p5 electric circuits electric charge – objects become charged when electrons are transferred to or...
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P5 Electric Circuits
Electric charge – objects become charged when electrons are transferred to or from them, for example, by rubbing
Two types of charge are positive and negative (these names are just labels)
Two objects with the same charge repel each otherTwo objects with different charges attract each other
+ metal ions
electrons
The electrons experience resistance when they flow through the metal.
The potential difference (voltage) provides energy which makes the electrons move through the metal ie it generates a current.
potential difference
The symbol for voltage is V
The symbol for current is I
The symbol for resistance is R
Metal wire Normally the free electrons in a metal move around slowly at random.
current =voltage
resistance
V
v1 v2
I
i1
i2
I = i1 = i2 The current is the same everywhere
V = v1 + v2
SERIES
The sum of the voltages across each component equals the supply voltage
R = r1 + r2Resistance
I
i1
i2
PARALLEL Current
I3
i4
i5
I = I3 I1 = I4 I2 = I5 I = I1 + I2
Current does not get used up
Total current = the sum of the currents through each component
V
v1
v2
V = v1 = v2 The voltage across each component is the same as the supply voltage.
PARALLEL
Voltage = energy per unit of charge
If more bulbs are added in parallel to a circuit then they will all be as bright as normal and more current is drawn from the power supply
The potential difference is largest across the component with the greatest resistance, because more energy is transferred by the charge flowing through a large resistance than through a small one
The current is smallest through the component with the largest resistance, because the same battery voltage causes more current through a smaller resistance than a bigger one
12 V
4 V 5 V
2A
i3
SERIES
v3
1 ohm 3 ohm r3
i3 = 2 A v3 = 3 V r3 = 2 ohm
12 V
v2
PARALLEL
1 A
1 A
i3
4 A
r3
i3 = 2 A
v2 = 12 V
12 ohm
12 ohm
Current is a flow of electrons
Electrons have charge (negative)
So current is a flow of charge
How do we quantify current ?
Current is the amount of charge flowing in a particular amount of time
Voltage provides energy to the electrons
Electrons have charge (negative)
So Voltage provides energy to the charge
How do we quantify voltage ?
Voltage is the amount of energy a particular amount of charge has
What about resistance ?
All components will offer resistance to a flow of electrons
How do we quantify resistance ?
If a current of 1A flows through a component when the voltage across it is 1V then the component is said to have a resistance of 1 ohm [ 1 W ]
IV
R=
Multiply both sides by R IV
R=R x x R
R I = V Or V = I R
Take V = I R and divide both sides by I V I R
I I=
V
I= R or
V
I=R
IV
R= V = I R
V
I=R
V
I R
Q. A current of 4 A flows through a circuit with resistance 3 . W What is the voltage ?
use V = I R V = 4 x 3 Voltage = 12
current = voltage / resistance voltage = current x resistance resistance = voltage / current
V
Q. A current of 5 A flows through a circuit with voltage 10 V. What is the resistance ?
V
I R
V
I=Ruse R =
10
5resistance = 2
Q. A circuit with voltage of 6 V has a resistance of 2 W . What current should flow ?
useV
R=I I =
6
2current = 3
W
A
Q. A current of 4 A flows through a circuit with voltage 12 V. What is the resistance ?
V
I R
V
I=Ruse R =
12
4resistance = 3
Q. A circuit with voltage of 8 V has a resistance of 2 W . What current should flow ?
useV
R=I I =
8
2current = 4
W
A
Q. A current of 60 A flows through a circuit with resistance 4 . W What is the voltage ?
use V = I R V = 60 x 4 Voltage = 240 V
IV
R= V = I R
V
I=R
Q. A current of 2 A flows through a circuit with voltage 16 V. What is the resistance ?
V
I R
V
I=Ruse R =
16
2resistance = 8
Q. A circuit with voltage of 230 V has a resistance of 5 W . What current should flow ?
useV
R=I I =
230
5current = 46
W
A
Q. A current of 25 A flows through a circuit with resistance 3 . W What is the voltage ?
use V = I R V = 25 x 3 Voltage = 75 V
IV
R= V = I R
V
I=R
The higher the temperature the lower the resistance
The greater the light intensity the lower the resistance
Variable resistorsResistors are used in circuits to control the sizeof the current
Two resistors in series have a larger resistance thanone on its own.Connecting two resistors in parallel makes a smallertotal resistance
Two resistors in series make a potential divider
Voltage (V)
Cur
rent
(A
)
Current through a filament bulb
Current is less here due to the extra resistance of the heating effect
Power = current X voltage(watt,W) (ampere, A) (volt, V)
If you know the power, it is easy to calculate how muchwork is done (or how much energy is transferred) in a given period of time:
Work done (or energy transferred) = power x time (joule, J) (watt, W) (second, s)
AC generator
DC generatorThe size of the induced voltage can be increased by:• increasing the speed of rotation of the magnet or electromagnet or coil;• increasing the strength of its magnetic field;• increasing the number of turns on the coil;• placing an iron core inside the coil
Generators produce a voltage by a process called electromagnetic induction
AC = alternating current
If a magnet is moving out of the coil, or the other pole of the magnet is moving into it, there is a voltage induced in the opposite direction
Vp / Vs = Np / Ns
Voltage across primary coil
Voltage across secondary coil
Number of turns primary coil
Number of turns secondary coil
=
8 turns 4 turns
Transformer
Rate/speed of rotation
Strength of magnet/ magnetic field
Number of turns/coils of wire
a.c / alternating current
Energy = power x time Power = energy / time
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30 ohm