S2 Chapter 13: Electrical Systems

What does an electric current consist of?

Electric charge

S.I unit for electric charge

coulombs (C)

symbol for electric charge

Q

Formula to determine electric current

I = Q/t

Definition of current

Current is the rate of flow of electric charge.

symbol for current

I

Si for current

amperes (A)

What is an ammeter used for?

An ammeter is used to measure the current flowing through an electric circuit. It must be connected in series.

Definition of potential difference

Potental difference is the work done per unit charge in driving charges through a component.

SI for potential difference

volts (V)

symbol for potential difference

V

What is a voltmeter used for?

A voltmeter is used to measure the potential difference across a component. must be connected in parallel to the component.

formula for potential difference

V = Work done (W) / Q

Definition of resistance

Resistance of an electrical current is a measure of how much the component opposes the flow of electric current.

Symbol for resistance

R

SI for resistance

ohm

What is a fixed resistor

A resistor with fixed & non-adjustable resistance

What is a variable resistor (rheostat)

A resistor with adjustable resistance ranging from 0 ohm to a certain value.

main idea of resistance:

Resistance opposes the flow of current. As resistance in a circuit increases, current flowing decreases, and vice versa.

Electrical conductors: low or high resistance to current?

low

Electrical insulators: low or high resistance to current?

high

When a resistor is added to a circuit in series..

the ammeter reading decreases and the bulb becomes less bright, showing that the overall resistance of the circuit has increased.

When a resistor is added to a circuit in parallel..

the ammeter reading increases and the bulb becomes brighter, showing that the overall resistance of the circuit has decreased.

Adding a resistor in series causes..

increase in the overall resistance across the resistors, thus the bulb becomes less bright.

Adding a resistor in parallel causes..

decrease in the overall resistance across the resistors, thus causing the bulb to become brighter.

Formula for resistance

R = V/I

Calculating total resistance of resistors in series:

R total = R1 + R2 + …

Calculating total resistance of resistors in parallel:

Rt = (1/R1 + 1/R2 + …)-1

When the slider on a rheostat is shifted to the left:

• rheostat is at minimum resistance

• ammeter reading shows large increase in current flowing through circuit

• bulb is bright

When the slider on a rheostat is shifted to the right:

• rheostat resistance increases

• ammeter reading decreases

• bulb becomes less bright