Unit 2: Chapter 4 - Current Electricity

Question 1

Electrical Charges

Answer 1

CHARGING BY FRICTION

Some INSULATORS become charged by rubbing them with a dry cloth:
- Rubbing polythene rod with dry cloth allows it gain electrons so it become negatively charged.

• Rubbing a Perspex rod with a dry cloth transfers electrons to the surface. The Perspex rod becomes positively charged.

Two objects with the SAME charges REPEL.
Two objects with DIFFERENT charges ATTRACT.

Question 2

Electric Circuits

Answer 2

ELECTRIC CURRENT

An ELECTRIC CURRENT is a flow of charge of electrons. Electrons carry negative charges. The electrons pass through the filament of the bulb because it is made of metal.

THE SIZE OF THE CURRENT IS THE RATE OF FLOW OF ELECTRIC CHARGE.

Electric charge is measured in COULOMBS (C) and the electric current is measured in AMPERES (A) or ‘amps’.

Current = Charge / time

Question 3

Resistance

Answer 3

AMMETERS AND VOLTMETERS

Ammeters measure the current going through an appliance or just the wire.

A voltmeter measures the potential difference (Pd) across the appliance. This is the amount of work done or energy transferred to the appliance for each coulomb of charge that passes through.

Pd = Work done (J) / Charge (C)

RESISTANCE

Resistance is measured in OHMS.

Resistance (Ohms) = Pd / Current

A resistor in a circuit limits the current. For a given pd, the larger the resistance of the resistor, the smaller the current is.

CURRENT - POTENTIAL DIFFERENCE GRAPHS

The current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.

Question 4

More Current - Potential Graphs

Answer 4

FILAMENT BULB

• Similar to an ‘x3’ graph. The line curves away from the current axis. The current is NOT directly proportional to the potential difference. The filament bulb is a non-ohmic conductor.
• Resistance increases as current does. The resistance of a filament bulb increases and the filament temp increases.

THE DIODE

• Current is NOT directly proportional to the potential difference. A diode is not an ohmic conductor.
• In ‘reverse’ direction, current is negligible so resistance is much higher than forward direction.

LED (Light-Emitting Diode)

LEDs emit light when a current passes through in a forward direction.
For LEDs, when a bright light is shone upon it, more current passes through.

THERMISTOR

• At a consistent temp, the line is straight so the resistance is constant.
• If temperature increases, resistance decreases.

Question 5

Series Circuits

AMMETER?

PD?

RESISTORS?

Answer 5

CIRCUIT RULES (SERIES)

A series circuit is circuit that is drawn as a rectangle.

• The same current passes through components in series with each other.
• The total potential difference in cells in series is the sum of potential difference of each component of the cell.
• The total resistance of components in series in equal to the sum of the resistance of each component.

Question 6

Parallel Circuits

AMMETER?

PD?

Answer 6

CIRCUIT RULES (PARALLEL)

• The total current through the whole circuit is the sum of the currents through the separate components.
• For components in a parallel circuit, the potential difference across each component is the same.