Download Year 9 Electrical Circuits summary sheet

Year 9 Electrical Circuits summary sheet
Electricity is a flow of electrons. Electricity can flow through conductors but not through insulators. Metals
are good conductors of electricity.
Circuits
A complete circuit is needed for electricity to flow.
We use symbols when we draw circuits:
Component
Symbol
Component
cell
variable resistor
battery of cells
ammeter
bulb
voltmeter
switch
motor
Symbol
resistor
The current is a measure of how quickly the electrons flow around a circuit. The units for current are amps
(A). Current is measured using an ammeter. An ammeter is placed in series with the other components in
the circuit.
The potential difference (voltage) is a measure of the difference in energy between two parts of a circuit.
The units for potential difference are volts (V). Potential difference is measured using a voltmeter. A
voltmeter is placed in parallel with the component being measured.
Circuits can be series or parallel circuits.
Series circuit
• If one bulb breaks, all the others go off.
• The current is the same everywhere.
• If you put more bulbs in they will be
dimmer, because it is harder for the
electricity to get through. The resistance of
the circuit is higher.
• The voltage from the cell or power pack is
divided between the components.
Parallel circuit
• If one bulb breaks, the bulbs in the other
branches stay on.
• The current splits up when it comes to a
branch. The current in all the branches adds
up to the current in the main part of a
circuit.
• If you add more bulbs they stay bright. It is
easier for the current to flow with more
branches, because there are more ways for
the electrons to go.
• The voltage is the same across all the
branches of the circuit.
Electrical current
Current in a series circuit:
The reading at A1 is the same as A2
E.g. if A1 = 2A then A2 = 2A
Current in a parallel circuit:
The reading at A1 is the sum of the readings at A2
and A3.
A1 = A2 + A3
E.g. if A1 = 3A then A2 and A3 both = 1.5A each
(assuming that the bulbs are identical
Potential difference
Potential difference in a series circuit:
The sum of the readings at V1 and V2 is the total
potential difference from the cell (or battery)
E.g. If the cell supplies a potential difference of 3V
then V1 = 1.5V and V2 = 1.5V (assuming the bulbs
are identical)
Potential difference in a parallel circuit:
The reading at V1 is the same as the readings at V2
and V3.
V1 = V2 = V3
E.g. If the battery supplies a potential difference of
4.5V (read at V1), then the readings at V2 and V3
will both be 4.5V also.
Resistance
The wires and the other components in a circuit reduces the flow of charge through them. This is called
resistance. The unit of resistance is the ohm, and it has the symbol Ω (an uppercase Greek letter omega).
For example, a 2 Ω component has a greater resistance than a 1 Ω component, and will reduce the flow of
charge through it more effectively.
The resistance increases when you add more components in series. For example, the resistance of two
lamps is greater than the resistance of one lamp, so less current will flow through them.
The resistance of a circuit is a way of saying how easy or difficult it is for electricity to flow.
•
high resistance = hard for electricity to flow = small current
•
low resistance = easy for electricity to flow = large current
The units for resistance are ohms (Ω) and can be calculated using the following equation:
resistance (Ω) =
potential difference (V)
current (A)
For example, 3 A flows through a 12 V bulb. What is the resistance of the bulb?
Answer: resistance = 12 ÷ 3 = 4 Ω