o level physics 5054 notes on current electricity

1. Current Electricity
Physics G9, Anwar Mohamed.

2. Electric Current
• Electric current (I) is the rate of flow of charges.(Q)
• SI unit: Ampere (A)
• Can be measured by an ammeter (must be connected in SERIES to the circuit)
• Formula: I= Q / t
• A current of one ampere is a flow of charge at the rate of one coulomb per second.
• If the direction of the current (charge flow) is fixed, it is known as a direct current.
If the motion of the electric charges is periodically reversed; it is called an
alternating current

3. Electromotive Force
• Electromotive Force (e.m.f.) of a source is the energy converted from non-electrical to
electrical form when one coulomb of positive charge passes through the source.
• SI unit: Volt (V)
• E=W/Q, where E = e.m.f., W = work done by source, Q = amount of positive charges
• Potential difference between two points is defined as the energy converted from electrical to
other forms when a coulomb of positive charge passes between the two points.
• SI unit: Volt (V)
• V=W/Q , where V = potential difference, W = work done in driving the charge between the two points, Q = amount of positive charges
• IMPORTANT: There can be e.m.f. without a closed circuit. BUT there cannot be a potential
difference without a closed circuit.

4. Sources In Series
• When two or more sources are arranged so that the positive terminal of one
is connected to the negative terminal of the next, they are said to be in series
and their e.m.f.s add up. This arrangement gives increased e.m.f. because, the
charge flowing round a circuit will pass through more than one source and
gains electrical potential energy from each of them.
• Cells can also be arranged in parallel. In this, all the positive terminals are
connected together and all the negative terminals are connected together.
The combined e.m.f. in parallel connection will not increase like in the series
connection. But the battery will last longer before going flat.

5. Potential Difference
• When a torch bulb is connected to a battery, the torch bulb gets lit. The battery converts chemical
energy into electrical energy and is therefore a source of electrical energy. The torch bulb converts
electrical energy into heat and light and is therefore a sink of electrical energy.
• Dissipation of electrical energy between two points (e.g. across torch bulb) in an electrical circuit
causes potential difference (p.d.) between those two points
• The potential difference (p.d.) between two points in a closed circuit is defined as the energy converted
from electrical to other forms when a unit positive charge passes between the two points.
• SI unit of p.d. is the volt (V). It is the same as that of e.m.f.. (Both are measures of electrical potential
energy, e.m.f. is gained electrical energy while potential difference is lost electrical energy.)
• V=W/Q

6. Resistance
• By increasing p.d. across the ends of a conductor, current flow can be increased. But the increase
in the amount of current flow depends on the conducting ability of the conductor. Some
conductors offer some resistance to current flow than others.
• Resistance (R) of a conductor is defined as the ratio of potential difference (V), across the
conductor to the current (I), flowing through it.
• SI unit of resistance is the ohm Ω.
• V = IR
• Ohm’s law states that, the current flowing in a metallic conductor is directly proportional to the
potential difference applied across its ends, provided that all other physical conditions, such as
temperature, are constant. Comparing with V=IR, thus, R must be constant for a metallic
conductor under steady physical conditions.

7. Resistivity
• Besides temperature, experimental results shows that the resistance (R of a given conductor) also
depends on the composition and size.
• Resistance, R is found to be:
• directly proportional to its length, L
• inversely proportional to its cross-sectional area
• dependent on the type of material
• For the similar lengths and material,
Thinner wires have higher resistance than thicker wires.
• For similar thickness and material,
Longer wires have higher resistance than shorter wires.

8. Effective resistance of resistors
• In many situations, several electrical devices are connected to the same power
supply. There are two basic methods of connecting resistors or other devices
together. They are called series and parallel connections.

9. Resistors In Series
• If individual resistors are connected from end to end, the resistors are said to be
connected in series. The effective resistance, R, of three resistors of resistances R1,
R2; and R3 connected in series (shown in the figure) is given by:
• R=R1+R2+R3
• In general, if there are n resistors in series, the effective resistance R is given by:
• R=R1+R2+….+Rn
• Note: In a series connection, the effective resistance, R, is always larger than the
largest of the individual resistances.

10. Resistors In Parallel
• If each end of individual resistors are
connected together to one another as one,
the resistors are said to be connected in
parallel.
• The effective resistance, R, of three resistors
of resistances R1, R2 and R3 connected in
parallel is given by:
• 1R=1R1+1R2+1R3
• In general, if there are n resistors in parallel,
the effective resistance R is given by:
• 1R=1R1+1R2+….+1Rn

11. I/V characteristic graphs For O Level
• Metallic conductors at constant temperature
• The I-V graph of metallic conductor is a
straight line pass through the origin. They
obey Ohm’s Law, having resistance that is
independent of current. (Ohmic conductors)
• Reason: Resistance in metal is the reduction
of the drift velocity of electrons due to
collision with the lattice ions. If the
temperature of the conductor is kept
constant, the magnitude of the vibration of
the lattice ions remain the same, and hence
it’s resistance would remain the same.

12. • Filament lamp
• From the I/V graph, the ratio V/I increases as
current increases.
• Resistance of the filament lamp increase with
temperature.
• Reason: As the potential difference across a
filament lamp increases, the current increases and
the energy dissipated, as heat, increases, resulting
in a higher temperature. As the temperature
increases, resistance of the filament increases. The
collision between the free electrons and the lattice
ions increases due to more rigorous vibration of
the lattice ions.