Electric Circuit

OBJECTIVES

1)

State the relationship between voltage, current and
resistance;

Apply ohm’s law in solving problems

problem

2)

Electric Current

Current is the rate of flow of
electrons/charge

- It is abbreviated as I
- It is measured in amperes
- One ampere is defined as one
coulomb of electrons flowing past a
point each second (Q/s)

VOLTAGE

- Voltage is a force that pushes/drives the electrons/
charge

- It is also referred to as electromotive force or difference in potential.
- It is abbreviated as E or EMF
- Voltage is measured in volts (v)

Voltage source will have a polarity (negative and
positive side)
Current flows from negative to positive (changing
conventions)
AC/DC: Alternating current (polarity of source
reverses) or Direct current (polarity is constant)

RESISTANCE

Resistances are the barriers to
the flow of charge
It is abbreviated as R
It is measured in ohms

ELECTRICAL CIRCUITS

The CELL

The cell stores chemical energy and transfers it to

electrical energy when a circuit is connected.

When two or more cells are

connected together we call this

a Battery.

The cells chemical energy is

used up pushing a current round

a circuit.

What is an electric current?

An electric current is a flow of microscopic particles

called electrons flowing through wires and

components.

+
-

In which direction does the current flow?

from the Negative terminal to the Positive terminal of a
cell.

simple circuits

Here is a simple electric circuit. It has a cell, a

lamp and a switch.

To make the circuit, these components are connected together with metal connecting wires.

cell

lamp

wires

switch

simple circuits

When the switch is closed, the lamp lights up. This is
because there is a continuous path of metal for the
electric current to flow around.

If there were any breaks in the circuit, the
current could not flow.

circuit diagram

cell

switch

lamp

wires

Scientists usually draw electric circuits using symbols;

circuit diagrams

In circuit diagrams components are represented by the following symbols;

cell

battery

switch

lamp

motor

ammeter

voltmeter

buzzer

resistor

variable
resistor

types of circuit

There are two types of electrical circuits;

SERIES CIRCUITS

PARALLEL CIRCUITS

The components are connected end-to-end, one after the other.

They make a simple loop for the current to flow round.

SERIES CIRCUITS

If one bulb ‘blows’ it breaks the whole circuit and
all the bulbs go out.

PARALLEL CIRCUITS

The current has a choice of routes.

The components are connected side by side.

If one bulb ‘blows’ there is still be a complete circuit to
the other bulb so it stays alight.

measuring current

Electric current is measured in amps (A) using

an ammeter connected in series in the circuit.

A

measuring current

A

This is how we draw an ammeter in a circuit.

SERIES CIRCUIT

A

PARALLEL CIRCUIT

measuring current

SERIES CIRCUIT

PARALLEL CIRCUIT

• current is the same
at all points in the
circuit.

2A

2A

• current is shared
between the
components

2A

2A

1A

1A

2A

copy the following circuits and fill in the
missing ammeter readings.

?

?

4A

3A

?

1A

?

3A

measuring voltage

The ‘electrical push’ which the cell gives to the current

is called the voltage. It is measured in volts (V) on a

voltmeter

V

Different cells produce different voltages. The

bigger the voltage supplied by the cell, the bigger the current

measuring voltage

Unlike an ammeter a voltmeter is connected across the components

Scientist usually use the term Potential Difference (pd) when they talk about voltage.

measuring voltage

V

This is how we draw a voltmeter in a circuit.

SERIES CIRCUIT

PARALLEL CIRCUIT

V

V

measuring voltage

V

V

V

series circuit

1.5V

• voltage is shared between the components

1.5V

3V

• voltage is the same in all parts of the circuit.

3V

parallel circuit

3V

3V

measuring current & voltage

Assignment:
Copy the following circuits on the next two slides on 1/2 sheet of paper cross wise.

Complete the missing current and voltage readings.

Remember the rules for current and voltage in series and parallel circuits.

Submit next meeting.

measuring current & voltage

V

V

6V

4A

A

A

a)

measuring current & voltage

V

V

6V

4A

A

A

b)

A