Measurements in Electric Circuits

Gr. 9 Electricity Unit

• Electric current is the rate of electrons flowing past a certain point in a circuit.

• Current (I) is measured in Amperes (A), using an ammeter.

• Potential difference, or voltage (V), is the difference in electrical potential energy per unit of charge measured between two points.

• Voltage is measured in volts (V), using a voltmeter.

• Electrical resistance (R) is the opposition to the flow of electrons through a material. Resistance is measured in ohms (Ω) using an ohmmeter.

• All materials have some electrical resistance.• Materials with the least resistance are used as

conductors.• Materials with the highest resistance are used

as insulators.

• A multimeter is a device that allows us to measure currrent, voltage and resistance by changing the settings. These are hand-held devices used for the home.

How does an ammeter work?

• An ammeter is connected in series and measures the amount of electric current.

• Depending on the electric appliance, the current varies.

How does a voltmeter work?

• A voltmeter is connected in parallel in an electric circuit.

• It measures the voltage drop or rise (difference) between two points in the circuit.

Loads

• Loads are made of materials that have high electrical resistance. However, the electrical resistance must be less than that of the insulators used in the circuit. Otherwise, no electrons will flow through the load.

The movement of electrons

• Electrons flow in one direction in a circuit.• The cell produces an excess of electrons at the

negative terminal, producing an electric field in the conductor.

• The electric field causes the electrons to move in one direction through a wire.

• Electric current refers to the rate of movement of electric charge.

• The flow of water flowing in a river is an analogy for the flow of electrons through a conductor.

• The electric charge passing by a point in a circuit is measured in terms of a very large number of electrons.

• The unit of electric charge is the coulomb.

• One coulomb (C) is the quantity of charge that is equal to the charge of 6.25 x 10 E18 electrons.

Loads in Series

IT = I1 + I2 + I3

VT = V1 + V2 + V3

RT = R1 + R2 + R3

Loads in Parallel

VT = V1 + V2 + V3

IT = I1 + I2 + I3

RT R1

RT R2

RT R3