(f) Current Electricity

Learning Objectives

- · state that a current is a rate of flow of charge measured in amperes
  - · distinguish between conventional current and electron flow
  - · recall and apply the relationship charge = current x time to new situations or to solve related problems
  - · *recall and apply the relationship Q = ne to new situations or to solve related problems
  - · define electromotive force (e.m.f.) as the work done by a source in driving a unit charge around a complete circuit
  - · calculate the total e.m.f. where several sources are arranged in series
  - · state that the e.m.f. of a source and the potential difference (p.d.) across a circuit component is measured in volts
  - · define the p.d. across a component in a circuit as the work done to drive a unit charge through the component
  - · distinguish between e.m.f. and p.d. in terms of energy considerations
  - · recall and solve problems using the equation V = W/Q
  - · define resistance of a component as the ratio of potential difference across it to the current flowing through it
  - · apply the relationship R= V/I to new situations or to solve related problems
  - · describe an experiment to determine the resistance of a metallic conductor using a voltmeter and an ammeter and make the necessary calculations
  - · recall and apply the formulae for the effective resistance of a number of resistors in series and in parallel to new situations or to solve related problems
  - · recall and apply the relationship of the proportionality between resistance and length and the cross-sectional area of a wire to new situations or to solve related problems
  - · state Ohm’s law
  - · describe the effect of temperature increase on the resistance of a metallic conductor
  - · sketch and interpret the V-I characteristic graph for metallic conductor at constant temperature, a filament lamp and for a semiconductor diode
  - · show an understanding of the use of a diode as a rectifier
  - · *show an understanding of the effects of the internal resistance of a source of e.m.f. on the terminal potential difference and output power.

Electric Circuit

What is an electric circuit?

Moving charges constitute an electric current.

Electric current is the rate of flow of electric charges, or the flow of electric charges per unit time.

I = Q/t

where I, is the electric current measured in ampere (A), Q is the amount of charges measured in Coulomb (C) and t is the time taken measured in seconds (s).

An ammeter is used to measure electric current. It is always connected in series to the circuit.

Conventional current flows from the positive terminal to the negative terminal.

Electron current flows from the negative terminal to the positive terminal.

Electromotive force and potential difference

What causes charges to move?

The electromotive force (e.m.f) of the cell is defined as work done by an electrical source un driving a unit charge around a complete circuit .

E = W/Q

where E is the e.m.f, W is the work done and Q the amount of charge.

The SI unit for e.m.f is joules per coulomb (JC-1) or volt (V).

What is potential difference?

Work is done by the electric field if the electric force acting on the charge causes it to move from one point to another. These two points differ in their electric potential. The magnitude of the work done on the charge by the electric field is a measure of the difference in potential.

The potential difference (p.d.) between two points is defined as work done by a unit charge as it passes between the two points.

V = W/Q

where V is the p.d., W is work done and Q the amount of charge.

The SI unit for p.d. is joules per coulomb (JC^-1) or volt (V).

A voltmeter is used to measure e.m.f. of a cell and p.d. between two points in a circuit. The voltmeter is always connected in parallel to the electrical component.

Voltage and Current

Part 1

Part 2

Electric resistance

What is resistance?

Electrical resistance is the resistance to current flow.

Electrical resistance exists even in the best conductors.

There are fixed or variable resistors.

SI unit for electrical resistance is ohm.

Ohm's law

Ohm's law states that in a metallic conductor, the current flowing through it is directly proportional to the p.d. across its ends, provided that the physical conditions and temperature remain constant.

Conductor that obeys Ohm's law is known as an ohmic conductor.

Conductor that does not obey Ohm's law is known as non-ohmic conductor.

The resistance R, of an electrical component is defined as the ratio of the potential difference, V, across the component to the current, I flowing through it.

R = V/I

How do we determine the resistance of electrical component?

The resistance R of a given conductor is also affected by