Electric potential difference, current and resistance
5.1.1
5.1.2
5.1.3
5.1.4
5.1.5
5.1.6
5.1.7
5.1.8
5.1.9
5.1.10
What is electric potential difference?
What is the change in potential energy when
a) a proton moves between 5V and 25V
b) an alpha particle moves between earth and 500V
What is the electronvolt?
What can you say about the potential differences across X, Y and Z if
a) they all have the same resistance
b) Z has twice the resistance of X (X=Y)
What is electric current?
What is meant by resistance?
A cylindrical conductor of length l, diameter D and resistivity ρ has resistance R. A
different cylindrical conductor of resistivity 2 ρ, length 2l and diameter 2D has a
resistance. What is the resistance?
Two rectangular blocks, X and Y, of the same material have different dimensions but
the same overall resistance. What can you say about their resistivity?
What is Ohm's law?
List the differences between ohmic and non-ohmic behaviour.
A resistor of resistance 12 Ω is connected in series with a cell of negligible
internal resistance. The power dissipated in the resistor is P. The resistor is
replaced with a resistor of resistance 3.0 Ω. What is the power dissipated in this
resistor?
In the circuits below the cells have the same emf and zero internal resistance. The
resistors all have the same resistance.
a) what is the ratio of power dissipation X to Y?
b) what would the power dissipation be if circuit X had twice the emf of Y?
5.1.11
A resistor R and a filament lamp L are connected in series with a battery.
The battery has an emf of 12 V and internal resistance 4.0 Ω. The
potential difference across the filament of the lamp is 3.0 V and the
current in the filament is 0.25 A.
What is the power supplied by the battery?
What is the resistance of R?
A resistor of resistance 1.5 Ω is made from copper wire of radius 0.18 mm. The
resistivity of copper is 1.7 × 10–8 Ω m. Determine the length of copper wire used to
make the resistor.
Electric circuits
5.2.1
5.2.2
5.2.3
5.2.4
5.2.5
What is electromotive force (emf)?
Describe the concept of internal resistance.
Derive the equation for the total resistance of a circuit with
a) 3 resistors in series of value R
b) 3 resistors in parallel of value R
c) 2 parallel resistors of value R in series with 2 globes of resistance 2R each
Draw circuit diagrams for:
a) 3 globes and a battery but a switch that operates only one globe
b) a potential divider
c) measuring the VI characteristics of a thermistor
Describe the use of ideal ammeters and ideal voltmeters.
5.2.6
5.2.7
5.2.8
What is a potential divider.
Explain the use of sensors in potential divider circuits.
A particular filament lamp is rated at 12 V, 6.0 mA. It just lights when the
potential difference across the filament is 6.0 V. A student sets up an electric
circuit to measure the I-V characteristic of the filament lamp.
In the circuit, shown below, the student has connected the voltmeter and the
ammeter into the circuit incorrectly.
The battery has emf 12 V and negligible internal resistance. The ammeter has
negligible resistance and the resistance of the voltmeter is 100 k. The
maximum resistance of the variable resistor is 15.
a) can you move the slider to get the globe to light?
b) estimate the maximum current reading
c) what is the correct circuit?
Electromagnetic Induction
12.1.1
Describe the inducing of an emf.
12.1.2
Derive the formula for the emf induced in a straight conductor moving in a
magnetic field.
12.1.3
What is magnetic flux?
What is flux linkage?
12.1.4
Describe the production of an induced emf by a time-changing magnetic flux.
12.1.5
What is Faraday’s law?
What is Lenz Law?
12.1.6
In order to measure the rms value of an alternating current in a cable, a small
coil of wire is placed close to the cable.
The plane of the small coil is parallel to the direction of the cable. The
ends of the small coil are connected to a high resistance ac voltmeter.
Use Faraday’s law to explain why an emf is induced in the small coil.
A long solenoid is connected in series with a battery and a switch S. Several
loops of wire are wrapped around the solenoid close to its midpoint as
shown below.
The ends of the wire are connected to a high resistance voltmeter V that
has a centre zero scale (as shown in the inset diagram). The switch S is
closed and it is observed that the needle on V moves to the right and then
drops back to zero.
Describe and explain, the deflection on the voltmeter when the switch S is
re-opened.
Alternating current
12.2.1
Describe the emf induced in a coil rotating within a uniform magnetic field
12.2.2
Explain with diagrams how an a/c generator work?
12.2.3
Describe the effect on the induced emf of changing the generator frequency.
12.2.4
What is rms current?
What is rms voltage?
What is the relation between peak and rms values for sinusoidal currents and
voltages.
12.2.5
12.2.6
i) The rms current rating of an electric heater is 4 A. What direct current would
produce the same power dissipation in the electric heater?
ii) An AC generator produces a voltage of peak value V. The frequency of rotation
of the coil of the generator is doubled. What is the rms value of the voltage produced
12.2.7
A rectangular loop of conducting wire rotates in a region of magnetic field. The graph
shows the variation with time t of the induced emf in the loop during one cycle.
The resistance of the coil is 5.0 Ω. What is the average power dissipated in the
loop?
12.2.8
Describe the operation of an ideal transformer.
12.2.9
How can power losses in a transformer be reduced?
Transmission of electrical power
12.3.1
Why do power losses occur in transmission lines?
12.3.2
Why do we use high voltage step-up and down transformers?
12.3.3
12.3.4
12.3.5
In a particular power station the voltage generated is stepped up by a transformer.
The root mean square voltage is increased by a factor of 2 × 103. The output power
of the transformer is transmitted to a town by cables. Estimate the factor by which
the power losses are reduced.
Suggest how extra-low-frequency electromagnetic fields, such as those created by
electrical appliances and power lines, induce currents within a human body.
What are the health risks associated with living/working near high voltage power
lines?