measuring current through a bulb?
A charge of 90 C passes in 30 s. What is the current?
Why does a bulb not glow when the key is open?
Practice Problems
- A charge of 150 C passes a point in 50 s. Find the current. Solution: I = Q/t = 150/50 = 3 A.
- A current of 2.5 A flows for 4 minutes. Find the charge transferred. Solution: t = 4 × 60 = 240 s. Q = It = 2.5 × 240 = 600 C.
- How long will a current of 0.8 A take to transfer 240 C? Solution: t = Q/I = 240/0.8 = 300 s = 5 min.
- A current is reported as 750 mA. Express it in amperes and find the charge transferred in 20 s. Solution: 750 mA = 0.750 A. Q = It = 0.750 × 20 = 15 C.
- Estimate the number of electrons in 8 C of charge. Solution: N = Q/e = 8/(1.6 × 10⁻¹⁹) = 5 × 10¹⁹ electrons.
Key Takeaways
• Electric current measures the rate of flow of charge. • One ampere equals one coulomb per second. • Conventional current is opposite to electron drift in a metal. • Current flows only through a complete conducting path. • An ammeter is connected in series with correct polarity. • Unit conversion must be completed before using I = Q/t. • Charge moves around a circuit; it is not consumed by a component.
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Electric Potential and Potential Difference