Current Electricity
ICSE Class 9 · Physics · Chapter 9
What this chapter covers
- Electric charge and free electrons
- Conductors and insulators
- Electric current
- Potential difference
- Resistance and Ohm's law
- The simple electric circuit
- Detecting electric current
- Sources of direct current
Key points
- Charge comes in two kinds; like charges repel, unlike attract. SI unit: coulomb.
- Charge is quantised: Q = ne, with e = 1.6 × 10-19 C.
- Free electrons in metals carry current; good conductors have many, insulators almost none.
- Semiconductors (silicon, germanium) conduct moderately, more when hotter; superconductors have zero resistance below a critical temperature.
- Current I = Q/t; SI unit ampere (1 A = 1 C s-1).
- Conventional current flows from + to − in the external circuit; electrons flow from − to +.
- Potential difference V = W/q; SI unit volt (1 V = 1 J C-1). Current flows from higher to lower potential.
- Resistance opposes current; SI unit ohm (1 Ω = 1 V A-1).
- Ohm's law: V = IR, at constant temperature and physical conditions; the V–I graph is a straight line through the origin with slope R.
- A simple circuit has a cell, a key, a device, a rheostat for control, and connecting wires; it must be closed for current to flow.
- A galvanometer detects small currents and their direction; an ammeter measures current and is joined in series.
- Cells and batteries give direct current; dry cells are primary, accumulators are rechargeable secondary cells.
Questions answered
What are free electrons? Why do metals conduct electricity well?
Free electrons are the loosely bound outer electrons of metal atoms that move freely through the metal. Metals have a very large number of them, so charge flows easily.
What is meant by the quantum nature of charge? Can a body have a charge of 2.4 × 10-19 C?
Charge is always a whole-number multiple of e = 1.6 × 10-19 C. A charge of 2.4 × 10-19 C is 1.5e, so it is not possible.
Classify as conductor, insulator or semiconductor: copper, rubber, silicon, glass, aluminium.
Conductors: copper, aluminium. Insulators: rubber, glass. Semiconductor: silicon.
What is a superconductor? Give one use.
A substance that conducts with zero resistance below a certain very low temperature. Superconducting magnets are used in MRI scanners.
Define electric current and its SI unit.
Electric current is the rate of flow of charge, I = Q/t. Its SI unit is the ampere: 1 A is a flow of 1 C per second.
Distinguish between conventional current and electronic current.
Conventional current flows from the positive to the negative terminal through the external circuit; electronic current (the flow of electrons) flows from the negative to the positive terminal.
A current of 0.8 A flows for 5 minutes. How much charge passes?
Q = It = 0.8 × 300 = 240 C.
Define potential difference and the volt.
Potential difference is the work done in moving a unit positive charge between two points, V = W/q. One volt is one joule of work per coulomb.
Past board exam questions
ICSE 2019 sample paper · 2 marks
State the purpose of using the following in an electric circuit. (i) Ammeter (ii) Rheostat
Show answer
(i) An ammeter is used to measure the electric current flowing in the circuit (it is connected in series).
(ii) A rheostat (variable resistance) is used to change the resistance of the circuit and hence to regulate (vary) the current in the circuit.
ICSE 2019 sample paper · 2 marks
A and B are two metal spheres which are connected with the help of a metal wire. State the direction of flow of electrons in each case. (i) Sphere A is negatively charged and sphere B is uncharged. (ii) Sphere A is positively charged and sphere B is uncharged.
Show answer
(i) Electrons flow from A to B through the wire (A has excess electrons).
(ii) Electrons flow from B to A through the wire (A has a deficit of electrons and attracts electrons from B).
The flow continues until both spheres are at the same potential.
ICSE 2019 sample paper · 3 marks
(i) Define a secondary cell. (ii) Give one example of a secondary cell. (iii) State one advantage of a secondary cell over a primary cell.
Show answer
(i) A secondary cell is a cell in which electrical energy is first stored as chemical energy (by charging) and later given out as electrical energy; it can be recharged and used again.
(ii) Example: lead–acid accumulator (also nickel–cadmium or lithium-ion cell).
(iii) It can be recharged and reused many times (it also has low internal resistance, so it can supply a large current).